ECCO-build

ECCO-build Contract N°ENK-CT-2002-00656

Report ECCO-Ingelux-200305-01 User assessment of visual comfort: Review of existing methods

Author(s): Christophe Marty / Marc Fontoynont / Jens Christoffersen / Marie-Claude Dubois / Jan Wienold and Werner Osterhaus

Date: 21 November 2003 Contractor: Ingélux Espace Carco- 1 Rue Francis Carco 69120 Vaulx-en-Velin France With DBUR and ISE

ECCO-build project User assessment of visual comfort: Review of existing methods

Contents 1 1.1 1.2 1.2.1 1.2.2 1.2.3 1.2.4 1.2.5 1.2.6 1.3

Summary evaluation of the literature review Offices, Visual Display Unit (VDU) Tasks and Daylighting Key Factors in Visual Comfort Assessment for Daylit Offices Glare Luminance, Distribution and Ratios Illuminance Level, Uniformity and Daylight Factor View Colour and Colour Rendering Visual Ergonomics Overview Table

2 2.1 2.2 2.2.1 2.2.2 2.2.3 2.2.4 2.2.5 2.2.6 2.2.7 2.2.8 2.3 2.3.1 2.3.2 2.3.3

Literature review 15 Introduction: Lighting Quality, Visual Comfort and Daylighting15 Visual Comfort 16 Vision 17 Visual field 17 Visual perception 19 Daylight utilisation and daylight quality 20 Definition of daylight quality 21 Direct glare: disability glare and discomfort glare 22 Discomfort glare indices 24 Summary on glare indices 35 Assessing user’s visual comfort in offices 37 Tools for in-situ methods 37 Tools for experimental methods 40 User assessment methods: Free choice /Discriminative choice / convergent algorithms 43 Performance indicators to assess visual comfort in offices 45 Main parameters influencing visual comfort in offices 45 Absolute work plane illuminance 48 Illuminance uniformity on the work plane 50 Absolute luminance of surfaces in the room 51 Maximum luminance values 52 Minimum luminance values 53 Luminance ratios 54 Influence of the veiling luminance on Visual DisplayTerminal56 Direct reflections on a flat panel screen 58 Influence of the seasonal and the temperature background 58 Final performance indicators to assess daylight quality 58 Discussion on performance indicators’ accuracy 59 User’s behaviour towards facade shading systems 60 General uses of shading system 60 Building configuration 62

2.4 2.4.1 2.4.2 2.4.3 2.4.4 2.4.5 2.4.6 2.4.7 2.4.8 2.4.9 2.4.10 2.4.11 2.4.12 2.5 2.5.1 2.5.2

Report ecco.031121.d1wp21.cm.review 05 November 2015

4 4 4 4 5 6 6 7 7 7

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2.5.3 2.5.4 2.5.5 2.5.6

2.5.7 2.6 2.6.1 2.6.2 2.6.3 2.6.4 2.6.5 2.6.6

Relation between irradiance , sun position, and user behaviour63 Outside illuminance thresholds; hysteresis 63 Inside luminance thresholds and ratios 64 Users preferred positions for Venetian blinds; colours of the slats 64 Office position towards window, seasonal influence 65 Other concerns 65 Medical issues on Macular degeneration ; glaucoma 65 Age factor 66 Eye colouring factor vs. age factor 67 Exposure of eye to UV and short-wavelength light (“BlueLight”) 68 Sensibility to glare coming from the upper / lower part of the visual field 69 Non visual field: influence of light on melatonin secretions, influence on the pineal gland 69

3 3.1 3.2

Literature survey to come: CIE Survey Others

70 70 70

4 4.1 4.2

Appendix Appendix A. Assessment of glare (Aizlewood, BRE, UK) Appendix B. Configuration factor for element parallel to rectangle

71 71

References

77

5

Signature and Name of the Author

Signature and Name of second person in the company

CHRISTOPHE MARTY

MARC FONTOYNONT

75

Note: Some paragraphs of this review are direct reproductions of scientific research documents. The authors have choosen to quote them because of their relevancy for this literature review. Quotations are done with agreement of the original authors.

Beginning of Quotations will be indicated by an italic paragraph with the document references. End of Quotations will be indicated by an italic paragraph “end of quotation”


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1 Summary evaluation of the literature review 1.1 Offices, Visual Display Unit (VDU) Tasks and Daylighting The increasing use of digital technologies in offices and industrial settings can create considerable challenges for workers in receiving, processing and transferring information in short periods of time. Eye movements between VDU, keyboard and manuscript can occur up to 30,000 times per day [KGS 2001]. Often, a multitude of other visual tasks has to be completed in addition to the VDU work, likely requiring different lighting criteria. To function properly and maintain a focussed image, the worker’s eyes have to adapt again and again to changes in brightness (luminance), contrast and distance to a visual object. If visual and ergonomic conditions are inappropriate, visual tasks, especially VDU work, can quickly lead to complaints about difficulty focussing, double images, glare, or headaches. Muscular pain can add to these complaints when workers attempt to shift their posture in order to avoid perceived discomfort, eg reflections of a light source in the VDU screen. An appropriate visual environment is therefore of utmost importance. For a variety of reasons, including health and well-being, energy conservation and general worker preference, daylighting is again receiving increased attention from building owners, users and managers. However, daylight, because of its variability and intensity, poses additional challenges, and needs to be carefully considered to realise its potential in providing healthy and comfortable office environments, while at the same time decreasing the need for electrical lighting or heating/cooling energy. 1.2 Key Factors in Visual Comfort Assessment for Daylit Offices The critical factors affecting the level of visual comfort and quality in daylit office spaces include glare, room surface luminances and luminance ratios, illuminance and its uniformity across the space, colour and colour rendering, daylight factors, view through the window or daylight opening to the outdoor environment, the overall design of the daylit office in response to those factors and general visual ergonomics. These factors will form the foundation for subsequent user behaviour studies and the photometric characterisation of the luminous environment in the test offices used in this research project. 1.2.1 Glare A number of daylight glare prediction models exist, including the Daylight Glare Index, the New Daylight Glare Index, the Visual Comfort Evaluation Method, and the vertical illuminance at the eye. The Daylight Glare Index remains the most widely used despite its accepted limitations [Wilks and Osterhaus 2003, Velds 2001].


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Particular concerns also exist about the treatment of source and background luminance relationships in the Daylight Glare Index. In practical terms, this leads to the overestimation of the impact of the background luminance in scenes with large glare sources covering much of the observer’s visual field. Because of the glare / background relationship treatment, current glare prediction models cannot properly account for the adaptation of the human eye to the dynamics of daylighting conditions. None of the existing glare prediction models currently include provisions for the influence of work tasks on glare perception. Research suggests that it is critical to assess glare and other visual discomfort phenomena under relevant work task conditions. It is not sufficient that observers assess glare by simply looking at a potential glare source. A new technical committee (TC 3-39) of the International Commission on Illumination (CIE) is charged with advancing the knowledge about discomfort glare from daylight. It would be advantageous to develop a glare index applicable to both daylight and electric lighting applications, including the treatment of complex luminaire and fenestration systems (eg Venetian blinds). 1.2.2 Luminance, Distribution and Ratios The only visually perceptible unit of photometric measurement is luminance. When surfaces with large differences in luminance occur side-by-side, as is often the case in daylit environments, our eyes might have difficulty adapting to both luminances at the same time, leading to possible visual discomfort and a potential reduction in visual performance. Shading devices, if appropriately selected and controlled, can significantly reduce luminance differences. While it would make sense to use luminance as the basis for lighting recommendations or code requirements and their assessment in terms of lighting quality, its dependence on observer position and daylight variability makes it difficult to judge compliance with a simple set of numbers. Luminance and luminance ratios (and contrast) can perhaps be seen as a subset of glare, but they have implications beyond glare. The likely impact of a particular luminance ratio between surfaces is judged by whether or not it exceeds a recommended maximum [van Ooyen et al 1987]. Little research has been conducted specifically for daylit interiors. However, surveys appear to indicate that ratios of up to 1000:1 are frequently tolerated in daylit offices if views and other amenities compensate for possible glare experiences [Osterhaus 2001]. Veiling luminances and reflections, particularly on the VDU monitor screen, are an important factor in office environments and can contribute to much user dissatisfaction and discomfort. Flat panel displays with matte screen surfaces and light coloured backgrounds generally pose significantly lower risks for both concerns. We anticipate that this project will provide new insights into the impact of luminance distributions on visual comfort and user acceptance of daylighting systems. Luminances will be recorded using CCD camera-based luminance mapping technology. It is in our view critical to assess limits of acceptance


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for luminances and their ratios within the field of view for different sky and solar shading control conditions. 1.2.3 Illuminance Level, Uniformity and Daylight Factor While varying between countries, there are accepted standards and recommendations for minimum maintained illuminance levels. In those countries also requiring workplaces to be supplied with daylight and a view out, well-designed daylit offices will typically exceed the minimum maintained illuminance levels suggested. For others there will be a top-up with artificial light if some areas fall below the recommendations. To ensure the illuminance on the working plane is sufficiently uniform to avoid high contrast, a simple guide has been suggested – the average daylight factor in the front half of the room should not exceed three times that at the back of the room. Across desks in an open plan office, an illuminance uniformity ratio of 0.6 or better appears to be acceptable [Littlefair 1996]. It has been recommended that the daylight factor should exceed 1% in the worst lit working plane location of an office [Slater et al 1993]. 1.2.4 View It appears to be generally accepted that having a view is central to having a pleasant visual environment. A numerical assessment of the quality of the view, however, is difficult. Research [Chauvel et al 1982, Osterhaus, 2001] suggests that a greater view complexity will likely reduce the sensitivity of the observer to discomfort glare as he/she displays greater tolerance to glare when compensation in the form of a view is present. This could possibly be studied via an assessment of view complexity, which provides details on the information and enjoyment content of the observer’s view. It seems important to identify how many ‘layers’ of view are visible through the window and to determine the size of the window as a percentage of the window wall. Information might also be needed about whether the view content is relatively static or continuously changing, and whether the view content is likely to be enjoyable. Unclear is also whether and how much complex shading systems such as Venetian blinds affect users’ perceptions of view and other amenities as balancing factors for visual discomfort. We suggest that a good and stimulating view might, for example, reduce the discomfort glare rating by several steps on a glare index scale, although the magnitude of such potential reduction is still to be confirmed by appropriate experimental research outside of this project.


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1.2.5 Colour and Colour Rendering Yano et al [2001] suggest that observers are possibly more sensitive to discomfort glare from light sources with a high correlated colour temperature. Others [Chain et al 1999 and 1998] found that under uniform sky conditions, the Correlated Colour Temperature (CCT) was constant. This could suggest that colour temperature might not be a variable in glare assessments under overcast sky conditions. For clear sky conditions the CCT decreases with increasing luminance. This suggests that for the same average sky luminances the perceived degree of discomfort glare might be lower under clear sky conditions than under overcast sky conditions. It is similarly possible that large differences between the CCTs of the overall visual field and a glare source will increase the likelihood of glare perception and visual discomfort. This might also be the case when coloured Venetian blinds are introduced into an environment. For the time being, coloured Venetian blinds have been excluded from this study. 1.2.6 Visual Ergonomics There are various reference sources for appropriate ergonomic treatment of office spaces, eg [KGS 2001, Anshell 1998]. Typically, such sources include recommendations about furniture selection and adjustment for proper posture and minimal strain. On the visual side of ergonomics, critical factors include the placement of VDU screens and other surfaces on which visual tasks are performed in relation to daylight and electric light sources and the physical attributes and limitations of our visual system in the perception and analysis of stimuli. 1.3 Overview Table We attempt to display the factors considered in this literature review and our judgement of their relative and absolute importance in Table 1. From the review, we have derived a number of questions or issues to be addressed in the user assessment surveys and corresponding photometric characterisation of the daylit office spaces and solar shading control options studied as part of this research project. In selecting the measurement parameters and questionnaire components, we have focused on wellestablished and internationally recognised methods as much as possible and where possible. Potential measurement or assessments methods for new or relatively unknown relationships between user behaviour and photometric parameters have been selected on the basis of relevant existing research in closely related fields or subject areas. We have also listed some aspects that appear irrelevant or outside the scope of this project, but could affect other future studies in the field of visual comfort. Detailed explanations for the various factors can be found in the main body of this report.


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Table 1 : Overview table Glare sensation: Important aspect / hypothesis / property

What we believe to be known

Referen ce [name/ year /page]

To be

Important questions / issues

considered in WP2: y/n controller: WP4: y/n planning tool: WP5: y/n

Working on paper

 400 lx preferred on the paper, 100cd/m²

Working on modern VDU screen (TFT)

 300 lx preferred on the desktop,  from 200 to 500lx globally when windowless  minimum of 100lx  less than 300 lx

Berrutto 2003

To be considered / assessed in WP2: y/n controller: WP4: y/n planning tool: WP5: y/n

What we believe to be irrelevant or outside the scope of this project

Referen ce [name/ year / page]

To be considered in WP2: y/n controller: WP4: y/n planning tool: WP5: y/n

WP2 : y WP4: y WP5: y

Preferred values

Absolute values for luminan ce and illuminan ce in the field of view (not conside ring veiling reflectio ns)

Referen ce [name/ year/ page]

Berrutto 2003,

WP2 : y

Veitch et Al, 2001

WP5: y

WP4: y

 The illuminance next to the TFT will probably have an important role for the controller

Nutek, 1994 Escuyer, 2001

Having a meeting

 500lx preferred on the table

Walls

 140 cd/m² max for meetings  minimum of 30cd/m²

Berrutto WP2 : y 2003 Loe, WP4: y 1994

Ceiling

 Maximum of 850cd/m² (10times average VDT screen luminance, higher ceiling luminance for greater screen luminance)

Osterha us, 2002

Berrutto 2003


WP5: y



 The maximum ceiling luminance could also play a role in the controler

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To be


considered in WP2: y/n controller: WP4: y/n


planning tool: WP5: y/n

Window

Discomfort glare

Disability glare

 Maximum of 1800cd/m² tolerated, 1500 cd/m² accepted. But it is unsure if we are talking about maximum or average values. (average across large glare source configuration covering majority of visual field)

Velds, 1999 Osterha us, 2001


For interior lighting:  500 cd/m² in foveal region  1000 cd/m² in peripheral region

Nutek 1994

WP2 : y

CIBSE 1994

WP5: y

For interior lighting:  1000 cd/m² in foveal region  2000 cd/m² in peripheral region  Blackwell visual performance levels

Nutek 1994

WP2 : y

CIBSE 1994 Sutter 2003

WP5: y


WP4: y

WP4: y


 To be assessed: what is most important: maximum luminances (peaks), or mean luminance? – importance of the peaks in the shading systems?  Probable difference of acceptance between different positions of the observer: near and far from the window  The size of the window (solid angle) could have a major role on the luminance acceptance – see study of Mc Gowan  When more than one window in the same office : probable difference of acceptance of each window  Different roles of the window: Performance (illuminances...) Information (weather, time) View to the outside  Interaction between the window and the DVU position  Major role of the luminance of adaptation and of the veiling luminance (see VDU veiling luminance)  Difference of glare acceptance in case of daylighting (see luminance ratios)





Nelson, Mc Gowan, Royal society, April 2002


 Role of the specular properties of materials : glossy elements can cause disability glare (relevant for specular Venetian blinds).

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To be considered in WP2: y/n controller: WP4: y/n




considered / assessed in WP2: y/n

WP2 : y

 Min to max luminance ratio in the field of view : 10 to 50 or 1 to 20  Emin/Eaverage>0,8 (illuminance)  Emin /Emax>0,7 to 0,5 (illuminance)  Max luminance of 3:1 or 1 :3 between paper and adjacent VDT, or between task and adjacent surrounding  Max luminance of 10:1 or 1:10 between task and remote surfaces  Max luminance of 40:1 or 1:40 between points everywhere in the field of view  Usual accepted ratios of maximum luminances for artificial lighting between work area; task surrounding; remote elements: 10:3:1  Minimum uniformity on workplane: 0.6  Daylight Factor ratio between half room near the window and half room away from the window: less than 3  Daylight Factor should always be >1%

Loe et al 1994

WP2 : y

CIE 86, CIBSE 94

WP5: y

Discomfort  Low or no reflection asked on VDU glare  Objects that can be reflected should have an average luminance below 1000cd/m² for class I and II screens, and below 200cd/m² for class III  For screens providing 85cd/m², no surfaces within the peripheral view should exceed 850cd/m²

Veitch et al 1995

WP2 : y

WP2 : y

WP4: y

WP4: y

ISO 2000

WP5: y

WP5: y




Berruto 2003; Veitch 2000



controller: WP4: y/n

 Luminance of the walls equivalent to the working tool (in particular VDU)  Luminance of the surrounding walls equivalent to the one of the people faces

Luminance Walls ratios in the field of view [mean luminance of workplace in General ratios relation to max./mean luminance of surrounding s]

Veiling reflections on modern VDU screens (TFT)

To be

WP4: y WP5: y

WP4: y

Osterha us 2002 Littlefair, 1996 Slater, 1993

Sutter  Higher acceptance of luminances 2003 when space daylit. Usual ratios between work area : task surrounding : remote elements could become 20:6:1 - This point could be assessed in WP2  To be assessed: what is to be considered in the ratios: maximum luminances (peaks), or mean luminances? – importance of the peaks in the shading systems  A new Glare index should be established to include better treatment of glare source / background luminance relationship, daylight acceptance and dynamic adaptation of the eye.


 The existing Glare indices seem to be irrelevant for the Ecco-Build project because none of them include the dynamic adaptation of the eye, and daylight is not yet completely integrated. -The most useful one could be the Daylight Glare Index (DGI)


IES 1993

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To be





Disability glare

Position in the room

Single office

 Veiling luminance should stay below 10cd/m² for normal screen, and below 20cd/m² for good screens

Sutter 2003

Preferred positions:  Line of sight of the user parallel to the window  Seat near the window

Osterha us 2002

WP2 : y

Christoff ersen et al 1999

WP5: y

More than one workplace per room Threshold s for user reaction to

Manual control


WP4: n


 Wide inertia of the use of blinds : blinds can stay in the same position for weeks


WP2 : n WP4: n WP5: n


 The acceptance of the veiling luminance depends on the screen quality (can be five times more for high quality screen)  With today’s standard screens the veiling luminance should be the most selective criteria as it leads to disability glare (elements around VDU should be below 300cd/m²) With TFT screens and screens with high luminance, the veiling luminance could become a less selective criteria (acceptance of elements above 1000cd/m² around the screen).  The reflection of sun patches on walls in the screen can provoke a reaction of the user to close blinds. This disability glare could be mainly related to the sun position.azimuth, elevation  When more than one window for a single office: different use of the blinds for each window




Sutter 2003


 This configuration Is an issue for the ecco-build controller : will the controller adapt itself to a group of persons?  Wide differences of uses between individuals  Higher frequency of use of the


WP2 : n WP4: ? WP5: ?

Rienhart 2002

WP2 : y

Sutter

WP5: y

WP4: y

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To be





blinds Motorised (immediat blind e action/long term action)

Automatic control

Correlatio n of multiple occurrenc e of short term glaring situations and user satisfactio n

One day

 Closing the blinds: 37klx to 49klx  Re-opening the blinds: 18klx to 28klx  Existence of an hysteresis : Difference of thresholds: 18klx to 24klx  Slats positions: o totally up 18% o horizontal : 20% o protecting from the sun : 51% o intermediate height : 6.5%

Reinhart 2002

WP2 : y

Sutter 2003

WP5: y

 Rejection by the user of 45% of the actions of today’s controllers

Reinhart 2002, Iwata – Tokura 2002

WP2 : y

Velds 1999

WP2 : y

Sutter, 2003

WP5: y

 Reaction of the user within 30 to 60 minutes when window luminance exceeds 1800cd/m²  75% of the users maintain the luminance of the window under 1800cd/m²

WP4: y


controller: WP4: y/n


motorised blinds (3 times more than with manual blinds) with an average of 2,1 times per day  Colour of the slats: dark slats can give an impression of gloom  The view to the outside is a major issue


2003

WP5: y

WP4: y

Laurenti n 2001

WP2 : y

Escuyer 2001

WP5: y

Sutter 2003





Duration of time for which noticeable discomfort glare can be tolerated before the user takes action (20minutes or more?)

 Long-term evaluation of the benefits generated by the controller, in easily understandable units (e.g: hours of glare avoidance), over one year (1600h of work ) for instance.  More artificial light added by the user in winter than in summer, for identical daylight illumination levels (+200lx)  Higher levels of artificial light asked in winter and spring (300-350lx ) than in summer and autumn (200-250lx)  The temperature seems to have a clear influence when it is outside the normal comfort range: if T>26° in summer, the average slat angle of the blinds is

considered / assessed in WP2: y/n

WP4: y

In the long run

Influence of season and temperature on glare sensation (and other aspects of visual comfort)

To be

WP4: y

 Should the controller be configured differently in Northern and Southern countries?




 No clear relation have been shown between temperature and user preferences, unless temperature is outside of normal operating / comfort conditions

Nicol 2001


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To be









accentuated by a 10° (see fig. 2) and the thresholds for lowering and putting up again the blinds are then changed Correlation between acceptable luminance and visual contact with the exterior

Individual Drugs factors (alcohol) with impact on glare sensation Age

 View to the outside : users can be dissatisfied by the lighting conditions because of insufficient shading or limitation of the view.  Information (weather, time) : Users can prefer having sunshine entering their office sometimes during the year.  Well-being : the occupants of offices receives only 0,5 to 3% of the daylight received by someone working outside  Relation between melatonin (sleeping hormone) secretion and light, in some precise wavelength (480 nm)  500lx with classical fluorescent lamp seems to be insufficient to stop melatonin secretion

Christoff ersen et al 1999


Rea 2002 Lewy et al 1980 Rangi, Osterha us 1999

 Antidepressant pills, for example, provoke pupil dilation, and thus increases glare perception

 What is the slat angle corresponding to the minimum acceptable view to the outside?  Will windows shaded with Venetian blinds and unshaded windows of the same average luminance result in the same glare assessment, or does the shaded window receive a different rating due to ist partial obstruction of the view?


WP2 : n WP4: n WP5: ny check: y

 The influence of the age on light preferences and glare acceptance is important: the veiling luminance perceived is twice as important, and the critical contrast is three times as important between a 25 yearsold person and a 70 year old one.

Mood

CIE 1999

WP2 : y WP4: n WP5: n check: y



 Should the tests evaluate a category of age, or should they take a representative panel of users?

 A high motivation (high interest in scientific research projects for instance) tends to decrease the glare perception.

WP2 : y WP4: n WP5: n

WP2 : y WP4 : n WP5 : n check : y

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To be





Type of work

Colour of the eye

 Subjects generally report a higher incidence of glare for VDU-based tasks than for paper-based tasks which appears to be related to the vertical work plane of the VDU and the vertical surface of the potential daylight glare source

Osterha us 2001

 The acceptance threshold of the veiling luminance increases (+25%) from a person with light coloured eyes to a person with (dark) brown eyes.  No statistically significant differences were found in the glare assessments of subjects with light and dark coloured eyes in laboratory experiments and building user surveys.

CIE 1999






WP2 : y WP4: n WP5: y check: y

Osterha us 2001

Glasses/ contact lenses

WP2 : y WP4: n WP5: y check: y

Eyes irritated for any reason Eye diseases

Personal sensitivity without clear relation one of the above mentioned factors

WP2 : y WP4: n

 Eye irritation might increase glare perception

WP5: n check: y

 Macular degeneration, glaucoma drastically increase the perception of glare and change the lighting preferences.

Nelson et al 2002

 Wide differences of light preferences between individuals : photophile persons / photophobic persons.  On a same façade, people may or even may not activate their blinds.

Sutter 2003

WP2 : y

Lindsay et al 1993

WP5: y




WP4: y

 Users might have a higher sensitivity to glare coming from the lower part of the visual field (but no dedicated study yet)


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2 Literature review 2.1 Introduction: Lighting Quality, Visual Comfort and Daylighting Experts agree that there is currently no commonly accepted metric of lighting quality that predicts the effects of the luminous environment on building occupants [Veitch and Newsham 1995]. Miller [1998] appropriately describes the current approach to lighting quality as a “recipe”, implying that many ingredients contribute, and provides a list of those ingredients. Recommendations for the assessment of individual lighting quality components (eg discomfort glare, luminance distributions, etc) published in lighting handbooks, standards and recommended practices predominantly address physiological factors, those that directly affect how we see. In addition, indirect or psychological effects occur because lighting can affect attention, motivation and behaviour. While researchers are aware of these indirect effects, they have not been studied extensively and are at an early state of development. Furthermore, researchers are not yet sufficiently clear about how to use new research in a practical way in the design and analysis of lighting applications [Kanaya, 1998]. For the time being, most known physiological factors continue to be treated in isolation Miller believes that “quality” is in the eye of the beholder and that there is no single number to sufficiently describe it. She suggests that with dedicated research, numbers could perhaps be applied to some of the factors such as visual comfort, but doubts whether it would be worth the effort attempting to quantify other factors, eg visual interest. Daylighting is acknowledged as providing many benefits ranging from improvements in people’s health and well-being to increasing lighting quality [Rangi and Osterhaus 1999]. Many of the available methods for lighting quality and comfort assessment have been developed predominantly through controlled and electrically lit laboratory studies that do not necessarily account for the experiences reported by occupants of real daylit spaces. For example, discomfort glare from daylight appears to be tolerated to a much higher degree than predicted by available assessment methods if a pleasant view is provided by the window causing the discomfort glare [Chauvel et al 1982, Osterhaus 2001]. Daylighting quality has been described in the literature through “guidance on the craft aspects of design based on experience” or by recommending a series of “visual comfort criteria based on numerical calculations” such as luminance or illuminance ratios and glare indices [Papairi et al 2002]. To adequately address lighting quality and visual comfort in daylit buildings, more specific assessment tools, based on both photometric data and user responses are required. While we agree with Miller that a single numerical criterion for lighting quality cannot be achieved, we believe that it is possible to establish sufficient sets of criteria that can be appropriately combined to successfully address a specific lighting problem.


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2.2 Visual Comfort Some of the qualitative aspects of human requirements for the interior environment are often expressed through visual and thermal "comfort" criteria. At first glance, one would expect significant overlap in the definition and description of visual and thermal comfort parameters. Unfortunately, that is not the case. Indeedthere are few parallels between these two comfort criteria. An optimal thermal condition can be described as the neutral perception of the interior environment, where occupants do not feel any need for changes towards warmer or colder conditions. Visual comfort, on the other hand, is a more complex parameter related to receiving and processeing messages, instead of referring to a state of neutral perception of the environment (Baker 1993). The main difference between the two comfort criteria is that visual comfort can always be improved, unlike the optimal thermal condition of the neutral perception. Visual "comfort" must therefore be interpreted as that state in which the clear and unobstructed reception of visual messages from the visual environment is possible without affecting the person’s well-being or health. Hopkinson, a prominent lighting researcher, recognised the complex nature of the visual environment and suggested the following description for visual comfort: "The term visual comfort is

taken to mean the absence of a sensation of physiological pain, irritation or distraction. It is not intended to cover the aesthetic sensation of pleasure or dislike of the surroundings" (Hopkinson 1963a).

While the definition of visual comfort is relatively limited in this description, lighting practitioners and researcher know all too well, that visual comfort nevertheless presents huge challenges. The design of a comfortable and delightful visual environment depends on vision, perception and what we want to see in different room configurations and for different activities. An observer will receive visual information and perceive the interior in relation to the bounding surfaces subjected by their colour, texture and brightness, the furniture and its arrangement, and interaction between the interior and exterior environment together with a host of other details (Boyce 1981). The stimulus of the information received, and its interpretation or impression of the interior, is regarded as the visual environment (Canter 1975). Increased natural illuminance levels adequate for the intended task were usually one of the earlier solutions for improving visual comfort. However, this could lead to a false assumption revealed in a lack of understanding or knowledge of the function of the human vision and the complex relationship between human requirements and visual perception of the luminous environment. Therefore, "daylighting design" must emphasise the importance of daylight integration, not only with electric lighting, but with the complete window system as part of the building envelope. The building design must therefore be co-ordinated, using adequate design tools, to produce an efficient and aesthetically satisfying interior, without ignoring other aspects of the environment affecting human comfort. It is difficult to judge the quantity of light, so lighting design must be based on what one can perceive and what one wants to look at - the quality of the luminous environment (Lam 1977). Unfortunately, most design tools are predominantly concerned with the physical values or ratios using a variety of approaches, and practically none of these are associated with the design of the qualitative aspects of the visual environment. On the other hand,


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qualitative preferences and descriptors need to be expressed through those measurable photometric relationships that are likely to lead to a high-quality environment. How else could we provide guidance to designers, users and manufacturers of lighting and control equipment? 2.2.1 Vision Vision, as a perceptual system, is the eye's ability to sense and process the visible light admitted through the pupil. Admitted light rays are converted into electrical signals and processed by the brain to provide visual information of light, colours and shapes etc. The eye receives from an object form an inverted image at the light sensitive receptors in the retina. To protect the sensitive receptors and avoid excessively bright light striking the retina, the iris reduces the diameter of the pupil. Accommodation is the eye's ability to change the shape of the lens to focus light on the retina from near to distant vision (Baker 1993). The retina contains light sensitive photoreceptors, cones and rods, where cones are predominantly located at the centre while rods are more evenly distributed. Nerve cells transmit the signals received through the optic nerve from the stimulation of the photoreceptors. At the retina, cones contain colour sensitive pigments and perform the function of colour perception and accurate vision at normal daytime light levels(photopic vision). The eye's ability to see at low levels of light (scotopic vision) is provided by the rods, since rods are highly light sensitive, but not colour sensitive (Hopkinson 1969, CIE 1987, G. Christoffersen 1993 a-b). In the solar spectrum, only wavelengths between 380 nm to 780 nm cause a visual sensation that depends on the amount of radiant energy received by the retina. However, the eye is not equally sensitive to all radiation within the visible band. The relative spectral response is defined by the CIE standard observer as an ideal observer having a relative spectral response curve that conforms to the V(λ) function for photopic vision and to the V'(λ) function for scotopic vision, and that complies with the summation law implied in the definition of luminous flux (CIE 1987). Photopic vision, illustrated by V(λ) curve, peaks for the light-adapted eye (> 2-3 cd/m2) at 555 nm, which is the green-yellow region in terms of perceived colour. Low illuminance levels (night vision), cause the eye's sensitivity curve to be preferentially more sensitive to shorter wavelengths in terms of perceived colour (Purkinje phenomenon). Scotopic vision, displayed as the V'(λ), peak for the darkadapted eye (less than 10-2 cd/m2) at 507 nm because of the rod-dominated vision (i.e. red colours are perceived as dark). Intermediate vision between photopic and scotopic is called mesopic vision. 2.2.2 Visual field The extent of the visual field seen by a person when looking straight ahead must be divided into monocular and binocular portions. The monocular field is generally considered to extend approximately 90° temporally, 60° nasally (depending on the prominence of the nose), 70° inferiorly (restricted by the cheek), and 50 to 60° superiorly (restricted by the brow). The monocular visual fields overlap to form a combined binocular field, the central 120° of which is seen by both eyes (IES, 1993). According to Loe, Mansfield and Rowlands (1994), even if the visual field is almost 180° horizontally and 120° vertically, the part which has a significance for visual comfort evaluations is a


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band of 40° centred at normal eye height. It can be noticed that there are usually no restrictions in the horizontal direction, resulting in an almost panoramic view of roughly 180° view angle (Fry 1969, Robbins 1986, Lechner 1992). The field of vision can also be separated into central (foveal) vision and peripheral (foveal surround) vision. In a 2° cone around the centre of the retina, foveal vision endows the human eye with awareness and focus. In addition,information on details and colours is provided by the cones which are found in high density in the fovea. Peripheral vision (30° cone) provides quite high awareness and discrimination of brightness differences between an object and its background or foreground through the rodsr (Liljefors 1987). Fry tended to separate in the visual field a 2° cone in which we can analyse details, representing the task field, and a 180° cone (reduced by the 2° cone) representing the veiling luminance (here, the Fry luminance). The global luminance seen by the eye is called adaptative luminance : Ladapt= L2° + L Fry 90 360

Lumcos()sin() d d Fry (1953)  ( 1/38,2)  0

  

Lfry0,00396

1

Lum : luminance of the point spotted with λ and Φ (see figure 1), in cd/m². Φ : anglular height of spotted zone in reference with the line of sight, in degrees. λ : in the plan perpendicular to the line of sight, angle between the vertical and the spotted point.

Figure 1 : Fry integral

However, the Fry luminance model will not be possible to adapt to the Ecco-Build issue, as it does not include eye-movements

Quotation from Dubois, M.-C. (2001). Impact of Shading Devices on Daylight Quality in Offices: Simulations with Radiance. Report TABK--01/3062. Lund Institute of Technology, Lund, Sweden. ): Carter et al. (1994) demonstrated the importance of the surfaces in front of the subject, compared with surfaces to either side when assessing room brightness. The most important factors appeared to be the luminance of the walls, particularly those forming the background to the tasks as perceived by the subjects, which determined the relative brightness of the tasks. In general, ratings of brightness increased as the average luminance within a 40°-wide horizontal band centred about the eye increased.However, there is some evidence that surfaces in front of the subject had a greater


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influence than surfaces to the side, and also that dark room surfaces, notably ceilings, outside the 40° band adversely influenced the assessment of brightness. In a publication about shading screens, Fontoynont (2000) considers that a central cone of vision of 30° is significant for visual comfort. Other authors (Paule, 2000; Meyer, Francioli and Kerkhoven, 1996) consider that the visual field consists of two main parts: the ergorama and the panorama. The ergorama is a cone of 60°, centred about the line of sight while the panorama is a cone of 120-140° centred about the line of sight. According to Meyer, Francioliand Kerkhoven (1996), maximum luminance ratios of 1:3 in the ergorama and 1:10 in the panorama should be respected. 2.2.3 Visual perception Visual perception is an active, information-seeking process, partly conscious and partly unconscious, involving many mechanisms in a cognitive process interpreted by the eye and the brain. Lam described visual perception as a meaningful impression obtained through the senses and apprehended by the mind which involves the combination of incoming sensations with contextual information and past experience so that the object or events from which the stimuli arise are recognised and assigned a meaning (Lam 1977). The visually perceived information from the luminous environment, illuminated by natural and/or artificial lighting, is interpreted by a combination of incoming sensations with contextual information of brightness, colour, distance, size, movement, perspective, etc. Any interference in the pleasantness of the perceived information is considered to be visual noise, which is an undesirable or disagreeable stimulus confusing, obscuring or competing with relevant, desirable or needed sensory information (Lam 1977 and 1986, Lynes 1978, Boyce 1981). The brain's perceptual psychology enables an associative translation ability, constancies, where changes in the visual environment are perceived unchanged. This permits objects, especially colour, size and shapes etc., to be seen and experienced similarly under different conditions even if the incident light is changed. Shape constancy is the brain's ability to recognise the shape of an object or pattern even if viewpoint, illumination, or distance is changed. The optical size of an object is measured by the solid angle from the eye subtended by the object of interest. Size constancy estimates the perceived object correlated by the immediate surroundings in which they are seen. A visual environment may appear to be of constant brightness since the brain makes adjustments to what the eyes see and compares the perceived information of an object with the immediate brightness of the surroundings. Brightness constancy is the ability to ignore differences in luminances under certain conditions. Colour constancy is the ability to eliminate the differences in colour due to variation of the incident light. If more than one type of light source is used in the visual environment, colour constancy is not possible, since the brain does not adjust to the colour balance of each source simultaneously (Lam 1977, Corth 1987). Attractive brilliance is described as sparkle, but the brightness may interfere with the perception of other objects in the visual environment. The bright element can be evaluated as sparkling, romantic and desirable in one situation (candle, exterior view etc.) and glaring in a different situation, if the same element is evaluated to cause visual noise. These diverse, perceived effects illustrate the complex


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nature of psychological preferences combined with relevant physical variables, responded to and reacted upon, by the observer in the visual environment. A window design fulfilling human physical needs and psychological preferences depends on the correlation between the physical, psychophysical and the psychological component (MacGowan 1980). The physical component affects the physical indoor environment containing the window dimensions and location, the interior illuminance levels provided by natural and/or artificial light, the optical and thermal properties of the glazing etc. Psychophysics is the assessment of corresponding psychological magnitudes described by human observers - sensation - with the measured physical component - stimulus (Tiller 1990). Glare illustrates the magnitude of visible noise, interfering with the perception of visual information in the luminous environment, caused directly by an uncomfortably bright source of light or by reflections of the source in the line of sight. The discomfort glare index represents the concept of psychophysics combining physical values of the sky and interior luminances, the solid angle subtended by the glare source, manifesting the experienced psychological sensation. Interior visual discomfort caused by daylight, as a result of the luminance distribution, may make the window appear as "too bright", and the area at the back of the room as "gloomy". These visual descriptive impressions illustrate the complexity of the components influencing the visual perception of the luminous environment and the psychological preferences for view, daylight, sunlight, privacy, colour quality, geometry of the incident light, etc. 2.2.4 Daylight utilisation and daylight quality The main functions of a window are to allow for an appropriate interrelationship between the exterior and the interior, to provide adequate interior natural lighting levels, natural ventilation, acoustic interchange and protection from the thermal exterior climate all year round. Use of natural light has a major interest because of its aesthetic possibilities and its ability to satisfy biological needs and to address ecological concerns. Although daylight penetrating the thermal envelope, as a "free" natural resource, has an undisputed positive psychological impact on the occupants, the natural light is simultaneously associated with unavoidable side effects including the risk of overheating and glare. Most of the research in the complex nature of visual comfort has been conducted with respect to commercial buildings because of its potential applicability to a number of similar office configurations. Conscious design for use of daylight in the interior affects the shape and structure of the building, since the occupants typically have relatively static working positions with restricted individual movements (often due to the use of VDUs) and long working hours. Especially in the last decade, a renewed interest has emphasised the environmental and global issues, recognising that intelligent use of natural light and artificial lighting control strategies can contribute significantly to energy efficiency of buildings. The latest technology makes additional changes to the window envelope by integrating new daylight strategies, "enhancing" daylight penetration to improve the luminance distribution in the interior. However, little research has been conducted with the intention of acquiring a more profound understanding of the behaviour of natural light in the interior environment (Littlefair 1988 and 1990, Aizlewood 1993). Even fewer have evaluated the interior qualities and consequences of introducing "new" technologies, aiming at increased utilisation of daylight by manipulating the optical properties of the fenestration elements.


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2.2.5 Definition of daylight quality A comprehensive research programme was initiated a few years ago at the National Research Council (NRC) of Canada with the aim of defining lighting quality. This research, which focused on artificial lighting systems, can be used as a background for a discussion about daylight quality as well. Many articles have been produced from this research (Veitch and Newsham, 1995, 1996; Veitch, 2000; Tiller and Veitch, 1995, etc.). In one of these articles (Veitch and Newsham, 1995), the authors assert that lighting quality is the success or failure of a lighting design to meet the needs of end users. According to them, lighting quality exists when a lighting system: Creates good condition for seeing Supports task performance or setting Fosters desirable interaction and communication Contributes to situationallyappropriate mood Provides good conditions for health and avoids ill-effects Contributes to the aesthetic appreciation of the space Veitch and Newsham (1995) claim that lighting quality is not directly measurable but is an emergent state created by the interplay of the lit environment and the person in that environment. In the language of the environmental psychologist, lighting quality is a construct i.e. an intangible condition that has no physical counterpart. In other words, one cannot measure quality in the same sense as one measures length, mass, or lumen output (Veitch and Newsham, 1996). Therefore, lighting quality can only be assessed indirectly using behavioural measures (Veitch and Newsham, 1995). While this is fundamentally true, these statements provide little guidance on how lighting quality can be assessed in a real design situation, where the building is not yet erected or where a large number of design parameters must be evaluated. Behavioural studies have the drawback of being rather time-consuming since a large number of subjects is needed to evaluate a single situation. In the case of shading devices and glare control, it would be unrealistically demanding to use behavioural studies to evaluate all the existing types of shading devices under a few different sky conditions. It is evident that a simpler method must be developed. Veitch (2000) recently proposed a start for research-based lighting guidelines framed in terms of internal psychobiological (visibility, photobiology, stress and arousal) and psychological (attention and environmental appraisal, perceived control, effect and expectations) processes for achieving lighting quality. These lighting guidelines are still not fully developed and according to Veitch (2000), much work remains in “human factors research” (encompassing biological and psychological processes and effects) to fully understand the lighting effects on individual well-being and yield useful applications and specific guidelines for real situations. Thus, while the work of Veitch and Newsham on lighting quality is important and fundamental for this field, it has not yet resulted in concrete guidelines to evaluate lighting quality in office environments.


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Some more concrete tips were provided by the “Quality of the Visual Environment Committee” of the (Illuminating Engineering Society of North America – IESNA (Miller, 1994). This committee identified ten factors that contribute to lighting qualitywhich may be used to evaluate daylighting quality as well: Brightness (comparative luminance) of room surfaces Task contrast Task illuminance Source luminance (glare) Colour spectrum and colour rendering Daylight (view) Spatial and visual clarity Visual interest Psychological orientation Occupant control and system flexibility While aspects such as visual interest, psychological orientation, occupant control and system flexibility can hardly be assessed without using behavioural studies, factors such as comparative luminance, task contrast, task illuminance and source luminance can easily be studied by using computer simulations, scale models or even full-scale measurements. This provides a much simpler approach for evaluating lighting quality in a case where many alternatives must be studied. End of quotation 2.2.6 Direct glare: disability glare and discomfort glare The interference with visual performance caused by an unshielded light source or a bright window is called direct glare. There are two distinctly different forms of direct glare, disability glare and discomfort glare. The International Commisiion on Illumination (CIE) has described the differences as follows: Disability glare is glare that impairs the vision of objects without necessarily causing discomfort. Discomfort glare is glare that causes discomfort without necessarily impairing the vision of objects (CIE 1987). Discomfort glare is known as “psychologische Blendung” in German speaking countries or psychological glare. Disability glare is known as “physiologische Blendung”, or physiological glare. Discomfort glare produces a disagreeable sensation without necessarily disturbing the vision. The above definitions given by the CIE for discomfort and disability glare are quite precise, however the boundaries between the two stay quite fluid :


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The majority of the users are not able to identify discomfort glare (CIE 1987). They may, however, experience side or after effects of it in the form of head aches or fatigue at a later time. Disability glare generates a diminution of visual performance, evaluated with precise methods as the one of Blackwell (1946) which assesses the speed and accuracy in achieving a task. Disability glare can be caused by a reflection of light on a screen, or backlighting behind the visual task, for instance. Many studies have tried to identify the origins of each dicomfort category: the magnitude of discomfort (not permanent) is more a result of the luminance of the source than its apparent size. Generally the luminance of the source must be greater than 700 cd/m2 to cause discomfort glare (CIE 1983). Field studies of discomfort glare caused by daylight have indicated a more tolerant assessment of acceptability than from an artificial lighting installation. This is usually explained by the external view's mediating factor even if the glare sensation was not reduced (Hopkinson 1963a-b and 1970-71 and 1972). The discomfort glare index does not provide an absolute value that simultaneously covers the variability of external sky and sun conditions during the day and season and "all" the individual's subjective glare assessments in a specific luminous environment. This only illustrates the complex nature of daylight glare providing an illusion of an overall static discomfort glare model, describing the subjective assessment of the magnitude of the corresponding sensation.Discomfort glare indoors is also caused by reflection, especially specular, from external surroundings and/or interior surfaces. This may cause a secondary sensation of distraction and annoyance if the glare source (sky and sun) is reflected into the field of vision. Approaching a light coloured interior environment (Lambertian surfaces), sometimes reduces direct and indirect glare due to an increased adaptation luminance caused by reduced interior contrast discrepancies. Reducing the contrast effect between the sky, seen through the glazing and the window itself, by light coloured window frames and glazing bars, will reduce the magnitude of discomfort glare caused by daylight. In the early decades of the last century, investigations have been conducted to study luminaires appearing too bright in the field of vision and causing visual discomfort (Perry 1993c). Most of the recognised experimental research on subjective glare sensation was conducted in the 1940-50s at the Building Research Establishment BRE (England) and by Luckiesh and Guth (USA). In all experiments, trained observers were used to assess the sensation of glare. BRE used a scale-model simulating the glare from windows via a back-lit window-like diffusing surface with a fixed range of luminance levels. The assessment of glare sensation was evaluated by observers adjusting the general background luminance level to achieve a predetermined degree of sensation (Petherbridge 1950). Guth used a white hemisphere covering the field of vision with a single incandescent source at the apex, simulating the glare source. Different adaptation luminances were projected at the hemisphere and the luminance of the glare source was rated at the borderline between comfort and discomfort (Luckiesh 1949, Guth 1952 and 1959). The research described discomfort glare by the luminance of a small source and the interior adaptation luminance. It resulted in an index describing the subjective assessments of the degree of discomfort caused by a glare source subtending a solid angle ωs (CIE 1983 and 1992, Einhorn 1969 and 1979, Guth 1952 and 1959, Holladay 1926, Hopkinson 1963a and 1966, IES 1962, Luckiesh 1946 and 1949, Peterbridge 1950, Sørensen 1987).


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2.2.7 Discomfort glare indices

The aim of a good daylight design is first, to provide fully sufficient light for efficient visual performance, and second, to ensure a comfortable and pleasing environment appropriate to its purpose. The comfort aspect of a daylight design is closely related to the problem of glare (Hopkinson 1966). A glare index describes the subjective magnitude of glare discomfort with high values illustrating uncomfortable or intolerable sensation of discomfort. It also provides the designer with an indication of how to control and limit glare discomfort. However, most of the equations developed do not (unfortunately) predict the sensation of glare from daylight accurately (Chauvel 1982). In studies about visual comfort, it has been the custom to use a (discomfort) glare index to assess the degree of visual discomfort in a particular situation. A glare index is simply an empirical formula connecting directly measurable physical quantities (e.g. source luminance, solid angle of the glare source, background luminance, etc.) with the glare experienced by research subjects. Most glare indices are empirical formulas based on research with real human subjects. While the calculation of the glare index is complex, the important variables are (CIE, 1983; Moore, 1985): 

The luminance (Ls) of the glare source.



In the case of windows: the luminance of the sky as seen through the window (the brighter the source or sky, the higher the index);



The solid angle subtended by the source (s). In the case windows: the apparent size of the visible area of sky at the observer’s eyes (the larger the area, the higher the index);



The angular displacement () of the source from the observers line of sight. In the case of windows: the position of the visible sky within the field of view (the further from the centre of vision, the lower the index);



The general field of luminance (Lb) controlling the adaptation levels of the observer's eye (also called the background luminance). In the case of windows: the average luminance of the room excluding the visible sky (the brighter the room, the lower the index).

The subjective sensation of discomfort glare experienced by the observer can thus be related to the four parameters by a general expression of the following type (CIE, 1983):

G=

Luminance of the glare source angular subtense of the glare source at the eye  Luminance of the background  deviation of the glare source from the line of sight 

 Le  f  G   g s s   Lb  f   


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Where G is a quantity called the Glare Constant expressing the subjective sensation on a semantic/numerical scale and e, f and g are suitable weighting exponents while f(Ψ) is a complex function of the displacement angle, which takes separate account of its vertical and azimuthal components. A lot of glare indices have been conceived, and it is difficult to review all of them here. (Glare problems have been studied since the beginning of the last century). Nevertheless, some of the most common indices are listed and discussed below: 

BRS glare formula (BRS or BGI)



Cornell formula or Daylight Glare Index (DGI)



CIE Glare Index (CGI)



Unified Glare Rating (UGR)



Guth visual comfort probability (VCP)



Comfort, satisfaction and performance index (CSP)



Daylight Glare Perception Scale (DGPS)



Predicted Glare Sensation Vote (PGSV)



New Daylight Glare Index (DGIN)

Note that all but the DGI and DGIN have been developed for electric lighting systems and that the DGI and DGIN are based on an empirical (DGI) or purely theroretical (DGIN) modifications of data originally obtained from glare studies with artificial light sources (Osterhaus, 1996, 2003). BGI, DGI, CIE, UGR and VCP are supported by the simulation program Radiance (see Ward, 1996a), which is the main simulation tool used in the present study.

Quotation from Dubois, M.-C. (2001). Impact of Shading Devices on Daylight Quality in Offices: Simulations with Radiance. Report TABK--01/3062. Lund Institute of Technology, Lund, Sweden. : 2.2.7.1 BRS glare formula (BRS or BGI) The BRS glare formula was developed by Petherbridge and Hopkinson (1950) at the Building Research Station in England. Petherbridge and Hopkinson examined the effect of source and background characteristics for relatively small sizes of sources and produced formulas that appeared to describe the relationship up to a specific size, which subtended a solid angle of 0.027 sr. The


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sensation of glare was evaluated by the following degree of sensations: just noticeable, just acceptable, just uncomfortable and just intolerable. The empirical formula developed had the form: n

BGI 10 log 10 0.478  i 1

L1s.6  s0.8 Lb  P1.6

where 

P Guth’s position index, expressing the change in discomfort glare experienced relative to the azimuth and elevation of the position the observer’s line of sight



n number of glare sources.

The BGI is limited to small sources with solid angles inferior to 0.027 steradians (Osterhaus, 1996). According to Chauvel et al. (1982), the equation produced does not predict glare accurately for larger sources and does not take account of the effect of human eye adaptation. Moreover, Iwata et al. (1992, 1990/91) and Nazzal (2000), mentioned that this formula is mathematically inconsistent: a large glare source cannot be subdivided for the purpose of summing up glare contributions. Iwata et al. (1990/91) demonstrated that the BRS glare formula was the least accurate (compared with the DGI and CGI, see below) in a lighting environment with a wide light source. The BRS glare formula consistently predicted higher i.e. more severe glare votes than what actually occurred. Iwata et al. (1990/91) commented that this was due to the fact that this index was originally intended for pointsource light rather than wide-source glare. 2.2.7.2 Cornell formula and Daylight Glare Index (DGI) The Cornell glare formula (Hopkinson, 1963), which was developed at the Building Research Station (England) and at Cornell University (USA) is a modification of the BRS formula, which has been adapted to large sources. The formula was developed through experiments where a bank of closely packed fluorescent lamps behind an opal diffusing screen was set in a separately illuminated white surround extending to the limits of the observer’s view. The formula is expressed as follows: n

GI 10 log 10 0.48  i 1

L1s.6   0s .8 Lb  0.07  s0.5 Ls

where Ωs (sr) solid angle subtended by the glare source modified by the effect of the position of its elements in different parts of the visual field in the way put forward by Petherbridge and Longmore (1954). Validation studies of this formula involving physical measurements both in artificial and daylighting conditions showed that the correlation between observed glare from windows and the predicted calculated glare was not as strong as in the case of artificial lighting. There was a greater tolerance of


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mild degrees of glare from the sky seen through the window than from a comparable artificial lighting situation with the same value of glare index, but that this tolerance did not extend to severe degrees of glare (Boubekri and Boyer, 1992; Chauvel et al., 1982). The numerical relationship of the second of these two conclusions was derived from regression curves, which produced an equation for a Daylight Glare Index1 (DGI) as expressed below (Chauvel et al., 1982; Baker, Fanchiotti and Steemers, 1993):

DGI 

2 GI 14 3

This equation expresses the observed fact that there is a greater tolerance for glare from the sky, as seen through the windows, than from a comparable artificial lighting situation, provided that the glare index is not too high (Baker, Fanchiotti and Steemers, 1993). Chauvel et al. (1982) argued that the weak correlation between the GI and the observed glare from windows is compounded by other visual and aesthetic factors such as the quality of the view out, the appearance of the window as well as the visual and aesthetic interior qualities of the room. Iwata et al. (1991 in Velds, 2000) showed that the perceived glare under real sky conditions was smaller than that predicted by the DGI. However, they mentioned some discrepancies between the real sky and artificial sky evaluations such as different adaptation times, and cultural differences (one experiment with Japanese subjects, the other with Europeans and Americans). Through experiments with artificial lights, Iwata et al. (1990/91) showed that the Cornell formula was the most accurate (compared with the BGI and CGI, see below) to predict the glare vote from a wide-source glare. However, they maintained that it is inadequate for a range of wide-source glare conditions because it does not include parameters for adaptation and the luminance of the desk surface. They showed that there was a difference between early and late votes showing that adaptation occurs so that the subjects judged the light to be less uncomfortable even after only 30 seconds, suggesting that the most serious glare problems occur during the transition i.e. the time immediately after exposure to the glare source. Also, Osterhaus (1996) observed that the research subjects (32) in his experiment commented on becoming more sensitive to glare as the experiment progressed (2-2.5 hours) and that this impression was confirmed by experimental data. Osterhaus and Bailey (1992) also pointed out that the DGI does not include a measure of adaptation. In their experiment, they observed that subjects selected higher luminances when high initial presentation luminances preceded the adjustment of luminance for the background. They also observed that when glare severity was assessed immediately following the difficult letter-counting task, the subjects showed less sensitivity to glare. Moreover, they remarked that subjects became more sensitive to glare over the course of the 1.5-hour experiment, a result that agrees with other studies (Hopkinson, 1963 in Osterhaus and Bailey, 1992).

1

Note that the GI is often called the DGI (see for instance Boubekri and Boyer, 1992; Christoffersen, 1995; Iwata et

al., 1990/91). This is not without adding to the general confusion about discomfort glare indices.


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In a more recent study, Osterhaus (1998, 1996) compared subjective glare ratings (SGR) with calculated values derived from the CIE glare index (CGI), the Unified Glare Rating (UGR), the Daylight Glare Index (DGI) as well as measured direct vertical illuminance value at the observers’ eye. The subjects were presented with a large glare source of non-uniform luminance pattern. Calculations based on the CGI and UGR showed reasonable correlation with experimental results while the DGI – expected to be more appropriate for the large window-like glare sources – showed a weaker correlation. The best correlation was found for the direct vertical illuminance at the eye or the overall luminance of the visual field. The author concluded that these results suggest that luminance is a fundamental parameter in response to glare discomfort. Boubekri and Boyer (1992), Iwata et al. (1990/91) and Chauvel et al. (1982) also indicated that the Cornell formula is not directly applicable to a case where the window is parallel to the subject’s line of sight. Chauvel et al. (1982) concluded that, for most people, the discomfort glare will be less than that predicted for a window perpendicular to the line of sight. Osterhaus and Bailey (1992) pointed out that currently (as of 1992), no data is available on perceived comfort or discomfort and the relations between comfort and task performance under conditions in which the glare source borders or surrounds a work task, since all previous studies evaluated discomfort glare by directly viewing the glare source rather than focusing on a work task. They concluded that for relevance of today’s work environment, it seems important to more carefully consider situations in which the glare source occupies a substantial part of the visual field while the subjects actually perform work tasks. Waters, Mistrick and Bernecker (1995) also showed that non-uniform surfaces can cause more glare than uniform light sources when positioned perpendicular to the line of sight and less glare when located 10° to 20° from the line of sight. Since the DGI is based on experiments with uniform light sources, it should not be applied when discomfort glare is caused by non-uniform light sources (like in the case of a window with Venetian blinds, as an example). Finally, note that Chauvel et al. (1982) also observed that the discomfort glare resulting from the direct view through windows has been found to vary greatly from observer to observer and also to vary with factors associated with the appearance of the window, the view outside and the surroundings. Gall et al (2000) tested different glare protection systems for their suitability to prevent glare on VDU screens. Besides using a questionnaire-based survey, they took measurements with a luminance camera of the situation and calculated the DGI from the luminance distribution. They found good correlation (r=0.91) of the DGI using an artificial window, also in combination with different glare protection systems. For real windows under daylight they found a lesser correlation (r=0.53). In that case, the DGI overrates the situation i.e. people tolerate more glare than DGI predicts. They also made regression calculations between vertical illuminance at eye level and the glare rating. They found some correlations (0.5-0.7), but not as good as other studies. They also measured the cylindrical illuminance at the VDU position and found correlation towards glare in the same range than the vertical illuminance sensor at eye level. They found good correlation between cylindrical illuminance and room brightness.


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2.2.7.3 CIE glare index (CGI) The CIE adopted the following formula proposed by Einhorn (1969, 1979) as a unified glare assessment method:

 Ed  1 500  n L2   s s CGI  8 log 10 2    2 Ed  Ei i 1 P where 

Ed (lx) direct vertical illuminance at the eye due to all sources;



Ei (lx) indirect illuminance at the eye (Ei = πLb).

The CGI was developed in order to correct the mathematical inconsistency of the BRS formula for multiple glare sources. The formula provides the steps of glare sensation corresponding to the BRS scale. Iwata et al. (1990/91) showed that the CGI was less accurate than the Cornell formula to predict the glare vote from a wide-source glare. They also maintained that this formula is not adequate because it fails to take into consideration the adaptation factor as well. 2.2.7.4 CIE’s Unified Glare Rating system (UGR) Later, the CIE (1992) proposed a unified glare rating system (UGR), in which Sørensen combined the “best” aspects of the BGI and CGI (Osterhaus, 1996). The UGR incorporates Guth’s position index and combines the aspects of the CGI and BGI to evaluate glare sensations for an artificial lighting system, restricted to sources with a solid angle of 3·10-4 to 10-1 sr. The formula is expressed as follows:

UGR  8 log 10

0.25 n L2s  s  Lb i 1 P 2

This formula is intended for small sources of artificial lighting. However, Iwata et al. (1992) obtained a good correlation between the Glare Sensation Votes (GSV) and the UGR in central vision (centre of window corresponding to line of sight) and a weaker correlation for the peripheral vision in the case of rectangular windows. Note, however, that this experiment involved a simulated window with artificial lighting.


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2.2.7.5 Guth's visual comfort probability (VCP) The Visual Comfort Probability (VCP) method provides ratings of visual comfort in terms of the percentage of people who will consider a given lighting system to be acceptable (CIE, 1983). It takes into account all the key factors which influence visual comfort and is applicable to all types of interior lighting systems. The VCP is determined from the calculation of another factor called the discomfort glare rating or DGR, which is expressed as:

 n  DGR    M i   i 1 

n 0.0914

 0.5  Ls 20.4 s 1.52 s  0.075    M   P  Fv0.44    Lw  w  L f  f  Lc  c  Ls  s   Fv   5   where 

M index of sensation for the ith glare source;



Fv (cd/m2) average luminance for the entire field of view;



L (cd/m2) average luminance of the walls (Lw), floor (Lf), ceiling (Lc) and source (Ls);



ω (sr) solid angle subtended at the observer’s eye by the walls (ωw), floor (ωf), ceiling (ωc) and source (ωs).

The formula

VCP  279110 log10 DGR is a very good approximation for the main range of interest: VCP = 20 to 85, respectively DGR = 55 to 200. Beyond this range, the following correction must be added (CIE, 1983):

VCP  279 110 log10 DGR 350 log10 DGR  2.08

5

Equations for the calculation of the VCP were developed by Luckiesh and Guth (1949, 1959, 1961 in IES, 1993) who carried out experiments in simulated rooms using lensed direct fluorescent lighting systems only (IES, 1993). According to IES (1993), the VCP cannot be applied to very small sources such as incandescent and high intensity discharge, to very large sources such as the ceiling in


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indirect systems, or to non-uniform sources such as parabolic reflectors. The equations are developed for luminaries under standardised conditions of use. According to Veitch and Newsham (1996), several features of the VCP model limit its applicability as an indicator of discomfort glare. The original model was developed using flat-bottomed recessed luminaries only, and was initially restricted to that application. The validity of the curves for the wide range of luminaries and possible installation is unknown. That is, the model only makes predictions for a given line of sight, and probably does not hold for other viewing positions that occupants might reasonably adopt. Furthermore, Veitch and Newsham (1996) maintain that evidence (Water, Mistrick and Bernecker, 1995) shows that perceptual differences exist between uniform and non-uniform sources that render the VCP model ineffective in predicting glare ratings for non-uniform sources. 2.2.7.6 Relationship between BGI, CGI, UGR, DGI, DGR, Comfort VCP % The DGR system was used to define the percentage of people assessing an installation to be at or more comfortable than the borderline between comfort and discomfort, also called the visual comfort probability (VCP) (Guth 1959 and 1963 and 1966, MacGowan 1969, CIE 1983). High levels of VCP predict increasing acceptability of the glare performance from an installation. The VCP glare scale is inverted relative to the BGI scale (Perry 1993c). The scale defined by the British system demonstrates that one glare index unit is the least detectable step and three glare index units are the normally acceptable step (CIE 1992). However, some of the criticisms to the experiments conducted at BRE and by Luckiesh and Guth are: its applicability to ordinary observers, the time of adaptation to the experimental conditions before assessments of discomfort, the "leading" nature of the instructions given and the criterion technique of subjective appraisal (Hopkinson 1963, Boyce 1981). The criticisms regarding the criterion technique are simply that observers tend to match the middle of the rating scale with the middle of the conditions experienced [Poulton 1977, Boyce 1981]. Although the recognised empirical models of discomfort glare provide the designer with an indication of advice, they are based on lighting technology current at the time of developments, "reducing" their applicability of glare calculations of today's lighting technology, working conditions and activities, for example VDU work (Perry 1993c). Table 2 shows for different glare indices the magnitude of discomfort glare corresponding to the visual comfort probability (VCP).


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Table 2

Comparison of the corresponding magnitude of discomfort glare experienced for different glare indexes with the visual comfort probability (VCP)

Corresponding degree of Glare

BGI CGI

Comfort VCP DGI

DGR %

UGR No Glare

< 20

Unnoticeable

< 10

< 16

35

95

Just imperceptible

10

16

50

87

13

18

65

75

16

20

90

64

18.5

22

120

50

22

24

220

20

25

26

300

11

28

28

400

5

Acceptable but not imperceptible Just acceptable BCD Just uncomfortable Uncomfortable Just intolerable Intolerable

700

2.2.7.7 Comfort, satisfaction and performance index (CSP) The CSP index was developed, based upon existing data, current recommendations of the CIBSE Code for Interior Lighting (CIBSE, 1994) and detailed studies of over 650 individual workers and their offices. The CSP index is designed to be used in conjunction with the code and takes a value of zero to 100 which relates to the probability that office workers will be satisfied with their visual environment (Bean and Bell, 1992). The CSP is conceptually similar to the VCP system but the development followed a different path (Veitch and Newsham, 1996). The calculation of the CSP is very complicated and is therefore not reported here (see Bean and Bell, 1992 for details). The derivation of the CSP was limited to direct office lighting with or without visual displays. Note that Perry et al. (1995) attempted to replicate Bean and Bell (1992) and obtained a very low correlation between the subjective ratings of lighting acceptability and the photometrically derived CSP index. Even if there was daylighting in the rooms where the experiments were carried out to develop the CSP index, the light from daylight origin was neither recorded nor included in the formula. The CSP


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index was only intended to deal with artificial lighting. This means that the CSP index is not suitable for problems including daylight or sunlight. 2.2.7.8 Predicted Glare Sensation Vote (PGSV) The PGSV is a formula based on experiments with simulated windows with over 200 subjects encompassing 100 different test conditions (Tokura, Iwata and Shukuya, 1996; Tokura et al., 1993). The PGSV is expressed as follows according to Tokura, Iwata and Shukuya (1996):

PGSV  3.2 log10 Lwp  0.64 log10  s  0.79 log10  s  0.61log10 Lb  8.2

 Ev     Lwp   w  Lb     1  w    where 

Ev (lx) vertical illuminance at the eye;



Lwp (cd/m2) luminance visible within the window plane;



Фw configuration factor for the window.

The PGSV was based on glare assessments under artificial lighting conditions and uniform light source. Through experiments involving a simulated window with artificial lighting (Tokura, Iwata and Shukuya, 1996; Tokura et al., 1993), it was shown that the PGSV gave more plausible degrees of glare than the DGI, but generally the glare sensations predicted were too high. Moreover, a comparison was made between the glare sensation votes (GSV) and PGSV in two experiments with actual windows oriented towards two different directions. In the first experiment, the subjects were seated directly facing the window while in the second, the subjects were asked either to look up at the window located forward them diagonally or to look up at the other window located just to their left perpendicularly. The results of these experiments indicated that the actual GSV were generally lower than the PGSV and that few people involved in the experiments felt that the actual windows were uncomfortable because of glare. The authors attributed this to the fact that the PGSV cannot cover the effect of the luminance distribution of the window on glare sensation and that the luminance distribution of actual windows or the view out from the windows could bring some psycho-physical comfort to subjects. Velds (2000) mentioned that the PGSV does not include a position index and therefore only aims at the evaluation of glare from windows located in the line of sight. However, in contrast with the DGI, the PGSV takes into consideration the transition of the adaptation luminance level of the eyes and the total amount of light coming into the eyes.


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2.2.7.9 New Daylight Glare Index (DGIN) After arguing that there is still no valid glare index for daylighting, Nazzal (2001, 2000) recently proposed a new daylight glare index, which he called DGIN, where the N stands for “new”. The DGIN is expressed as: n     L2ext  pN    i 1 DGI N  8 log 10 0.25 0 . 5   n  Ladapt  0.07   L2window N     i 1  

where 

Lext (cd/m2) average vertical unshielded luminance of the outdoors;



Lwindow (cd/m2) average vertical shielded luminance of the window;



Ladapt (cd/m2) average vertical unshielded luminance of the surroundings;



ωN (sr) solid angle subtended by the glare source (window) to the point of observation;



ΩpN position factor depending on the geometry of the window and the distance from the observation place to the centre of the window area.

There was not sufficient evidence at the moment of writing the present report that this daylight glare index could be reliably used for the prediction of discomfort glare from windows. This index promises some improvements for quantitatively assessing daylight and sunlight glare but it does not include performance assessment and user investigations under such glare conditions (Osterhaus, 2001). 2.2.7.10 Other indices Meyer, Francioli and Kerkhoven (1996) introduced the “J” index, which expresses the relationship between the loss of relative visual acuity (AC) of a particular operator under given illumination conditions and the maximum possible visual acuity (ACmax) that this person can reach. It is expressed as:

J

V

a max

V a

a max

V



0  J 1

The input of data may find their origin through direct photometric measurements on the field, simulations in a laboratory, or computer simulations with Radiance. The J index is very promising as a


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way of measuring and computing visual acuity and, perhaps, visual comfort in a space. According to Velds (2000), the J indice is still in development but is promising. However, there was not enough information available at the time of preparing this study to be able to use it as a glare index. Moreover, it is unclear from the publication whether the index can be used as a measure of comfort in the same way as it measures visual acuity. End of quotation 2.2.8 Summary on glare indices After reviewing the literature for existing discomfort glare indices, it appears that there is currently no glare index which can reliably predict the level of discomfort glare from daylighting in an office room representing a normal working environment in which normal work activities are carried out (i.e. looking at a computer screen for a prolonged period of time). This opinion is also shared by Velds (2000), who claimed that the majority of existing glare formulas were developed for the evaluation of discomfort glare from small artificial light sources, such as the VCP, the BGI and the UGR methods. These formulas cannot be used for the assessment of discomfort glare from windows because the source size mostly subtends a solid angle at the eye that exceeds 0.01 steradians. In the case of daylighting, the glare source occupies a large part of the visual field raising the adaptation level of the eye and thus reducing the sensation of glare and the contrast effect (Hopkinson and Bradley, 1960, in Velds, 2000). Osterhaus (1996) also argued that existing glare evaluation methods primarily target small to medium size ceiling fixtures. For very large glare sources that occupy a substantial part of the visual field, formulae obtained from small source studies have been modified to fit data obtained with large sources, such as luminous ceilings. At the moment, only the DGI seems to predict the combined effect of the physical values of size and position of windows (large glare source), sky and background (adaptation) luminance, the observer’s line of sight, distance and position in relation to the window as showed by the work of Iwata et al. (1990/91). However, the work of Iwata et al. (1990/1991) was performed in a simulated room with an artificial light source. There is much evidence that the spectrum of the light source might have an effect on the tolerated glare (Boubekri and Boyer, 1992; Chauvel et al., 1982). For example, a recent study by Berman et al. (1996) involving only 12 subjects submitted to two spectrally different broad-band sources indicated that the scotopically deficient source (i.e. the source with more energy in the reddish end of the spectrum in this case than the cool-white lamp) elicited a higher level of subjective and objective discomfort. Velds (2000) found that glare sensations with an artificial sky could not be related with glare sensations with an equivalent natural sky concluding that other factors such as the spectrum of light and the view through the window might mitigate the experience of glare. Boubekri and Boyer (1992) and Nazzal (2000) also mentioned that none of the proposed discomfort glare methods predict discomfort glare from direct sunlight origin. According to Nazzal (2000), a


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single internationally acceptable phenomenological glare formula and evaluation method has not been attained and no standard monitoring procedure is available. Osterhaus and Bailey (1992) also pointed out that no data is currently available on perceived comfort or discomfort and the relations between comfort and task performance under conditions in which the glare source borders or surrounds a work task. All existing discomfort glare indices were developed by assessments of subjects directly viewing the glare source rather than focusing on a work task. The study by Osterhaus and Bailey (1992) indicated that subjects tolerated larger changes in glare source luminance when performing a letter-counting task than when just fixating the centre of the VDT screen without actual attention to the task. This identified attention to a work task as a relevant variable in the analysis of discomfort glare (Osterhaus, 1996). Christoffersen (1995) also mentioned that although the recognised empirical models of discomfort glare provide the designer with an indication of advice, they are based on lighting technology current at the time of developments, which reduces their applicability of glare calculations to today’s lighting technology, working conditions and activities (computer work). Osterhaus (1996) also suggested to carry out glare experiments with subjects exposed to the daylighting situation for at least the eight hours of a regular workday. Decreasing work performance would be expected due to fatigue and distraction induced by glare discomfort. Sivak and Flannagan (1991) found that task difficulty affected discomfort glare. In their study, smaller gapsizes in a gap-detection task resulted in more discomfort glare responses concerning a simultaneous presented light source. They concluded that the assessment of discomfort glare requires the inclusion of the relevant visual task the observer is involved in during the presentation of the glare stimulus. Finally, note that many researchers attribute the lack of strong correlation between glare indices and subjective evaluations to the nature of psychometric studies and to the difference between the human subjects themselves. According to Nunally (1978 in Boubekri and Boyer, 1992), the nature of human beings is far too complex to allow precise prediction, and their visual, emotional and psychological appraisals can be equally complex. Because of the above uncertainties with the currently available glare prediction models, we may utilise relevant models for comparison purposes where applicable, but will treat any results with caution. Conclusions from the experimental phase of this research project will therefore place higher weighting on user responses and photometric measurements taken during the experiments We anticipate that this project will make significant contributions to the advancement of our knowledge about user responses to glare in offices equipped with Venetian blinds.


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2.3 Assessing user’s visual comfort in offices Different studies have been carried out in the last years that put the user at the centre of their work. Assessing the user behaviour has been a major issue to establish preferred ambiences, including biological and functional needs as well as more subjective evaluations such as light colour temperature preference. Different types of studies led to different protocols and aimed at different parameters of the lighting field. Several methods have been confirmed to be efficient in the assessment of user behaviour, but the tools that each uses are different. The two main categories of user assessment can be described as in-situ methods, which study the users in their own offices, and experimental room (including virtual rooms) methods. The tools and methodology that each uses will be listed, and the general user assessment methods will be described afterwards. 2.3.1 Tools for in-situ methods Assessing the user behaviour in their own office environment often considers only one or few interventions to record the way an occupant reacts (e.g. how he/she uses lighting devices, and eventually how he/she adjusts it). Two main advantages are attributed to this method: a) long-term studies can be carried out to record a high number of interior and exterior measurements with the available weather conditions, b) the user behaviour is not influenced by the presence of the experimenter or by the artificial ambiance, and the collected information is more likely to reflect real behaviour. The general disadvantages of this method are that few persons can be assessed, few new devices can be studied at the same time and it is time-consuming. The first inconvenience is major since we know that users have very different reactions towards light, presenting a large individual spread (Cf Chapter 2.4.1). The user assessment in the field of lighting and shading systems requires a large number of subjects to be able to conduct a meaningful statistical analysis. The other inconvenience is that only a restricted number of changes can be introduced in the environment of the user if the experimenter wants to be able to identify the links between users and new devices. Another difficulty is that the experimenter cannot control the parameters, and cannot experiment all required situations. The tools for in-situ assessment are of different types:

Photographies / non-interfering methods The non-interfering in-situ methods are more efficient as they require little intervention from the experimenter. It is thus quite easy to let a camera (analogue or digital) take pictures of a room at regular time intervals. Rubin (1978) used this method and took pictures of the façade of a building under study, shooting from another building. His aim was to analyse the way people used their manual venetian blinds. He took 4 pictures per day for a period of two weeks. He could then analyse


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the blind occlusion rate via the pictures. Rea (1984) had the same protocol. He took pictures in the morning, at midday, and in the afternoon of different facades (with different orientations), with different sky conditions. He could not take into account the slat angle and only considered the blind occlusion. Sutter (2003) and Wienold (ISE building, Freiburg) used digital web cameras with remote control from computers. The images could be automatically analysed by the computer. This permitted recording with higher frequency, thus yielding more images for the analysis, especially under changing sky conditions when users might adjust their blinds more frequently This method is useful for manual blinds, which cannot be assessed with bus-linked sensors. The previously described tools have the advantage to be non-interfering, since they do not change the user’s environment. Thus the information gathered during the experiments more likely to be objective than with the following tools which interfere with the user’s immediate environment: the user is then aware of the experimentation which can lead him/her to change his/her behaviour.

Sensors / computer bus Inoue (1988) stood outside an experimental building, taking pictures of blind positions each hour as previously described, but also monitoring at the same time the outside direct and indirect irradiances for 3 weeks, on 4 facades. It is noted that the building was entirely air-conditioned. Lindsay et al (1993) also recorded the blind positions in individual and open-space offices by taking pictures every two hours over four months, and monitored at the same time the sky conditions, the inside temperature, the occupancy and the slat angles. The use of inside and outside sensors gave a number of measurements which could be correlated with the blind occlusion. Bülow-Hübe (2000) installed blinds and dimmable electric luminaires in offices, and let the users adjust their new environment, while she recorded interior illuminances and the exterior vertical illuminance. Sutter (2003) tested 8 offices over 30 weeks. The shading systems were motorised, and by using a LON-WORKS system, the height of the blind and the slat angle could be recorded. He also measured the exterior vertical illuminance on the facade, the illuminance on the VDT screen, the status of the artificial light (on-off), the inside temperature, and the presence of the user. However, he was not allowed to monitor the time the occupant used the computer. Galasiu and Atif (2002) assessed the use of blinds by installing an on-the-market motorized shading system. They monitored the horizontal illuminances on the ceiling (beside the commercial artificial light sensor) and on the desktop, the slats tilt angle, the power demand of each lighting circuit (on/off state and, respectively, the dimming percentage), and the artificial) lighting energy consumption each 12 hours. Another evaluation was experimented by Sutter (2003) by taking fish-eye pictures shot from the exact position of the user’s eye, and in the direction of the computer’s screen. He could then evaluate the luminance repartition in the user’s visual field and correlate the results with the other measurements taken at the same moment. . However, these pictures required to disturb the user and could not be automated. Escuyer (2001) wanted to assess the user’s light preferences in offices, and started with observing their behaviour: she monitored the ambiances (illuminance and luminances, with dataloggers or bus acquisition) and correlated the informations with the user’s reactions towards the artificial light


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system. She then needed to have direct user’s explanations on the reasons that drove them to act on the artificial lighting system, and used questionnaires. Iwata-Tokura (2002) reported a survey of workers response to automated lighting and shading systems. They monitored the occupancy, the illuminances, the solar radiation and the blinds positions and angle. They used questionnaire to correlate the measurements with the user response.

Questionnaires / interviews / subjective factor It is complicated and at the same time necessary to consider the subjective parameter of the user when assessing his comfort in-situ. On one side the informations have to be analysed with precaution and sometimes require a psychological or anthropologic point of view. Enrech-Xena (1999) wanted to assess the needs of daylighing in offices, and to evaluate whether it was a need of view to the outside, a biological need, or other needs. She thus needed to understand each of the user’s subjective motivations. Questionnaires and interviews were necessary to answer these issues. Interviews and questionnaires can be carried in different ways: by directive semi-directives, or opened questions. The interviews can be based on questions from a questionnaire, and permit to collect more informations from the same person. Questionnaires can be distributed to a larger number of persons, and to have a statistical analysis. Escuyer (2001) carried semi-directive interviews, before distributing a large number of questionnaires. Enrech-Xena (1999) considered the user’s own words as a sound basis for her study and had free-talking interviews with them. Hygge et al (1999) established a methodology for questionnaires, which was used afterwards by Sutter (2003) to assess the user’s impressions. Iwata-Tokura (2002) conducted three different methods to assess the user’s behaviour. She used simple observation of the worker’s behaviour; a questionnaire to the occupants and subjective experiment in the office with few subjects. The questionnaires were distributed to 171 persons, while only 12 subjects were finally assessed in experimental offices. There are few post-occupancy evaluations (POE) specifically focusing on the lighting conditions in a building. Most studies cover a much wider spectrum of interior environmental aspects (Collins 1990, Dillon 1987 a,b, Gillette 1986, 1987 a, b, Heerwagen 1991, Love 1995, Marans 1985 a,b, 1987 a,b, Vischer 1989). A POE study of a building produce indicators of, based on reactions and attitudes of the users, how successful the building is, where the problems are, and, to some extent, how the performance can be improved. However, very few POE’s are seriously evaluated and often is the method and the questionnaires used different from one POE to another. Also, most POE's are never used repeatedly, which reduce the gathered experience of using POE as a tool to evaluate the quality of a building. It is thus very difficult to assess which method is the best and to compare different buildings on the same scale (Hygge and Löfberg 1999). Christoffersen et al (1999) carried out a Post-Occupancy Evaluation Survey in 20 Danish office buildings during the spring and autumn of 1997, to ensure similar outdoor conditions, and to reduce any prejudice towards either the winter or summer conditions prior to the survey. A comprehensive questionnaire based on hypotheses focusing on windows, daylight, sunlight, and artificial lighting


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was handed out to 2340 office workers and the opinions and preferences of 1823 respondents were included in the analyses (78 % response rate). The main objective of the survey was to evaluate the working environment from the users’ point of view regarding windows, daylight and artificial lighting in their office rooms.

Duration / Number of assessed people The duration and the number of persons assessed can vary, depending on the informations needed. The existing methods show that, the sky conditions have an important influence on the use of shading systems. The duration of the methods should then cover a sufficient period to be able to correlate the results with the sky conditions. The first studies regarding blinds were carried only during couple of weeks (Rubin, 1978; Inoue, 1988). Lindsay et al (1993) extended this duration to 4 months. The latest studies have chosen to cover longer periods by automatic monitoring or by reporting the experimentations over the year : Sutter (2003) covered a period of 30 weeks of monitoring, Galasiu and Atif (2002) covered one year by two 2 months periods of monitoring (one in winter, the other in summer). Iwata-Tokura (2002) also covered almost one year by two days lasting observations, in three different seasons. We can notice here (see also 2.3.9) that even if seasonal changes can be correlated with user behaviour, we found no study correlating these data with the sun position (zenith and azimuth) under clear sky conditions though one of the major goals of the blinds is to stop direct sunlight. The number of offices and users assessed in each study decreased as we started understanding the user behaviour towards shading systems. Initial studies (Rubin, 1974; Rea, 1984; Inoue, 1984) were concerned with entire buildings, representing lots of offices. When monitoring tools where installed, the needed number of offices to be assessed was reduced to a much lower number (12 for IwataTokura; 8 for Sutter, 2003; 4 for Galasiu-Atif, 2002). Questionnaires and interviews have to answer statistical issues to be representative and exploitable: Iwata-Tokura (2002) reported 171 questionnaires and interviews, Escuyer (2001) reported 43 interviews and 63 questionnaires, EnrechXena (1999) carried out 35 interviews. 2.3.2 Tools for experimental methods Another solution to assess users’ behaviour consists of re-creating an environment which will point the way the user is going to react to specific conditions. This general method presents many advantages. First, the experimenter can control each parameter with precision, without being dependent of external conditions for example. A corollary is that one can obtain informations in a short time, and increase the number of persons to be assessed. Another advantage is to be able to present different scenes in a short time (or even simultaneously), the user being able to choose between them. The disadvantage is mainly the artificial character of the experimentation. The user is aware that he is being observed, and he can have different reactions than in a “normal” situation. To achieve that assessments of the different combinations is as equal as possible, the same user should


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be used for all combinations (also called a within-subject design). If not possible, a between-subject design can be made similar to the within-subject design, but since people are genuinely different the variance in a between-subject designs is greater than in within-subject design. Consequently more subjects are needed in the between-subject design to make a difference in means come out statistically significant (Hygge and Löfberg, 1999). Different tools can be used to assess user behaviour with experimental methods:

Experimental rooms, sensors In order to be able to evaluate the influence of precise parameter on user behaviour, different experimental rooms have been entirely constructed, or modifications to existing offices have been made. Most of them are conceived to appear like a “typical” work office, and some had peculiar characters, especially when researchers were looking for specific parameters. Boyce et al (2001) carried out a study in windowless offices, to prevent daylighting from interfering in the study. Veitch et al (1999) also experimented with user behaviour in open-plan offices without windows, to limit the interaction with daylight. In most cases, experimental offices were build with a lateral window, a desk and a computer. Dubois (2001) used two identical full-scale experimental rooms to investigate shading systems. Berrutto (1996) also conceived two similar test-rooms where he presented various scenarios to the users. Laurentin (2001) and Charton (2002) used the same test rooms to carry out their own study. Iwata andTokura (2002) modified the offices they had studied and re-assessed the user behaviour, considering the new devices. In those experimental rooms, monitoring is made easier by the possibility to include sensors in the conception of the room. The sensors are then linked to a computer, which stores the relevant data. All sensors can be integrated in the rooms and record the following: illuminance, luminance, temperature, air movement, solar shading device position, solar radiation, etc. A great advantage provided by this method is the possibility to reproduce the assessed conditions after the user evaluation, in order to take pictures from his/her point of view and in the same conditions (particularly in windowless rooms; in daylit situations the reproduced conditions may not be exactly identical to the assessed conditions). During experiments, different tasks may be completed by the user to evaluate the his/her performance under normal or simulated activities that closely resemble “normal” activities. Assessing the performance can be done with several tests, for example the AVT (Alphanumeric Verification Test) used by Boyce et al (2001). This test consists of identifying discrepancies between two alphanumerical lists of numbers, characters etc. on a piece of paper. The experimenter can also ask the user to achieve a specific task with the VDT, or he/she can let the user freely handle his/her activities.


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Virtual rooms Recent technological advances (computer simulation software, high-power video-projection, stereoprojection) allowed researchers to experiment with new tools for user assessments. A simulated room can now be presented to a subject, and evaluations of different scenes can be made. The virtual rooms, as the experimental rooms, present the advantage to allow more effective protocols, as they allow two-by-two comparisons, and are more likely to identify user preferences. They also permit easier exploitation of the results. Charton (2002) compared the reactions of users in front of various realistic scenes, and the reaction in front of the same virtual 3D projected scenes. Wienold et al (1998) also developed a virtual simulator at the Fraunhofer Institute for Solar Energy Systems (ISE) in Freiburg, using slides of stereo images of scenes rendered with the lighting simulation software RADIANCE. The process is called SVR. This stereo projection offers the opportunity to create realistic impressions, observed by the subject through magnifying glasses. The main advantage of the SVR system is the ability to offer equivalent test conditions to a number of subjects. While this system delivered promising first results, luminance restrictions of the projection system (max. luminance of 1,0000 cd/m²), limitations in the accuracy of simulating complex daylighting elements with RADIANCE, and lack of funding stopped the developments early on. Today, simulations are frequently used tools to assess user behaviour. It is possible to let people adjust their visual environment in real time by increasing or decreasing the output of luminaries. The software’s options permit to control each lighting characteristic, such as illuminance, luminance, reflectance, etc., and to change them almost immediately. This, of course, is impossible in real rooms. Under those conditions, new experimental procedures are possible as demonstrated by Reinhart (2002) who simulated typical offices, provided a user behaviour patterns (blinds position as a function of sun position) to a computer, and ran simulations with outputs like daylight autonomy, annual energy savings as a function of office orientation, office room depth restrictions, and user behaviour.

Questionnaires / interviews / subjective factor In experimental settings, subjective assessment of various conditions remains an important consideration. While we know that the reactions of users are influenced by the presence of an observer or by the experimental ambiance, users can nevertheless provide valuable clues regarding the potential acceptability of environmental conditions In general, the same tools are used as in the in-situ methods, but they can be more precisely linked to the assessed situations. Questionnaires, in particular, can easily be adapted to different scenes, and statistical exploitation is easier due to higher levels of control for the experimenter when compared to in-situ settings.


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Duration / Number of assessed people The time needed by a user to make an assessment of an experimental setting is a function of the item to be assessed and its complexity. It is generally recommended to allow the subject sufficient time to make a reasonable decision. However, time limits might be imposed to simulate decision making or visual performance under real-life conditions. Charton (2002) presented several lighting scenes and asked the people to evaluate them by giving a mark for 5 criteria, without imposing a time limit. Users generally took less than a minute to evaluate a scene. Laurentin (2001) assessed three different situations: people had to sit in an experimental office, and could adjust the power of several luminaries until they felt comfortable for a specific task. The total time of assessment for one person and for a given task was half an hour. Veitch et al (2000) left the user for a whole day in a windowless experimental office where he could freely adjust each luminaire’s power depending on the task he was achieving (which was not imposed to him), and Berrutto (1996) made one-hour assessments, also asking people to adjust the power of the luminaries of the experimental office until they felt comfortable with a specific task. The number of subjects used for an experiment has to permit statistical analysis. As a rule of thumb, 30 subjects or more are needed in each group for between-subject design, if they are to be properly matched. This rule of experimentation consists in comparing the reactions of different subjects for a same situation. It is then possible to identify a trend of reaction of the users in the experimented situation. Around 15 subjects are a minimum for within-subject design (Hygge and Löfberg, 1999). This experimental design permits to analyse the behaviour of a subject in a situation, by a precise study of each user’s reactions. This second design requires a long time of experimentation for each user, while the first one allows shorter times of experimentation. The experimental methods previously described generally matches with both between-subject and within-subject methods, as the minimum number of persons for statistical analysis is easily achieved : Berrutto (1996) assessed 73 persons, and Veitch et al (2000) reported 94 subjects, both with experimental offices in which people could adjust the light. The in-situ method is more difficult to carry out as it is often not possible to find 30 subjects in the same site, available for the whole duration of the experimentation. 2.3.3 User assessment methods: Free choice /Discriminative choice / convergent algorithms Field research shows that people are unlikely to speak about lighting on their own. Most of the time they do not know what kind of luminaire they have, and they generally cannot evaluate their visual comfort unless severe problems occur. Choosing an in-site or an experimental method is thus a first step for a strategy of assessment. It is also necessary to choose a method to make people express themselves in the experimentation. To assess users’ preferred luminous environments, several methods have been suggested. The first one, which we will call free choice method consists in presenting a virtual scene or an experimental room to the user, and in letting him/her adjust the ambiance until he/she feels comfortable with it. It is then possible to record the final adjustments from different users, and to extract preferred


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ambiances. This method can have variations, by changing the initial ambiance presented: by presenting specific ambiances such as very dark ambiances, the experimenter can assess the user’s first intentions. It is also possible to present ambiances with “average” adjustments and to record the modifications wished by the user. Veitch et al (2000) asked the users to work in a environment where no adjustments where allowed for one day, and then asked them at the end of the day to make corrections to the lighting conditions. This permitted to record the imperfections of the initial adjustments. Other parameters can be included in this method: Berrutto (1996) let the user adjust his/her ambiance, without any constraint at first, and then again with an electrical total power limitation of the lighting system. This second test permitted to record which luminaire the user chose to dim, and which one was maintained, being thus the most important for the ambiance With the free choice method, the experimenter is able to let the user express his/her wishes by adjusting the luminaries, but the user still does not talk about lighting. The communication is still distant, using the dimming system as the communication medium. A second method called discriminative choice method permits to let the users express themselves directly, with words. This method consists in showing two different scenes at the same time or in a short time period, so as to let the subject compare the scenes. Experiences of Veitch and Newsham (2000) and Charton (2003) showed that the subjects are then much more comfortable in choosing their preferred ambiance and to explain the reasons for their choice. The discriminative choice method can be carried out with real scenes (presentation of two real rooms close to each other), with photographies (comparison of the photography of the same place with two different light conditions), or with virtual simulation rendering (comparison of pictures of the same place with different light conditions obtained by simulation rendering) : . Charton (2002) validated this virtual comparison method by simulating light ambiances, similar to the real experimentations carried by Veitch and Newsham (2000). By presenting different ambiances two by two, people could express their preferences, and Charton could correlate his results with the ones of Veitch and Newsham obtained with the real scenes. The discriminative choice is a way to help the users express their wishes towards luminous environment, and is based on Thurstone’s comparative judgement laws (Thurstone, 1927). Thurstone explored the fluctuation of the thresholds for reactions, and established laws of analysis for the “pair comparison”, and the “comparative judgement”. The discriminative choice method can be optimised with the convergent algorithms that select, from a high number of initial possibilities, the preferred ones with only a restricted number of questions asked of the subject. For lighting scenarios, these questions can easily be presented using the discriminative choice method. Three different mathematical tools are used for the convergent algorithms: 

The fuzzy logic permits the translation of expert and “common sense” basic knowledge into rules that corresponds to the way human beings manage their environment. This model is a translation of our behaviour into mathematic rule, but these rules are definitive, and do not adapt themselves to each specific user’s behaviour.


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

The artificial neural networks builds adaptive models for various parameters to be controlled. It thus creates models of reaction for different parameters (thermal model, daylighting model, weather evolution, etc.), which adapt themselves to the user’s habits.



The genetic algorithm represents an evolutionary optimisation algorithm, which maximizes the convergence within several generations. This model takes basis on the genetic evolution and discrimination : instead of testing criteria one by one, the system generates “populations” representing a global solution, and simulates mutations in that population. It then generates a new global solution called mutant generation, the mutation happening on various criteria. Depending on the reaction of the user to the mutant generation, the model accepts or rejects the mutation, and then chooses another relevant mutation for the next generation.

Newsham et al (2002) presented simulated ambiances on a screen, and applied the genetic algorithm technique to assess preferred ambiance of the subjects. The users could evaluate the ambiance by using rating scales for each parameter (desktop, ceiling, etc.). With a number of possible images of over one billion, the system could reach the end conditions within 22 images, and final best image was rated 8,2 (s.d.= 1,4) on a scale of 10, putting in evidence the efficiency and the accuracy of the discriminative method when used with the genetic algorithm model. Morel and Guillemin (2002) are also carrying out an experiment on a thermal controller tested in a reference room without genetic algorithm, and in another room with genetic algorithm in order to evaluate the efficiency of the genetic algorithm. 2.4 Performance indicators to assess visual comfort in offices In the passed 70 years, starting from 1928 when the IESNA (IESNA, 1928) started talking about satisfactory ambiances under artificial and natural lighting, many searchers had been collecting information and data on visual comfort. They could identify indicators providing performance evaluation. As a societal issue, offices have been the most frequently studied spaces and a report on the main parameters influencing visual comfort in offices can be raised to achieve performance indicators. 2.4.1 Main parameters influencing visual comfort in offices The initial analysis of visual issues in offices should be addressed by identifying the different elements of the office that are likely to have an impact on the visual comfort of the user. The following list will describe these likely parameters, starting with the closest elements to more distant ones, noticing that their importance is not necessarily decreasing or increasing. In fact, different parameters can have different importance at different times. 

Desktop (or working plane)

The desktop is often referred to as the working plane or task zone. The user can be looking at it when reading, for example, or it can remain in the user’s visual field with a great importance because


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of its proximity. The indicators for this item are mainly uniformity (I assume of illuminance) and illuminances levels. Large brightness contrasts possibly creating glare by reflection is today quite exceptional. The indicators for this item will be discussed in more detail in the next paragraph. We can here notice that the reported needs for illuminances will vary from an one activity to another: Berrutto (1996) recorded preferences for 500 lx when having a meeting, 400 lx when reading, and 300 lx when working on the computer. A study by Newsham et al. (2001) in a windowless environment showed that the range of preferred work plane illuminances is 200 to 500 lx, which corresponds well with the range recommended for most offices (IESNA 1993, CIBSE 1994). Newsham et al. (2001) also point out that the desktop illuminance should be less than 500 lx in VDT spaces, which is confirmed by the study of Berrutto (1996). 

Working tools : Paper / Visual Display Terminal / Meetings

Visual needs usually vary with tasks, for example, when having a meeting, when reading or writing and when working on a computer. These three activities represent most of the possible working tasks and are usually sufficient to assess the requirements for luminous environments for offices . When having a meeting, the users need to see each other and to be able to read or present papers. The vertical illuminances on faces are important. Berrutto (1996) showed that the luminance of the faces had to be equivalent to the one of the surrounding walls. He also reported preferred illuminances of 500 lux on the desktop. When writing or reading, the user needs sufficient contrast between a task and its background (for example ink on paper) and to experience no visual stress. Berrutto (1996) reported preferred luminances of 100 cd/m² from the paper, and illuminances of 400 lx on the desktop. When looking at a VDT (Visual Display Terminal) during computer tasks, luminances, reflections and contrasts can be indicators. Berrutto (1996) recorded preferred luminances of 70 cd/m² for the selfemitting VDT, and Sutter (2003) reported preferred veiling luminances under 20 cd/m². Veitch et al (1995) established that users preferred very low or no reflected images. The veiling luminance will be an important indicator for visual performance during VDT activity. 

Close walls / distant walls

Close walls or partitions and distant walls represent an important percentage of the visual field. Their importance is thus high for the visual comfort, particularly when the user is working in a relatively static position, for example when completing computer tasks. Berrutto (1996) reports that users prefer the luminance of the walls to be equivalent to the luminance of the working tool. The luminance of the wall behind the VDT was preferred to have the same luminance as the screen, which was confirmed by Veitch et al (2000). When reading, users preferred walls with the same luminance as the paper. In meetings, a wall luminance of 140 cd/m² was preferred when the desktop’s illumination was classically 500lx. Loe et al. (1994) suggested that the minimum-tomaximum luminance ratio in the field of view should be between 1:10 and 1:50, while Newsham et al (2001) suggested a minimum-to-maximum luminance ratio of 1:20..


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

Ceiling

Just as the walls, the ceiling represents an important surface, but it is almost always outside of the 2° cone of central vision and only contributed mostly to peripheral or global visual field. His presence is generally asked to be sober but not too subdued. Veitch (1995) reports that direct / indirect artificial lighting is appreciated by the users with 40% of indirect light. Osterhaus (2002) suggests that a maximum of 850 cd/m² for the ceiling is likely to be accepted by the users for VDT office situations with average screen luminances of about 85 cd/m² and an assumed 1:10 ratio between the screen and the not immediately adjacent ceiling surface. 

Window

A window is a considerably attractive element in an office (Veitch, 1995), due to daylight access and the view to the outside. It has been recorded that users accept higher levels of glare when the glare was created by natural light coming from a window, rather than an electric light source (Osterhaus, 2001). However, the studies on shading systems (see chapter 2.4.4) reported that the maximum acceptable window luminance tolerated by the users was 1500 cd/m² . It is also necessary to consider situations where the shading systems occlude the window: how high are the new luminances, are there glare problems related to small defaults of the shades or to the reflection on the slats? The sun factor is thus major : potential glare from a shaded or unshaded window are related to the sky vault luminance. The inside luminance thresholds have to be related to the sunlight penetration (sun path). The interaction between the sun position and other elements (sun patch on the wall or on the desktop) can be a visual discomfort parameter, and the outside luminance also has to take into account the sun position (zenith, azimuth). 

General office organisation

It has been shown that for standard offices with a window, the preferred viewing position of users requires a line of sight parallel with the window wall (eg Steffy, 1995). Other studies on glare and shading systems have confirmed these positions. Furthermore, Christoffersen et al (1999) asked the subjects to report their position of their desk and VDT in their office room. When working at their VDT, 33% faced the windows, while only 12% said their view direction was towards the window when working at their desk. The remaining subjects had a view direction parallel with the widow or looked towards the back wall. Interestingly, the study also showed that almost 70% of the subjects sat near the window. Therefore, user assessments, locations and actual furniture arrangements should pay special attention to the area near the window and the view direction. Other issues still have to be evaluated, in particular in offices presenting two separated windows, or in open-spaced offices. Potentially, studies on the use of lighting and shading systems can provide preferred situations that have not yet been evaluated by detailed studies. As an example, Sutter (2003) reported that in offices with two windows, people tended to close the nearest blind and to leave the more distant one open, in order to avoid glare on the desktop but to


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maintain a view to the outside and a daylight penetration with the second window. This instance is not taken into account by classical performance indicators.

Quotation from Dubois, M.-C. (2001). Impact of Shading Devices on Daylight Quality in Offices: Simulations with Radiance. Report TABK--01/3062. Lund Institute of Technology, Lund, Sweden. : 2.4.2 Absolute work plane illuminance Although recent research (e.g. Loe, Mansfield and Rowlands, 1994; Loe, 1997) points out that the luminance in the visual field is the most important determinant of lighting quality in a space, sufficient levels of illuminance on the work plane are required to ensure visibility and visual performance, especially in offices. A pilot study by Berrutto, Fontoynont and Avouac-Bastie (1997) indicated that the horizontal illuminance appeared to be a major lighting quality parameter. When the lighting power was limited in their experiment, people tended to reduce wall luminances more drastically than horizontal illuminances, suggesting that horizontal illuminance was a more crucial parameter. For offices containing computer screens, the IESNA (IES, 1993) recommends to maintain illuminance levels at or below 500 lx on the horizontal work plane. Similarly, the CIBSE (1994) recommends to maintain the illuminance on the horizontal work plane of the VDT installations in the range 300-500 lx as far as practicable. Illuminances toward the lower end of this range are appropriate where the task is wholly, or substantially screen-based; where the task involves working on paper and on screen in roughly equal proportions, illuminances toward the higher end of the range are suitable. This is supported by the pilot study of Berrutto, Fontoynont and Avouac-Bastie (1997), which indicated an average preferred horizontal illuminance of around 325 lx for work on computer, while 425-500 lx were preferred for other tasks (reading/writing, receiving visitors). In that study, none of the 73 subjects chose a horizontal illuminance higher than 550 lx for work with a VDT screen. For all tasks other than computer work, NUTEK (1994) requires that the average illuminance directly on the (reading) task be at least 500 lx and the average work plane illuminance be at least 300 lx. Moreover, the overall lighting shall not be less than 100 lx at any point located 0.5 m from the room’s inner walls on a plane located 0.85 m above the floor. They also recommend limiting the illuminance on the computer screen to 200 lx. The Danish standard DS 700 “artificial lighting in work rooms” (Arbejdstilsynet, 2000) is similar: a horizontal illuminance of 500 lx is required on the task area and the illuminance should not be below 100 lx at any point in the room. According to this standard, it is not generally desirable that the lighting be 200 lx in the whole room. It is only necessary that the task lighting provide the recommended lighting levels at the task area. In their field study, Christoffersen et al. (1999) measured the illuminance in 20 Danish office buildings to be between 150 and 200 lx and found that 75 % of the workers in those offices rated the artificial lighting as acceptable. However, it should be pointed out that some studies indicate that the minimum illuminance levels required in most standards may not be accepted by some users. A review conducted by the IESNA (IES, 1993 in Velds, 2000) indicated that judgements of optimum illuminances increased with age and decreased with task contrast. Most subjects preferred 1000 lx with high contrast and 1800 lx


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with low contrast. On average, the younger subjects (< 50 years) preferred 2000 lx, and the older subjects, 5000 lx, for both contrast levels. Studies by Begemann, Tenner and Aarts (1994) and by Begemann, van den Beld and Tenner (1995) showed that the average preferred (total) desk illuminance varied between 1100 to 3000 lx as a function of outdoor daylight conditions and sky type. Begemann, van den Beld and Tenner (1995) concluded that meeting biological lighting needs is very different from meeting visual needs, which form the basis of today’s indoor lighting standards. They concluded that indoor artificial lighting levels, which have been lowered following several energy crises, should probably be classified as “biological darkness”, as suggested by medical research. Research about the so-called non-visual effects of light has indicated the clear link between light and health. Light influences biochemical processes at single cell level, the activation of the central and autonomic nervous systems, the entrainment of diurnal rhythms and the secretion of hormones, particularly the secretion of sleep (melatonin) and stress (cortisol) hormones (Küller and Wetterberg, 1995, 1993). Light also has an effect on behaviour and emotions as indicated by the work of Küller et al. (1999) and Küller and Lindsten (1992). Boyce and Kennaway (1987) showed that illuminance levels as high as 2500 lx did not suppress melatonin (the “sleep” hormone) to daytime levels, which contradicted Lewy et al. (1980 in Boyce and Kennaway, 1987) who found that bright artificial light of 2500 lx was able to suppress melatonin to daytime levels and that 500 lx was insufficient to do so while 1500 lx provided an intermediate amount of inhibition. Moreover, Boyce (1973) carried out a study into the effect of age on visual performance and showed that significant improvements in performance, in terms of time taken to achieve a visual task, can be obtained when illuminance is raised from 500 to 750 lx for subjects in the 46 to 60 year age group. Also, Saunders (1969) carried out a series of experiments in which workers were asked to judge lighting levels for performing simple office tasks. From his results, it was clear that significant improvements ceased once 800 lx was reached. Bean and Bell (1992) set the optimum illuminance level for office lighting without VDTs at 800 lx and at 500 lx for offices with VDTs, since it is known that higher levels create too great a contrast between written material and the VDT screen. Finally, Inui and Miyata (1973 in Collins, 1994) found that the perception of spaciousness increases as the horizontal illuminance increases – as well as percentage of window area, room volume, and sky luminance – and that the more spacious an area is considered to be, the more “friendly” subjects find it. In summary, most lighting standards require an illuminance of at least 500 lx on the task in offices where traditional (paper) work is carried out. In offices where the work in mainly computer-based, the work plane illuminance should be lower i.e. preferably between 300-500 lx. In any case, the work plane illuminance should never be below 100 lx. Much research in the field suggests that these lighting levels are low and that some users may prefer more light. Also, research about the non-visual effects of light (on health, behaviour) indicates that the lighting levels recommended in most standards are not sufficient to maintain health of humans. These illuminance requirements should thus be regarded as minimum lighting requirements.


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2.4.3 Illuminance uniformity on the work plane Illuminance uniformity has been said to be highly desirable, both across the working surface and across rooms (Veitch and Newsham, 1995). Excessive variation in horizontal illuminance may contribute to transient adaptation problems and should be avoided (CIBSE, 1994). Therefore, lighting standards often contain recommendations regarding the uniformity of illuminance on the work plane. These recommendations are expressed as the quotient of the minimum to the average or to the maximum illuminance on the work plane. Note, however, that Bean and Bell (1992) found that illuminance uniformity was far less important than illuminance level when they tried to correlate judgements of lighting quality by office workers and lighting performance index. Slater and Boyce (1990) provide a list of some uniformity ratios recommended in some standards (Table 3). As shown in Table 3, the CIE (1986) and the CIBSE (1994) recommend that the uniformity of illuminance (minimum/average) over any task area and immediate surroundings should not be less than 0.8. When the precise size of the task area is not known, calculations can be based on an area measuring 0.5 m by 0.5 m located immediately in front of the observer at the edge of the desk or working surface. Similarly, Berrutto, Fontoynont and Avouac-Bastie (1997) found that illuminance uniformity on the desk of 0.8 (minimum/average) was preferred for reading/writing and that the preferred ratio was somewhat higher for receiving visitors in the room. Slater, Perry and Carter (1993) observed that the ratings of the difference in illuminance rise sharply when the illuminance ratio between desks in an office room is less than around 0.6. They concluded that an illuminance ratio of at least 0.7 (minimum/maximum) between work areas was unlikely to pose problems, confirming previous results of Saunders (1969). Slater and Boyce (1990) and Boyce and Cuttle (1994 in Velds, 2000) focused on the uniformity of the desk and suggested a minimum to maximum illuminance ratio of 0.7 or 0.5 if the work is primarily done in the central area of the desk. In most offices, the actively worked area on most desks is the central part, which is about 1 m in width (Slater and Boyce, 1990). In a further study, Carter and Slater (1992 in Carter et al., 1994) investigated the acceptable illuminance differences between working areas and adjacent ancillary areas in a simulated office and demonstrated that an illuminance ratio between the two of at least 0.5 is likely to be satisfactory. Table 3

Illuminance uniformity recommended in some standards (in Slater and Boyce, 1990)

Source CIBSE Code for Interior Lighting (1984) CIE Guide on Interior Lighting (1986) British Standards Institution BS 8206: Pt (1985) Code of Practice for Artificial Lighting Deutsches Institut für Normung. DIN 5035 Innenraumbeleuchtung mit künstlichem Licht (1979 later edition available) Standards Association of Australia. AS 1680 Code of Practice for Interior Lighting (1976 later joint AS/NZS edition available) Nederlandser Stichting von Verlichtingskunde Aanbevelingen vor Binnenverlichting (1981)


Illuminance uniformity over task Emin/Eav > 0.8 Emin/Eav > 0.8 Emin/Emax > 0.7(Emin/Eav > 0.8) Emin/Eav > 0.67

Emin/Eav > 0.67

Emin/Emax > 0.7

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These uniformity ratios have been recommended mainly for artificial lighting. Slater and Boyce (1990) mention that the proposed criteria may not be appropriate for interiors lit by side windows. Slater, Perry and Carter (1993) maintain that daylit spaces often exhibit a much greater illuminance variation without causing significant complaints. Slater and Boyce (1990) also argue that although the illuminance across a desk placed perpendicular to the window will vary smoothly, it may well be that people’s expectations about illuminance uniformity are different in the case of daylighting from side windows than for electric lighting from regular arrays of luminaries. The proposed criteria may only apply to illuminance distributions that vary smoothly over space. Sudden changes of illuminance in the space, such as are produced by some types of local lighting unit, may arouse greater sensitivity to illuminance non-uniformity. In summary, many lighting standards require a uniformity ratio of 0.8 (minimum/average) or 0.7 (minimum/maximum), but some research indicates that a ratio of 0.5 (minimum/maximum) may perhaps be acceptable. Some authors have argued that these criteria may not be appropriate for interiors lit by side windows, where the tolerance to illuminance non-uniformity may be greater than in the case of artificial lighting. 2.4.4 Absolute luminance of surfaces in the room According to Rowlands et al. (1985), the criteria of task illuminance and its uniformity do not provide sufficient guidance on the adequacy and qualitative aspects of the visual environment. The luminance of different parts of the field of view must also be considered. The importance of the luminance of elements located in the visual field is increasingly recognised as a major determinant of visual comfort. Many studies have indicated the importance of the distribution of luminance within a space and in particular the luminances of vertical surfaces: the walls are especially significant but the ceiling may also need to be included depending on the size and height of the room (Loe, 1997). Already in the 1980s, a team of researchers surveyed photometric conditions and conducted a survey of occupants’ opinion of the lighting in 912 workstations in 13 buildings across the United States. Five reports were generated from these data. Overall, it appeared that the pattern of luminance in the space (created by indirect ambient lighting systems with integrated furnituremounted task lighting) resulted in low ratings of the lighting system. This called attention to luminance distribution as an important element in good lighting design (Veitch and Newsham, 1996). Some years later, in a study involving 180 subjects, Van Ooyen, van de Weijgert and Begemann (1986) concluded that wall luminance contributes most to the way a room is experienced. Later, in a study about electrically lit spaces, Carter et al. (1994) demonstrated the importance of wall and vertical surfaces luminances. They found significant differences between the ratings of both adequacy and comfort of the lighting of an area in the office room with different lighting installations affecting the luminance of walls although the horizontal illuminance in the area was the same under


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all lighting installations. The same year, the National Institute of Science and Technology (NIST) conducted post-occupancy evaluations (POEs) in more than 20 buildings (Collins, 1994). They found that subjective brightness was clearly an important contributor to perceived lighting quality. Occupants rated spaces with lower average luminances as dim, while rating those with higher average luminances as bright. Furthermore, the data indicated that the relationship between subjective brightness and average room luminance was stronger than that between brightness and task illuminance. In other words, the occupants based their judgements of brightness on room luminance rather than task illuminance. A recent pilot study by Berrutto, Fontoynont and AvouacBastie (1997) confirmed these findings. They found that whatever the task, wall luminance seemed to have a significant effect on users' satisfaction and appeared to deserve more attention. A pilot study carried out in Sweden (Bülow-Hübe in Wall and Bülow-Hübe, 2001) also suggests that luminance in the field of view is important for visual comfort. In this study, the author found no correlation between the measured illuminance in experimental rooms and the way the research subjects adjusted the shading device in the window. However, she observed some correlation between the use of the shading device and the presence of a bright sunlight patch in the room. 2.4.5 Maximum luminance values Regarding luminance, the first rule is to avoid bright light patches in the visual field, which can cause disability and discomfort glare. According to Veitch (2000), direct glare and excessive luminance contrast can create undesired arousal and stress. In Sweden, NUTEK (1994) requires that luminance values in an office space be kept below 1000 cd/m2 (preferably below 500 cd/m2) in the normal visual field1 and below 2000 cd/m2 (preferably below 1000 cd/m2) outside the normal visual field. In America, the ANSI/IESNA RP-1 VDT Lighting Standard (IES, 1993 in Moeck, Lee and Rubin, 1996) recommends that all room surfaces within the peripheral view, including the window, shall not exceed 850 cd/m2 given an average VDT screen luminance of 85 cd/m2 (respecting maximum luminance ratios of 1:10 between the VDT tasks, and the room surfaces within the peripheral view). For paper or reading tasks, this Lighting Standard recommends to maintain luminance levels below around 255 cd/m2 for surfaces within close visual proximity (thus respecting maximum luminance ratios of 1:3 between the VDT and the directly adjacent surfaces). The CIBSE (1994) and Perry (1993) recommend2 that surface luminances should not exceed 1500 cd/m2 where work on computer is performed and that the luminance of the surfaces and objects facing the screen be kept low, preferably below 500 cd/m2. Thus, although the recommended values vary according to lighting standard, at least three sources recommend to avoid luminance values above around 1000 cd/m2, and 500 cd/m2 appears preferable, especially in offices with VDTs.

2

The normal visual field is defined as the area that extends 90° each side horizontally, 50° upward and 70° down from the horizon.


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2.4.6 Minimum luminance values Research also indicates that there are minimum and preferred luminance values for the walls. Van Ooyen, van de Weijgert and Begemann (1987, 1986) observed that wall luminance contributes most to the way a room is experienced. With increasing wall luminance, the room is felt to be more stimulating, and it is easier to concentrate on a task. According to Loe (1997), people prefer an interior to have a measure of “visual lightness” combined with a degree of “visual interest”. The visual lightness refers to the brightness of the major surfaces within the main field of view, particularly the vertical surfaces. Rowlands et al. (1985) also showed that the adequacy of a space relates to the overall “brightness” of the space and that the pleasantness and attractiveness of the space relates to the luminance of the area of the binocular vision. Their experiment showed that as the luminance of this area increased, the assessment of quality also increased (except when the luminance of the luminaries was as high as 8000 cd/m2). Note also that the ISO Standard 9241-6 (ISO, 2000) recognises this aspect since it states that, “apart from work plane illuminance, it is essential to consider

vertical illuminance, especially when the impression of depth in the room plays an important role. In general, the impression of depth can be increased by increasing the illuminance of vertical surfaces”.

In the experiment of Carter et al. (1994), lighting installations where the wall was darkest was considered both less adequate and less comfortable. Miller (1994) also reported that, in a pilot study with artificial lighting, 60 % of the 74 subjects preferred a scene where there was approximately equal lighting energy applied to the walls and the horizontal work plane. In that study, most people preferred the middle-to-high range of wall luminance (58-157 cd/m2) but the author warns that there was some serious bias in the experiment making it somewhat unreliable. Van Ooyen, van de Weijgert and Begemann (1986, 1987) observed a marked relationship between the preferred work plane luminance and the wall luminance. The preferred wall luminance was dependent upon the task performed. Reading, writing and interviewing a person resulted in preferences that were all in the same range i.e. 30-60 cd/m2. Work on a computer screen (with bright text against a dark background) called for somewhat lower luminances i.e. 20-45 cd/m2. The preferred working task luminance was also dependent upon the task performed. The areas of preference for reading writing and interviewing were between 45-105 cd/m2 while a somewhat lower desktop luminance (40-65 cd/m2) was preferred when working on a computer screen. Note that these observations were made strictly under artificial lighting conditions. The authors admit that the experiments that included daylight had to be carried out under such a wide variety of weather conditions that no regions of preferred luminance could be established (van Ooyen, van de Weijgert and Begemann, 1987). One study (Loe, Mansfield and Rowlands, 1994), where a commercial type interior was investigated, indicated that for the room to be assessed as “bright”, the average luminance within a horizontal band of 40° centred about the eye needed to be at least 30 cd/m2. In a review of research, Collins (1994) reports that scenes considered to be bright tend to have high surface luminances (above 100 cd/m2) in the central field of view but that there is a point beyond which brightness becomes excessive: luminances above 800 cd/m2 are considered glaring rather than bright. A pilot study by Berrutto, Fontoynont and Avouac-Bastie (1997) indicated preferred average wall luminances of around 120 cd/m2 (60 cd/m2 at eye level) for reading/writing tasks, 130 cd/m2 (65 cd/m2 at eye level) for receiving a visitor in the office room. They also found that for work on VDT screen, wall luminances inferior or at most equal to the VDT luminance were preferred and that a balanced (i.e.


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symmetrical) luminance was preferred for the walls surrounding the subjects on each side. Rothwell and Campbell (1987 in Tiller and Veitch, 1995) observed that subjects reported that the light was getting “dim” when the luminance on a simple visual acuity task ranged from 28 to 110 cd/m2; luminances between 3.6 and 28 cd/m2 were judged as “gloomy”. Shepherd, Julian and Purcell (1989 in Tiller and Veitch, 1995) studied subjective judgements of three different ambient lighting levels in a complex realistic visual field. They found that ambient lighting was described as “gloomy” only when the adaptation luminance in the field of view ranged from 5 to 9 cd/m2. The two other adaptation luminance conditions used in the experiment (6-11, and 38-60 cd/m2) were not judged gloomy. In summary, there is plenty of evidence suggesting that low wall luminance may be unacceptable. However, there appears to be no consensus as to which minimum luminance values should be accepted. In this review, the preferred wall luminance ranged from 20-157 cd/m2 and the minimum wall luminance appeared to be somewhere between 20-100 cd/m2. However, a minimum luminance value around 30 cd/m2 has been mentioned by some authors (e.g. Loe, Mansfield and Rowlands, 1994; Rothwell, Campbell, 1987 in Tiller and Veitch, 1995; van Ooyen, van de Weijgert and Begemann, 1986, 1987). This is approximately the luminance of a white diffusing surface which receives 100 lx of illuminance. Since many lighting standards recommend 100 lx as minimum illuminance value, it makes sense to use 30 cd/m2 as the minimum acceptable luminance value for the walls. But it is evident that more research is needed in this area to establish minimum acceptable luminance values, especially in situations with daylighting. 2.4.7 Luminance ratios Although the eye's adaptation ability can handle large variations in luminance levels, it cannot handle very different luminance levels simultaneously. Visual stress and fatigue is often experienced when working with visual display units (VDU). This is often caused by continuous and rapid eye movements between surfaces with high contrasting luminances (Perry 1993b). The eye's sensitivity to luminance ratios is characterised by a higher sensitivity near the centre of vision, and lower sensitivity at the edge or peripheral vision. Acceptable luminance ratios indoors require knowledge of all the factors involved, from the light source itself to the reflectances of the interior surfaces (Boyce 1981, Baker 1993). Controlling the luminance ratios in the field of vision can be accomplished by adjusting the reflectance factors of the surfaces, the illumination of surfaces and by avoiding dark backgrounds and/or distracting bright surroundings (Lynes 1978). When designing an adequate interior for high visual performance in a normal work area, the submitted luminance ratios should not be greatly exceeded since both uniformity and excessive contrasts are not desirable. Uniform luminance (monotony) may support visual efficiency but also emotional fatigue, while excessive luminance may provide emotional acceptance, but impair visual performance. The task of interest should therefore be slightly brighter than the immediate surroundings to ensure attention and avoid distraction (Lechner 1993). The luminance design ratios should ensure a comfortable balance between the interior luminances and the surface reflectances of the room (Boyce 1981).


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As mentioned in the previous section, the importance of the luminance of elements located in the visual field is increasingly recognised as a major determinant of visual comfort. Office interiors should be lighted to provide for good visibility with no distracting glare. Thus large luminance variations creating direct and reflected glare should be avoided (Hopkinson, Petherbridge and Longmore, 1966). This is also stated in the ISO Standard 9241-6 (ISO, 2000): “the most important factors for ensuring good lighting is an even distribution of luminance and contrasts in the office room”. Carter et al. (1994) found that the (lighting) installations rated more even were also rated more acceptable. The most uneven installations were rated most unpleasant and least acceptable. The perception of unevenness appeared to be adversely influenced by both excessive luminance contrasts between adjacent surfaces within the 40° band and by relatively dark surfaces immediately outside the band. The importance of luminance ratios was also pointed out by Loe (1997), who claimed that the visual performance can be enhanced by highlighting the task area with an illuminance ratio of 3:1 between the immediate task area and the surrounding area. Van Ooyen, van de Weijgert and Begemann (1986, 1987) came to an almost equivalent conclusion i.e. if the luminance of a visual task was fixed at 10, then the preferred ratio of task luminance to work plane luminance to wall luminance is 10:4:3. Two separate phenomena are influenced by the luminance ratios within the field of view: transient adaptation and discomfort glare. To limit transient adaptation and discomfort glare, the IESNA (IES, 1993) recommends that the luminance ratios (maximum luminances) should not exceed the following:  3:1 or 1:3 between the paper task and adjacent VDT screen;  3:1 or 1:3 between the task and adjacent surroundings;  10:1 or 1:10 between the task and remote (non-adjacent) surfaces;  40:1 or 1:40 between points anywhere in the field of view. Lechner (2001) reports that these luminance ratios were slightly modified as follows:  3 : 1 between the task and the adjacent surfaces (e.g.: between the book and the top of the desk)  5 : 1 between the task and adjacent surrounding (e.g.: between the book and the adjacent partitions)  10 : 1 between the task and the remote surfaces (e.g.: between the book and the walls of the room  20 : 1 between a light source and the main adjacent surfaces (e.g.: between the window and the adjacent walls)


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In Sweden, NUTEK (1994) also recommends that the luminance ratios within the work area between the task, the direct surrounding and the remote surrounding do not exceed 10:3:1. Moreover, this norm recommends that the luminance ratios between any points within the field of view should not exceed 20:1, which is stricter than the IESNA recommendation mentioned previously. The CIBSE (1994) has similar recommendations: the luminance ratios should not exceed 3:1 between the task and immediate surroundings and 10:1 between the task and general background. While large luminance ratios should be avoided, it is not either desirable to create totally even lighting distributions. Dull uniformity in lighting, though not harmful, is not pleasant, and can lead to tiredness and lack of attention (Hopkinson, Petherbridge and Longmore, 1966). According to Loe (1997), people prefer an interior to have a measure of “visual lightness” combined with a degree of “visual interest”. The visual interest applies to the non-uniformity of the light pattern. Therefore, according to IES (1993), it is important to provide enough variation in luminance (or colour) to contribute to a stimulating, attractive environment. Small visual areas that exceed the luminance-ratio recommendations are desirable for visual interest and distant eye focus (for periodic eye muscle relaxation throughout the day). Interiors should thus have elements of light and shade rather that the even light pattern provided by regular array of ceiling mounted luminaries (Loe, 1997). Loe, Mansfield and Rowlands (1994) showed that for a room to be visually interesting, the ratio of the maximum to minimum luminance within a 40°-wide horizontal band, needed to be at least 13:1. Veitch (2000) recommends using meaningful luminance patterns to create interest, to keep vertical surfaces bright, and to use daylighting and windows where possible. She suggests creating interest by integrating luminance variability with architecture to satisfy attention and appraisal processes. However, she also mentions that the acceptable upper limit for luminance contrast that is desirable to provide interest without the maximum value becoming a glare source is still not known. It is also probable that the acceptable luminance ratios are larger for natural than for artificial lighting. In summary, it is generally acknowledged that large luminance contrasts in the field of view should be avoided. Most standards recommend that the luminance ratios between the task (paper or VDT screen) and immediate surroundings should not exceed 3:1 and that the ratio between the task (paper or VDT screen) and remote surfaces should not exceed 10:1. Moreover, NUTEK (1994) recommends that the luminance ratios between any points within the visual field do not exceed 20:1. While researchers claim that dull, uniform luminance distribution is not either desirable, the acceptable luminance contrast that is desirable to provide interest without the maximum value becoming a glare source is still not known. End of quotation 2.4.8 Influence of the veiling luminance on Visual DisplayTerminal Finding favourable visual conditions for working on Visual Display Terminals (VDT) has been a real challenge in the last years. The technological advances allowed to work with dark letters on light backgrounds instead of the contrary when terminals were not powerful enough to provide the necessary self-emitted luminance. However, users still complain about poor visual conditions when


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working at their computer. Many of the reasons for their complaints can be addressed with today’s advanced screen or lighting solutions, but reflections and veiling luminances on the screens are not fully controlled yet. The ISO norms (ISO 13406-2, 2002) related to the flat-panel displays provide the basic framework : emitted luminance is considered separately from the luminance reflected from diffuse illumination. Flat panels must therefore fulfil the following conditions: The luminance of information (either foreground or background) shall satisfy the following inequality, aver all viewing directions (6 viewing directions are described for measurements, varying the position and inclination of the head. For further informations, please report to ISO 13406-2:2001(E) p24 ) :

LHS  LD 20cd / m² where LHS, is the emitted luminance component in the High State LD is the luminance component reflected from diffuse illumination The total emitted luminance can vary from 20 to 150 cd/m², LHS going from 20 to 120 cd/m² and LD from 0 to 30 cd/m². The ISO norm indicates a link between emitted illuminance and veiling luminance, taking into account the illumination of the screen resulting from the general office illumination. This illumination ES ranges from 250 to 750 lux, with a default value of 500 lux. The ISO Standard 9241-6 (ISO, 2000) recommends to limit the average luminance of lighting fixtures, windows or surfaces which can be reflected in the computer screen to 1000 cd/m2 for screens of class I and II and to 200 cd/m2 for screens of class III. While plasma television screens are listed to produce 780 cd/m² at peak panel brightness, 400cd/m² at peak set brightness and contrast ratios of 1 to 3000, the classical computer VDT presents much inferior luminances: Rea (1991) reported luminance (“brightness”) levels of about 5 to 10 cd/m² for dark background displays (“positive contrast displays”), and about 100 cd/m² or more for “negative contrast displays”, both regarding self-luminous displays. Sutter (2003) monitored luminances from 15 to 105 cd/m², depending on the age of the VDT, and on the user adjustment of the screen. The influence of the screen performance is thus important for the user judgement of glare, reflections and other visual comfort criteria. New screens (plasma screens, flat screens, auto-adaptive screens with sensors, etc.) have to be monitored to be able to establish a glare index limit for these screens. Sutter reported that all users maintained veil luminances under 20cd/m². He referred to the Blackwell (1946) visual performance levels to evaluate the user visual situation: with poor quality screens (luminance under 35 cd/m²), people worked with poor visual performance levels most of the time. With classical screens, people worked with rather normal visual performance levels. With good screens, people worked with optimal visual performance levels most of the time. The visual


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performance thus decreases with the background luminance of the screen. Sutter established that for a normal or poor screen, the veiling luminance should be less than 10 cd/m². With a good screen, veiling luminance should stay under 20 cd/m². 2.4.9 Direct reflections on a flat panel screen Gall et al (2000) tested different types of displays to find limits for noticeable and disturbing reflections. They mounted a self luminous surface on a wall, that reflections can be seen at the display. For a general illuminance level of 500 lux, they found no disturbing reflections up to 3300 cd/m² (max. possible luminance in set-up), also for positive contrast. For the negative contrast, no noticeable reflections up to a source luminance of 3300 cd/m² were found (positive contrast: 1800 cd/m²). Those results show that the danger of direct reflections on a flat panel display is low. The limiting luminance cannot be determined exactly from this study, since the 3300 cd/m² was only the limit value of the experimental installation. 2.4.10 Influence of the seasonal and the temperature background The influence of external parameters could have be important for the results of user assessments. The seasonal effects and the influence of temperature backgrounds have thus been explored. The seasonal effect seems to be a relevant parameter for assessing reactions of the users in the lighting field. Laurentin (Laurentin, 2001) noticed that users added more artificial lighting (+200 lx) in winter than in summer, starting from identical daylighting conditions. Escuyer (2001) also reported that higher levels of artificial lighting in offices were requested in winter and spring (300-350 lx) than in summer and autumn (200-250 lx). The temperature background is less clearly identified as an issue for lighting devices : Nicol (2001) could not show any relation between temperature and illuminance levels when the temperature was maintained at an “acceptable” level. Laurentin (2001) did not find any noticeable effects of thermal conditions on occupant preferences either. A correlation seems to exist when the indoor temperature is outside the normal operating and thermal comfort conditions. Sutter (2003) has shown that for inside temperatures over 26°C in summer, the user behaviour towards blinds and artificial lighting was changed. 2.4.11 Final performance indicators to assess daylight quality In the ECCO-Build study, the quality of daylighting in the space should be evaluated by considering a number of factors, which we call “performance indicators”. The performance indicators considered are: 

The absolute work plane illuminance (at specific points and the average across the work plane)


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

The illuminance uniformity on the work plane



The absolute luminance of surfaces in the room, including the window, seen from the user’s point of view, and the luminance distribution across the visual field of the user, excluding punctual luminance sources



Puncltual luminance sources



The luminance ratios between the work plane (paper task), VDT (video display terminal) screen and surroundings.



The absolute illuminance on the VDT



Veiling luminance evaluation

These performance indicators are also in the list provided by the “Quality of the Visual Environment Committee” of the IESNA (previous section) although some of them were given different names. The last five items in the IESNA’s list cannot be taken into consideration within the scope of this project since it is not possible to do so with the tools and methods available for this project. We also discarded colour rendering to simplify the study and only included monochrome (black, grey or white) shading devices instead. 2.4.12 Discussion on performance indicators’ accuracy Finally, we should mention that we considered using the (daylight) glare index or another formula of this type as a lighting quality indicator. However, this should be investigated since reviewing the literature about glare indices has shown that there is a lack of supporting evidence that any of these glare indices can be used in the present context (i.e. with shading devices). Recognised Difficulties There will always be problems in trying to describe and correlate subjective responses of occupants with objective measures such as photometric data. Fontoynont (2002) gives several examples of where the correlation may not be suitable. A stained glass window, for example, could be objectively assessed in terms of its luminous transmittance, spectral change in transmitted daylight, daylight factors and luminance, whereas a subjective analysis would likely include the aesthetics of the stained glass window and its suitability to the function of the room. Thus the objective analysis does not truly reveal what the occupants would feel about the lighting conditions. Even if the measurements taken fairly reflect the opinions of the occupants, the calibration of the objective measure will always be difficult because of the the inevitable wide range of responses from the occupants. The suggested assessment measures are all based on the average response of a large group of people. Thus, even if the assessment reveals that the office is theoretically extremely comfortable, there could still be a few particularly sensitive occupants that are uncomfortable. It thus raises the question whether the level of visual comfort should be set so that everyone in the office is


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comfortable, or the majority of the people in the office are comfortable, or the theoretical ‘average’ person is comfortable. Should the office have to ‘pass’ or ‘score well’ for every aspect of visual comfort for the office to be deemed visually comfortable? What if the office has a magnificent view and excellent illuminance and a good degree of uniformity but terrible glare problems? Should some of the considerations regarding visual comfort be of more importance than others and if so how should the weightings be calculated? Is the same format going to be applicable for every office? The debate on these issues will likely continue. We expect a reasonable spread of individual user responses and intend to consider as many approaches to the classification of visual comfort for the situations studied within the framework of ECCO-Build. 2.5 User’s behaviour towards facade shading systems Façade shading systems can be used as a protection for many different reasons, for sun or glare protection, over-heating protection, prevention of reflections on VDT screens, or simply for more privacy. Visual comfort in offices is strongly influenced by the use of shading systems, as they generally have direct interactions with artificial lighting and cooling or heating devices. The first studies dealing with manual blind systems were carried out in the late seventies, and considered only general concerns like building orientation or seasonal parameters, and did generally not take into account the effective occupancy of the offices. The latest studies are considering the user behaviour in response to many other parameters like façade illumination, activity in the office, or colour of the slats. Reviewing the existing information about the use of shading systems will point to how we can manage the integration of different operating systems in the room, such as heating/cooling devices, artificial lighting, blinds, etc. 2.5.1 General uses of shading system Christoffersen et al (1999) showed that at some times during the year, the occupants of Danish offices exhibited a higher desire for sunlight when entering their offices. More than 60% said they wanted sunlight sometimes during the year and sunlight was accepted more often during the winter than summer. Interestingly, more than 70% said they were never or only sometimes bothered by sunlight in their offices, but 70% said they also used their solar shading device (typically curtains or Venetian blinds) often or always when the sky was clear. However, almost 30% were dissatisfied with their solar shading device and the most frequently reported reasons for dissatisfaction were insufficient shading and restrictions of the view to the outside. Use of the shading device was higher if the glazing area was larger than 35% of the facade or the offices were orientated towards South. Even though the subjects used their shading device to reduce


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problems with sunlight, especially when the VDT was positioned near the window, more than 20% reported that they cope with the problems of insufficient shading and screen reflections by turning their VDT. Moving the VDT further away from the window caused significantly fewer problems. Large windows (glazing area above 35%) and especially if the work place (desk and VDT) were near the window caused significantly more heat and indoor temperature problems, or problems with sunlight at the work place and reflections in the screen. Other studies show that users tend to use their manual blinds in an inefficient way. Rubin (1978) reported that blinds could stay in the same position for weeks, even if this position was initially decided by the experimenter to be fully down. Rea (1984), who considered that this phenomenon was related to a long-term way of managing the solar radiation (by considering that soon or later the sun would penetrate in the room if the blinds were not fully closed), confirmed these results. He compared this inertia with the one observed in rooms equipped with artificial lighting devices where users changed the artificial lighting settings only once or twice. Foster (1999) observed that there had to be a radical change in the weather conditions to provoke the movement of the solar shade by the user, and Inoue (1988) reported that people activated the blinds only when situations became intolerable. Blinds can also be an answer to uncomfortable reflections or glare on the VDT screens, and the shading use can in some cases be related to this only reason. Pigg (1996) observed that 87% of the occupants used their blinds to reduce glare on the computer screen. Most research projects show that a great majority of users of a manual blind do not manage to prevent uncomfort situations, as they move the blinds with a low frequency. This low frequency of use leads to situations of sun penetration, and also to situations where the blinds are closed while no sun penetration is to be feared. . Sutter (2003) linked this purpose to the manual characteristic of the blind system: he recorded that blinds were moved three times more often when they where equipped with motorisation than with manual systems, for the same offices. The average number of moves recorded was 2,1 times per day with the motorised shading system than with manual one.( The general idea raised by Reinhart (2002) is that people tend to use their blinds mainly to block direct sunlight. This idea was first highlighted by Rubin (1978). Rea (1984) emitted a doubt, wondering whether solar heat or daylight reduction was the motor of user reaction. Lindsay et al (1993) answered this issue, observing that blinds were used for solar radiation under 50W/m², which is not very high as far as heat gain is concerned. As Pigg (1996), he considered that blinds were activated to reduce glare. Bülow-Hübe (2000) found the same answers through questionnaires. Most researchers reported huge differences of use between offices occupants. Rea (1984) observed that each user clearly has a preferred use of blinds, but that it is very difficult to generalize and find a characteristic pattern for the “average user“. Sutter (2003) reports wide differences of use between “photophobic“ and “photophil“ users: each one has a coherent way of using the shading system, with a tendency of photophobic persons to close more often the blinds, but the gap with photophil users is too important to allow an average description of utilisation. Lindsay et al. (1993) described that on the same façade, each day’s blind manipulation could vary from 0% (never) to 100% (daily), with an average of 40%, showing wide differences between users preferences.


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It is important to notice that automatic control of blinds has to face this issue : Reinhart (2002) reported that 45% of the actions decided by the controllers of blinds were rejected by the users. Iwata and Tokura (2002) also observed than in open-spaced offices where blinds were controlled automatically, people were dissatisfied with their visual situation and disconnected the controller. Blinds were thus manually moved, and the artificial light dimming system that was linked to the controller was also disconnected. No energy savings could be achieved with this controller. Obviously, the control system of blinds has to answer user’s wishes before answering energy saving issues. Finally, some accordances appeared on the fact that globally user prefer having their blinds opened than closed (Rubin, 1978), and that individual control is preferred to automatic systems (Inoue, 1988 / Bülow – Hübe, 2000). In offices, the avoidance of reflection on the VDT seems to play a major role in the use of blinds : (Sutter, 2003), and Osterhaus (2001) reported that the level of glare perceived by the user was higher if there were reflections on the VDT. Bülow-Hübe (2000) also reported that sunlight patches tended to trigger the use of blinds. Another accordance seems to indicate that considering shading systems assessment, a large number of individuals is necessary to be able to observe real trends, due tothe important gap between user behaviours towards shading systems. Vine (1998) and Bülow – Hübe (2000) related that they needed more subjects because of the large individual spread, more numerous measurement points, and longer measurement periods.

2.5.2 Building configuration Some issues are directly related to the building itself and have important consequences on the use of solar shading devices. The way of using the blinds depends for example of the orientation of the façade leads for example to a difference of use : Rubin (1978) showed that occlusion was higher on southern than on northern offices, which was confirmed by Reinhart (2002). Rea (1984) also reported good correlations between occlusion and window orientation. Adjustments of blinds are thus different from an orientation of the facade to another orientation. Generally, experiments on shading devices have been carried out in buildings benefiting from a homogeneous access to daylight:, and the influence of close or distant obstructions such as high trees or other building close to the experimented one for instance was not included in the study. Sutter (2003) reported that such obstructions had an important influence on the blind occlusion, and on the frequency of blind movements. Reflection on bright buildings can even provoke the use of shading devices innorth oriented offices. When the obstruction is provoked by a close building, the importance of the obstruction depends on the height of the experimented floor : in a same building, and on the same façade, the shading will depend on the height of the office. Another parameter of the building can influence the use of shading devices : overheating in offices (due to high surfaces of glazing for instance) was proved to provoke a more important use of blinds.


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Inoue (1988) pointed the relation between overheating and blind use, and Reinhart (2002) also concluded that the use of blinds was more important in buildings presenting general overheating problems. 2.5.3 Relation between irradiance , sun position, and user behaviour The relation between the use of blinds and the sky conditions is still an issue to be resolved by research. Rea (1984) found a good correlation between occlusion and sky condition, while BülowHübe (2000) could not show this correlation, and Rubin (1978) was not able to establish this link, due to the restricted number of situations he could observe with his protocol (Cf. Chapter 2.2.2). Reinhart (2002) established that the manipulation rate was lower on western facades than on southern ones. The sun position and the sky condition could be an explanation for this, but there is a lack of information about the influence of the sun position (angle to zenith, azimuth) on the user behaviour. All observations regarding this issue report the influence of sun patches, or are related to irradiance or illuminance rates, without establishing direct relations between the sun position and the blinds move. The reported studies from now to paragraph 2.7 will show relations between the use of shading systems and the outside or inside conditions, without taking into account the sky conditions. Rea (1984) was the first to record the influence of incident irradiance and blinds adjustments, even though he could not show this relation throughout the day, but with a larger scale. Inoue (1988) shown that the rate of closed blinds had a good correlation with the amount of solar radiation, and that beyond 50W/m², the blind occlusion was proportional to the depth of sunlight penetration in the room. 2.5.4 Outside illuminance thresholds; hysteresis Closing the blinds: After those first evaluations, Reinhart (2002) intended to find a relation between the use of blinds and illuminances on the facades, and found that people tended to close their blinds when vertical illumination on the façade was over 49 000 lux. Sutter established this threshold at 37 000 lux, by taking into account the occlusion due to the angle of the slats. Opening the blinds: Reinhart (2002) found that people opened their blinds when vertical illumination on the façade was under 28 000 lux. Sutter (2003) recorded this threshold at 18 000 lux. Hysteresis: Reinhart observed a difference of threshold between closing and opening the blinds. Inoue (1988) already established this phenomenon of hysteresis, reporting that the correlation between the rate of blinds closed and the solar radiation followed an arc: even if the radiation was decreasing, the


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number of blinds closed could still grow. Reinhart (2002) reported a difference of 24 000 lux between the two thresholds, and Sutter (2003) found a difference of 28 000 lux. 2.5.5 Inside luminance thresholds and ratios The studies previously reported in this chapter deled with external measurements (façade illumination, sky conditions etc.). User assessments gave informations on the behaviour of users by recording the inside preferred conditions. Velds (1999) shown that if the window produced more than 1800cd/m² , the user would react within 30 to 60 minutes. Sutter (2003) also reported that 75% of the users maintained the luminance of their window under 1800cd/m², validating this inside threshold. Inside of the offices, the luminance ratios reported by Osterhaus (2002) of 1:3:10 (luminance of the task : luminance of the immediate surround : luminance of the general surroundings) has also been studied as a function of window presence and blind use. Sutter (2003) agreed with this repartition in the case of absence of a window in the field of view. If the user can see a window, the rates become 1:6:20, traducing a higher tolerance for natural light coming from a window. The surface of this window was always under 5% of the view field. Sutter (2003) also reported that the Glare Index (GI) did not reflect exact visual comfort conditions when natural light was coming from a window. The glare was then over-estimated, and the index was quite difficult to calculate. Vine (1998) observed that even with non-retractable blinds (slat angle being the only possible adjustment), people preferred illuminance over 500lux, but couldn’t identify if this was an answer to a visual or a biological need. He also reported that people had a higher acceptance for blinds when their operating system was grouped with the artificial lighting system. 2.5.6 Users preferred positions for Venetian blinds; colours of the slats In his work, Sutter (2003) studied the preferred positions of the blinds, by monitoring the frequency of position of the motorized blinds when the offices where occupied. He reported that 3 main positions where monitored (see Figure 2) o

Totally up : 18%

o

Totally down, alpha (α)= 0° -horizontal slats, saving the view to the outside- : 20%

o

Totally down, α>0° -protecting from direct sun illumination- : 51%

Figure 2 : slats angle

The other positions monitored where: all intermediate heights of the blinds representing 6,5% of the uses; all positions with α26°) in summer, the average blind’s angle was accentuated by 10°, and threshold slipped by a 15%. But this temperature happened only in summer, and temperature and seasonal influence could not be separated. 2.6 Other concerns 2.6.1 Medical issues on Macular degeneration ; glaucoma Age-related macular degeneration is the most common eye disease among people 65 years and older. Macular degeneration implies destruction of the central part of the retina and thus loss of central vision. Glaucoma is the 2nd leading cause of vision loss in the world (1st one being Macular Degeneration, and cataract 3rd). We here report the results of a study titled « Helping Glaucoma Patients Design


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their Lighting » from the Visual Impairment Group of the Princess Alexandra University, Eye Pavilion, Edinburgh (Nelson et Al, 2002). Basics, handicaps Patients attacked by Glaucoma have a limited Visual Field. The restriction happens on the surround of the Visual field while the centre of the foveal field remains safe. Subjects thus bump into objects, this being the first complaining from the patients. Patients also complain of too much glare from large windows and supermarket lighting. They stumble when going from light to dark spaces, and experience too much glare when driving at night. In a common room configuration, with a working on a table with a window near to the desk, patients experience too much veiling glare over the table and suffer of imbalance of high and low contrasts in the field of view. Experiences on Quality of life, results Experiences have been carried with 80 patients. First results shown that people still have wide differences of preferences on light. The experiences found the best correlation on contrast sensitivity tests and glare tests. The main reported results are that visual performance (reading skills) improves with higher illuminance; patients also prefer considerably higher illuminances than average domestic and CIBSE recommended lighting levels ; patients with cataract in addition to glaucoma set lower illuminances People with visual impairment due to glaucoma need glare reduction and improvement of lighting, and improved contrast task visibility . 2.6.2 Age factor Discomfort glare frequently arises in the working areas, and is a common source of annoyance from both electric lighting and windows. A wide variability between individuals in reaction to discomfort glare is a common finding and is consistent with other studies of discomfort in sensory systems. Bennet (1976) has found a positive correlation between age and discomfort sensitivity such that subjects of 70 years age tend to be seven times and subjects of 20 to 30 years twice as sensitive as 10 year olds. Many mathematical models have been proposed, but they often neglect the physiological, psychological and age factors. In the experiments the subject's task is generally to increase the intensity of a light source, via a control knob, to that level at which it is judged to be at the borderline between comfort and discomfort (BCD). Such settings are made at several background luminance levels, but even in normal subjects, Stone and Harker (1973) found very great differences and various trends in the settings of discomfort glare criteria.


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2.6.3 Eye colouring factor vs. age factor The veiling luminance perceived by the eye depends on the eye colour and on the age of the user. Those issues have strong consequences on the preferred contrasts and illuminance levels. Visual comfort is particularly important for security reasons when driving on the road and is pointed in the artificial lighting system of tunnels or in the night-lighting of roads. Based on the CIE method which integrates the results of Fry (Fry, 1976), Adrian and Schreuder’s works, the French Organisation for Tunnel Studies (CETU, 1998) gives a method for the evaluation of visual comfort, depending on the age of the user and on the colour of his iris. The iris is ranked from 0 –dark eyes- to 1,2 –clear blue eyes-. The expression given by Cie (CIE 24th Session, Warsawa 99) is: 4 Lveil luminance  10   5 0.1 p 1 age   0.0025 p Eglare  3  2     62.5  

E glare : vertical illuminance received by the eye, for a given lighting source θ : angles in degrees between lighting source and vision direction p : from 0 (dark brown eyes) to 1.2 (clear blue-eyes) Formula is valid from small angles (0.1°) to great angles (100°) Table 4 shows the differences of contrast acceptance between 4 users, by changing the age or the colour of the eyes, in the same visual situation in a car (equal luminance in the 2° cone and on the visual field). Table 4

User age and eye colour 70 years old iris =0 (dark) 70 years old iris =1,2 (clear) 25 years old iris =0 (dark) 25 years old iris =1,2 (clear)

veiling luminance and critical contrast on road

Luminance in the 2° cone (cd/m²)

Veiling luminance (cd/m²)

Critical contrast

195

41

0,535

195

55

0,534

195

22

0,18

195

30

0,18

Note 1: Critical contrast is the contrast above which the user will not see a fence on the road in normal conditions.


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The veiling luminance is almost twice more important for the older persons, and difference exists between clear and dark eye people of the same age, in a range of 25%. The critical contrast is 3 times higher for older persons. Differences are not significant between too persons of the same age. The age of the users has a wide influence on their visual perception of situations, when considering the veiling luminances. It is also admitted that age factor is a main parameter for visual acuity and perception of contrasts. The colour of the eye is significant in a same category of age, influencing the veiling luminance but not the contrast perception. Influence of age prevails on the influence of eye colouring, first one having important consequences on the general dispersion of user preferences, and the second one having it’s influence inside a same age category. 2.6.4 Exposure of eye to UV and short-wavelength light (“Blue-Light”) Quotation from Sliney and Marshall (2002), CIE x024:2002, Proceedings of the CIE/ARUP Symposium on Visual Environment The eye is well adapted to protect itself against optical radiation (ultraviolet, visible and infrared radiant energy) from the natural environment. The adverse effects of viewing bright light sources, such as the sun, has been studied for decades and guidelines for limiting exposure to protect the eye have been developed (see Sliney and Marshall, 2002 for additional references). Hazards to the eye from optical sources are: o

Ultraviolet photochemical injury to the cornea (photokeratitis) and lens (cataract) of the eye (180-400 nm).

o

Thermal injury to the eye (400-1400 nm)

o

Blue-light photochemical injury to the retina of the eye (principally 400-550 nm; unless aphakic 310-550 nm)

o

Near-infrared thermal hazards to the lens (approximately 800-3000 nm)

o

Thermal injury of the cornea of the eye (approximately 1400 nm to 1 mm)

Retinal hazard is a result of viewing bright light sources, causing photoretinitis (e.g. solar retinitis is a result from staring at the sun). In recent years it has become clear that photoretinitis is also caused by an intense exposure of the retina to shorter wavelengths in the visible spectrum, i.e. violet and blue light (referred to as “blue-light”). Blue-light retinal injury (photoretinitis) can result from viewing either an extremely bright light for a short time, or less bright light for longer exposure periods. For an


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outdoor situation, the sun has a strong blue and UV component only near midday. The spectral irradiance at a wavelength of 300 nm (UV-B) is ten times grater that at either three hours before or after noon. When the sun altitude is at angels of about 5º to 20º above horizon, one can stare at it for short periods without risk of injury. The natural aversion to bright light and the use of solar protection, i.e. Venetian blinds, prevents hazardous acute retinal exposure from the sun. End of Quotation 2.6.5 Sensibility to glare coming from the upper / lower part of the visual field No dedicated study was found around this thematic. Nevertheless, observations carried on during light experimentations, correlated with medical observations tend to show that users have a higher sensibility to glare coming from the lower part of the visual field. This could be related to the natural adaptation of the eye to higher luminances coming from the sky. 2.6.6 Non visual field: influence of light on melatonin secretions, influence on the pineal gland Considering daylighting, , a standard user working in offices will spend 8 hours at 150lux and 2 hours at 2000lux . He receives around 8 000 lxh a day, while somebody having exterior activities will typically receive 100 000 to 200 000 lxh per day. The occupant of an office only reaches 0,5 to 3% of the daylight he would receive in exterior conditions. This difference has important consequences on non-visual fields. Rea (2002) reports that the amount of light needed for vision (spectral composition, spatial distribution, timing and duration) is very different from that needed for circadian (rhythm of day and night) functioning. This difference is related to the melatonin hormone, which is secreted by the pineal gland. The melatonin level in blood is the primary measure of the status of the “master biological clock”. When melatonin secretion is stopped, we wake up, and we fall asleep when secreting. Light is the primary stimulus for controlling, through the suprachiasmatic nucleus, the timing and the amount of melatonin produced by the pineal gland. The typical office illuminance levels (500lx) from fluorescent lights were shown to be ineffective on melatonin suppression (Lewy et al, 1980), while very low light levels (3,5lx) can affect the circadian system (Kronauer et al, 1999). This can be explained by the spectral light composition : blue Leds (460nm peak ) and red (630 nm peak) produces the same photopic illuminance, but their relative effectiveness for the circadian system is about 1200 to 1. We thus have to be careful, though medical issues are still experiencing these concerns, that visual comfort reached by balancing daylight and artificial light with shading systems does not necessarily means optimal lighting when considering the circadian rhythm issues. The assessed solutions for visual comfort will only be also relevant for circadian system if we achieve optimising daylight penetration, even by diffusion.


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3 Literature survey to come: 3.1 CIE Survey Survey of the Technical Committee 3-39 of the International Lighting Commission (CIE) - with the objective of proposing a new model for daylight glare evaluation

3.2 Others To be completed during ECCO-build Project.


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4 Appendix 4.1 Appendix A. Assessment of glare (Aizlewood, BRE, UK) The potential to control daylight glare is an important feature of the daylighting systems under test. There are a number of glare indexes that have been developed. The monitoring protocol has been developed to provide the necessary data for calculation of the Daylight Glare Index (DGI)(1). The DGI is given by the formula: DGI 10 log10 0.478

n

L i 1

b

L1s.6   0.8  0.07  s0.5  Lw

Ls is the average luminance of each glare source in the field of view [cd/m2] Lb is the average luminance of the background excluding the glare source [cd/m2] Lw is the average luminance of the window [cd/m2] s is the solid angle of the source seen from the point of observation [sr]  is the solid angle subtended by the source, modified for the position of the light source with respect to the field of view and Guth's position index P [sr]. n is the number of glare sources Rather than attempting to make a difficult series of frequent spot luminance measurements in the test rooms, the protocol calls for continuous measurement of shielded and unshielded vertical illuminances from which Ls, Lb and Lw can be derived3. Example vertical sensors are shown in Figure 3. One sensor is unshielded and measures the vertical illuminance at that point in the room. A cone of black material shields the other sensor such that it only receives direct light from the window. The glare source luminance Ls is determined from: Ls 

E shielded  

where  is the configuration factor of the glare source with respect to the measurement point (see appendix B). The background luminance Lb excludes the area of the glare source and is given by: Lb 

EUnshielded  E Shielded   1   

The value Lw is the average luminance of the window. If the shielding cone is shaped such that the shielded sensor sees the whole window but nothing else, then Lw is the same as Ls. This is the preferred configuration, and results in a “cone” with an irregular pyramidal shape. 3

Velds & Christoffersen would like to add that it has not been proved yet if this experiment is reliable in situations with daylighting systems (for more information see Velds 2000).


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Alternatively, it is easier to build a simple cone or cylinder. In this case the shielded sensor sees only part of the window. Care should be taken that the sensor sees an average portion of the window such that Lw can be treated as Ls. s is the solid angle subtended by the glare source (window) to the point of observation. It can be calculated using the following equation:

s 

A  cos  cos d2

A is the window area [m2] d is the distance from the viewpoint to the centre of the window area [m] , are the angles between the line of sight and the centre of the window area

Figure 3 . Example of the shielded and unshielded illuminance sensors used to calculate the DGI  is slightly more difficult to calculate. It is the solid angle subtended by the window, modified by the position index of the window, P. The basis of this modification is that the original data collected by Hopkinson, was for a glare source 10o above the line of sight. A glare source directly in the line of sight would be significantly more glaring, while at the periphery of vision it would be notably less glaring. (A table of values for P is given in Figure 4).  is calculated using: 

 P  d i

s

ds the solid angles of elements of the window and Pi the position indexes of those elements.


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Calculating  with a large number of elements may generate a small increase in accuracy. The DGI formula is expressed as a sum of the individual glare sources in the field of view. One of the problems with the DGI is that if you divide a large glare source into a number of smaller sources and then sum them up at the end you would not get the same result as if you treated the source as one large element. This lack of mathematical consistency exists because the DGI is an experimental fit to data. For the purposes of this protocol, the summation in the DGI formula can be ignored and the daylighting systems can be treated as one large source. The DGI (and this method of calculating it) only considers average daylight glare. It does not take into account direct sunlight striking an occupant. Nor does it consider small areas of high brightness within the overall window area such as the bright vertical lines that can be produced by prismatic systems in direct sunlight. For this reason, spot measurements and photographs should be used to record additional glare phenomena.

Figure 4 : Table of position index

Measurement advice The DGI is supposed to be “substantially independent” of the measuring position. Thus, the positioning of the vertical sensors is not critical. A distance from the window of 3 m or more is recommended. The position should be chosen such that the vertical cells do not directly shade any horizontal cells. The DGI is a daylight glare index, not a sunlight glare index. At any time that direct sunlight falls on the vertical cells, the formula can become unreliable. Cells should be far enough from the window relative to the window head height and solar elevation so that


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direct sunlight does not strike the cells too often. If the shielding cone has been chosen to align with the corners of the window, then care should be taken to line up the shielded sensor correctly. One method is for an observer to look from the corners of the window towards the shielded sensor and get an assistant to adjust the shielding and sensor position until the whole sensor head is only just visible from each position. Alternatively, if the shielding construction allows it, a small light source within the cone can be used to align the sensor with the window corners at night.


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4.2 Appendix B. Configuration factor for element parallel to rectangle The configuration factor i of the examined part of the window from the viewpoint of the observer can be found for instance by assuming the area of where the measurements are recorded as a small area F1 parallel to the examined rectangular part of the window area Fa. The small area F1 lies on a line perpendicular to one corner of the examined rectangular part Fa. In order to adjust to different windows and reference location combinations that do not match the requirements that F1 lies on line perpendicular to one corner of Fa, the configuration factor i can be determined by adding and/or subtracting the i of two or more hypothetical rectangles. The configuration factor i can be estimated by the following equation or by the graph in Figure 5, derived from [ETSU 1993]. Input

Parameter X and Y where

X 

Equation

a b and Y  as in diagram c c

A arctan B  C arctan D 2 X Y Y X where A  , B , C , D 2 2 2 1Y 1Y 2 1 X 1 X i 

Note Angles are expressed in radians Reference: Siegel R, Howell J R, Thermal Radiation Heat Transfer, McGraw Hill, 1972 ETSU, Daylighting Algorithms, Prepared by Tregenza P, Sharples S, Renewable Energy Research and Development Programme, Energy Technology Support Unit (ETSU), 1993


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Figure 5


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5 References Aizlewood M. E. (1993). Innovative Daylighting Systems: An Experimental Evaluation. Lighting Research and Technology, Vol.14, No.4 Anshell, J: Visual Ergonomics in the Workplace. Taylor and Francis, London, 148 pp, 1998. Arbejdstilsynet (2000). Om Lys. www.indeklima.at.dk/kontor/lys_b.html. Baker, N., Fanchiotti, A. and Steemers, K. (1993). Daylighting in Architecture: A European Reference Book. James and James. London (UK). Bean, A. R. and Bell, R. I. (1992). The CSP Index: A Practical Measure of Office Lighting Quality as Perceived by the Office Worker. Lighting Research and Technology. 24 (4). pp. 215-225. Begemann, S. H. A., Tenner, A. and Aarts, A. (1994). Daylight, Artificial Light and People. Proc. of the 39th IES Convention. Sydney Lights. Sydney (Australia). Begemann, S. H. A., van den Beld, G. J. and Tenner, A. D. (1995). Daylight, Artificial Light and People, Part 2. Proc. of the 23rd session of the Commission Internationale de l’Éclairage (CIE). New Delhi (India). pp. 148-151. Bennet, H. S., Wyrick, D., Lee, S.W., McNeill, J.H. (1976): Science and art in preparing tissue embeded in plastic for light microscopy with special reference to glycol methacrylates, glass knives and special stains. Stain Technology; 51(2): 71-79." Benton, C: Lockheed 157 Monitoring Project Phase II: The Lighting Control System, Final Report. Lawrence Berkeley National Laboratory, Berkeley, CA, 67 pp, 31 March 1989. Berman, S. M., Bullimore, M. A., Bailey, I. L. and Jacobs, R. J. (1996). The Influence of Spectral Composition on Discomfort Glare for Largesize Sources. Report LBL-37007 UC-1600. Lawrence Berkeley National Laboratory. Berkeley, California (USA). 22 pages. Berrutto V (1996) Monitoring the visual quality of lighting ambiances. Application to office lighting. Laboratory for Building Sciences, ENTPE-CNRS, France Berrutto, V., Fontoynont, M. and Avouac-Bastie, P. (1997). Importance of Wall Luminance on Users' Satisfaction: Pilot Study on 73 Office Workers. Proc. of Lux Europa 1997: the 8th European Lighting Conference. May 11-14, Amsterdam (The Netherlands). pp. 82-101. Blackwell (1946), A unified framework of methods for evaluating visual performances aspects of lighting, Technical report of the CIE 1972, CIE publication nr. 19.


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Boubekri, M. and Boyer, L. L. (1992). Effect of Window Size and Sunlight Presence on Glare. Lighting Research andTechnology. 24 (2). pp. 69-74. Boyce, P. R. (1973). Age, Illuminance, Visual Performance and Preference. Lighting Research and Technology. 5 (3). pp. 125-144. Boyce, P. R., Cuttle, C. (1994). Lighting evaluation tools: a review. Lighting Research Center, Rensselaer Polytechnic Institute, Troy, New York, USA. Boyce, P. R. and Kennaway, D. J. (1987). Effects of Light on Melatonin Production. Biol. Psychiatry. No. 22. pp. 473-478. Boyce P. R. (1981). Human factors in lighting. Applied Science Publisher, London, UK Boyce P, Ngai P (2001). The effect of overhead glare on visual discomfort. Journal of the Illuminating Engineering Society, 29 (2), 29-38 Boyce PR, Eklund NH, Simpson SN (1999), Individual lighting control: task performance, mood and illuminance. Lighting Research Center, Rensselear Polytechnic Institute, NY Bülow-Hübe, H (2000) Office worker preferences of exterior shading devices. Proceedings of the EUROSUN in Copenhagen, Denmark. Canter et al (1975). Environmental interaction - psychological approaches to our physical surroundings. Surrey University Press, London, UK Carter, D. J., Slater, A. I., Perry, M. J., Mansfield, K. P., Loe, D. L. and Sandoval, J. (1994). The Influence of Luminance Distribution on Subjective Impressions and Performance within a Non-uniformly Lit Office. Proc. of the CIBSE National Lighting Conference. pp. 61-74. Centre d'Etudes des Tunnels (CETU) (1998), Method for road tunnel entry's lighting dimensioning. French ministry for equipment, housing and transportation. Chain, C, D Dumortier, and M Fontoynont: Integration of the Spectral and Directional Component in Sky Luminance Modelling. Proceedings of the 24th Session of the Commission Internationale de l’Eclairage (CIE), 24 – 30 June 1999, Warsaw, Poland, Vol 1, pp 248-252. Chain, C, D Dumortier and M Fontoynont: Consideration of Daylight’s Colour. Energy and Buildings, Vol 33, No 3, pp 193-198, 1998. Charton V (2002) Assessing the perception of lighting ambiances in real and in virtual conditions. Doctoral dissertation (PhD thesis), Laboratory for Building Sciences, ENTPE-CNRS, France Chauvel, J., Collins, B., Dogniaux, R. and Longmore, J. (1982). Glare from Windows: Current View of the Problem. Lighting Research andTechnology. 14 (1). pp. 31-46.


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Christoffersen, G. (1993 a). Sansning af lyskontraster. Louis Poulsen NYT No. 540 Christoffersen, G. (1993 b). Øyet som lysmåler. Louis Poulsen NYT No. 541 Christoffersen, J. (1995). Daylight Utilisation in Office Buildings. PhD thesis. April 1995. Energy and Indoor Climate Division, Danish Building Research Institute and Thermal Insulation Laboratory and Technical University of Denmark. Hørsholm (Denmark). 157 pages. Christoffersen, J., Petersen, E., Johnsen, K., Valbjørn, O, and Hygge, S. (1999). Vinduer og dagslys - en feltundersøgelse i kontorbygninger (Windows and Daylight - a Post-occupancy Evaluation of Offices). SBI-rapport 318. Statens Byggeforskningsinstitut. Hørsholm (Denmark). 71 pages (in Danish). CIBSE (1993). Lighting for offices (Lighting Guide LG7). Chartered Institution of Building Services Engineers (CIBSE). London (UK). CIBSE (1994). Code for Interior Lighting. Chartered Institution of Building Services Engineers (CIBSE). London (UK). CIE (1983). Discomfort Glare in the Interior Working Environment. Publication CIE No 55 (TC-3.4). Commission Internationale de l’Éclairage (CIE). Vienna (Austria). 43 pages. CIE (1986). Guide on Interior Lighting. Second Edition. Publication No 29.2 (1986). Commission Internationale de l’Éclairage (CIE). Vienna (Austria). 113 pages. CIE (1987). International Lighting Vocabulary. Publication CIE No. 17.4. Commission Internationale de l’Éclairage (CIE). Vienna (Austria). CIE (1992). Discomfort Glare in the Interior Lighting. Commission Internationale de l’Éclairage (CIE), Technical committee TC-3.13, Division 4, Interior Environment and Lighting Design. Vienna (Austria). Collins, B. L. (1994). Subjective Responses to Lighting: A Review of the Research. The Construction Specifier. October. pp. 82-90. Corth, R. (1987). Human visual perception - its implications to the illuminating industry." Lighting Design + Application, Vol. July DS 700 (1997). Retningslinier for kunstig belysning i arbejdslokaler. 5. edition (in English: Directions for artificial lighting in workrooms), Dansk Standardiseringsråd, Copenhagen, Denmark Dubois, M.-C. (2001). Impact of Shading Devices on Daylight Quality in Offices: Simulations with Radiance. Report TABK--01/3062. Lund Institute of Technology, Lund, Sweden. 157 pages Einhorn, H. D. (1969). A New Method for the Assessment of Discomfort Glare. Lighting Research andTechnology. 1 (4). pp. 235-247.


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Einhorn, H. D. (1979). Discomfort Glare: A Formula to Bridge Differences. Lighting Research and Technology. 11 (2). pp. 90-94. Enrech-Xena C (1999). Simulating natural light with artificial sources: architectural issues. Doctoral dissertation (PhD thesis), Laboratory for Building Sciences, ENTPE-CNRS, France Escuyer S (2001) In-site assessment of lighting control devices in offices, aiming at ergonomic improvement. Laboratory for Building Sciences, ENTPE-CNRS. ETSU, Daylighting Algorithms, Prepared by Tregenza P, Sharples S, Renewable Energy Research and Development Programme, Energy Technology Support Unit (ETSU), 1993 Fontoynont, M. (2000). Archi: Confort thermique - confort visuel, Hexcel Fabrics. Brochure distributed by Hexcel Fabrics. Villeurbanne (France). Foster M, Oreszczyn T (1999) Occupant control of passive systems: the use of venetian blinds. The Barlett School of Architecture, University college London. Fry G. A. (1969). Limits of the field of view." Illuminating Engineer, Vol. May Fry, G. A., and Alpern, M. (1953). The effect of a peripheral glare source upon the apparent brightness of an object. Journal of the Optical Society of America 43, 189-195. Galasiu A, Atif M. (2002) Field performance of daylight-Linked lighting controls and window blinds: a pilot study. National Research Council, Canada. IEA Task 31 and CIE division 3 mini-conference, Ottawa, Canada, October 2nd 2002. Gall D., Vandahl C., Jordanow W., Jordanowa S. (2000) Tageslicht und künstliche Beleuchtung: Bewertung von Lichtschutzeinrichtungen, Schriftenreihe der Bundesanstalt für Arbeitsschutz und Arbeitsmedizin, Fb 882, Dortmund/Berlin 2000 Guth S. K. (1951). Comfortable brightness relationships for critical and casual seeing. Illuminating Engineering, Vol. February Guth S. K. (1952). BCD brightness ratings in lighting practice. Illuminating Engineering, Vol. April Guth S. K. (1955). Quality of lighting. Illuminating Engineering, Vol. June Guth S. K., McNelis J. F. (1959). A discomfort glare evaluator. Illuminating Engineering, Vol. June Guth S. K., McNelis J. F. (1961). Further data on discomfort glare from multiple sources. Illuminating Engineering, Vol. January Guth S. K. (1963). A method for the evaluation of discomfort glare. Illuminating Engineering, Vol. May


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Guth S. K. (1966). Computing visual comfort ratings for a specific interior lighting installation. Illuminating Engineering, Vol. October Holladay L. L. (1926). The fundamentals of glare and visibility. Journal of the Optical Society of America and Review of Scientific Instruments, Vol.12, No.4 Hopkinson, R. G., Bradley, R.C., (1960). A study of glare from very large sources. Illuminating Engineering, Vol. 55, No. 5, pp. 288-294. Hopkinson R. G., Longmore J. (1959). The permanent supplementary artificial lighting of interiors. Transactions of the Illuminating Engineering Society (London), 24, pp. 121-148 Hopkinson R. G. (1963a). Architectural Physics: Lighting. Her Majesty's Stationary Office, UK Hopkinson R. G. (1963b). Daylight as a cause of glare. Light and Lighting, 54, pp. 222-226 Hopkinson, R. G., Petherbridge, P. and Longmore, J. (1966). Daylighting. Heinemann. London (UK). 606 pages. Hopkinson R. G. (1969). Lighting and seeing. Heineman, London, UK Hopkinson R. G., Collins J. B. (1970). The ergonomics of lighting. Macdonald Technical and Scientific, London, UK Hopkinson R. G. (1970 -71). Glare from windows. Construction Research and Development Journal (Conrad), Three parts: Vol. 2 No. 3, pp. 98-105; Vol. 2 No. 4, pp. 169-175: Vol. 3 No. 1, pp. 98-105 Hopkinson R. G. (1972). Glare from daylighting in buildings. Applied Ergonomics, 34, pp. 206-215 Hygge, S and H A Löfberg: User Evaluation of Visual Comfort in Some Buildings of the Daylight Europe Project. Proceedings of Right Light Four, The Fourth European Conference on EnergyEfficient Lighting, Copenhagen, Denmark, November 1997, Vol 2, pp 69-74. Hygge and Löfberg 1999. POE – Post Occupancy Evaluation of daylight in buildings. A Report of IEA SHC Task 21 / ECBCS Annex 29. IES (1928). Illuminating Engineering Society (North America - IESNA), (1928) Code of lighting: factories, mills and all other work places, Transactions of the illuminating Engineering Society, vol. 13, Nr. 10, December 1928. IES (1962). The development of the IES glare index system. Contributed by the Luminance Study Panel of the IES Technical Committee, Transactions of the Illuminating Engineering Society (London), Vol.27 IES (Rea, M. S., ed.) (1993). Lighting Handbook: Reference and Application.


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