a very believable way. This offers both an op- portunity and a risk. The opportunity is that daylight in buildings can be far more accurately designed than before.
LATEST DEVELOPMENTS IN DAYLIGHTING MODELLING DESIGN
RIGHT
BY DR MICHAEL DONN, SCHOOL OF ARCHITECTURE, VICTORIA UNIVERSITY OF WELLINGTON
Optimum daylighting DAYLIGHTING DESIGN HAS COME A LONG WAY IN THE LAST 20 YEARS, BUT THE DESIGNER NEEDS TO UNDERSTAND HOW TO USE SKY AND CLIMATE MODELLING DATA TO DELIVER OPTIMUM LIGHTING IN A 21ST CENTURY OFFICE SPACE.
Computers today can simulate the appearance of buildings under different light conditions in a very believable way. This offers both an opportunity and a risk. The opportunity is that daylight in buildings can be far more accurately designed than before. On the flip side, the tools are extremely easy to use and can produce convincingly realistic pictures even when the input data is inaccurate.
Perez sky-based renders of traditional clear sky (left), cloudy sky (centre) daylight plus the real world partly cloudy sky (right).
The key areas to get right are sky definition and accurate material and physical dimensions.
Data needs to be accurate Less than 10 years ago, books describing how to produce realistic images of buildings included sections on faking reality. The focus was on tweaking settings to achieve a realistic appearance, assisting operators to represent what they thought the room should look like, not on how real lights and real building surfaces might function. Over the past 2 decades, computer models of light have begun to use physics-based renderers and interfaces, and increasingly programs require real-world values for parameters defining real light sources and for building properties. With the earlier ‘artist’ approach to rendering light, the calculations might look realistic but
Accurate daylight render by Jake Osborne as part of his Master of Building Science from Victoria University.
not represent reality. Although using the new physics-based approach with accurate material
Daylight design has in the past focused on cloudy
properties and dimensions can produce realistic
skies because they are simpler to model. Before
outside illuminance on cloudy days. For example,
The DF is the ratio between the inside and
daylight predictions, use inaccurate data, and the
the 1950s, a cloudy sky was assumed to be of
in a climate where the outdoor illuminance is
results can be just as unreal as before.
uniform brightness. This was replaced by the CIE
greater than 6,000 lux, a 5% DF predicts that the
standard overcast sky model, and techniques
light level indoors will be greater than 300 lux.
Real skies as models
and formulae were developed to assist specialist
If 6,000 lux is the minimum available outdoors
The new software can model a far wider range of
daylight designers to estimate daylight factors
for 80% of the working hours in a year, then a
real daylight conditions experienced in buildings.
(DF) for new building designs.
minimum of 300 lux will also be available
Build 133 — December 2012/ January 2013 — 27
indoors for 80% of these hours – so electric lights
While the Perez sky is a huge improvement over
posing two targets – a minimum and a maximum
can be replaced for 80% of the working year in
the CIE standard clear and cloudy skies, there are
for the task. In an office, these might be 300 lux
offices where 300 lux is the target.
two caveats:
minimum and 2,000 or 3,000 lux maximum.
For much of the last century, it was assumed
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Developed for weather data from Berkeley,
Variations in designs can now be compared on
this addressed a worst-case scenario, and as long
California, it may not be suited to desert or
all the hours in a year, rendering the traditional
as the office had sunshades keeping out direct
tropical humid climates, but it should suit New
cloudy day DF approach obsolete.
sun, it would produce good daylight design. With
Zealand.
physics-based daylight calculation software, designers can examine more than the worst case.
●●
Combined with the UDI, statistics can be
It produces significant errors at very low sun
calculated about how often anywhere in a
angles so should not be used for calculation
building daylight might be sufficient for the task
when the sun is below 5 degrees above the
and not too great as to cause glare. With the
From one calculation to 15 skies
horizon, for example, before 9 am and after 6
Perez All-Weather Model of the sky built into a
At its simplest, the DF approach comprised one
pm in winter in our southern cities.
program, it is now possible to calculate the UDI
calculation or a scale model study for cloudy
for every hour of the working day all year by
conditions to represent the whole year. Making
Green building schemes
scale models is time consuming, so the calcula-
In green building rating schemes, daylight auton-
tion was made simple enough to be repeated on
omy (DA) is used to rate daylighting approaches.
different designs. More complex schemes tried to
DA reports not just that a particular light area is
Modern daylight design – what’s in the toolbox?
incorporate estimates of the CIE standard clear
achieved, but what percentage of the floor area
Software currently available can be reliably used to
sky on representative days and times throughout
achieves this. For example, a point in a room near
examine new and innovative ideas about daylight
the year.
a window might be lit to a target of 300 lux for
design. Used to its potential, it can accurately
80% of the working year and therefore its DA is
predict performance at a much more profound
Sunny sky mathematical definitions from the
80%, but a point a long way from the window
level than achievable with traditional methods.
1950s with 13 more skies representative of those
may only have a DA of 20%. The best daylight
in real climates around the world. The challenge
schemes have a high DA across a large propor-
address presented at the International
for the CIE is educating designers on using these
tion of the floor area and do not have excessive
Commission for Illuminance (CIE) Congress
15 skies. Until this guidance is available, designers
contrasts between parts of the room.
in Shanghai in September 2012.
The CIE recently supplemented the Cloudy and
are unlikely to incorporate these skies into an analysis of daylight in their designs. The standard mode of operation of 3D
Note
This article is based on a keynote
In 2000, new research proposed an improvement on this DF/DA approach. The researchers noted that the DA approach assumed
computer render programs today is unfortunately
that any DF that led to an illuminance indoors
stuck in the last century, selecting only the
greater than the target was good. The corollary of
simplistic CIE standard overcast sky approach for
this is no illuminance is too bright.
modelling. There is an alternative.
linking in the appropriate weather file.
Buildings designed using this traditional
Perez sky software Software that can use Perez sky to model
approach, even with the sun excluded, risk being
daylight includes:
Perez All-Weather model
so overglazed that the shades are pulled all the
●●
Known as the Perez sky after the researcher who
time to control excess light, defeating the original
Max Products. 2011. See usa.autodesk.
developed it, the Perez All-Weather model auto-
purpose of the DF study.
com/3ds-max
matically selects and generates the most suitable
●●
climate datasets used for dynamic thermal
Minimum and maximum daylight useful
modelling, for example, those for New Zealand
The Useful Daylight Illuminance (UDI) index
developed by NIWA for EECA.
brings common sense to these calculations, pro-
sky type from parameters found in the annual
28 — Build 133 — December 2012/ January 2013
Autodesk 3ds Max Design. Autodesk 3ds
LBNL. Radiance Software. See radsite. lbl.gov/radiance/
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Welcome to Daysim. 2011. See daysim. com/index.html.