Optimum daylighting

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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/

●●

Welcome to Daysim. 2011. See daysim. com/index.html.