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One of the aim of daylighting design is to provide enough daylight in a room to realize several tasks and to create a luminous comfortable space.

There are different ways to predetermine daylight availability in a building. You can work with simplified models (tables, diagrams,…), complex models (computer tools, …) and scale models (under real sky or artificial sky).

To quantify daylight in a space, you can choose to work with static or dynamic metrics.

The most familiar static metric is the daylight factor. Daylight factor (DF) expresses the ratio between illuminance on the work plane and illuminance available outdoor, under an overcast sky. It is easily calculated (scale model under artificial sky, split flux method, computer tools, …) but is based on the worst sky conditions : the overcast luminance distribution. That is why it is necessary to couple illuminance information obtained from the calculation of DF with the penetration of sun in the room and with risks of glare, which change over the year.

Some dynamic metrics called climate-based daylight metrics (CBDM) were developed recently to counter the limitations of daylight factor and take into account some important parameters (evolution of availability of light over the year, orientation of the building and penetration of sun, location of the building, …). These dynamic metrics will give information about the ability of the designed room to reach a minimum level of illuminance with only daylight like the daylight autonomy (DA). Or the ability of the room to reach an illuminance goal, like the useful daylight index (UDI) and the illuminance metric develop in Lightsolve.

Calculations and design methods


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Daylight factor (DF)

“The daylight factor is the ratio of the interior natural illumination received at a point of a reference plane with the simultaneous external illumination of a horizontal surface in a perfectly unobstructed site under overcast sky.

These two illumination values are due to the light received from a same sky, whose distribution of brilliances is estimated or is known, with direct sunlight being excluded. The daylight factor is expressing as a %.

DF = 100 * Einterior / Eexterior (%)

Under overcast sky conditions, daylight factor values are independent of the orientation of glass windows, of the season and of the time of day. In this way, they give an objective and easily comparable measurement of illumination quality inside a building. A room's daylight factor values can then be compared with the values of the minimum daylight factor reference values.

However, the daylight factor (DLF) does not make it possible to immediately see whether the recommended illumination levels for a visual task have been reached.

This being the case, once the daylight factor at a point in a room is known, the illumination reached at this point can be calculated at any moment of the year, under overcast sky conditions, from exterior horizontal illumination.“[1]


Extract from : [1] www.energies-renouvelables.com

© Architecture & Climat, UCL



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Daylight factor presents some limitations:- It does not take into account the location of the studied room in

terms of climate (which percentage of the year really presents an overcast sky ?)

- It does not take into account the location of the studied room in terms of latitude (what is the course of the sun? is there some sun penetration in the building?)

- It gives no information about risk of glare- It gives no information about quality of light

Daylight factor should be considered as an indication of the minimum level of illuminance reached under an unfavourable sky. It should be completed by maximal illuminances reached under sunny sky.

If the architect does not verify sun penetration, illuminance under more favorable sky and risks of glare, a design resulting from the only consideration of DF can suggest buildings with too much glazing which lead to overheating and risks of glare.

Nevertheless, calculation of DF is nowadays the basis of daylighting design in the French HQE standard (High environmental quality) and in LEED.


Sources : www.energies-renouvelables.com



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Climate-based daylight metrics (CBDM)

Climate-based daylight metrics (CBDM) are dynamic metrics calculated on the basis of weather data files. Contrary to static metric, dynamic illuminance, luminance and glare metrics evolve over the year according to the outdoor daylight availability. The amount of data simulated for the whole year is quite large and should be reduced with the development of CBDM.

Evaluation of CBDM can be realized through dynamic computer simulations or measurement in scale models. For each point in the space and for each moment (generally one hour) illuminance, luminance or glare can be calculated. Some CBDM suggest a spatial analysis while some others a temporal analysis.




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Dynamic autonomy (DA)

Developed by C. Rheinhart

Daylight Autonomy (DA) at a point of interest is the percentage of yearly occupied time for which a certain light level is reached through the only use of daylight. All sky conditions are considered through weather data files. Dynamic autonomy can be easily calculated for an area of interest with DAYSIM software. DA gives a spatial information.

Continuous daylight autonomy (DAcon) is similar to DA but partial credits are attributed when illuminance is between 0 lux and the minimum illuminance threshold.

Maximum daylight autonomy (DAmax) gives the percentage of time for which an exceeding illuminance can lead to risks of glare.


Simulations : Magali Bodart, Architecture & Climat



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Useful Daylight Illuminance (UDI)

Developed by J. Mardaljevic

Useful Daylight Illuminance (UDI) is a goal-oriented metric which informs about how fixed satisfying illuminances are spatially reached. UDI is the annual occurrence of illuminances, at a point, which are within the useful fixed range of illuminances.

In DAYSIM, ranges of UDI are fixed as following:

- < 100lux, illuminances are considered as too low;

- between 100 and 2000lux, illuminances are considered as useful;

- > 2000lux, there is to much light and risk of glare and overheating.


Simulations : Magali Bodart, Architecture & Climat



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Goal-oriented metrics in Lightsolve

Developed by M. Andersen

Illuminance metric developed in Lightsolve is, like the UDI, a goal-oriented metric. It fixes a rage of target values and evaluates the percentage of space whose performances falls within the range. So, contrary to UDI which evaluates the percentage of time for which fixed illuminances are reached on the workplane, in Lightsolve, it is the percentage of space satisfying criteria which is evaluated over the year.

Satisfying illuminance range is fixed as well as minimum and maximum acceptable illuminance. Partial credits are given if satisfying range illuminance is not reached but that tolerable illuminances (minimum or maximum) are not exceeded.

Results are plotted on temporal map graph (J. Mardaljevic) which presents on x-axis, the date and on y-axis, the time of day. The entire annual dataset can thus be viewed in one glance.

The triangular scale suggested in Lightsolve is an interesting way to inform about percentage of area of interest whose illuminance is within the satisfying range (yellow) or, when illuminance does not reach this criteria, if it is because illuminance is too high (red) or too low (blue). The availability of daylight over the year can moreover be analyzed as the distinction is done between night (black) and day (colored).

The Lightsolve methodology suggest a division of the year in 56 periods of time, for a matter of speed calculation, as Lightsolve is addressed to architects in the pre-design phase. But the methodology could be modified for an hourly application.


Simulations : Coralie Cauwerts, Architecture & Climat


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Mirror box

Illuminance under overcast sky


Sources: Architecture & Climat



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Mechanical sun

Penetration of sun in the scale model





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Single patch sky and sun

Based on the division of the sky in 145 patches (Tregenza)


Sources: CSTC



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Daylighting software

Several softwares (free or not) are nowadays available to realize daylight simulations (sun penetrations, illuminance calculations, luminance renderings, risks of glare calculations, lighting autonomy evaluation, …).

Google SketchUp – Google – free – Sun path

Ecotect – Autodesk – $ – Sun path – Split-flux formula

Dial-Europe – Estia – $ – Simplified formula

Dialux - DIAL GmbH – free – Radiosity

Relux - Relux Informatik – free – Radiosity + Raytracing

Velux Daylight Visualizer – Velux – free – Backward and forward raytracing (photon mapping)

3DSmax – Autodesk – Sun path – Backward and forward raytracing (photon mapping)

Desktop Radiance & Radiance – LBNL – free – Backward raytracing

Lightsolve - software under development

These softwares can be completed, for the UDI and DA calculations, by:

Daysim – Harvard Graduate School of Design, the Lighting Group of the National Research Council Canada and at the Solar Building Design Group of the Fraunhofer Institute for Solar Energy Systems – free




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Some interesting links to tools and design aids:

http://www.daylighting.org/designaids.php

Help for the construction of model scales :

M. Bodart, A. Deneyer, V. Gilbert, Validation of the Belgian single-patch sky and sun simulator, Building and Environment, 43 (2008), 1892-1901.

M. Bodart, R. de Peñaranda, A. Deneyer, Photometry and colorimetry characterisation of materials in daylighting evaluation tools, Building and Environment 43 (2008), 2046-2058.




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Homeaboutpartnersnewsdownloads principlesin depthapplicationsteaching resourceslearning resourcesprinciples Natural Lighting One of the aim of daylighting