As discussed in the Sky Illuminance topic, it is possible to model the distribution of light over the sky dome and establish a worst-case Design Sky illuminance level to design for. The next step is to be able to work out how much of the light from the sky dome actually ends up passing through the apertures of a building and into the spaces within. Unlike electric lighting, the total available light is fixed (in this case the worst-case Design Sky illuminance), thus control over the amount of light is possible only by changing the means of transmission into the space through its apertures, and then to points deeper within the space by its distribution system.

This means that architectural elements such as windows, skylights, light-shelves and even the reflectivity of internal surfaces are very important factors in daylighting design. So too are external elements such as site obstructions and applied shading devices.

As lighting conditions are continually changing and absolute sky illuminance levels are often hard to find or calculate, it is more useful to design based on the relative amount of light entering a space. This way internal light levels can be expressed as a percentage of external levels. Thus, even though the sky may get momentarily darker or brighter, the percentage will remain the same. This means that the percentage value is a property of the building itself, providing an objective measure for the comparison of alternative design options and allowing for direct comparison with other buildings. This percentage value is known as a Daylight Factor.

Daylight Factor Definition

The Daylight Factor is defined as the ratio of the illuminance at a particular point within an enclosure to the simultaneous unobstructed outdoor illuminance under the same sky conditions, expressed as a percentage. Once both the Daylight Factor and Design Sky are known, simply multiplying the two together gives the illuminance level (in either lux or foot candles) due to daylight at the point.

Daylight Factor Calculations

Working out the Daylight Factor in different areas of a building can be a time consuming and laborious process. In most cases it is done using a computer program, of which there are quite a few to choose from. However, a good knowledge of manual calculation methods is very important if you are to fully understand the processes involved and therefore apply these computer programs in the most appropriate ways. There are a number of ways to calculate the Daylight Factor for a space:

• Average Daylight Factor
  This is quite a simple equation that requires only a few parameters and makes quite a few assumptions about the nature of your space. The result is a single value room-average daylight factor.
• Daylight Factor Protractors
  Also known as the Split Flux Method, this involves overlaying protractors onto the plans and sections of your building. This can be done directly on print-outs or, using the new Square One DF Protractor tool, directly over a scanned image or within your favourite CAD tool.
• Projecting Points of Equal Sky Illuminance
  This is a simplified method involving the projection of points over the sky dome within a 3D view of your model or in a Sun-Path diagram. You can then simply count the points you can 'see' through windows and skylights.
• Geometric Analysis using ECOTECT
  This method involves generating or importing a 3D geometric model of the building and it's apertures into the ECOTECT software, within which daylight factor contours can be calculated and plotted.
• Radiosity Analysis using RADIANCE
  The RADIANCE software is a physically accurate lighting simulation tool from Lawrence Berkeley Laboratories. You can generate models for RADIANCE within AutoCAD (if you have the free Desktop Radiance toolkit), from ECOTECT or, if you are really keen, using a simple text editor.

Useful Links

Building Daylight Models - Quick Tips (1.4Meg - slow download)
http://www.lightingdesignlab.com/pdf/New_Model_Study_Quick_Tips.pdf

Application of Daylighting Computer Modeling in Real Case Studies:
Comparison between Measured and Simulated Daylight Availability and Lighting Consumption
http://www.iea-shc.org/outputs/task21/nrcc42862.pdf

Lighting - the Electronic Textbook
http://www.saud.ku.edu/book/contents.htm