QUESTION:

How come when I change the date and time in my ECOTECT model and re-run the daylight factor calculation the results do not change ? Surely changing the position of the sun (by changing the date and time) will alter the amount of light I get in my building...?

ANSWER:

Regarding daylight calculations, the daylight factor is a worst-case scenario based on the CIE overcast/uniform sky. Thus it assumes a completely overcast sky in the middle of winter. The actual day and date will make no difference. The CIE Overcast Sky varies only with altitude, being 3 times brighter at the zenith than at the horizon.

If you want to simulate actual dates and times, simply export your model to Radiance and it will generate a full analysis with the selected sky condition and at the currently selected date and time.

Unfortunately, the BRE Daylight Factor method is always based on an overcast sky as that is the worst case you are likely to find -- hence good for designing with. There is no point for legislation to specify light levels on a sunny day in summer as natural light is usually quite abundant. Thus regulations are usually based on minimum daylight factors which is why ECOTECT does lighting it this way.

If you want to see how the space will look or how the light level is distributed over surfaces under a particular sky condition, then Radiance is the best way. The good thing about using Radiance is that ECOTECT (once setup) will run the program for you, so will take away a lot of the hassle of running Radiance on its own... you can even re-import Radiance numbers back onto the ECOTECT Analysis Grid if you prefer that type of graphical display/consistency.

The following are a good set of references on our web site which goes in to more detail about the intricacies of the DF calc, it's limitations and also the implementation in ECOTECT:

• The Daylight Factor
• The Split Flux Method
• ECOTECT Daylight Factor Analysis
• Points of Equal Sky Illuminance