There are a number of different sky types in Radiance, but two are of particular importance - the CIE Overcast Sky and the Sunny Sky. The CIE Overcast Sky is used to give the worst-case scenario (in design terms) for illuminance. The Sunny Sky is used to give the worst-case scenario for glare analysis (luminance images). Note that the CIE Overcast Sky does not have a fixed brightness everywhere on the sky dome (usually quoted as 10,000 Lux). In reality, the brightness of an overcast sky varies from the horizon to the zenith, by site latitude, by date and by the time of the day. The most commonly used overcast sky in Radiance is the 10K Lux CIE overcast sky which is fixed at 10,000 lux at the zenith.
