Average daylight factor for the 15 CIE standard skies

For daylighting calculations, the design criteria are often expressed in terms of average daylight factors (DFave) with the computations being based on the CIE (International Commission on Illumination) overcast sky, which is generally considered to provide the worst daylight condition. The daylight illuminance of a room is mainly influenced by the luminances and patterns of the sky in the direction of view of the window at any given time. Overcast skies may not always be the appropriate reference sky types for daylighting analysis. Recently, Kittler et al. have proposed a new range of 15 standard sky luminance distributions including five clear, five partly cloudy and five overcast sky types. These 15 sky luminance models have been adopted as the CIE General Standard Skies. This paper presents an approach to computing the DFave for the 15 standard skies. The techniques for calculating the two configuration parameters for the DFave determination, namely; (a) light received directly from the sky above the horizon on a vertical surface, and (b) light received directly from the ground below the horizon on a vertical surface are established and described. The performance of the proposed method was assessed against the results obtained by other independent calculation methods and computer simulations. It is shown that the results predicted by the proposed approach are in reasonably good agreement with those produced from the other two calculation tools. The findings provide architects and building designers with a reliable and simple method for estimating the interior daylight illuminance under various standard overcast and nonovercast sky conditions.