Simple software tool for determining internal daylight illuminance

The traditional daylight factor approach with the calculation being based on an isotopic overcast sky cannot cater for the dynamic variations in sky luminance and illuminance as the sun's position changes under non-overcast skies. In 2003, the International Commission on Illumination (CIE) adopted a range of 15 standard skies covering the whole probable spectrum of usual skies found in the world. Currently, we propose a numerical method that considers the changes in the sky luminances to predict the interior daylight illuminance under the CIE Standard Skies. This paper extends the study to develop a simple computer tool for determining the indoor daylight. The daylight illuminances in a living/dining room of a typical domestic flat were computed and compared with those based on a lighting program, namely, RADIANCE. It was shown that the interior daylight illuminances estimated by the simple program were in reasonably good agreements with the results produced from RADIANCE. The findings also revealed that daylighting designs using the CIE overcast sky only would considerably underestimate the indoor daylight availability.