Predicting daylight illuminance by computer simulation techniques

The first step in evaluating the visual performance and energy efficiency provided by daylight requires an accurate estimation of the amount of daylight entering a building. Traditionally, the daylight performance of a building is often evaluated in terms of daylight factors with the calculations being based on the CIE overcast sky. In general, the daylight factor approach is quite simple to use but it cannot predict the dynamic variations in interior illuminance as sky conditions and the sun's position change. The recently introduced concept of the daylight coefficient, which needs sky luminance data, provides an alternative approach. With the advances in computer technology, the computation of daylight illuminance using the two prediction methods can be conducted by a simulation program. This paper presents a study of the daylight factor and daylight coefficient approaches. The interior daylight illuminance data measured in a scale model and a classroom were compared with the simulated results using RADIANCE computer simulation software. It was found that, in general, the daylight coefficient approach performs better than daylight factor approach. Both methods tend to estimate the daylight illuminance less accurately when the measurement points in the classroom are far away from the window façade. It seems that the parameters for inter-reflection calculations including the furniture layout and external obstructions cannot be input into the RADIANCE program with sufficient detail for accurate simulation. © The Chartered Institution of Building Services Engineers 2004.