An analysis of all-sky zenith luminance data for Hong Kong

Sky luminance distribution data are of great importance to estimate internal daylight illuminance for building energy modelling. Sky luminance of a particular sky patch is always expressed as a ratio of the corresponding zenith luminance. For places where measured zenith luminance is not available, generating the required data from conversion models would be an appropriate alternative. This paper describes the sky luminance measurements made in 1999 at the City University of Hong Kong and reports the findings. A simple model relating zenith luminance and horizontal diffuse illuminance has been proposed. An evaluation of the proposed model and the Perez zenith luminance model has been carried out. It has been found that the proposed model gives reasonably good agreement with the measured data and outperforms the Perez model. The simplicity nature of the proposed model provides a convenient and reliable approach for building designers in estimating the zenith luminance as well as the whole sky luminance distribution.