Models of sky radiance distribution and sky luminance distribution

Abstract

The aim of this research work is to propose sky radiance and luminance distribution models for all sky conditions from clear sky to overcast sky. The classification method of sky conditions is examined. Clear sky index is defined based on the global irradiance. Cloudless index is defined based on the global irradiance and the diffuse irradiance. The sky conditions are classified according to both indices. The data of the average sky radiance (luminance) distributions are obtained based on the classified sky conditions. The average relative sky radiance distributions are compared with the average relative sky luminance distributions. It has been confirmed that both radiance/luminance distributions can be shown by the same equation. An equation that shows the relative sky radiance (luminance) distribution for all sky conditions and equations that show the zenith radiance and the zenith luminance are obtained. The absolute value of the sky radiance (luminance) distribution is shown by multiplying the relative sky radiance (luminance) distribution and the zenith radiance (luminance). The equations to show the absolute values of the sky radiance distribution called the All Sky Model-R and the sky luminance distribution called the All Sky Model-L are proposed. Both models are compared with the previous models based on the measurement distributions concerning the region of sky vault and the sky conditions and good results are obtained. In addition, the vertical irradiance and illuminance calculated by the proposed models and the previous models are compared with the measurements, and the proposed models are confirmed their accuracy.