Estimation of horizontal illuminance by measured sky luminance distribution

Horizontal diffuse illuminance and direct illuminance were estimated by measured sky luminance distribution to investigate the consistency among global, diffuse and direct illuminance for a long-term IDMP data in Kyoto, Japan. In order to estimate the diffuse and direct illuminance by sky luminance distribution, it is necessary to identify the direct circumsolar region of the sky. This study introduced a new method to identify the direct region by comparing the measured diffuse illuminance and the calculated illuminance came from outside the supposed region. As a result, angular distance of the direct region was identified as 10 degree from the sun. The range was verified by comparing two diffuse luminous efficacies: one was calculated by measured sky luminance and radiance, and another was calculated by illuminance and irradiance. They were modeled by clearness index and confirmed to vary similarly as clearness varied.