Investigation of the overcast skies luminance distribution using 35 sensors fixed on a dome

For the luminance measurements reported in the present work, one sensor was used to measure zenith luminance, and 34 sensors were positioned on a dome with their collimators pointing toward different positions P on the celestial sphere, which can be specified by the values of their zenith Z and azimuth θ angles. Overcast skies values were used. The final model proposed for the mean luminance distribution for overcast skies normalized to the zenith luminance LZθ/Lz has been developed in four steps. For each value of Z, the mean value of LZθ/Lz is expressed as a ratio of gradation functions that depend on θ. Only for θ = 180°, a good coincidence is obtained with the model proposed by the Commission International de l’Eclairage (CIE) for a wide range of values of Z.