Scattered sunlight determining sky luminance patterns

Recently in sustainable solar energy utilisation and daylighting studies via building apertures and photovoltaic panels take into account also radiance and luminance gains from the sky vault under any weather conditions. The luminance distribution on homogeneous sky types is today based on mathematically defined ISO/CIE standards of relative gradation and indicatrix functions in the range between overcast and cloudless skies. The review of the development and the different methodologies of categorising typical sky luminance patterns is discussed and summarised in this paper.