Analysis of diffuse irradiance from two parts of sky vault divided by solar meridian using portable spectral sky-scanner

•Irradiances on arbitrary inclined surfaces from the diffuse solar radiation are calculated.•Measurements are provided by the self-constructed sky scanner with spectroradiometer.•Methodology is based on the CIE sky standard.•Disadvantages of the CIE standard are discussed.

A self-constructed prototype of a portable sky scanner with calibrated spectroradiometer was used for scanning of the sky vault. Methodology widely used in daylight in combination with the spectral measurements in a broader range of the electromagnetic spectrum can improve calculations of incoming diffuse radiation usable in various electricity saving systems like light-transporting devices, building-integrated systems and with some limitations also PV panels. Determining sky radiance gradation and scattering indicatrix function based on CIE standard are applicable to some extent for computations of the immediate diffuse irradiance on the arbitrary inclined surfaces. Various sky conditions were considered with emphasis on different cloud coverage. Disadvantages of CIE sky standard and its applicability in partly cloudy conditions are discussed.