[Fundamentals of Solar Radiation, Lucien Wald] The diffuse fraction kD of the radiation (irradiance, irradiation) is the ratio of the diffuse component to the global radiation received on a horizontal plane. The diffuse fraction is dimensionless. It quantifies the part of the diffuse radiation in the global radiation. It can be defined for a wavelength, a range of wavelengths, or the entire spectrum.
Diffuse fraction
Radiative transfert property