Transmittance for diffuse radiation

Radiation incident on a collector also consists of scattered solar radiation from the sky and possibly reflected solar radiation from the ground. In principle, the amount of this radiation that passes through the cover system can be calculated by integrating the transmitted radiation over all angles. However the angular distribution of this radiation is generally unknown. For incident isotropic radiation the integration can be performed. The presentation of the results can be simplified by defining an equivalent angle for the beam of radiation that gives the same transmittance as for diffusion radiation. For a wide range of conditions encountered in solar collector applications the angle is taken as 60 degrees. Circumsolar diffuse radiation can be considered as having the same angle of incidence as the beam radiation; diffuse radiation from the horizon is usually a small contribution to the total and as an approximation can be taken as having the same angle of incidence as the isotropic diffusion radiation.