Diffuse attenuation factor

The sky diffuse component is affected by the near shading obstacles, as well as the incidence angular modifier (IAM).

In order to take it into account, we suppose as simplification that the diffuse sky irradiation is isotropic. A shading factor on this diffuse irradiation (independent of the sun's position, and therefore constant over the year) can be calculated, by integrating the shading and/or IAM factors over the visible part of the sky hemisphere, that is the dihedron between the collector plane and the horizontal plane.