Diffuse attenuation factor

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

To account for this, the diffuse sky irradiation is approximated as isotropic. A shading factor for this diffuse irradiation (independent of sun position and therefore constant throughout the year) can be calculated by integrating the shading and/or IAM factors over the visible portion of the sky hemisphere—the solid angle between the collector plane and the horizontal plane.