Diffuse sunlight (B), reflected from clouds, the ground, and nearby objects, and direct sunlight (A) falling onto flat-plate solar panels. Credit: US Dept. of Energy.
Diffuse insolation is sunlight received indirectly as a result of scattering due to clouds, fog, haze, dust, or other obstructions in the atmosphere or on the ground. Unlike direct insolation, diffuse light cannot be focused by the optics of a concentrator photovoltaic system, though it can be used by a flat-plate collector. See also insolation.