Flat-plate collectors, which typically contain a large number of solar cells mounted on a rigid, flat surface, can make use of both direct sunlight and the diffuse sunlight reflected from clouds, the ground, and nearby objects.
Direct insolation is solar radiation that comes straight from the Sun, without reflecting off clouds, dust, the ground, or other objects. Scientists also talk about direct-normal radiation, which refers to the portion of sunlight that comes directly from the Sun and strikes a surface at a 90-degree angle. The distinction between direct insolation and diffuse insolation is important because flat-plate solar collectors can use both forms of light, whereas concentrator systems can only use direct light. See also insolation.