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.