Radiation Components and Their Measurement

In the previous section we studied the geometry of what is called the direct beam component of the radiation, i.e. that component of the solar irradiance that reaches the collector directly from the sun's apparent position in the sky. But this does not represent all the radiation that reaches the surface. There is also a diffuse component that comes from other parts of the sky after multiple scattering from water vapor molecules, dust particles and clouds. On an overcast day the diffuse component dominates the direct beam, but it can be substantial even on a clear day.