Irradiance Definition

This page is intended to provide a layman's definition of irradiance, more rigorous definitions can be found in a good physics textbook. Irradiance is the amount of energy per unit time per unit area. To take an example, think of a black flat plate lying in the sun. The irradiance is the solar energy input heating the plate. If the sun is high in the sky there will be more energy hitting the plate and heating it than if the sun were low, assuming atmospheric effects are neglected. Although the sun is putting out the same amount of energy in both instances, the projected area of the plate is smaller and hence less energy is absorbed by the plate. The projected area is the actual area times the cosine of the viewing angle and describes the phenomena of the apparent decrease in area of a plate as you tilt it. The graphic below illustrates the irradiance on two plates with differing angles of illumination. The graphic is linked to a larger version (27K GIF) if increased clarity is needed.