When the Sun's rays are perpendicular to an absorbing surface, the irradiance incident on that surface has the highest possible power density. As the angle between the sun and the absorbing surface changes, the intensity of light on the surface is reduced. When the surface is parallel to the sun's rays (making the angle from perpendicular to the surface 90°) the intensity of light falls to zero because the light does not strike the surface.
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Bonneville Environmental Foundation
1500 SW 1st Avenue, Suite 885
Portland OR 97201