4 NONIMAGING OPTICAL SYSTEMS

The relationship between the concentration ratio and the angular field of view is a fundamental one that merits more than one demonstration. We shall give a thermodynamic argument in the context of solar energy concentration. Imagine the sun itself as a spherically symmetric source of radiant energy (Figure 4.1). The flux falls off as the inverse square of the distance *R* from the center, as follows from the conservation of power through successive spheres of area 4π*R*^{2}. Therefore, the flux on the earth’s surface, say, is smaller than the solar surface flux by a factor (*r/R*)^{2}, where *r* is the radius of the sun and *R* is the distance from the earth to the sun. By simple geometry, *r/R* = sin ...

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