ABSTRACT
As light travels through an optical system, it requires area and angular space. Figure 3.1 shows a spherical light source SR (e.g., the sun) of radius r emitting light into space. As the emitted light expands, it will eventually illuminate the inner face of a spherical surface A1 of radius d1. When it reaches the surface, the angular spread of the light is confi ned to angle θ1 defi ned by the tangents to SR on the points of A1. This angle θ1 can be obtained from r/d1 = sin θ1. The area of the spherical surface A1 is given by A1 = 4π d 12, or by using the expression obtained for sin θ1 we get A1 sin2 θ1 = 4πr2 = AS, where AS is the area of the source SR.
