In addition to the obvious need for transparency, a critical requirement for a window in a hot environment is that it does not emit light that would obscure the scene being viewed. In this chapter we will explore transmission, emission and the related properties of reflection, refraction and scatter.
Consider what happens when light passes through an optical window with thickness b in Fig. 1.1. Suppose for the moment that the material can transmit, absorb or reflect the light, but it cannot scatter light away from the incident direction. Radiant power P0 (Wâm2) strikes the first surface, where radiant power R1 is reflected and radiant power P1 enters the sample. Some of P1 is absorbed, so power P2 (<P1) arrives at the second surface. Some power is transmitted through the second surface, and R2 is reflected. This process of partial reflection and partial transmission continues ad infinitum as the light ray bounces back and forth inside the window and eventually dies to near zero intensity. The net transmitted power, P, is the sum of all the partially transmitted light at the second surface.
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