Since radiance is the fundamental radiometric quantity, this chapter is devoted to some of its properties. Radiance is invariant through an optical system, but when there is a change in the medium the radiance divided by the square of the refractive index is invariant (reduced radiance). All the other radiometric terms can be obtained from radiance by appropriate integrations. Not only is reduced radiance invariant, but the throughput is also invariant. Both of these invariance assertions will be proven in more than one way.
5.1 The Invariance of Radiance in a Vacuum
Consider the transfer of radiation from a small source of area A at the left in Figure 5-1. The distances of the several areas are d 1 , d 2 , etc. By the first law of thermodynamics, the power must be the same on area A 2 as it is on A 1 as it is on A 3 as it is on all of the areas. Only the flux densities change.
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