Abstract
Publisher Summary Kirchhoff's law of heat radiation states that the emissivity of radiating bodies is equal to their absorptivity. Kirchhoff's law seems to remain valid in the case of nonequilibrium if induced emission is considered as negative absorption as was first shown for freely radiating weakly absorbing bodies. Absorptivity is redefined as induced absorption minus induced emission, whereas emissivity is understood as being due to spontaneous emission only. In addition, from studying the original concepts of Kirchhoff, it can be inferred that the quantities compared in Kirchhoff's law are the spontaneous emission and the net absorption. The emissivity is exclusively due to spontaneous emission and is consequently independent of the environmental radiation field. Therefore, one expects Kirchhoff's law to hold as well for freely radiating bodies, provided that the distribution of the material states of the sample is the equilibrium distribution.
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