On the Validity of Kirchhoff's Law in a Nonequilibrium Environment

In the heat radiation literature, the words absorptivity and emissivity have both been used in different senses. This can lead to confusion, and in one case it was argued that Kirchhoffs law of equality between emissivity and absorptivity does not apply to the radiation from a material for which the stimulated emission depends on the temperature of the environment. We clarify the proper thermodynamic definition of absorptivity (and emissivity) and show that Kirchhoff's law stated in terms of those definitions correctly predicts the results of experiments. This follows even when there are large deviations from equilibrium, or when there is nonlinear absorption or emission.