Classical vs generalized Kirchhoff's law in anisothermal structures

Abstract

We quantitatively analyze the thermal emission by anisothermal structures. By comparing the heat flux radiated by simple multilayered systems calculated using the classical Kirchhoff's law to the exact flux derived from a fluctuational-electrodynamics theory, we highlight the limits of validity of the radiometric theory to predict the thermal emission with respect to the geometric configuration, the temperature gradient, and the type of materials and also highlight discrepancies as high as 50%.