Experimental Verification of Kirchhoff’s Thermal Radiation Law on Surfaces With Emittance Spectra Characterized by Optical Interference Phenomena

This paper discusses Kirchhoff’s law on thermal radiation. The logic of derivation of the law is reconsidered, first. Then, spectra of normal emittance ε_N and normal incidence hemispherical reflectance R_NH are measured on surfaces whose microstructure changes in a non-equilibrium experimental system to examine the validity of the complementary relation between ε_N and R_NH, which is the suggestion of Kirchhoff’s law. As the results of the examination, it is illustrated experimentally on a variety of surfaces that the complementary relation is valid within an experimental uncertainty. Provided this conclusion is admitted, the followings are suggested. If a surface is designed so that it does not reflect a narrow spectral region of radiation to any direction and reflects the other spectral regions of radiation much over the hemisphere, then the surface can be a new spectrally-functional emitter of radiation. It is also suggested that thermal radiation emitted at a surface is considered as the emission of plane waves at the surface rather than as the emission of spherical waves by electric dipoles.