Experimental verification of Kirchhoff's law on thermal radiation

Abstract

This paper discusses the Kirchhoff's law on thermal radiation. The logic of derivation of the law is reconsidered, first. Then, spectra of normal emittance ∈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 ∈N and RNH, which is the suggestion of the 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.