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) arc 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.
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