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It has been observed experimentally, both at NBS and elsewhere, that the emittance of polished metals can be markedly increased by roughening the surface, by as much as a factor of 2 or 3. For nonmetals and particularly white ceramic materials, on the other hand, the emittance appears to be essentially independent of surface roughness, at least for wavelengths below 7 or 8 μ. This apparant anomaly is explained on the basis of the differences in the optical properties of the two types of materials.
Joseph C. Richmond (Tue,) studied this question.