This work determines the spectral emissivity of a semi-infinite uniaxial medium in vacuum. If the optic axis is normal to the surface, then, for many materials and wavelengths, such as rutile between 10 and 25 μm, the directional and hemispherical spectral emissivities of the medium can be approximated, with an error of less than 10 percent, as those of an isotropic medium possessing the ordinary optical constants. In contrast, if the optic axis is parallel to the surface, the directional and hemispherical spectral emissivities can be predicted only by accounting for the optical anisotropy of the medium. Measurements of the directional emissivities of rutile crystals conform to the theoretical predictions.
Spectral Emissivity of Optically Anisotropic Solid Media
Malone, C. G., Choi, B. I., Flik, M. I., and Cravalho, E. G. (November 1, 1993). "Spectral Emissivity of Optically Anisotropic Solid Media." ASME. J. Heat Transfer. November 1993; 115(4): 1021–1028. https://doi.org/10.1115/1.2911356
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