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The emissivity of a material (usually written ) is the ratio of energy radiated by the material to energy radiated by a black body at the same temperature. It is a measure of a material's ability to absorb and radiate energy. A true black body would have an while any real object would have . This emissivity depends on factors such as temperature, emission angle, and wavelength. However, a typical engineering assumption is to assume that a surface's spectral emissivity and absorptivity do not depend on wavelength, so that the emissivity is a constant. This is known as the grey body assumption. When dealing with non-black surfaces, the deviations from ideal black body behavior are determined by both the geometrical structure and the chemical composition, and follow Kirchhoff's law of thermal radiation: emissivity equals absorptivity (for an object in thermal equilibrium), so that an object that does not absorb all incident light will also emit less radiation than an ideal black body.
Astrophysical Greybody The monochromatic flux density radiated by a greybody at frequency through solid angle is given by where is the Plank function for a blackbody at temperature T and emissivity . For a uniform medium of optical depth radiative transfer means that the radiation will be reduced by a factor giving . The optical depth is often approximated by the ratio of the emitting frequency to the frequency where τ=1 all raised to an exponent β. For cold dust clouds in the interstellar medium β is approximately two. Therefore Q becomes, See also | ||||||||
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