spectral-directional emissivity

[] ε(λ,θ,ϕ,T) (of an element of surface of a temperature radiator at a given wavelength and in a given direction)
The ratio of its spectral radiance at that wavelength and in the given direction to that of a blackbody at the same temperature and wavelength:

epsilon left ( lambda ,theta ,phi ,T right ) equiv L_{lambda }left ( lambda ,theta ,phi ,T right )/L_{lambda, blackbody } left ( lambda ,T right )
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