[2.13.8.3] \varepsilon(\lambda, \theta, \phi, 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, \mathrm{blackbody} }\left ( \lambda ,T \right )

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