= Directional emissivity
{title2=$\epsilon_\Omega(T)$}
<Directional emissivity> is the ratio of emitted <radiance> to <blackbody> <radiance> at the same <temperature> in a specified outgoing direction. It can be total over <wavelength> or additionally spectrally resolved. Normal-direction <emissivity> is not automatically <total hemispherical emissivity>: the latter integrates all directions with the <projected area> weighting. A <diffuse thermal emitter> removes this directional dependence of emitted <radiance>.
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