= Emissivity
{title2=$\epsilon$}
{wiki}
<Emissivity> compares thermal emission from a surface with emission from an ideal <blackbody> at the same absolute <temperature>, using the same <wavelength> and angular averaging convention. For ordinary passive surfaces in the far field at local <thermal equilibrium>, the ratio lies between zero and one. A <blackbody> has <emissivity> one. Unless a spectral or directional qualification is given, the $\epsilon$ in $M=\epsilon\sigma T^4$ is <total hemispherical emissivity>: a ratio of emitted <radiant exitances>, not of all outgoing <radiant flux> including reflected radiation.<Thermodynamics and Propulsion>[§19.2, definition of emittance and equilibrium absorption/emission balance]{ref}
<Emissivity> belongs to a specified surface state, not just a bulk material name. Coating, oxidation and surface finish can change it; <wavelength>, viewing direction and <temperature> also matter. A surface that looks white in visible <light> need not have low <infrared> <emissivity>. This distinction is useful when choosing a radiator coating or interpreting a radiation-based <temperature> measurement: a narrow-band detector needs the appropriate <spectral emissivity>, and its signal can include reflected surrounding radiation. The <grey body> and <diffuse thermal emitter> assumptions are separate simplifications, addressing <wavelength> and direction respectively.
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