= Grey body
= Gray body
{synonym}
A <grey body> has <spectral emissivity> independent of <wavelength> over the range in which the approximation is used. Under <Kirchhoff's law of thermal radiation>, the corresponding <radiometric absorptance> is also wavelength-independent. This allows the same factor to weight emission at $T$ and black-enclosure irradiation at a different $T_s$ in <net radiative heat exchange>. The factor can still depend on <temperature>; treating it as constant over a cooling curve is an additional assumption. Wavelength independence alone does not imply direction independence.<Thermodynamics and Propulsion>[§19.2, final paragraph: wavelength-independent absorption and emission factors]{ref}
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