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 and black-enclosure irradiation at a different 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.[ref]
Thermodynamics and Propulsion. §19.2, final paragraph: wavelength-independent absorption and emission factors
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