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(or nonselective radiator), a body whose absorptivity is less than 1 and is independent of the wavelength A of the radiation. The absorptivity αλ,T of all real bodies depends on A; in other words, such bodies absorb selectively. For this reason, they are regarded as gray bodies only in ranges of A where αλ, T is approximately constant. Substances having the properties of a gray body in the visible region of the spectrum include coal, with αλ,T = 0.80 at 125°-625°C; carbon filaments of incandescent lamps, with αλ,T = 0.526 at 1040°-1405°C; and carbon black, with αλ,T = 0.94–0.96 at 100°-200°C. Black platinum and bismuth absorb and radiate as gray bodies in the widest range of λ—from visible light to 25–30 micrometers (αλ,T = 0.93–0.99).
The heat radiation of a gray body is identical to the radiation of a blackbody in spectral composition but has a lower radiance. The laws governing the radiation of a blackbody are applicable, with only the constants being replaced, to the radiation of a gray body. These laws include Planck’s law, Wien’s law, and the Rayleigh-Jeans law. The concept of a gray body is used in optical pyrometry.