a term used in the theory of thermal radiation for a body that completely absorbs the whole flux of radiation falling on it. The absorption coefficient of a black body is equal to unity and does not depend on the wavelength of the radiation. The closest approximation to a black body is an opaque vessel with a small opening, the walls of which are all at the same temperature (Fig.). A ray that gets into such a vessel undergoes multiple reflections and is partly absorbed at each of them. After some time the walls of the vessel absorb it completely. Soot and platinum black have absorption coefficients close to unity.
The intensity of the radiation of a black body is higher than that of all other (“non-black”) bodies at the same temperature (see Kirchhoff's radiation law). The chief feature of black-body radiation is that its properties do not depend on the nature of the substance and are determined only by the temperature of the walls, that is, the radiation of a black body is in thermodynamic equilibrium with the substance, the distribution of its density over wavelengths is given by Planck's radiation law, and the total radiation density over all wavelengths is determined by the Stefan—Boltzmann radiation law. The regularities that govern black-body radiation are used in optical pyrometry to measure high temperatures; black bodies are also used as light standards.