black body A perfectly black body is one which absorbs completely all radiation which falls on it. As it neither reflects nor transmits any radiation, it appears black, whatever be the color of incident radiation. It is also perfect radiator, its emission being the greatest possible for every wavelength at any given temperature.

A lamp blacked surface may be considered as perfectly black for all practical purposes. Kirchhoff established that the radiation inside a uniformly heated enclosure is perfectly black. Based on this principle Fery designed a black body. It consist of a double walled hollow metallic sphere with narrow opening. The interspacing between the walls is evacuated to prevent loss of heat by conduction and convection. When used as a source of radiation the enclosure is heated in a bath of fused salt. Care must be taken, so that only radiation from the opening falls on the measuring instrument.