The conditions of a black body can be realized in a cavity made of a material of high emissivity which is as isothermal as possible, and with an outlet sufficiently small that its presence affects as little as possible the black body nature of the source. The arrangement described of a graphite tube, with appropriately shaped inner screens. A thorough study of the shape allows one to obtain a good approximation to an isothermal state. In this way the conditions of a black body are realized with a precision of 0.2% in the absolute energy distribution. The temperature used can easily exceed 2000K for long use, and a temperature control based on the radiation emitted allows one to obtain a stability to better than 1K during several hours. The arrangement described was conceived for the needs of astronomy, but can be very useful in other fields whenever one desires to have precise information on the radiation emitted by a body at a high temperature.