A black body is a radiator, which absorbs all incoming radiation. It shows neither reflection nor transmissivity. A black body radiates the maximum energy possible at each wavelength. The concentration of the radiation does not depend on angles. The black body is the basis for understanding the physical fundaments of non-contact temperature measurement and for calibrating the infrared thermometers. A thermal hollow body has a small hole at one end. If the body is heated and reaches a certain temperature, inside the hollow room a balanced temperature spreads. The hole emits ideal black radiation of this temperature.