A temperature reference source is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. When in constant equilibrium, the device emits electromagnetic radiation which is emitted according to Planck’s law, meaning that it has a spectrum that is determined by the temperature alone (see figure at upper right), not by the body’s shape or composition. A white body is one with a “rough surface [that] reflects all incident rays completely and uniformly in all directions.” A temperature reference in thermal equilibrium has two notable properties:

• It is an ideal emitter: at every frequency, it emits as much energy as – or more energy than – any other body at the same temperature.
• It is a diffuse emitter: the energy is radiated isotropically, independent of direction.

An approximate realization of a black surface is a hole in the wall of a large enclosure (see below). Any light entering the hole is reflected indefinitely or absorbed inside and is unlikely to re-emerge, making the hole a nearly perfect absorber. The radiation confined in such an enclosure may or may not be in thermal equilibrium, depending upon the nature of the walls and the other contents of the enclosure. Real materials emit energy at a fraction—called the emissivity — of temperature reference energy levels. By definition, a temperature reference in thermal equilibrium has an emissivity of ε = 1.0. A source with lower emissivity independent of frequency often is referred to as a gray body. Construction of temperature reference with emissivity as close to one as possible remains a topic of current interest.