A black body is a theoretically perfect absorber of electromagnetic radiation, absorbing all wavelengths falling upon it. At normal room temperature it appears black because is it does not reflect radiation in the visible spectrum (light). A black body is also a perfect emitter of radiation, and the radiation is consequently known as "black body radiation". However, this is a purely theoretical concept. In the real world even the best black bodies are less than perfect.

The intensity of radiation emitted by a black body at various wavelengths is dependent upon temperature only. The emitted spectrum usually incorporates infrared, visible light and ultraviolet wavelengths.

The animation below may be used to illustrate how the emitted radiation moves across the wavelengths of the visible spectrum as the temperature changes. This physical phenomenon forms the basis of colour temperature measurement.