Infrared radiation is electromagnetic in nature and has a wavelength from about 750 nm to 400 000 nm, rather longer than that of visible light, and so is invisible to the human eye. All bodies emit radiation, the intensity and wavelength distribution depending on the nature of the body itself and its temperature.

Prévost suggested that all bodies radiate energy, but that those with a higher temperature radiate more energy than those at lower temperatures.

An ideal absorber is one that absorbs all the radiation that falls on it and also emits the maximum amount of radiation possible for that area at that temperature. Such a body is known as a black body and the radiation emitted by it as black body radiation.