Solid bodies, liquids and gases radiate energy on the basis of their temperature. The higher the temperature (in Kelvin), the greater the amount of energy emitted.

Thermal radiation is a form of Continuous Radiation, ranging from the shortest to the longest wavelengths. The first weak shine of a glowing solid is visible in the dark at about 800 K (approx. 527°C). As the temperature rises the solid reaches red heat, yellow heat and finally white heat. In addition, a certain amount of irradiation is also reflected, depending on the surface properties of the solid.

Full radiator

In the case of a full radiator, i.e. a primary radiator that absorbs all incident radiation (also called a black body or Planckian radiator), the spectral composition of the radiation is determined by temperature alone. The temperatures of full radiators can therefore be used to define the spectral composition (color appearance, color temperature) of any light source. In the standard chromaticity diagram the chromaticity of the full radiator lies on a curve.

The spectral properties of thermal radiators for artificial lighting (incandescent lamps, mains voltage tungsten halogen lamps, Low Voltage Tungsten Halogen Lamps) are very close to those of a full radiator, and very good color points can therefore be assigned to them.

The chromaticity of other light sources discharge lamps, on the other hand, only lies near the curve. Such chromaticity can be assigned to the nearest color temperature.