Luminescence sources
Radiometry and non-coherent sources

Any rise in the temperature of a body increases its radiation (Stefan's law) and shifts its spectrum towards shorter wavelengths (Wien's displacement law). If, by means of selective excitation, the atoms of a material can be moved to higher energy levels without significantly altering its temperature, the return of these atoms to the fundamental level can be accompanied (in the case of radiative returns) by the emission of photons, known as luminescence emission.

The distribution of electrons in the various energy levels of the medium is then no longer dictated by Boltzmann's statistics, which prevail at thermal equilibrium, and the energy of photons emitted by luminescence between the two energy levels E₁ and E ₂ is equal to :