is the process by which the energy
of a photon
is released by another entity, for example, by an atom
make a transition between two electronic energy levels
. The emitted energy is in the form of a photon.
The emittance of an object quantifies how much light is emitted by it. This may be related to other properties of the object through the Stefan–Boltzmann law.
For most substances, the amount of emission varies with the temperature and the spectroscopic composition of the object, leading to the appearance of color temperature and emission lines. Precise measurements at many wavelengths allow the identification of a substance via emission spectroscopy.