The emission of light by a substance for any reason other than a rise in its temperature. In general, atoms of substances emit photons of electromagnetic energy when they return to the ground state after having been in an excited state (see excitation). The causes of the excitation are various. If the exciting cause is a photon, the process is called photoluminescence; if it is an electron it is called electroluminescence. Chemiluminescence is luminescence resulting from a chemical reaction (such as the slow oxidation of phosphorus); bioluminescence is the luminescence produced by a living organism (such as a firefly). If the luminescence persists significantly after the exciting cause is removed it is called phosphorescence; if it does not it is called fluorescence. This distinction is arbitrary since there must always be some delay; in some definitions a persistence of more than 10 nanoseconds (10 −8 s) is treated as phosphorescence.