Luminescence is the term used for all light radiation that is not thermal radiation. The most important light generation process involving luminescence is gas discharge on passage of an electric current through a gas or metal vapour (gas discharge lamp). Such lamps take advantage of the phenomenon of photoluminescence in the form of fluorescence or phosphorescence, in which invisible short-wave radiation is converted into visible light through processes within the fluorescent material. Fluorescence persists only for a fraction of a second after excitation, whereas phosphorescence persists for some time as a visible afterglow.