 Fluorescence

Fluorescence is photon emission following photon absorption by matter: an excited state releases some of its energy as light. A fluorophore can therefore mark where something is, while its fluorescence emission spectrum and fluorescence lifetime report different aspects of its environment. Fluorescein is one example; FRET exploits a competing route for excitation energy rather than requiring a second emitted photon.

The usual molecular sequence is excitation, relaxation, then emission, as in the Jablonski diagram. The emitted photon commonly has lower energy and longer wavelength, giving a Stokes shift; the exceptions are discussed below.^[ref]

Fluorescence is a process in which a substance absorbs light or other electromagnetic radiation and then re-emits light. Specifically, it refers to the emission of visible light by a substance that has absorbed ultraviolet or other high-energy radiation. The process usually occurs very quickly, typically within nanoseconds.