= Fluorescence
{wiki}
<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.<What is the Stokes Shift? by Edinburgh Instruments>[Stokes Shift in Fluorescence Spectroscopy, Figure 4 and relaxation discussion]{ref}
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