Fluorescence is a subtype of photoluminescence / luminescence. After excitation of material by light, fluorescence can occur by spontaneous emission. Usually, the photons emitted by the material are of longer wavelength, i.e. lower energy, than the incident photons. Physical systems capable of fluorescence are called fluorophores. Light-excited states have typical lifetimes characteristic of certain materials. While phosphorescence, a related type of radiation, often shows afterglow, fluorescent radiation is depleted after only a few micro-, nano-, or picoseconds. A special form of fluorescence is the so-called laser-induced fluorescence (LIF), which is used in many medical, bio- and environmental analytical applications.