Luminescence is a collective term for different phenomena where a substance emits light without being strongly heated, i.e. the emission is not simply thermal radiation. This definition is also reflected by the term “cold light”.
Important kinds of luminescence are:
- Fluorescence is light emission caused by irradiation with light (normally visible or ultraviolet light) and typically occurring within nanoseconds to milliseconds after irradiation. It involves the excitation of electrons into states with a higher energy, from where radiative decay is possible. Typically, the emitted wavelengths are longer than the excitation wavelengths; otherwise one speaks of upconversion fluorescence.
- Phosphorescence is a light emission which can occur over much longer times (sometimes hours) after irradiation. It involves storage of energy in metastable states and its release through relatively slow (often thermally activated) processes. The phenomenon was discovered early on for phosphorus.
- Photoluminescence is a less specific term which embraces both fluorescence and phosphorescence.
- Chemiluminescence is light emitted during (cold) chemical reactions.
- Bioluminescence is chemiluminescence from living organisms.
- Thermoluminescence is a type of phosphorescence which occurs at elevated temperatures. It is not to be confused with thermal radiation: the thermal excitation only triggers the release of energy from another source.
- Electroluminescence is light emission triggered by electric influences. In the case of excitation by an electron beam, this is also often called cathodoluminescence.
- Radioluminescence is caused by excitation with nuclear radiation or X-rays.
- Triboluminescence can occur when a material is mechanical treated, e.g. fractured or polished.
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