Luminescence

right|thumb|[[Luminol reacting with haemoglobin is a familiar demonstration of chemiluminescence.]]

thumb|UV-induced [[photoluminescence used in microbiological diagnostics.]]

Luminescence is the emission of optical radiation (ultraviolet, visible, or infrared) by a substance due to a process other than heating. In the broad chemical/photochemical sense, luminescence is the spontaneous emission of radiation from an electronically or vibrationally excited species that is not in thermal equilibrium with its environment.

Luminescence contrasts with incandescence, where light is produced as thermal radiation from hot matter (approximately described by black-body radiation). Because luminescent emission may be produced at relatively low temperatures, it is often described informally as cold light.

Fluorescence and phosphorescence

In common usage, luminescence is frequently discussed in terms of fluorescence and phosphorescence.

Fluorescence is luminescence that occurs essentially only while a substance is being irradiated (i.e., promptly following excitation).
Phosphorescence is luminescence involving a change in spin multiplicity (commonly emission from a triplet state), which often makes the radiative transition comparatively slow; the term has also been used phenomenologically for long-lived afterglow.

Note that practical materials marketed as "glow-in-the-dark" often rely on persistent luminescence (trap-mediated afterglow) rather than strictly spin-forbidden phosphorescence; persistent emission can last minutes to hours after excitation, depending on trap depth and release kinetics.

Spectral characteristics

A luminescent material is often described by its:

Emission spectrum (spectral distribution of emitted photons)
Excitation spectrum (incident spectral distribution that produces luminescence at a specified emission wavelength)

For many photoluminescent systems, emission occurs at longer wavelength (lower photon energy) than absorption. The difference between absorption and luminescence band maxima arising from the same electronic transition is known as the Stokes shift.

Efficiency and kinetics

Two common figures of merit are:

Quantum yield (fraction of excitations that produce emitted photons)
Lifetime (typical time scale of excited-state decay)

Both depend on the relative rates of radiative and non-radiative deactivation pathways and can be strongly affected by the local environment (solvent, temperature, oxygen, defects, etc.). It includes most molecular fluorescence/phosphorescence and many semiconductor and nanomaterial emissions.

In photoluminescent systems, prompt emission is typically termed fluorescence, while slower emission involving a change in spin multiplicity is termed phosphorescence.
Electrochemiluminescence (also called electrogenerated chemiluminescence) is luminescence initiated by electrochemical reactions, typically in solution near electrodes.
Galvanoluminescence is a historical term for luminescence observed at an electrode during electrolysis (e.g., at an anode in an electrolytic cell).

Chemically and flame-driven luminescence

Chemiluminescence is emission produced by a chemical reaction (e.g., luminol).
Lyoluminescence is light emission observed when certain irradiated solids are dissolved in a solvent (historically developed in radiation dosimetry).
Candoluminescence is non-thermal emission from solids excited by flames; it has been studied both as a basic phenomenon and as a flame-based analytical signal for trace analysis.
Pyroluminescence (flame emission) is characteristic spectral radiation from a gas or vapor excited by high temperature (e.g., vaporized salts in a flame).

Excitation by particles and ionizing radiation

Radioluminescence is luminescence excited by high-energy particles or ionizing radiation.
A brief, event-by-event flash is termed scintillation. Scintillators coupled to photodetectors are central to many radiation detectors.
Cathodoluminescence is luminescence caused by electron impact, used in materials characterization and microscopy.
Ionoluminescence (often described operationally as ion beam-induced luminescence, IBIL) is luminescence excited by fast ions and is used as an analytical probe of defects and bonding in materials.

Mechanical and acoustic excitation

Mechanoluminescence is emission produced by mechanical action on a solid (rubbing, cracking, pressing).
Triboluminescence and fractoluminescence are commonly used for emission associated with rubbing and/or fracture.
Piezoluminescence is light emission induced by (typically dynamic) pressure or elastic deformation in certain solids.
Sonoluminescence is light emission from collapsing bubbles driven by intense sound fields.

Thermally stimulated, phase-change, and temperature-associated phenomena

Thermoluminescence arises when heating releases trapped charge or trapped excited species in a rigid matrix, producing delayed emission.
Crystalloluminescence is luminescence produced during crystallization (often reported during rapid precipitation or nucleation).
Cryoluminescence has been used for light emission observed upon cooling or freezing of certain luminescent materials (sometimes discussed as an "opposite" to thermally released emission).

Materials and emitters

Luminescent emission can arise from a wide range of physical emitters:

Molecules (organic dyes, coordination complexes) where transitions occur between discrete electronic states; many are designed as fluorophores for imaging and sensing.
Phosphors (often inorganic solids) in which activator ions (e.g., rare earths, transition metals such as Cr<sup>3+</sup>) emit in host lattices; crystal-field environments can tune spectra and thermal stability, enabling applications from lighting to bioimaging.
Semiconductors where radiative recombination of electrons and holes produces light (LEDs, laser diodes).
Fluorimeters (a class of luminescence instruments) for fluorescence intensity and spectral distribution, often used for trace analysis.
Time-resolved methods (e.g., pulsed excitation, photon counting) to extract lifetimes and distinguish overlapping emissions.

Radiation detection and dosimetry

Scintillators convert high-energy radiation into visible photons detected by photomultipliers or photodiodes, enabling spectroscopy and imaging in nuclear medicine, high-energy physics, and security screening.

History

The term luminescence was introduced in 1888 by the German physicist Eilhard Wiedemann in the context of classifying forms of light emission not explained by heating alone. Early systematic studies of fluorescence and phosphorescence predate quantum theory and motivated later excited-state models and photochemical terminology.