Luminescence

Luminescence refers to the spontaneous emission of light from electronically or vibrationally excited species that are not in thermal equilibrium with their surroundings. In contrast to incandescence, where light is produced only when matter is heated to high temperatures, luminescent materials emit cold light. This phenomenon encompasses a wide range of physical, chemical and biological processes, making it an important subject in physical chemistry, materials science, mineralogy and biological imaging.
Luminescence often results from electrons transitioning between different energy levels within atoms or molecules following excitation by an external stimulus. Although the mechanism for vibrationally excited species is less well understood, the broad principles underpinning electron transitions remain central. Luminescent coatings have been historically used in aviation and navigational instruments through a process known as luminising, allowing key indicators to remain visible in darkness.

Types of Luminescence

Luminescence can be classified according to the nature of the excitation source or the mechanism of emission. The principal categories include:

• Ionoluminescence – produced when a substance is bombarded with fast ions, which deposit energy that is subsequently released as light.
• Radioluminescence – resulting from excitation by ionising radiation, commonly utilised in older luminous paints.
• Electroluminescence – arising when an electric current passes through a substance, the basis for light-emitting diodes (LEDs).
• Cathodoluminescence – generated when a luminescent material is struck by high-energy electrons, frequently used in geological analysis of minerals.
• Chemiluminescence – light emission from a chemical reaction, exemplified by the luminol reaction or the oxidative reactions that cause haemoglobin to glow under certain conditions.
• Bioluminescence – a type of chemiluminescence occurring within living organisms, such as fireflies, deep-sea organisms or certain fungi.
• Electrochemiluminescence – produced during electrochemical reactions where excited states form at electrode surfaces.
• Solvated luminescence – occurring when heavily irradiated solids are dissolved in a liquid, releasing stored energy as light.
• Glow at elevated temperatures (non-blackbody luminescence) – light emitted by certain materials at high temperatures without following normal blackbody radiation patterns.
• Triboluminescence – produced when bonds in a material are ruptured because of scratching, crushing or rubbing; observed in materials such as sugar crystals or adhesive tapes.
• Fractoluminescence – generated when crystal bonds break during fracturing, common in brittle minerals.
• Piezoluminescence – produced by applying mechanical pressure to certain solids; for example, crystals that glow when compressed.
• Sonoluminescence – light emission from imploding bubbles in a liquid when subjected to intense sound waves.
• Crystalloluminescence – light generated during crystallisation processes.
• Thermoluminescence – the release of stored energy as light when a substance is heated, used extensively in archaeological dating.
• Cryoluminescence – the emission of light upon cooling, observed in minerals such as wulfenite.
• Photoluminescence – light emitted after photons are absorbed. Traditionally defined as emission occurring only while the excitation source is on; quantum mechanically, it corresponds to transitions in which spin multiplicity is preserved.
• Phosphorescence – light emitted persistently after excitation has ceased; quantum mechanically, associated with transitions involving a change in spin multiplicity.

These categories demonstrate the diversity of processes that can give rise to luminescence, each characterised by distinct excitation sources and emission behaviours.

Applications

Luminescence underpins a wide spectrum of scientific, industrial and technological applications:

• Light-emitting diodes (LEDs) employ electroluminescent materials to create efficient, durable light sources used globally in electronics and lighting systems.
• Scintillation materials exploit radioluminescence to detect high-energy electromagnetic or particle radiation, essential in nuclear physics and medical imaging technologies.
• Temperature measurement using phosphorescence relies on temperature-dependent emission lifetimes in specialised phosphors.
• Cell biology and medical diagnostics use bioluminescence to track biochemical pathways, monitor cellular processes and detect pathogens with high sensitivity.
• Mineral identification benefits from photoluminescence techniques, particularly ultraviolet-induced fluorescence, to recognise mineral specimens in the field or laboratory.
• Electric lighting industries incorporate luminescent coatings in various lamp technologies.

The versatility of luminescence across scientific disciplines underscores its importance in both experimental and applied contexts.

History

The term luminescence was introduced in 1888 by German physicist Eilhard Wiedemann, who sought a unified term for forms of light emission not derived from heat. His definition captured a broad array of phenomena that had previously lacked consistent terminology. Luminescence subsequently became an established concept within physics and chemistry, guiding research into fluorescence, phosphorescence and numerous related processes.
Over time, luminescence has evolved into a sophisticated field incorporating quantum mechanical interpretations of excited-state dynamics. These advances have allowed more precise distinctions between types of luminescent processes, facilitating improvements in imaging, analytical technologies and material science.