Activator

ionic impurity in a host material which acts as a luminescing centre, also called dopant.

Afterglow

general emission of light after a source of excitation energy is removed, usually by phosphorescence.

Anti-Stokes

a material which converts low energy light to high energy light by photoluminescence. Also called up-converter.

Candela

unit of luminous intensity.

Cascading

multiple absorption/emission cycles in a material where the emission from one cycle is matched to the absorption of the next.

Cathodoluminescence

luminescence where the initial energy comes from fast moving electron bombardment, such as on a television screen or computer monitor.

Charging

excitation of a phosphorescent material, usually by incident light.

Chemiluminescence

luminescence where the initial energy comes from chemical reactions, such as phosphorous burning in oxygen.

Correlated colour temperature

the colour of white light sources, the temperature of the black body radiator which produces the chromaticity most similar to the light source. Unit: °K.

Daylight fluorescence (DF)

fluorescence where the emission is in the visible spectrum.

DIN67510 Part 1

specification describing afterglow performance in a standard way, in the form a/b - c - d - e, where a is afterglow brightness in mcd/m2 after ten minutes, b is afterglow brightness after sixty minutes, c is the decay period in minutes to 0.3 mcd/m2, d is excitation colour code, e is emission colour code. Parts 2 and 3 refer to in situ testing of photoluminescent items.

Dopant

ionic activator commonly used in inorganic phosphors.

Excited state

condition of a charged phosphor before emission.

Electroluminescence (EL)

luminescence where the initial energy comes from electric fields, usually alternating.

Fluor

a substance exhibiting fluorescence.

Fluorescence

very fast absorption and emission of photons where there is no appreciable afterglow. No electron spin inversion is involved.

Flux (light)

luminous intensity, usually of a light source, per unit solid angle; Unit: lumen.

Illuminant A, B, C

incandescent illumination in the range 380nm to 770nm, respectively 2856K (yellow), 4874K (mean noon sunlight) and 6774K (average daylight, blue).

Illuminant D

daylight illuminants defined from 300-830nm, designated with a two digit subscript to describe Correlated Colour Temperature, e.g. D65 indicates 6500K, close to Illuminant C.

Illumination

luminous flux, usually of incident light. Units: lux or lumens per square metre.

Infrared

part of the electromagnet spectrum immediately less energetic than visible light, ranging from around 700 nanometres to 10 microns wavelength.

Intersystem crossing

transfer from one molecular angular momentum state to another by electron spin inversion. Principal of physical phosphorescence.

Killing

quenching.

Light output

quantum efficiency multiplied by amount of absorbed radiation.

Lumen

unit of light flux. One lumen equals the flux emitted into a solid angle of one steradian by a point source of one candela.

Luminance

brightness, usually of a surface, i.e. luminous intensity per unit area. unit: candelas per square metre, usually expressed in millicandelas per square metre.

Luminescence

emission of light from a substance unaccompanied by heat.

Luminophor

luminescent material.

Luminous directional reflectance

reflectance of a surface in given directions of illumination and view. The ratio of the brightness of a surface to the brightness that an ideally diffusing, perfectly white surface would have if illuminated in the same way. Units: none.

Luminous intensity

a fundamental unit derived from black body radiation at set conditions in a given direction. Unit: candela.

Luminous efficiency (L)

luminous flux emitted by a source, per unit of power consumed. Unit: lumens per Watt.

Lux

unit of illuminance, lumens per square metre. In imperial units, one footcandle is approximately 10 lux.

Optically active

a) luminescent, b) able to change the polarity of incident light during reflection.

Phosphor

a substance exhibiting the property of phosphorescence.

Phosphorescence

slower absorption and emission of photons where afterglow is usually apparent, involving electron spin inversion allowing absorbed energy to be trapped for a period before being released as photons.

Photoluminescence

luminescence where the energy comes from incident light. Includes fluorescent and phosphorescent processes.

Quantum yield (q)

ratio of energy emitted by a luminescent substance to that absorbed, expressed as a percentage or decimal part of unity. Units: none.

Quenching

the loss of luminescent emissions to absorbing centres, or the addition of an agent to do this. Also called killing.

Radiant efficiency

ratio of emitted luminescent power to power absorbed from exciting radiation.

Radioluminescence (RL)

luminescence where the initial energy comes from radioactive decay, e.g. as with tritium. Products relying on RL are also called self emitters.

Resonance radiation

fast fluorescence with no internal loss of energy.

Saturation

charging of a phosphorescent material to maximum.

Scintillator

photoluminescent material with absorption at very low wavelengths, i.e. gamma or X rays.

Self emitter

radioluminescent material.

Stokes shift

difference in wavelength peaks between absorption and emission curves in photoluminescent materials, positive where wavelength increases, negative where wavelength decreases. Unit: nanometres.

Strontium Aluminate

a collective term for a group of crystalline phosphors derived from Strontium Oxide and Alumina (and silica) singally or doubly doped with rare earths Europium and Dysprosium.

Thermoluminescence

luminescence where heat energy triggers emission of photons from internal energy previously stored.

Triboluminescence

short lived luminescence caused by the violent breaking of chemical bonds, often associated with frictional forces.

Ultraviolet

part of the electromagnet spectrum immediately more energetic than visible light, ranging from approx. 100 nanometres (VUV) to 400 nanometres (UVA) wavelength.

Up-conversion

photoluminescent process converting lower energy incident light to higher energy emitted light. Also called anti-Stokes.