TERMINOLOGY

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
uminescence 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.

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