Absolute Zero

The total absence of any heat;  -273oC 0 Kelvin.


Alternating Current


A small amount of an element added to a very pure chemical to bestow luminescent properties.


The emission of light from a phosphor after the excitation has ceased.

Alpha Particles

a Particle.  These are Helium nuclei (two neutrons plus two protons). They are a product of radioactivity but only penetrate a few centimetres in air due to their high charge.


A luminescent process that does not obey Stokes law. e.g. The excitation wavelength is longer than the emission wavelength.

Beta Particle

b Particle.  A high-energy electron created when a neutron turns into a proton within a radioactive nucleus; it is negatively charged.

Beta Ray

Another term for Beta particles

Black Light

Long UV from a very dark blue glass tube or bulb (Wood’s glass)

Black Body

A theoretical material that when heated to incandescence will emit radiation according to Planck’s equation.  The colour of the emission moves from deep red through orange, white and finally blue-white with increasing temperature.

Black Body Curve

A curve on the chromaticity diagram connecting the colour points of black bodies at increasing Kelvin.

Body Colour

The natural colour of a material in daylight


The symbol for the velocity of light.  ~ 3×108 m/sec


Charge-Coupled Device; an array of light-sensitive electronic detectors.


A unit of luminous intensity defined as the brightness of a black body, at the temperature of the solidification of Platinum, is 60 candela cm-1.


An ion has either a positive or a negative charge

Charge Compensation

The addition of an extra ion, which enables an activator ion to enter a crystal lattice when it does not have the correct charge. e.g. Cu+ in Zn2+ in ZnS:Cu:Cl

Chromaticity Diagram

A chart that permits the definition of colours using x and y co-ordinates.


Commission Internationale d’Eclairage. 1931.  An International body that standardised the definition of colour in 1931.  It uses x y co-ordinates on a chromaticity diagram.

Colour Point

The conjunction of the x y co-ordinates of a phosphor.

Colour Rendering

The ability of a light source to accurately reflect the colours of an object.

Colour Rendering Index

A scale of colour rendering ability; 100% is perfect colour rendering.  Tri-chromatic phosphor lamps achieve about 85% CRI.

Colour Temperature

The definition of near white colours using K. e.g. 3000 K warm white, 3500 K white and 4500 K daylight.

Co precipitation

The incorporation of an activator into a host by precipitating them simultaneously from a mixed solution.


Cathode Ray Tube.

Crystal Field Effect

The effect of the surrounding crystal lattice on the activator of a phosphor.  Substitution with similar ions can cause subtle changes or shifts on emissions. e.g. Mg, Ca, Sr, Ba.


The emission of light from a phosphor after the excitation has ceased.


Direct Current.


Another term for an activator.


A conventional phosphor that obeys Stokes law (as against an up-converter).

Edge Emission

Emission that commences immediately after the excitation wavelength with zero Stokes Shift.

Energy Level Diagram

A graphic illustration of the excitation and emission processes of a phosphor.  The specific energy levels are indicated together with any intermediate stages or traps.


Electron Volt.  The energy acquired by an electron when it is accelerated through a potential difference of one Volt.  This unit is used in semiconductor physics and it can be converted to the equivalent wavelength thus: 1240/λnm = eV

Exponential decay

Afterglow or persistence that decreases exponentially with time.

Eye sensitivity curve

This is a bell-shaped curve with its peak at 555nm for photopic vision (day) and 507nm for scotopic vision (night).


Field Emission Display.  Flat display screen technology using pixel sized transistor cells.


A chemical added to pre-cursors before firing to enable crystal growth.


symbol υ…in optics it is the number of vibrations of light/sec.  frequency * wavelength = the speed of light.

Gamma Rays

They may be emitted from radioactive materials.  They are quanta of electromagnetic wave energy and are similar to X rays but of much higher energy.  They may penetrate several centimetres of lead. λ = 0.14~.0005nm.


The full extent of colours available from red, green and blue Phosphors.

Gaussian Distribution

After Karl Friedrich Gauss (1777-1855) a physicist and mathematician.  It is represented by a symmetrical bell-shaped curve; emission peaks are usually Gaussian.

Hard X Rays

The shortest wavelength X Rays from 0.1~0.01nm


These are ions that have lost electrons.  Holes can move through a crystal lattice by taking an electron from an adjacent ion and repeating the process indefinitely.  A phenomena discovered in semi-conductor technology.

Host Lattice

The basic material into which an activator is added.


The emission of light by a heated object. e.g. Light bulb filament.

Infra Red

Part of the electromagnetic spectrum extending from the limits of red visibility into heat. e.g. 750nm~8µm.


An Ion is an atom or radical that has either lost or gained valence electrons.

Ionic Radius

The radius of an ion.  Used to check if an activator will substitute into a host lattice; ± 10~15% is a guide figure.


Joint Electron Device Engineering Council.  A classification system for CRT phosphors.


After Lord Kelvin (1824-1907) an Irish physicist.  A temperature scale starting from absolute zero using a centigrade scale.


An impurity within a phosphor which destroys luminescence. e.g. Ni.


Greek letter Lambda the symbol for wavelength.

Lamp Phosphor

A phosphor used for general fluorescent lighting; excited by 254nm and unaffected by mercury vapour.


A laser is a device which produces light at a specific wavelength, this light is known as coherent light which means that all the individual waves are in phase and resonate together. A similar analogy is that of the sound produced by a tuned organ pipe. The initial resonance is produced by materials as diverse as YAG , dyes or metal vapours.

Light Emitting Diode

A doped semiconductor that emits light when passing current.


A unit of continuous luminous flux.  It is only related to the eye’s perception of brightness and the visible spectrum.  It cannot therefore be used to measure UV or IR.  One candle produces 4π Lumens.


Lumens emitted per Watt of input power.


The ability of a lamp or device to maintain its initial output.


Mega electron volts.


Another term for flux.


The atomic weight of an element or compound in grams.

Mole %

A way of defining the activator concentration in a host. e.g. Y1.9Eu0.1O3. 0.1*100% / 2 = 5 mole % or 5m/o


The shape and structure of a crystal.

Non-Radiative Process

Internal energy transitions that do not result in light, generally referring to losses to heat rather than sensitiser actions.

Planck’s Equation

E(λ) = Aλ-5 /exp(B/Tc)-1   Where A & B are constants, λ is the emission wavelength and Tc is the temperature of the Black Body.


A plasma is an ionised gas or vapour which is formed at low pressures when high voltages are placed across the gas. When gases are ionised they conduct electricity and emit characteristic lines of radiation.


Plasma Display Panel. A flat display panel consisting of two flat sheets (at least one is glass), sandwiching spacers which confine individual cells filled with an inert gas mixture. The phosphor is deposited inside the outer glass sheet and is illuminated by the radiation produced by the inert gas plasma and electrode in a similar fashion to a neon tube.


Another term for afterglow.


A packet of electromagnetic energy with wave-like properties


This refers to the region of physical effects that are on the level of individual electrons and quanta of light, on an atomic or molecular level.


A single “packet” of light radiation of any wavelength. Light behaves as both a wave and a particle in some situations. The exact nature of light has not been completely defined.

Quantum Efficiency

This is the simple expression;  Quanta of light out x100%/ Quanta of light in. e.g. 90 quanta of visible light emitted/ 100 quanta of UV 365nm absorbed means the phosphor has a quantum efficiency of 90%. N.B. please note that this applies to light of all wavelengths and has nothing to do with the eye’s perception of brightness.


Resistance of a capacitor or inductor to alternating current


The process whereby excited electrons reach equilibrium in their excited state before they return to their original energy level.


A secondary activator which absorbs additional excitation energy and transmits it directly to the primary activator to enhance efficiency.  For example: 30ppm of Tb added to Y2O2S:Eu


The release of trapped electrons in a storage phosphor; usually by IR.

Soft X Rays

The longer wavelength X Rays from 1~10nm


Exact proportions and relationships of the elements of a substance.

Stokes Law

The energy of absorption is higher than the energy of emission.

Stokes Shift

The distance between the excitation peak and the emission peak of a phosphor.  Measured in nm, eV or Wavenumber (cm-1).

Storage properties

The ability of a material to store electrons in traps until they are released by stimulating radiation such as infra red.


Tube Engineering Panel Advisory Council. A classification system for CRT phosphors.


Sites within a crystal where impurity ions or vacancies exist.  They trap electrons for variable lengths of time, depending on the phosphor: the decay time is thus directly related to the degree of trapping.


The release of stored light by mechanical action. e.g. ZnS:Mn


Another name for an anti-stokes phosphor.


Ultra Violet radiation between 320nm and 400nm


Ultra Violet radiation between 260nm and 320nm


Ultra Violet radiation between 200nm and 260nm


These are imperfections or discontinuities within a phosphor crystal lattice that may affect the performance of the material.


Vacuum Ultra Violet.  Ultra Violet radiation between 100nm and 200nm. This is absorbed by the Oxygen in the air and so can only radiate within an Oxygen-free gas or a vacuum.


λ. The wavelength of visible light is measured in nm.  N.B. Wavelength is inversely proportional to energy


υ = υ/c or 1/λ it is the number of waves/cm.  It is often used by spectroscopists as it is proportional to energy.


Yttrium Aluminium Garnet