Presented here is a glossary of terms related to luminescence and phosphors.

Absolute Zero

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

AC

Alternating Current

Activator

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

Afterglow

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

Alpha Particles

α 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.

Anti-Stokes

A luminescent process that does not obey Stokes’ law, i.e. the excitation wavelength is longer than the emission wavelength.

Beta Particle

β 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.

Blacklight

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

Black Body

A theoretical material that will emit radiation according to Planck’s equation, i.e. the emission spectrum depends only on the temperature of the material.  The colour of the emission moves from infrared through red, orange, white and blue-white as temperature increases.

Black Body Curve

A curve on the chromaticity diagram describing how the colour point of black body radiation will change with temperature (given in Kelvin).

Body Colour

The natural colour of a material in daylight.

c

The symbol for the speed of light in a vacuum.  ~ 3×108 m/s

C.C.D.

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

Candela (cd)

The SI unit of luminous intensity that is roughly equal to the light given off by a candle.

Charge

The property of a particle governing how much it is affected by an electromagnetic field. Ions will have a positive or 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 Diagram1931 CIE Chromaticity Chart

A chart that permits the definition of colours using x and y coordinates.

C.I.E.

Commission Internationale d’Eclairage. 1931.  An international body that standardised the definition of colour in 1931.  It uses x, y coordinates on a chromaticity diagram.

Colour Point

The point on a chromaticity diagram that matches a specific colour and defined by its x, y coordinates.

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 the Kelvin scale. 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.

C.R.T.

Cathode Ray Tube. A device used to generate and guide a beam of electrons onto a screen coated with a phosphor.

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.

Decay

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

DC

Direct Current.

Dopant

Another term for an activator.

Down-Converter

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

Edge Emission

Emission that takes place within the lower energy edge of the absorption band.

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.

eV

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).

FED

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

Flux

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

Frequency

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

Gamma Rays

These may be emitted by 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.

Gamut

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

Holes (or Electron Holes)

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 phenomenon discovered in semi-conductor technology.

Host Lattice

The basic material into which an activator is added.

Incandescence

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

Infrared

Part of the electromagnetic spectrum extending from the limits of red visibility, i.e. 700nm~1mm.

Ion

An ion is an atom, molecule 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.

JEDEC

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

Kelvin Scale

Named after Lord Kelvin (1824-1907). A temperature scale starting from absolute zero and with the same magnitude as Celsius.

Killer

An impurity within a phosphor that destroys luminescence, e.g. Ni.

λ

Greek letter Lambda, the symbol for wavelength.

Lamp Phosphor

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

Laser

A laser is a device that produces light at a specific wavelength. This light is emitted coherently, allowing the beam to stay narrow and focus on a tiny spot. 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 electric current passes through it.

Lumens (lm)

A unit of luminous flux that measures the total quantitiy of visible light emitted by a source. The value is weighted by the response of the human eye.  It cannot therefore be used to measure ultraviolet or infrared light. One candela of light emitting uniformly in all directions produces 4π lumens.

Lumens/Watt (lm/W)

A measure of Luminous Efficacy, lumens emitted per watt of input power. It is an indication of how well a light source is converting power into light.

Maintenance

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

Mev

Mega electron volts.

Mineraliser

Another term for flux.

Mole (mol)

The amount of a substance containing a number of constitutive particles (e.g. atoms or particles) equal to the Avagadro constant (~6.022 x 1023).

Mole % (m/o)

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

Morphology

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.

Plasma

A plasma is an ionised gas or vapour that 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.

P.D.P.

Plasma Display Panel. A flat display panel consisting of two flat sheets (at least one is glass), sandwiching spacers that 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.

Persistence

Another term for afterglow.

Photon

A packet of electromagnetic energy with wave-like properties.

Quanta

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

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.

Quantum Efficiency

This is the simple expression, given as a percentage:  [Quanta of light out] / [Quanta of light in], e.g. 90 quanta of visible light emitted / 100 quanta of UV 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.

Reactance

Resistance of a capacitor or inductor to alternating current.

Relaxation

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

Sensitiser

A secondary activator which absorbs additional excitation energy and transmits it directly to the primary activator to enhance efficiency.

Stimulation

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

Soft X-Rays

The longer wavelength X-rays from 1~10nm

Stoichiometry

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 infrared.

TEPAC

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

Traps

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.

Triboluminescence

The release of stored light by mechanical action.

Up-Converter

Another name for an anti-stokes phosphor.

UVA

Ultra Violet radiation between 320nm and 400nm

UVB

Ultra Violet radiation between 260nm and 320nm

UVC

Ultra Violet radiation between 200nm and 260nm

Vacancies

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

VUV

Vacuum Ultraviolet.  Ultraviolet 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.

Wavelength

λ. The distance between successive crests of a wave, measured in nm.  N.B. Wavelength is inversely proportional to energy.

Wavenumber

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.

YAG

Yttrium Aluminium Garnet.