Glossary of lighting terms

Words and phrases it might be useful to know when discussing light pollution.

Try also the following internet pages:

• Common Light Measurement Terms (with diagrams)

Ambient Light

The total light level or effect, or amount of light perceived, in one's surroundings.

Baffle

A plate inserted within or just outside a luminaire to shield the light from direct view.

Ballast

Electrical devices used in conjunction with a discharge lamp to start and control it.

Candela (cd) or standard candle

The SI unit of luminous intensity.

CIE

The Commission Internationale de l'Eclairage (International Lighting Commission), based in Vienna.

Colour rendering/rendition

The perceived effect on objects of different colours of lights of different types.

Colour temperature

This term describes the actual colour of the light source itself, as opposed to that of the light issuing from it.

Column

The post upon which a lamp is mounted.

Cones

See Rods and cones.

Dark adaptation

The transition of visual processes within the eye to darker surroundings. See Rods and cones.

Disability glare (Veiling Luminance)

Glare causing reduced visual performance.

Discomfort Glare

Glare producing discomfort or annoyance without necessarily interfering with visual performance.

FCO

This stands for full cut-off, referring to a lamp with a flat glass panel beneath which, when mounted horizontally, emits no light above the horizontal. See also SCO.

Fluorescent

A long-life, relatively cheap whitish light source based on a gas discharge process, where electrons pass through a tube and interact with a phosphor coating.

Flux

Luminous flux is the rate of flow of particles of light energy, measured in watts or ergs/sec.

Fovea

A small central depression in the back of the retina containing cone cells: the area of sharpest vision.

Full cut-off

See FCO.

IDA

The International Dark-Sky Association.

IESNA

The Illuminating Engineering Society of North America. The USA's professional guidance body for lighting engineers.

ILE

The Institution of Lighting Engineers. The UK's professional guidance body for lighting engineers

Incandescent

Describes a light source based on electricity passing through a thin filament (usually tungsten) which glows brightly.

kWh Kilowatt-hour

unit of energy equal to the work done by one thousand watts of power acting for one hour.

Light spill

The emission of light outside the premises which the lighting is supposed to illuminate.

Light trespass

Troublesome light entering areas or premises outside the boundary of the premises to be illuminated.

Lumen (lm)

The SI unit of luminous flux, being the flux emitted in a solid angle of 1 steradian by a point source with uniform intensity of 1 candela (q.v.).

Luminaire

A word not found in many dictionaries, but widely used in the lighting community to denote the lamp and its surrounding casing and optics.

Lux (lx)

The SI unit of illumination, being a luminous flux of 1 lumen (q.v.) per square metre. The value for the full Moon is about 0.2 - 0.3 lux.

Photometry

The measurement of the level and distribution of light.

Reflectance

The amount of light reflected by a given surface (the ratio of the reflected flux to the incident flux).

Reflectivity

The ability of a surface to reflect radiation (technically, equal to the reflectance of a layer of material sufficiently thick for the reflectance not to depend on the thickness).

Rods and cones

Cells in the retina of the eye. Rods are cylindrical cells containing rhodopsin ('visual purple'), and are sensitive to dim light but not to colour. Cones are conical cells which are sensitive to colour and bright light. The process of dark adaptation involves the rods taking over visual duties from the cones. Interestingly, there are no rods in the centre of the fovea (q.v.), which explains the astronomer's 'averted vision' trick (objects appearing more distinct if you look slightly to one side of them).

SCO Semi-cut-off

a lamp type which has a shallow bowl beneath, and emits little or no light skywards.

SON

Another name for high-pressure sodium sources.

SOX

Another name for low-pressure sodium sources.

Skybeam, Sky Beam

A concentrated beam of light sent into the sky deliberately, usually for the purposes of advertising. (Often erroneously called a 'laser'.) Such lights have been banned in several regions of Switzerland and Belgium.

Sky Glow, Skyglow

Unwanted light emitted into the night sky from poorly aimed lamps.

Stray light

See Light spill.

Street furniture

All manufactured items commonly seen along roadsides. e.g. lighting columns, telephone poles.

Veiling luminance

See Disability glare.

Visibility

Clarity of vision; how well we see something. The purpose of a good light should be to increase visibility: to reveal and not conceal.

UWLR, ULR, Upward flux

The abbreviations stand for Upward (Waste) Light Ratio. All these terms refer to the relative amount of the light emitted above the horizontal.