Glossary

Ballast

A component of conventional control gear. It controls the current through the lamp, and is used with discharge lighting, including fluorescent, sodium, mercury and metal halide lamps. The term is sometimes used loosely to mean control gear. Also called a choke.

Building lighting

The lighting of the main surfaces of a building, particularly walls; the surround to work stations and in large rooms, the ceiling. Building lighting design will depend on the required visual amenity and the architectural design and should be considered alongside the design of task lighting.

Choke

Alternative name for ballast.

Colour rendering

An indicator of how accurately colours can be distinguished under different light sources. The colour rendering index compares the ability of different lights to render colours accurately with a Ra of 100 being ‘ideal’. Colour rendering properties of a light source are specified by the colour rendering index (CRI). See also colour temperature and CRI.

Colour temperature

Also known as colour appearance, the colour temperature is the colour of ‘white’ the light appears. It is measured in Kelvin, and ranges from 1800k (very warm, amber) to 8000k (cool). 6500K is daylight. There are many colours of ‘white’ available. For general use these are: a warm white (2600 to 2700 degrees Kelvin), a medium white (3000 to 3500 degrees Kelvin) and a cool white (blue-y 4000 degrees Kelvin).

Control gear

A ‘package’ of electrical or electronic components including ballast, power factor correction capacitor and starter. High frequency electronic control gear may include other components to allow dimming etc.

CRI

Colour rendering index has been defined by the CIE. (Commission Internationale de l’Eclairage). The CRI is specified for individual colours, Rj, or, for eight reference colours, Ra. Good colour rendering equates to a high CRI (CRI 100 = daylight), poor colour rendering equates to a low CRI. See also Ra, colour rendering, colour temperature.

Diffuser

A translucent screen used to shield a light source and at the same time soften the light output and distribute it evenly.

Discharge lamp

A lamp which produces illumination via electric discharge through a gas, a metal vapour or a mixture of gases and vapours.

Efficacy (luminous efficacy)

The ratio of light emitted by a lamp to the power consumed by it, that is, lumens per Watt. When the control gear losses are included, it is expressed as lumens per circuit Watt.

Filament lamp

A lamp which produces illumination by heating a filament until it glows.

Illuminance

The amount of light falling on an area, measured in lux. 1 lux is equal to one lumen per square metre.

Kelvin

A measure of colour temperature for lamps. Kelvin is oC + 273.

Light output ratio (LOR)

The ratio of the total amount of light output of a lamp and luminaire to that of just the bare lamp.

Luminaire

A light fitting and lamp including all components for fixing and protecting the lamps, as well as connecting them to the supply.

Lumen

Unit of luminous flux, used to describe the amount of light produced by a lamp or falling on a surface.

Lux

An international unit of measurement of the intensity of light. 1 lux is equal to the illumination of a surface one metre away from a single candle.

Maintained illuminance

The minimum illuminance averaged over the reference surface at the time maintenance has to be carried out (by replacing lamps and/or cleaning luminaires and room surfaces).

Power density (W/m2)

The power density of a lighting installation is the total power it consumes measured in Watts. It includes the lamps and any lamp ballasts, divided by the total floor area of the installation in square metres.

Ra

The colour performance of a lamp is described by its general colour rendering index (Ra) which defines its ability to show surface colours accurately. It is described by a number — 100 is considered to be excellent, a value of 80 and above is good and appropriate for most situations where people are present. Where colour identification is important, a value of 90 or above should be used.

Rated average lamp life

The time when half the number of lamps in a batch fail under test conditions.

Restrike

The time taken for a lamp to illuminate after being switched off and then on again.

Start up

The time taken for a lamp to illuminate after being switched on from cold.

Task lighting

The lighting provided for specific tasks within a lighting design. For example, task lighting design will depend on the particular tasks undertaken and the building lighting design.

Universal operating position

Refers to a lamp that can be oriented in any way without affecting light quality.

Utilisation factor (UF)

For a particular installation, the UF is the proportion of lamp light output that reaches the working plane e.g. desktop, including both direct and reflected light.

Lamp abbreviations

GLS

General lighting service (incandescent lamps).

High pressure sodium lamps

SON

Used in this text as a general abbreviation for high pressure sodium lamps and would have either a ‘T’ (tubular) or ‘E’ (elliptical).

SON-DL

Used for SON deluxe lamps, which have an improved colour rendering.

White sodium

High pressure sodium lamp which achieves high Ra rating and around 2500 degree Kelvin.