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Lighting Glossary Of Terms

lighting glossary of terms

Arc discharge (in a gas or vapor)

Electric discharge characterized by a cathode fall that is small compared with that in a glow discharge.

Average illuminance (Eav)

Over a surface, Illuminance averaged over the specified surface.


Device used with discharge lamps for stabilizing the current in the discharge.

Beam axis

The direction in the center of the solid angle which is bounded by directions having luminous intensities of 90% of the maximum intensity of a luminaire.

Beam spread

The angle (in the plane through the beam axis) over which the luminous intensity drops to a stated percentage of its peak intensity.

Cold-start lamp (instant-start lamp, USA)

Discharge lamp designed to start without preheating of the electrodes, for example 'TL'S fluorescent lamp.

Color rendering

General expression for the effect of an illuminant on the color appearance of objects in conscious or subconscious comparison with their color appearance under a reference illuminant.

Color temperature

Temperature of the black body that emits radiation of the same chromaticity as the radiation considered. Unit Kelvin, K.


A device in the electrical circuit for varying the luminous flux from lamps in a lighting installation.

Discharge lamp

Lamp in which the light is produced, directly or indirectly, by an electric discharge through a gas, a metal vapor, or a mixture of several gases and vapors.

Downlight (downlighter)

Small luminaire concentrating the light, usually recessed in the ceiling.

Emergency lighting

Lighting provided for use when the supply to the normal lighting fails.


Impression of fluctuating luminance or color.


Projector designed for floodlighting, usually capable of being pointed in any direction and of weatherproof construction.

Fluorescent lamp

Discharge lamp of the low-pressure mercury type in which most of the light is emitted by a layer of fluorescent material excited by the ultraviolet radiation from the discharge. Note: This term is most commonly applied to low ­pressure tubular fluorescent lamps, for example 'TL', 'TL'D, 'TL'E lamps.

Fluorescent mercury lamp

A high-pressure mercury lamp in which the light is produced partly by the mercury vapor and partly by a layer of fluorescent material on the inner surface of the outer bulb excited by the ultraviolet radiation of the discharge - for example: HPL-N, HPLR-N lamps.


Condition of vision in which there is discomfort or a reduction in the ability to see significant objects, or both, due to an unsuitable distribution or range of luminance or to extreme contrasts in space or time.

Halogen lamp

Gas-filled lamp containing a tungsten filament and a small proportion of halogens.

High-pressure mercury (vapor) lamp

Mercury vapor lamp, with or without a coating of phosphor, in which during operation the partial pressure of the vapor is of the order of 105 Pa - for example: HPL and HPL-N lamps.

High-pressure sodium (vapour) lamp

Sodium vapour lamp in which the partial pressure of the vapour during operation is of the order of 104 Pa - for example, SON and SON-T lamps.These are very popular in our low bay lights and our floodlight range.

Illuminance (E)

At a point on a surface. Quotient of the luminous flux incident on an element of the surface containing the point, and the area of that element. Unit lux, lx. Note: The orientation of the surface may be defined, e.g. horizontal, vertical, hence horizontal illuminance, vertical illuminance.

Incandescent (electric) lamp

Lamp in which light is produced by means of an element heated to incandescence by the passage of an electric current.

Low-pressure mercury (vapor) lamp

Mercury vapor lamp, with or without a coating of phosphor, in which during operation the partial pressure of the vapor does not exceed 100 Pa - for example a 'TL' lamp.

Low-pressure sodium (vapor) lamp

Sodium vapor lamp in which the partial pressure of the vapor during operation does not exceed 5 Pa - for example a SOX lamp.

Luminance (L)

In a given direction, at a given point of a real or imaginary surface. Quotient of the luminous flux transmitted by an elementary beam passing through the given point and propagating in the solid angle containing the given direction, and the product of the solid angle, the area of a section of that beam containing the given point, and the angle between the normal to that section and the direction of the beam. Unit candela per square meter, cd/m2.

Metal halide lamp

Discharge lamp in which the major portion of the light is produced by the radiation from a mixture of a metallic vapor (for example, mercury) and the products of the dissociation of halides (for example, halides of thallium, indium or sodium) - for example: HPI-T lamps.These are very popular in our lowbay lights and our floodlights ranges

Metal vapor lamp

Discharge lamp such as the 'mercury (vapor) lamp' and the 'sodium (vapor) lamp' in which the light is mainly produced in a metallic vapor.


Luminaire using reflection and/or refraction to increase the luminous intensity within a limited solid angle.

Reflector lamp

Lamp in which part of the bulb is coated with a reflecting material, either diffuse or specular, so as to control the light, for example: HPL-R, MLR, and 'TL'F lamps.


A (small) projector giving concentrated light of usually not more than 20° divergence.


Quantity of a luminaire to indicate the extent to which the light is 'spread out' Luminaires are classified as being of narrow, average or broad spread.


Device for starting a discharge lamp (in particular a fluorescent lamp) that provides for the necessary preheating of the electrodes and/or causes a voltage surge in combination with the series ballast.

Switch-start fluorescent lamp

Fluorescent lamp suitable for operation with a circuit requiring a starter for the preheating of the electrodes, for example 'TL'D type.

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