GLOSSARY

Depreciation factor
The reciprocal of the maintenance factor.

Diffuse reflection
Diffusion by reflection in which, on the macroscopic scale, there is no regular reflection.

Diffused lighting
Lighting in which the light on the working plane or on an object is not incident predominantly from a particular direction.

Diffuser
Device used to alter the spatial distribution of radiation and depending essentially on the phenomenon of diffusion.

Direct lighting
Lighting by means of luminaires with a light distribution such that 90 to 100 per cent of the emitted luminous flux reaches the working plane direct, assuming that this plane is unbounded.

Directional lighting
Lighting in which the light on the working plane or on an object is incident predominantly from a particular direction.

Disability glare
Glare that impairs the vision of objects without necessarily causing discomfort.

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.

Discomfort glare
Glare that causes discomfort without necessarily impairing the vision of objects.

Downlight (downlighter)
Small luminaire concentrating the light, usually recessed in the ceiling.

Drip-proof luminaire
Luminaire so constructed that, when mounted in its designed position, it will withstand drops of water falling in a substantially vertical direction.

Dustproof luminaire
Luminaire so constructed that dust of specified nature and fineness cannot enter it when it is used in a dust-laden atmosphere.

Efficacy
see Luminous efficacy

Electric discharge in a gas
The passage of an electric current through gases and vapors by the production and movement of charge carriers under the influence of an electric field.
Note
This results in the emission of electromagnetic radiation, which plays an essential part in all its applications in lighting.

Flashed area
Of a luminaire in a given direction. The area of the orthogonal projection of the luminous surface on a plane perpendicular to the specified direction.

Flicker
Impression of fluctuating luminance or color. Floodlight Projector designed for floodlighting, usually capable of being pointed in any direction and of weatherproof construction.

Fluorescence
Photoluminescence that persists for an extremely short time after excitation.

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 low­pressure tubular fluorescent 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.

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