Glossary - G

gas-filled lamp an incandescent lamp in which the filament operates in a bulb filled with one or more inert gases.

gaseous discharge the emission of light from gas atoms excited by an electric current.

general color rendering index, *R*a Measure of the average shift of eight standardized colors chosen to be of intermediate saturation and spread throughout the range of hues. If the color rendering index is not qualified as to the color samples used, *R*a is assumed. general diffuse lighting lighting involving luminaires that distribute 40 to 60% of the emitted light downward and the balance upward, sometimes with a strong component at 90° (horizontal). See direct- indirect lighting.

general lighting lighting designed to provide a substantially uniform level of illuminance throughout an area, exclusive of any provision for special local requirements. See *ceiling area lighting, direct lighting, direct-indirect lighting, general diffuse lighting, indirect lighting, localized general lighting, semidirect lighting,* and *semi-indirect lighting.*

general-purpose floodlight (GP) a weatherproof unit so constructed that the housing forms the reflecting surface. The assembly is enclosed by a cover glass.

germicidal effectiveness† See bactericidal (germicidal) effectiveness.

germicidal efficiency of radiant flux† See bactericidal (germicidal) efficiency of radiant flux.

germicidal exposure† See bactericidal (germicidal) exposure.

germicidal flux and flux density† See *bactericidal (germicidal) flux* and *bactericidal (germicidal) flux density.*

germicidal lamp a low-pressure mercury lamp in which the envelope has high transmittance for 254-nm radiation. See bactericidal lamp.

glare the sensation produced by luminances within the visual field that are sufficiently greater than the luminance to which the eyes are adapted, which causes annoyance, discomfort, or loss in visual performance and visibility. See *blinding glare, direct glare, disability glare,* and *discomfort glare.*

Note The magnitude of the sensation of glare depends on such factors as the size, position, and luminance of a source; the number of sources; and the luminance to which the eyes are adapted.

globe a transparent or diffusing enclosure intended to protect a lamp, to diffuse and redirect its light, or to change the color of the light.

glossometer an instrument for measuring gloss as a function of the directionally selective reflecting properties of a material in angles near to and including the direction giving specular reflection.

glow discharge an electric discharge characterized by a low, approximately constant current density at the cathode (on the order of 10 *µ*A/mm2) at low cathode temperature and a high voltage drop (typically 50 V or more). Secondary emission from the cathode is much greater than the thermionic emission.

Note A distinction is made between the normal cathode drop (potential difference due to space charge near the cathode) that occurs when the glow does not cover the cathode completely (with constant current density) and that is independent of the discharge current, and the abnormal cathode drop that occurs when the glow covers the cathode completely (with increased current density) and that depends on the discharge current.

glow factor a measure of the visible light response of a fluorescent material to black light. It is equal to *π* times the luminance in cd/m2 produced on the material divided by the incident black-light flux density in mW/m2. It can be measured in lm/mW.

glow lamp an electric-discharge lamp whose mode of operation is that of a glow discharge and in which light is generated in the space close to the electrodes.

goniophotometer a photometer for measuring the directional light distribution characteristics of sources, luminaires, media, and surfaces.

graybody a temperature radiator whose spectral emissivity is less than unity and the same at all wavelengths.

ground-area open floodlight (O) a unit providing a weatherproof enclosure for the lamp socket and housing. No cover glass is required.

ground-area open floodlight with reflector insert (OI) a weatherproof unit so constructed that the housing forms only part of the reflecting surface. An auxiliary reflector is used to modify the distribution of light. No cover glass is required.

ground light visible radiation from the sun and sky reflected by surfaces below the plane of the horizon.

group flashing light a flashing light in which the flashes are combined in groups, each including the same number of flashes, and in which the groups are repeated at regular intervals. The duration of each flash is clearly less than the duration of the dark periods between flashes, and the duration of the dark periods between flashes is clearly less than the duration of the dark periods between groups.