Glossary - G

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, Ra 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, Ra 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 directindirect lighting. 

general 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. 

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 µA/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. 

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.