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Ambient LightThe general overall lighting in an area.
BrightnessStrength of the sensation that results from viewing surfaces which the light comes to the eye.
Bulb or LampThe source of electric light. To be distinguished from the whole assembly (see fixtures or luminaries).
Color RenderingEffect of a light source on the color of objects in comparison with their color under normal daylight.
Cutoff AngleAngle measured up from nadir (i.e. straight down) to the point which one can first view the bare light source (lamp).
Cutoff FixtureA fixture, which provides a cutoff (shielding) of the emitted light.
Dark AdaptionThe process by which the eye becomes adapted to low levels of light.
Disability GlareGlare that impairs the ability to see detail without necessarily causing visual discomfort.
Discomfort GlareGlare that causes visual discomfort without necessarily impairing the ability to see detail.
EfficacyThe ability of a lighting system to produce the desired result.
EfficiencyA measure of the useful output of a system or fixture compared to the input of the system.
FootcandleMeasurement of the illuminance produced on a surface, 1' X 1'.
Full Cutoff FixtureFixture that allows no light emission above a horizontal plane through the fixture.
GlareThe discomfort or impairment of vision experienced when parts of the visual field are excessively bright in relation to the general surroundings.
High Pressure Sodium (HPS)HID lamp with high lumen package but poor color rendering.
Illuminating Engineering Society (IES)Professional society of lighting engineers, manufacturing companies, and others professionally involved in lighting.
Light PollutionAny adverse effect of manmade light. Often used to denote urban sky glow.
Light TrespassLight falling where it is not wanted or needed. Spill light or obtrusive light.
LumenMeasurement of light, one footcandle is one lumen per square foot.
Metal HalideHID lamp with a white/blue light output. Best lamp for color rendering.
PhotometryThe quantitative measurement of light level and distribution.
Quality of LightSubjective ratio of the pluses to the minuses.
ReflectorControlling light output and distribution by means of reflection.
RefractorControlling light output and distribution by means of refraction lens' plastic or glass.
Semi Cutoff FixtureFixture that provides some cut-off, but less than a cut-off or full cut-off.
SpotlightFixture designed to light a specific area, well defined.
Stray LightEmitted light that falls away from the areas where it is needed or wanted. Light Trespass.