Ambient Light
The general overall lighting in an area.

Brightness
Strength of the sensation that results from viewing surfaces which the light comes to the eye.

Bulb or Lamp
The source of electric light. To be distinguished from the whole assembly (see fixtures or luminaries).

Color Rendering
Effect of a light source on the color of objects in comparison with their color under normal daylight.

Cutoff Angle
Angle measured up from nadir (i.e. straight down) to the point which one can first view the bare light source (lamp).

Cutoff Fixture
A fixture, which provides a cutoff (shielding) of the emitted light.

Dark Adaption
The process by which the eye becomes adapted to low levels of light.

Disability Glare
Glare that impairs the ability to see detail without necessarily causing visual discomfort.

Discomfort Glare
Glare that causes visual discomfort without necessarily impairing the ability to see detail.

Efficacy
The ability of a lighting system to produce the desired result.

Efficiency
A measure of the useful output of a system or fixture compared to the input of the system.

Footcandle
Measurement of the illuminance produced on a surface, 1' X 1'.

Full Cutoff Fixture
Fixture that allows no light emission above a horizontal plane through the fixture.

Glare
The 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 Pollution
Any adverse effect of manmade light. Often used to denote urban sky glow.

Light Trespass
Light falling where it is not wanted or needed. Spill light or obtrusive light.

Lumen
Measurement of light, one footcandle is one lumen per square foot.

Metal Halide
HID lamp with a white/blue light output. Best lamp for color rendering.

Photometry
The quantitative measurement of light level and distribution.

Quality of Light
Subjective ratio of the pluses to the minuses.

Reflector
Controlling light output and distribution by means of reflection.

Refractor
Controlling light output and distribution by means of refraction lens' plastic or glass.

Semi Cutoff Fixture
Fixture that provides some cut-off, but less than a cut-off or full cut-off.

Spotlight
Fixture designed to light a specific area, well defined.

Stray Light
Emitted light that falls away from the areas where it is needed or wanted. Light Trespass.​