We include in this glossary definitions for a number of the basic terms and words used in the lighting community. For further information and formal definitions please see discussions in standard dictionaries, encyclopedias, the IES Lighting Handbook, and other lighting industry books.

Note that some of these definitions are quite subjective, and are offered here as guidance only.


Accent lighting: Lighting used to emphasize or draw attention to a special object or building.

Adaptive Controls: Devices such as motion sensors, timers and dimmers used in concert with outdoor lighting equipment to vary the intensity or duration of operation of lighting.

Ambient light: The general overall level of lighting in an area.

Angstrom: A unit of wavelength often used in astronomy, equal to 10-10 meters or 0.1 nanometers.


Baffle: An opaque or translucent element to shield a light source from direct view.

Ballast: A device used with a discharge lamp to obtain the necessary voltage, current, and/or waveform for starting and operating the lamp.

Beam spread: The angle between the two directions in the plane in which the intensity is equal to a given percentage (usually 10 percent) of the maximum beam intensity.

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

Bulb or lamp: The source of electric light. To be distinguished from the whole assembly (see luminaire). Lamp often is used to denote the bulb and its housing.


Candela (cd): Unit of luminous intensity. One candela is one lumen per steradian. Formerly called the candle.

Candlepower distribution curve: A plot of the variation in luminous intensity of a lamp or luminaire.

Candlepower: Luminous intensity expressed in candelas.

CIE: Commission Internationale de l’Eclairage. The international light commission. Sets most lighting standards.

Coefficient of Utilization (CU): Ratio of luminous flux (lumens) from a luminaire received on the “work plane” [the area where the light is needed] to the lumens emitted by the luminaire.

Color rendering: Effect of a light source on the color appearance of objects in comparison with their color appearance under normal daylighting.

Color Rendering Index (CRI)  A measure of the accuracy with which a light source of a particular CCT renders different colors in comparison to a reference light source with the same CCT. A high CRI provides better illumination with the same or lower lighting levels. It is important not to mix lamps with different CCTs and CRIs. Specify both the CCT and CRI when purchasing lamps.

Cones and rods: Groups of light-sensitive cells in the retinas of animal eyes. Cones dominate the response when the luminance level is high, and provide color perception. Rods dominate at low luminance levels, but give no significant color perception.

Conspicuity: The capacity of a signal to stand out in relation to its background so as to be readily discovered by the eye (as in lettering on a sign, for example).

Correlated Color Temperature (CCT): A measure in degrees Kelvin (°K) of light’s warmness or coolness. Lamps with a CCT of less than 3,200 °K are pinkish and considered warm. Lamps with a CCT greater than 4,000 °K are bluish-white and considered cool.

Cosine law: Illuminance on a surface varies as the cosine of the angle of incidence of the light. The inverse square law and the cosine law can be combined.

Cut off angle, of a luminaire: The angle, measured up from the nadir (i.e. straight down), between the vertical axis and the first line of sight at which the bare source (the bulb or lamp) is not visible.

Cutoff fixture: An IES definition is “Intensity at or above 90° (horizontal) no more than 2.5% of lamp lumens, and no more than 10% of lamp lumens at or above 80°”.


Dark adaptation: The process by which the eye becomes adapted to a luminance less than about 0.03 candela per square meter (0.01 foot-lambert).

Diffuser: A device used to distribute light from a source.

Dimmer: Dimmers can reduce the input power requirements and the rated lumen output levels of incandescent and fluorescent lights. Fluorescent lights need special dimming ballasts. Dimming incandescent lights reduces their efficiency.

Disability glare: Glare resulting in reduced visual performance and visibility. It is often accompanied by discomfort.

Discomfort glare: Glare that produces discomfort, but does not necessarily diminish visual performance.


Efficacy  The ratio of light output to its consumption of power, measured in lumens per watt (lm/W), or the ability of a lighting system to produce the desired result.

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

Electromagnetic (EM) spectrum: The distribution of energy emitted by a radiant source, arranged in order of wavelength or frequency. Includes gamma-ray, X-ray, ultraviolet, visual, infrared, and radio regions.

Energy (radiant energy): Unit is erg, or joule, or kWh.


Façade lighting: The illumination of the exterior of a building

Fixture: The assembly that holds the lamp in a lighting system. It includes the elements designed to give light output control, such as a reflector (mirror) or refractor (lens), the ballast, housing, and the attachment parts.

Fixture Lumens  A light fixture’s light output after processing of emitted light by optics in that fixture.

Fixture Watts: The total power consumed by a fixture. This includes the power consumed by the lamp(s) and ballast(s).

Floodlight: A fixture designed to “flood” a well-defined area with light.

Flux (radiant flux): Unit is erg/sec or watts.

Footcandle: Illuminance produced on a surface one foot from a uniform point source of one candela.

Footlambert: The average luminance of a surface-emitting or reflecting light at a rate of one lumen per square foot.

Full-cutoff fixture: An IES definition; “Zero intensity at or above horizontal (90° above nadir) and limited to a value not exceeding 10% of lamp lumens at or above 80°”.

Fully Shielded fixture: A fixture that allows no emission above a horizontal plane through the fixture.


Glare: Intense and blinding light that reduces visibility. A light within the field of vision that is brighter than the brightness to which the eyes are adapted.


HID lamp: In a discharge lamp, the emitted energy (light) is produced by the passage of an electric current through a gas. High-intensity discharge (HID) include mercury, metal halide, and high pressure sodium lamps. Other discharge lamps are LPS and fluorescent. Some such lamps have internal coatings to convert some of the ultraviolet energy emitted by the gas discharge into visual output.

High-Pressure Sodium (HPS) lamp: HID lamp where radiation is produced from sodium vapor at relatively high partial pressures (100 torr). HPS is essentially a “point source”.

House-side Shield: Opaque material applied to a fixture to block the light from illuminating a residence or other structure being protected from light trespass.


Illuminance: Density of luminous flux incident on a surface. Unit is footcandle or lux.

Illuminating Engineering Society of North America (IES or IESNA): The professional society of lighting engineers, including those from manufacturing companies, and others professionally involved in lighting.

Incandescent lamp: Light is produced by a filament heated to a high temperature by electric current.

Infrared radiation: Electromagnetic radiation with longer wavelengths than those of visible light, extending from the nominal red edge of the visible spectrum at 700 nanometers to 1 mm.

Intensity: The degree or amount of energy or light.

International Dark-Sky Association (IDA, Inc.): A non-profit organization whose goals are to build awareness of the value of dark skies, and of the need for quality outdoor lighting.

Inverse-square law: Illuminance at a point varies directly with the intensity, I, of a point source and inversely as the square of the distance, d, to the source. E = I / d2



kWh: Kilowatt-hour: A unit of energy equal to the work done by one kilowatt (1000 watts) of power acting for one hour.


Lamp Life: The average life span for a specific type of lamp. Half of lamps will perform longer than the average; the others will fail before the average.

LED: Light-emitting diode.

Light Pollution: Any adverse effect of artificial light.

Light Quality: A measurement of a person’s comfort and perception based on the lighting.

Light Spill: Unwanted spillage of light onto adjacent areas and may affect sensitive receptors particularly residential properties and ecological sites.

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

Lighting Controls: Devices used for either turning lights on and off or for dimming.

Photocells Sensors that turn lights on and off in response to natural light levels. Some advanced mode can slowly dim or increase the lighting. See also: Adaptive Controls.

Low-Pressure Sodium (LPS) lamp: A discharge lamp where the light is produced by radiation from sodium vapor at a relatively low partial pressure (about 0.001 torr). LPS is a “tube source”. It is monochromatic light.

Lumen: Unit of luminous flux; the flux emitted within a unit solid angle by a point source with a uniform luminous intensity of one candela.

Lumen depreciation factor: Light loss of a luminaire with time due to the lamp decreasing in efficiency, dirt accumulation, and any other factors that lower the effective output with time.

Luminaire: A complete lighting unit that usually includes the fixture, ballasts, and lamps.

Luminaire Efficiency: The ratio of the light emitted by the luminaire compared to the light emitted by the enclosed lamps.

Luminance: At a point and in a given direction, the luminous intensity in the given direction produced by an element of the surface surrounding the point divided by the area of the projection of the element on a plane perpendicular to the given direction. Units: candelas per unit area.

Lux: One lumen per square meter. Unit of illuminance.


Mercury lamp: An HID lamp where the light is produced by radiation from mercury vapor.

Metal-halide lamp: An HID lamp where the light is produced by radiation from metal-halide vapors.

Mounting height: The height of the fixture or lamp above the ground.


NadirA point on the celestial sphere directly below the observer, diametrically opposite the zenith.

Nanometer (nm): 10-9 meter. Often used as the unit for wavelength in the EM spectrum.


Occupancy Sensors

  • Passive Infrared: A lighting control system that uses infrared beams to sense motion. When beams of infrared light are interrupted by movement, the sensor turns on the lighting system. If no movement is sensed after a predetermined period, the system turns the lights off.
  • Ultrasonic: A lighting control system using high–frequency sound waves pulsed through a space to detect movement by depth perception. When the frequency of the sound waves change, the sensor turns on the lighting system. After a predetermined time with no movement, the system turns the lights off.

Optic: The components of a luminaire such as reflectors, refractors, and protectors that make up the light-emitting section.


Photometry: The quantitative measurement of light level and distribution.

Photocell: An electronic device that changes the light output of a luminaire dynamically in response to the ambient light level around the luminaire.


Quality of light: A subjective ratio of the pluses to the minuses of any lighting installation.


Reflector: An optic that achieves control of light by means of reflection (using mirrors).

Refractor: An optic that achieves control of light by means of refraction (using lenses).


Semi-cutoff fixture: An IES definition; “Intensity at or above 90° (horizontal) no more than 5% of lamp lumens and no more than 20% at or above 80°”.

Shielding: An opaque material that blocks the transmission of light.

Skyglow: Diffuse, scattered skylight attributable to scattered light from sources on the ground.

Source Intensity: This applies to each source in the potentially obtrusive direction, outside of the area being lit.

Spotlight: A fixture designed to light only a small, well-defined area.

Stray light: Emitted light that falls away from the area where it is needed or wanted. Light trespass.


Task Lighting: Task lighting is used to provide direct light for specific activities without illuminating the entire area.


Ultraviolet light: Electromagnetic radiation with wavelengths from 400 nm to 100 nm, shorter than that of visible light but longer than X-rays.


Veiling luminance: A luminance produced by bright sources in the field-of-view superimposed on the image in the eye reducing contrast and hence visibility.

Visibility: Being perceived by the eye. Seeing effectively. The goal of night lighting.


Wallpack: A luminaire, typically affixed to the side of a structure, used for area lighting.




Zenith: An imaginary point directly “above” a particular location, on the imaginary celestial sphere.