International Dark-Sky Association (IDA)

3545 N. Stewart Ave.; Tucson, AZ 85716; U.S.A.



Glossary of Basic Terms and Definitions

Information Sheet 9, July 1996

We include in this information sheet definitions for a number of the basic terms and words used in the lighting community. For further information and formal definitions, 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 a guidance, not as a formal definition.

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

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

Angstrom: A unit of wavelength often used in astronomy, equal to 0.0000000001 meter or 0.1 nanometer.

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 wave form 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 many 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.

Cones and rods: Retinal receptors. Cones dominate the response when the luminance level is high, and provide color perception. Rods dominate at low luminance levels. No rods are found in the central part of the fovea. Rods have no color perception ability.

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).

Cosine law: Illuminance on an surface varies as the cosine of the angle of incidence of the light. The inverse-square law and the cosine law can be combined: E = (I cos theta)/D, where D = d*d.

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: A fixture that provides a cutoff (shielding) of the emitted light.

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

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 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, through radio regions.

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

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.

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

Glare: Intense and blinding light. Never helps visibility.

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".

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: EM radiation just to the long wavelength side of the visual.

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 lighting. (See IDA Information Sheet No. 14.)

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/D, where D = d*d.

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

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. Obtrusive light.

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. One footcandle is one lumen per square foot. One lux is one lumen per square meter.

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: The complete lighting unit, including the lamp, the fixture, and other parts.

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. L = dE/(dw cos theta). 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.

Nanometer (nm): 0.000000001 meter. Often used as the unit for wavelength in the EM spectrum.

Photometry: The quantitative measurement of light level and distribution.

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

Reflector: Controlling light output by means of reflection (mirror).

Refractor: Controlling light output by means of refraction (lens).

Semi-cutoff fixture: A fixture that provides some cutoff, but less than a full-cutoff fixture.

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: Lighting designed for a specific purpose or task.

Ultraviolet "light": The energy output by a source which is of shorter wavelengths than the eye can see. Some photographic films are sensitive to ultraviolet energy, as are many electronic detectors. "Black Light."

Urban sky glow: The brightening of the night sky due to manmade lighting.

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.

This material is copyrighted (©) by the IDA, or others as noted. It may be reproduced for non-commercial usage provided that credit is given to IDA.

A key source for information on outdoor night lighting is the International Dark-Sky Association (IDA).