Lighting Definitions

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 equal to 0.1 nanometer. The human eye is sensitive between 4000 angstroms (blue light) and 7000 angstroms (red light).

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’Éclairage): The International Commission on Illumination, headquartered in Vienna, Austria; sets many 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 index (CRI): Effect of a light source on the color appearance of objects in comparison with their color appearance under normal daylighting. The CRI for monochromatic light is nearly 0, and for sunlight it's 100. A standard "cool white" fluorescent lamp has a CRI near 63, while "triphosphor" fluorescent lamps often claim a CRI of 80 to 90.

Cones and rods: Retinal receptors in the human eye. 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 (E) on a surface varies as the cosine of the light's angle of incidence (theta).

Cut-off angle: 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.

Cut-off 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, microwave, and 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 (fc): Illuminance produced on a surface that is1foot from a uniform point source of 1candela.

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. (IESNA recently deemed this definition obsolete; use "fully shielded" instead.)

Glare: Intense and blinding light. Never helps visibility.

HID lamp: A "high-intensity discharge" lamp, in which the emitted light is produced by passing an electric current through a gas. HID sources include mercury vapor, 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. Abbreviated E.

IESNA or IES (Illuminating Engineering Society of North America): 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 at longer wavelengths than the eye can see.

Intensity: The degree or amount of energy or light.

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

Inverse-square law: Illuminance (E) at a point is reduced by the square of the distance, D, from a source of intensity I. (algebraically, E = I / D²). The inverse-square law and the cosine law can be combined: E = I × (cos theta) / D².

KWH: Kilowatt-hour. A unit of energy equal to the work done by 1 kilowatt (1000 watts) of power acting for 1 hour.

Light pollution: Any adverse effect of artificial 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-vapor (MV): An HID lamp where the light is produced by radiation from mercury vapor.

Metal-halide (MH): 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): A unit of wavelength equal to 0.000000001 meter, or 10 angstroms. The human eye is sensitive between 400 nm (blue light) and 700 nm (red light).

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: A device for controlling light output by means of reflection (mirror).

Refractor: A device for 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 radiation: Electromagnetic radiation at shorter wavelengths than the eye can see. “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 eyes' field-of-view that reduces contrast and hence visibility.

Visibility: Being perceived by the eye. Good visibility, allowing for effective seeing, is the goal of night lighting.

Watt: A rate of electricity consumption equal to 1 joule of energy per second. A 60-watt incandescent light bulb uses about four times more electricity than a 13-watt compact-fluorescent bulb, even though they produce roughly the same amount of light (840 vs. 880 lumens, respectively).