{"id":3302,"date":"2015-07-28T14:58:27","date_gmt":"2015-07-28T21:58:27","guid":{"rendered":"http:\/\/summitrise.com\/?page_id=3302"},"modified":"2025-05-07T12:29:48","modified_gmt":"2025-05-07T19:29:48","slug":"glossary","status":"publish","type":"page","link":"https:\/\/darksky.org\/resources\/glossary\/","title":{"rendered":"Glossary"},"content":{"rendered":"\n

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 ANSI\/IES LS-1-22, Lighting Science: Nomenclature and Definitions for Illuminating Engineering<\/em><\/a> (published by our partner<\/a> the Illuminating Engineering Society<\/a>), and other lighting industry books.<\/p>\n\n\n\n

Skip below to:<\/strong><\/p>\n\n\n\n

A<\/a> | B<\/a> | C<\/a> | D<\/a> | E<\/a> | F<\/a> | G<\/a> | H<\/a> | I<\/a> | J<\/a> | K<\/a> | L<\/a> | M<\/a> | N<\/a> | O<\/a> | P<\/a> | Q<\/a> | R<\/a> | S<\/a> | T<\/a> | U<\/a> | V<\/a> | W<\/a> | X<\/a> | Y<\/a> | Z<\/a><\/p>\n\n\n\n

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A<\/strong><\/h1>\n\n\n\n

accent lighting<\/h3>\n\n\n\n

Lighting used to emphasize or draw attention to a special object or building.<\/p>\n\n\n\n

adaptive controls<\/h3>\n\n\n\n

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.<\/p>\n\n\n\n

ALAN<\/h3>\n\n\n\n

Acronym for artificial light at night<\/em>.<\/p>\n\n\n\n

ambient light<\/h3>\n\n\n\n

The general overall level of lighting in an area.<\/p>\n\n\n\n

angstrom<\/h3>\n\n\n\n

A unit of wavelength often used in astronomy, equal to 10-10<\/sup> meter or 0.1 nanometer.<\/p>\n\n\n\n

artificial light<\/h2>\n\n\n\n

Light produced by humans. <\/p>\n\n\n\n

Examples include electric light as well as the light from campfires, candles, and fireworks.<\/p>\n\n\n\n

astrophotography<\/h3>\n\n\n\n

Also known as astronomical imaging, astrophotography is the photography or imaging of astronomical objects, celestial events, or areas of the night sky.<\/p>\n<\/div>\n\n\n\n

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B<\/strong><\/h1>\n\n\n\n

baffle<\/h3>\n\n\n\n

An opaque or translucent element used to shield a light source from direct view.<\/p>\n\n\n\n

ballast<\/h3>\n\n\n\n

A device used with a discharge lamp to obtain the necessary voltage, current, and\/or wave form for starting and operating the lamp<\/a>.<\/p>\n\n\n\n

beam spread<\/h3>\n\n\n\n

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.<\/p>\n\n\n\n

brightness<\/h3>\n\n\n\n

The strength of the sensation that results from viewing surfaces from which light comes to the eye.<\/p>\n\n\n\n

bulb or lamp<\/h3>\n\n\n\n

The source of electric light. To be distinguished from the whole assembly (see luminaire<\/a><\/em>). Lamp<\/em><\/a> often is used to denote the bulb and its housing.<\/p>\n<\/div>\n\n\n\n

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C<\/strong><\/h1>\n\n\n\n

candela (cd)<\/h3>\n\n\n\n

Unit of luminous intensity. One candela is one lumen<\/a> per steradian<\/a>. <\/p>\n\n\n\n

candlepower<\/h3>\n\n\n\n

Luminous intensity expressed in candelas<\/a>.<\/p>\n\n\n\n

candlepower distribution curve<\/h3>\n\n\n\n

A plot of the variation in luminous intensity of a lamp<\/a> or luminaire<\/a>.<\/p>\n\n\n\n

CIE<\/h3>\n\n\n\n

Commission Internationale de l\u2019Eclairage<\/a> (International Commission on Illumination). An international technical, scientific, and cultural non-profit organization that sets lighting standards.<\/p>\n\n\n\n

clutter<\/h3>\n\n\n\n

Bright, confusing, and excessive groupings of light sources.<\/p>\n\n\n\n

color rendering<\/h3>\n\n\n\n

Effect of a light source on the color appearance of objects in comparison with their color appearance under normal daylighting.<\/p>\n\n\n\n

color rendering index (CRI)<\/h3>\n\n\n\n

A measure of the accuracy with which a light source of a particular correlated color temperature (CCT) renders different colors in comparison to a reference light source with the same CCT. <\/p>\n\n\n\n

color spectrum<\/h3>\n\n\n\n

The portion of the electromagnetic spectrum that is visible to the human eye, i.e., from about 380 nm to about 760 nm.<\/p>\n\n\n\n

cones and rods<\/h3>\n\n\n\n

Two types of light-sensitive cells in the retinas of animal eyes. Cones dominate the response when the ambient light level is high, and provide color perception. Rods dominate at low ambient light levels and give no color perception.<\/p>\n\n\n\n

conspicuity<\/h3>\n\n\n\n

The capacity of a signal to stand out in relation to its background such that it may be readily discerned by the eye (as in lettering on a sign, for example).<\/p>\n\n\n\n

correlated color temperature (CCT)<\/h3>\n\n\n\n

A measure (in kelvins, K) of the warmth or coolness of the appearance of a light source. Light sources with a CCT of less than 3200 K are yellowish, pinkish, or orange-white in tone and considered “warm” in appearance. Light sources with a CCT greater than 4000 K are bluish-white and considered “cool” in appearance.<\/p>\n\n\n\n

cosine law<\/h3>\n\n\n\n

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.<\/p>\n\n\n\n

cut off angle of a luminaire<\/h3>\n\n\n\n

The angle, measured up from nadir (straight down), between the vertical axis and the first line of sight at which the bare source (the bulb<\/a> or lamp<\/a>) is not visible.<\/p>\n\n\n\n

cutoff fixture<\/h3>\n\n\n\n

Old IES<\/a> definition: Luminaire light distribution where the candela values per 1,000 lumens do not exceed 25 (2.5%) at or above an angle of 90 degrees above nadir, and the candela values per 1,000 lumens do not exceed 100 (10%) at or above 80 degrees above nadir. Note: <\/em>The cutoff system for describing outdoor luminaires is deprecated. It was replaced in 2007 with the IES Luminaire Classification System and BUG ratings (refer to ANSI\/IES TM-15-20<\/a>, Technical Memorandum: Luminaire Classification System for Outdoor Luminaires<\/em>).<\/p>\n<\/div>\n\n\n\n

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D<\/strong><\/h1>\n\n\n\n

dark adaptation<\/h3>\n\n\n\n

The process by which the eye becomes adapted to a luminance less than about 0.03 candela<\/a>s per square meter (0.03 nits).<\/p>\n\n\n\n

dark sky<\/h3>\n\n\n\n

Denoting or located in a place where the darkness of the night sky is relatively free of interference from artificial light.<\/p>\n\n\n\n

DarkSky<\/h3>\n\n\n\n

A nonprofit organization building awareness of the value of dark skies, and of the need for quality outdoor lighting. The awesome organization whose website you are looking at, formerly known as the International Dark-Sky Association<\/a>. Learn more about us<\/a>.<\/p>\n\n\n\n

diffuser<\/h3>\n\n\n\n

A device used to distribute and spread light from a source.<\/p>\n\n\n\n

dimmer<\/h3>\n\n\n\n

A device used to reduce the light output of many kinds of light sources. <\/p>\n\n\n\n

disability glare<\/h3>\n\n\n\n

Glare<\/a> that results in reduced visual performance. It is often accompanied by discomfort.<\/p>\n\n\n\n

discomfort glare<\/h3>\n\n\n\n

Glare<\/a> that produces discomfort, but does not necessarily diminish visual performance.<\/p>\n<\/div>\n\n\n\n

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E<\/strong><\/h1>\n\n\n\n

efficacy<\/h3>\n\n\n\n

In lighting: the ratio of light output to its consumption of power, measured in lumens<\/a> per watt (lm\/W), or the ability of a lighting system to produce the desired result.<\/p>\n\n\n\n

efficiency<\/h3>\n\n\n\n

A measure of the effective or useful output of a system compared to the input of the system, expressed as a percentage.<\/p>\n\n\n\n

electromagnetic (EM) spectrum<\/h3>\n\n\n\n

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.<\/p>\n\n\n\n

energy (radiant energy)<\/h3>\n\n\n\n

Unit is erg, or joule, or kilowatt-hour (kWh).<\/p>\n<\/div>\n\n\n\n

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F<\/strong><\/h1>\n\n\n\n

fa\u00e7ade lighting<\/h3>\n\n\n\n

The illumination of the exterior of a building.<\/p>\n\n\n\n

Five Principles<\/h3>\n\n\n\n

Refers to the Five Principles of Responsible Outdoor Lighting<\/a> developed jointly by the IES<\/a> and DarkSky.<\/p>\n\n\n\n

fixture<\/h3>\n\n\n\n

As used on this site, a light fixture or luminaire. It includes the elements designed to give light output control, such as a reflector<\/a> (mirror) and\/or refractor<\/a> (lens), the ballast<\/a> or driver, housing, and the attachment parts.<\/p>\n\n\n\n

fixture lumens<\/h3>\n\n\n\n

A light fixture\u2019s emitted light output.<\/p>\n\n\n\n

fixture watts<\/h3>\n\n\n\n

The total power consumed by a fixture. This includes the power consumed by the light source(s) and the ballast<\/a>(s) or driver(s).<\/p>\n\n\n\n

floodlight<\/h3>\n\n\n\n

A fixture designed to \u201cflood\u201c a well defined area with light.<\/p>\n\n\n\n

flux (radiant flux)<\/h3>\n\n\n\n

Unit is ergs per second or watts.<\/p>\n\n\n\n

footcandle (fc)<\/h3>\n\n\n\n

A unit of illuminance<\/a>. One lumen<\/a> per square foot. <\/p>\n\n\n\n

footlambert<\/h3>\n\n\n\n

An older, deprecated unit for luminance, replaced by candelas per square meter, or nits.<\/p>\n\n\n\n

full-cutoff fixture<\/h3>\n\n\n\n

Old IES<\/a> definition: Luminaire light distribution where the candela values at 90 degrees or above are zero. Additionally, the candela values per 1,000 lumens do not exceed 100 (10%) at or above 80 degrees above nadir. Note:<\/em> The cutoff system for describing outdoor luminaires is deprecated. It was replaced in 2007 with the IES Luminaire Classification System and BUG ratings (refer to ANSI\/IES TM-15-20<\/a>, Technical Memorandum: Luminaire Classification System for Outdoor Luminaires<\/em>).<\/p>\n\n\n\n

fully shielded<\/h3>\n\n\n\n

A luminaire<\/a> fabricated or shielded in such a manner that it has achieved a U0 rating. To achieve this rating, the light source shall not be visible above a horizontal plane passing through the lowest light-emitting part of the luminaire, and the maximum amount of stray or indirect reflected light that is measured between 90 and 180 degrees from nadir (i.e., LCS zone UL and UH combined) shall not exceed 0.5% of the total luminaire lumens up to a maximum of 50 lumens.<\/p>\n<\/div>\n\n\n\n

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G<\/strong><\/h1>\n\n\n\n

glare<\/h3>\n\n\n\n

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.<\/p>\n<\/div>\n\n\n\n

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H<\/strong><\/h1>\n\n\n\n

HID lamp<\/h3>\n\n\n\n

A high-intensity discharge (HID) lamp is a type of discharge lamp<\/a> where the emitted light is produced by the passage of an electric current through a gas concentrated in an arc tube. HID lamp types include mercury, metal halide, and high-pressure sodium. <\/p>\n\n\n\n

high-pressure sodium (HPS) lamp<\/h3>\n\n\n\n

An HID<\/a> lamp where radiation is produced from sodium vapor at relatively high partial pressures (100 torr). <\/p>\n\n\n\n

house-side shield<\/h3>\n\n\n\n

Opaque material applied to the rear side of a fixture to minimize the light emitted behind the fixture.<\/p>\n<\/div>\n\n\n\n

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I<\/strong><\/h1>\n\n\n\n

IES<\/h3>\n\n\n\n

The Illuminating Engineering Society<\/a>.<\/p>\n\n\n\n

illuminance<\/h3>\n\n\n\n

Density of luminous<\/a> flux<\/a> incident on a surface. Unit is lux<\/a> or footcandle<\/a>.<\/p>\n\n\n\n

Illuminating Engineering Society (IES)<\/h3>\n\n\n\n

In North America, the primary professional society of lighting engineers, designers, and other lighting professionals; headquartered in New York City, U.S.. Visit their website<\/a>.<\/p>\n\n\n\n

incandescent lamp<\/h3>\n\n\n\n

A lamp that produces light<\/a> from a filament heated to a high temperature by electric current.<\/p>\n\n\n\n

infrared radiation<\/h3>\n\n\n\n

Electromagnetic radiation with longer wavelengths than those of visible light, extending from the nominal red edge of the visible spectrum at 760 nanometers to 1 mm.<\/p>\n\n\n\n

intensity<\/h3>\n\n\n\n

As used on this site: luminous intensity, i.e., the concentration of the light emitted from a point source or from a source that is far enough away to act as a point source. Unit: candela.<\/p>\n\n\n\n

International Dark-Sky Association (IDA)<\/h2>\n\n\n\n

The former name of DarkSky<\/a>.<\/p>\n\n\n\n

inverse-square law<\/h3>\n\n\n\n

The equation used to determine illuminance on a surface from a light source (assumed to be a point source) a given distance away. The illuminance<\/a>, E<\/em>, at a point varies directly with the intensity, I<\/em>, of the point source and inversely as the square of the distance, d<\/em>, to the source. E <\/em>= I<\/em> \/ d<\/em>^2. This assumes that the light is arriving normal to the surface. If the light is arriving at an incident angle other than 90 degrees, the resulting illuminance varies with the cosine of the angle.<\/p>\n<\/div>\n\n\n\n

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J<\/strong><\/h1>\n<\/div>\n\n\n\n
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K<\/strong><\/h1>\n\n\n\n

kelvin (K) <\/h2>\n\n\n\n

The unit of thermodynamic temperature. 1 kelvin is equal in size to 1 degree on the Celsius scale. 0 K is absolute zero, or -273 degrees C. Learn more at Britannica<\/a>.<\/p>\n\n\n\n

kilowatt (kW)<\/h2>\n\n\n\n

A measure equal to 1,000 watts of electrical power.<\/p>\n\n\n\n

kilowatt-hour (kWh)<\/h3>\n\n\n\n

A measure of electrical energy equivalent to a power consumption of 1,000 watts for 1 hour.<\/p>\n<\/div>\n\n\n\n

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L<\/strong><\/h1>\n\n\n\n

lamp<\/h3>\n\n\n\n

The source of light<\/a> in an electric light fixture or luminaire. To be distinguished from the whole assembly (see luminaire<\/a>). Lamp<\/em><\/a> often is used to denote the bulb<\/a> and its housing.<\/p>\n\n\n\n

lamp life<\/h3>\n\n\n\n

The median life span for a specific type of lamp<\/a>. Half of the lamps in a large sample will perform longer than the rated life; the others will fail sooner.<\/p>\n\n\n\n

LED<\/h3>\n\n\n\n

Light-emitting diode.<\/p>\n\n\n\n

light pollution<\/h3>\n\n\n\n

As used in this site: The human-made alteration of outdoor light levels from those occurring naturally. Learn more<\/a>.<\/p>\n\n\n\n

light quality<\/h3>\n\n\n\n

A measure of likely visual comfort and visual performance based on the lighting; considers the distribution, glare, color rendering, and other characteristics of the light.<\/p>\n\n\n\n

light spill<\/h3>\n\n\n\n

Unwanted spillage of light onto adjacent areas, particularly residential properties and ecological sites.<\/p>\n\n\n\n

light trespass<\/h3>\n\n\n\n

Light falling where it is not wanted or needed. Sometimes called spill light or obtrusive light.<\/p>\n\n\n\n

lighting controls<\/h3>\n\n\n\n

Devices used for controlling when and how much light is emitted from a light fixture or lighting system; examples include dimmers, switches, and timers.<\/p>\n\n\n\n

lighting zones (LZ)<\/h3>\n\n\n\n

The five base (or ambient) light levels desired by a community. See Lighting zones<\/a> for more info.<\/p>\n\n\n\n

Low-pressure sodium (LPS) lamp<\/h3>\n\n\n\n

A type of discharge lamp<\/a> where the light is produced by radiation from sodium vapor at a relatively low partial pressure (about 0.001 torr). LPS is a \u201ctube source.\u201d It is monochromatic yellow light.<\/p>\n\n\n\n

lumen<\/h3>\n\n\n\n

A measurement unit for the luminous<\/a> flux<\/a> generated by a light<\/a> source, defined as the flux emitted within a unit solid angle by a point source with a uniform luminous<\/a> intensity of one candela<\/a>. Essentially, a measure of the total flow of light from a lamp, luminaire, or other light source, without regard to its directionality.<\/p>\n\n\n\n

lumen depreciation factor<\/h3>\n\n\n\n

Light loss of a luminaire<\/a> with time due to the lamp<\/a> decreasing in efficiency as it ages.<\/p>\n\n\n\n

luminaire<\/h3>\n\n\n\n

A complete lighting unit that usually includes the light source, the electrical driver for the source, the optical reflectors and\/or refractors, and the housing to protect the light source. Also called a \u201clight fixture<\/a>.\u201d<\/p>\n\n\n\n

luminaire efficiency<\/h3>\n\n\n\n

The ratio of the light emitted by the luminaire<\/a> compared to the light emitted by the enclosed lamps<\/a>.<\/p>\n\n\n\n

luminance<\/h3>\n\n\n\n

At a point and in a given direction, the luminous<\/a> intensity 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. Essentially, a measure of the brightness of an area, whereas luminous intensity (or just intensity<\/em>) is a measure of the brightness of a point source. Units: candelas<\/a> per square meter, or nits.<\/p>\n\n\n\n

lux<\/h3>\n\n\n\n

A unit of illuminance<\/a>: one lumen<\/a> per square meter.<\/p>\n<\/div>\n\n\n\n

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M<\/strong><\/h1>\n\n\n\n

mercury vapor lamp<\/h3>\n\n\n\n

A type of HID lamp<\/a> where the light is produced by radiation from mercury vapor. Mercury vapor lamps are no longer manufactured or sold in many parts of the world.<\/p>\n\n\n\n

metal halide lamp<\/h3>\n\n\n\n

A type of HID lamp<\/a> where the light is produced by radiation from metal-halide vapors.<\/p>\n\n\n\n

mounting height<\/h3>\n\n\n\n

The height of the bottom of a fixture above the ground.<\/p>\n<\/div>\n\n\n\n

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N<\/strong><\/h1>\n\n\n\n

nadir<\/span><\/h3>\n\n\n\n

In lighting, nadir<\/em> indicates the direction straight down from a luminaire (vertical angle 0 degrees), whereas zenith<\/em> indicates the direction straight up from a luminaire (vertical angle 180 degrees).<\/p>\n\n\n\n

nanometer (nm)<\/h3>\n\n\n\n

10-9<\/sup> meter. Often used as the unit for wavelength in the electromagnetic (EM) spectrum.<\/p>\n<\/div>\n\n\n\n

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O<\/strong><\/h1>\n\n\n\n

occupancy sensors<\/h3>\n\n\n\n

A type of lighting control system that turns lights on and\/or off based on the presence of people or other animals. There are two types:<\/p>\n\n\n\n