We include in this glossary of 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.
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Accent lighting: Lighting used to emphasize or draw attention to a special object or building.
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Ambient light: The general overall level of lighting in an area.
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Angstrom: A unit of wavelength often used in astronomy, equal to 10-10 meter or 0.1 nanometer.
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Baffle: An opaque or translucent element to shield a light source from direct view.
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Ballast: A device used with a discharge lamp to obtain the necessary voltage, current, and/or wave form for starting and operating the lamp.
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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.
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Brightness: Strength of the sensation that results from viewing surfaces from which the light comes to the eye.
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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.
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Candela (cd): Unit of luminous intensity. One candela is one
lumen per steradian. Formerly called the candle.
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Candlepower distribution curve: A plot of the variation in luminous intensity of a lamp or luminaire.
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Candlepower: Luminous intensity expressed in candelas.
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CIE: Commission Internationale de l'Eclairage. The
international light commission. Sets most lighting standards.
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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.
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Color rendering: Effect of a light source on the color appearance
of objects in comparison with their color appearance under normal daylighting.
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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.
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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).
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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.
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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.
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Cutoff fixture: An IES definition "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°".
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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).
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Disability glare: Glare resulting in reduced visual performance and
visibility. It is often accompanied by discomfort.
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Discomfort glare: Glare that produces discomfort, but does not
necessarily diminish visual performance.
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Efficacy: The ability of a lighting system to produce the
desired result.
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Efficiency: A measure of the effective or useful output of a
system compared to the input of the system.
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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.
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Energy (radiant energy): Unit is erg, or joule, or kWh.
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Façade lighting: The illumination of the exterior of a building
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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.
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Floodlight: A fixture designed to "flood" a well defined area
with light.
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Flux (radiant flux): Unit is erg/sec or watts.
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Footcandle: Illuminance produced on a surface one foot from a
uniform point source of one candela.
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Footlambert: The average luminance of a surface emitting or
reflecting light at a rate of one lumen per s: The average luminance of a surface emitting or
reflecting light at a rate of one lumen per square foot.
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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°".
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Fully Shielded fixture: A fixture that allows no emission above a horizontal
plane through the fixture.
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Glare: Intense and blinding light that reduces
visibility. Alight within the field of vision that is brighter than the brightness to which the eyes are
adapted.
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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.
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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".
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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.
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Illuminance: Density of luminous flux incident on a surface.
Unit is footcandle or lux.
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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.
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Incandescent lamp: Light is produced by a filament heated to a high
temperature by electric current.
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Infrared radiation: EM radiation just to the long wavelength side of
the visual.
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Intensity: The degree or amount of energy or light.
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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.
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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
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kWh: Kilowatt-hour: A unit of energy equal to the work done by one
kilowatt (1000 watts) of power acting for one hour.
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Light pollution: Any adverse effect of artificial light.
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Light trespass: Light falling where it is not wanted or needed.
Spill light. Obtrusive light.
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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.
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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.
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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.
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Luminaire: The complete lighting unit, including the lamp,
the fixture, and other parts.
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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.
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Lux: One lumen per square meter. Unit of illuminance.
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Mercury lamp: An HID lamp where the light is produced by
radiation from mercury vapor.
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Metal-halide lamp: An HID lamp where the light is produced by
radiation from metal-halide vapors.
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Mounting height: The height of the fixture or lamp above the
ground.
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Nadir: A point on the
celestial sphere directly below the observer, diametrically opposite the zenith.
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Nanometer (nm): 10-9 meter. Often used as the unit for wavelength
in the EM spectrum.
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Photometry: The quantitative measurement of light level and
distribution.
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Quality of light: A subjective ratio of the pluses to the minuses of
any lighting installation.
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Reflector: Controlling light output by means of reflection
(mirror).
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Refractor: Controlling light output by means of refraction
(lens).
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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°".
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Shielding: An opaque material that blocks the transmission of
light.
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Spotlight: A fixture designed to light only a small,
well-defined area.
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Stray light: Emitted light that falls away from the area where
it is needed or wanted. Light trespass.
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Task lighting: Lighting designed for a specific purpose or task.
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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."
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Urban sky glow: The brightening of the night sky due to manmade
lighting.
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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.
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Visibility: Being perceived by the eye. Seeing effectively.
The goal of night lighting.
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Zenith: An imaginary point directly above a particular location, on the imaginary celestial sphere.