Glossary of Lighting Terms

[ A | B | C | D | E | F | G | H | I | J | K | L | M ]
[ N | O | P | Q | R | S | T | U | V | W | X | Y | Z ]

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- A -

Accent Lighting

Directional lighting used to highlight or draw attention to a particular object.

Acrylic

Material used in making fixture lenses; does not yellow with age.

Alternating Current (AC)

Electron flow that periodically reverses polarity and direction of travel through a circuit. The common example is a 60-cycle AC power, used in most homes and offices, where voltage polarity and current go through 60 cycles of reversal per second.

Ambient Lighting

The general, overall lighting in a row or area, as opposed to "accent lighting."

Ampere (amp)

Standard unit for measuring the strength of an electric current defined as rate of flow of charge.

Argon

Inert gas used in incandescent and fluorescent lamps. In incandescents, it helps retard evaporation of tungsten filament.

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- B -

Ballast

Electrical device used with fluorescent and high intensity discharge lamps to provide the necessary starting and operating conditions.

Base

End (or ends) of a lamp that makes electrical and mounting connections with the luminaire.

Beam Spread

The angle of a beam from a light source to the main area lighted; more specifically, the angle between two vectors in which the light output is equal to a stated percent (usually 10%) of the maximum candlepower of the source.

Brightness

The light emitted, transmitted or reflected from a unit area of the source of a reflecting surface; expressed either in candles per square inch or in "Lamberts."

Bulb

Common term for an electrical light source or "lamp"; more specifically, the glass outer shell of the lamp.

Bulb Darkening

Discoloration of an incandescent lamp caused by tungsten particles collecting on the glass bulb as the filament evaporates.

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- C -

Candela meximum

A unit of luminous intensity.

Candlepower

The luminous intensity of a light source expressed in candles.

Color Corrected

Refers to a lamp with a special phosphor or coating (applied internally or externally) to give it a color rendering profile more like that of natural daylight.

Color Rendering Index

A measure of the color shift created by artificial light. Color rendering is measured on a scale from zero to 100, with both natural daylight and incandescent light equal to 100. The higher the index, the more true to life colors appear.

Color Spectrum

Same as the "visible light" spectrum: electromagnetic energy between 380(violet) and 700(red) nanometers, which can be seen by the human eye.

Color Temperature

Expressed in degrees Kelvin (see "Kelvin Scale"), color temperatures indicate the color of a light source. For example, incandescent lamps are approximately 3000 degrees Kelvin (K) and appear yellowish. Fluorescents range from 3000 degrees Kelvin up to 7500 degrees, with those at the high end appearing blue-white, or "cool." Rather than being the physical temperature of the light itself, the Kelvin number refers to the color a theoretical "black body" would be heated to that temperature.

Cove Lighting

Refers to light sources mounted above a ledge or in a recess (usually around the perimeter of a room) that distribute light upward for ambient lighting.

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- D -

Diffusion

The scatter, spreading or redirection of light in an even manner, by a lens, to achieve some intended effect such as reduced glare.

Downlight

A direct lighting unit, usually relatively small, that aims light downward. Can be recessed, surface mounted or suspended.

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- E -

Efficacy

May refer to either the efficiency of the lamp (lumens per watt) or that of the luminaire (lumens emitted by the fixture compared to those emitted by the unshielded lamp).

Efficiency

Used synonymously with "Efficacy."

Electric Discharge Lamp

A light source that produces light by passing a current between electrodes through a vapor or gas; includes fluorescent, high intensity discharge, and other lamps.

End Blackening

Darkening around the ends of a fluorescent tube caused by the emitter paste on the electrodes evaporating and collecting on the glass.

Energy Survey

Synonymous with "cost calculations," "energy audit," etc; a systematic cost comparison of an existing system to a proposed system; accounts for electrical, material and possibly maintenance/manpower cost.

Etch

Logo, brand name, and product information that appears on the glass envelope of the lamp.

Eyeball

Fixture usually recessed, loosely resembling an "eyeball"; can be rotated to point in desired direction.

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- F -

Filament

Wire element, often coiled, in a lamp that becomes incandescent and emits light when heated by an electric current.

Fixture

A luminaire with the lamps.

Fluorescent Lamp

A glass tube coated on the inside with a fluorescent substance that gives off light when mercury vapor in the tube is acted upon by a stream of electrons from the cathode.

Footcandle

A unit of direct illumination; the amount of light produced by a one-candlepower source on a square foot of surface every point of which is one foot from the candle. A lumen per square foot.

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- G -

General Lighting

Essentially uniform light throughout an area, with no special provisions for individual tasks or objects.

Globe

A transparent or translucent spherical lamp enclosure.

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- H -

Head

Luminaire for a track-lighting system.

High Intensity Discharge (HID) Lamp.

Generally speaking, mercury vapor, high pressure sodium, and metal halide lamps.

High Hat

A luminaire that is recessed, usually into a ceiling.

High Output Fluorescent

A fluorescent lamp designed for use with an 800 milliampere (ma) ballast; will usually operate at low temperatures (down to zero) while still producing high light levels.

High Pressure Sodium (HPS) Lamp

An HID light source that uses sodium vapor at relatively high pressure to produce light.

High Voltage

Voltages of 208 and higher.

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- I -

Illumination

The lighting in an area, or the result of the use of light. Also, the intensity of light per unit of area, sometimes called "illuminance"; usually measured in footcandles.

Incandescent Lamp

A light source in which illumination is produced by a filament of conducting material contained in a vacuum and heated to incandescence by an electric current.

Indirect Lighting

Luminaires that distribute 90% to 100% of their light upward to be reflected by the ceiling.

Infrared

Radiant energy wavelength between 700 and one million nanometers, i.e., longer than the wavelength of visible light. Applications include photography, heat lamps, industrial drying and medical therapy.

Instant Start (Fluorescent Lamp)

A fluorescent light source that will start from a high voltage with no preheating of the electrodes. Instant start lamps cannot be interchanged with rapid start lamps.

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- K -

Kelvin Scale

A scale of temperature measured in degrees Celsius from absolute zero (See "Color Temperature").

Kilowatt (kw)

A unit of electrical power, equal to 1000 watts.

Kilowatt Hour (kWh)

One thousand watts of electricity consumed in one hour; the measure of consumption generally used in billing for electricity. For example: a 100-watt lamp operated for 1000 hours consumes 100kWh (100 watts x 1000 hours = 100kWh). At 10¢/kWh, this bulb costs $10.00 to operate (100kWh x 10¢).

Krypton

A heavy inert gas used in incandescent lamps; allows the filament to glow hotter and brighter, yet last longer.

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- L -

Lamp

Industry term for a light bulb.

Lamp Life

Rated life of a lamp, as established through laboratory testing during which a sample group of lamps is burned, including being subjected to a scheduled number of starts per day. The length of time required for half the lamps to fail is the rated lamp life.

Lens

Transparent or translucent cover for the face of the lighting fixture; diffuses or focuses light passing through it and protects internal components. Also called "refractor" or "diffuser."

Lexan

Brand name for polycarbonate material used in lenses that will not break.

Light

The term generally applied to visible energy emitted by a source; usually measured in lumens or candlepower. When light strikes a surface it is absorbed, transmitted or reflected.

Light Loss Factor (LLD)

Used to calculate or project lighting system performance after a given period of time under certain conditions; takes into account variations in temperature and voltage, dirt, lamp depreciation and maintenance. Also called "maintenance factor."

Louver

In lighting fixtures, an assembly of slats or crosspieces used to control light intensity and glare.

Low Pressure Sodium Lamp

A discharge lamp using pressurized sodium vapor to produce light; distinguished by the most efficient light source and the only single color (yellow) light source.

Lumen

Unit of luminous flux. Radiometrically, it is determined from the radiant power. Photometrically, it is the luminous flux emitted within a unit of solid angle by a point source having a uniform intensity of one candela.

Lumen Depreciation

The term for the gradual decline in a source's light output. Except for the low pressure sodium lamps, all sources lose lumen output over their life due to gradual deterioration of the filaments or electrodes, accompanied by lamp blackening.

Lumen Maintenance

Same as "Light Loss Factor."

Lumens Per Watt (LPW)

A measure of the efficiency of a light source, equal to the lumens produced by the wattage. For example, a 100-watt lamp producing 1750 lumens yields 17.5 lumens per watt.

Luminance (or Brightness)

The light emitted, transmitted or reflected from a unit area of the source or a reflecting surface; expressed either in candles per square inch or in "Lamberts."

Luminaire

A complete lighting unit consisting of a lamp, housing, lens, and related components; also referred to as a "fixture."

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- M -

Metal Halide Lamp

A discharge source producing light by radiation of various metal halides in combination with metallic vapors, including mercury and others.

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- O -

Ohm's Law

A scientific law which states that current (amperes) in a circuit depends on resistance (ohms) and applied electromotive force (volts). Current (I) = Voltage(E)/Resistance(R). O: I=E/R.

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- P -

Photopic Vision

Refers to vision involving the cones of the eye; used in reference to the adaptation of the eye to illuminance of more than 3.4 candelas per square meter. See "Scotopic Vision."

PL

Designation for a type of compact fluorescent lamp.

Polycarbonate

Material used in lenses that will not break (see "Lexan").

Power Factor

The ratio of actual power used (watts) to the power apparently being drawn from the line (volt-amperes). Utilities bill for actual power used.

Preheat

Refers to fluorescent lamps and systems that use a "starter" to warm the lamp electrodes, causing a delay of several seconds before the lamp produces light.

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- Q -

Quartz

Obsolete term for a tungsten halogen lamp.

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- R -

Rapid Start

A fluorescent system that does not use a "starter." (see "Preheat"). Except for "slimline" lamps, all modern fixtures using 40-watt and higher lamps are equipped with rapid-start ballasts.

Rated Average Life

Same as "lamp life."

Reflector

A piece of glass or metal, usually concave, with a reflective surface that directs radiant energy in a desired direction.

Reflector Lamp

A light source with an integral reflector, usually a silver or aluminum coating on the bulb.

Refraction

The bending of a ray or wave of light.

Refractor

A translucent or transparent fixture covering, like a "lens" or "diffuser," that refracts.

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- S -

Scotopic Vision

Sight involving the eye's rods, which respond to low levels of lighting, below .024 candela per square meter.

Socket

Electro-mechanical connection in the fixture for the lamp.

SON

Another term for a high pressure sodium (HPS) lamp.

SOX

Another term for a low pressure sodium (LPS) lamp.

Starter

A device used in conjunction with a ballast to start electric-discharge lamps, usually older fluorescents.

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- T -

Trigger Start

A circuit used to eliminate the starter and allow for instant starting of preheat lamps.

Troffer

A long recessed lighting fixture, usually flush with the ceiling

Tungsten Halogen

A gas-filled, usually compact, tungsten-filament incandescent lamp that contains halogens; a tungsten-halogen lamp retains most of its initial efficiency over its life, with little lumen loss.

Two-by-Four

A commonly used fluorescent troffer measuring two feet by four feet, with two ballasts, and sockets for four lamps

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- U -

Ultraviolet

Radiant energy between 10 and 380 nanometers in wavelength, beyond the violet end of the visible spectrum. Typically used in ozone-producing light sources(180-220 nanometers), germicidal lamps (220-300 nanometers), and "black lights" (320-400 nanometers).

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- V -

Very High Output (VHO) Lamps

A fluorescent that operates from a 1500 milliamp ballast; also referred to as Ultra High Output (UHO) or Super High Output (SHO).

Visible Spectrum

Electromagnetic energy between 380 (violet) and 700 (red) nanometers in wavelength, which can be seen by the human eye. Same as the "color spectrum."

Voltage Rating

The recommended operating voltage for a lamp. For example, a 120-volt lamp operated at 125 volts will lose 40% of its lamp life; conversely, operating that lamp at 115 volts will virtually double its life.

Volt

The unit of electromotive force or potential difference in an electric current.

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- W -

Watt

The unit of power consumption or use; one watt (W) is equal to one ampere of current flowing through one ohm of resistance.

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