Lighting Glossary

Accent Lighting

Directional lighting to emphasize a particular object or to draw attention to a part of the field of view.


The dissipation of light within a surface or medium.

Alternating Current (AC)

Flow of electricity which cycles or alternates direction many times per second. The number of cycles per second is referred to as frequency. Most common frequency used in this country is 60 Hertz (cycles per second).

Ambient Lighting

Background or fill light in a space.

Amperes (amps)

The unit of measurement of electric current.

Back Lighting

The illumination provided for scenery in off-stage areas visible to the audience.


An opaque or translucent element that serves to shield a light source from direct view at certain angles, or serves to absorb unwanted light.


An auxiliary device consisting of induction windings wound around a metal core and sometimes includes a capacitor for power correction. It is used with fluorescent and HID lamps to provide the necessary starting voltage and to limit the current during operation.


The unit of measurement of luminous intensity of a light source in a given direction.


Luminous intensity expressed in candelas.

Class "P" Ballast

Contains a thermal protective device which deactivates the ballast when the case reaches a certain critical temperature. The device resets automatically when the case temperature drops to a lower temperature.

Cold Cathode Lamp

An electric-discharge lamp whose mode of operation is that of a flow discharge.


The measurement of color.

Color Rendering Index (CRI)

Measure of the degree of color shift objects undergo when illuminated by the light source as compared with the color of those same objects when illuminated by a reference source of comparable color temperature.

Color Temperature

The absolute temperature of a blackbody radiator having a chromaticity equal to that of the light source.

Cool Beam Lamps

Incandescent PAR lamps that use a special coating (dichroic interference filter) on the reflectorized potion of the bulb to allow heat to pass out the back while reflecting only visible energy to the task, thereby providing a "cool beam" of light.

Dimming Ballast

Special fluorescent lamp ballast, which when used with a dimmer control, permits varying light output.

Direct Current (DC)

Flow of electricity continuously in one direction from positive to negative.

Direct Lighting

Lighting involving luminaires that distribute 90 to 100% of emmited light in the general direction of the surface to be illuminated. The term usually refers to light emitted in a downward direction.

Discharge Lamp

A lamp in which light (or radiant energy near the visible spectrum) is produced by the passage of an electric current through a vapor or a gas.

Emergency Lighting

Lighting system designed to provide minimum illumination required for safety, during power failures.

Discharge Lamp

A lamp in which light (or radiant energy near the visible spectrum) is produced by the passage of an electric current through a vapor or a gas.

"ER" (Elliptical Reflector)

Lamp whose reflector focuses the light about 2" ahead of the bulb, reducing light loss when used in deep baffle downlights.

Extended Life Lamps

Incandescent lamps that have an average rated life of 2500 or more hours and reduced light output compared to standard general service lamps of the same wattage.


A system designed for lighting a scene or object to a luminance greater than its surroundings. It may be for utility, advertising or decorative purposes.

Fluorescent Lamp

A low-pressure mercury electric-discharge lamp in which a phosphor coating transforms some of the ultraviolet energy generated by the discharge into light.

Footcandle (fc)

The unit of illuminance when the foot is taken as the unit of length. It is the illuminance on a surface one square foot in area on which there is a uniformly distributed flux of one lumen.


A set of striplights at the front edge of the stage plateform used to soften face shadows cast by overhead luminaires and to add general toning lighting from below.

High Intensity Discharge (HID) Lamp

A discharge lamp in which the light producing arc is stabilized by wall temperature, and the arc tube has a bulb wall loading in excess of three watts per square centimeter. HID lamps include groups of lamps known as mercury, metal halide, and high pressure sodium.

High Output Fluorescent Lamp

Operates at 800 or more milliamperes for higher light output than standard fluorescent lamp (430MA).

High Pressure Sodium (HPS) Lamp

High intensity discharge (HID) lamp in which light is produced by radiation from sodium vapor. Includes clear and diffuse-coated lamps.


The self-emission of radiant energy in the visible spectrum due to the thermal excitation of atoms or molecules.

Incandescent Lamp

A lamp in which light is produced by a filament heated to incandescence by an electric current.

Instant Start Fluorescent Lamp

A fluorescent lamp designed for starting by a high voltage without preheating of the electrodes.


Unit of measurement for color temperature. The Kelvin scale starts from absolute zero, which is -273° Celsius.

Kilowatt-Hour (KWH)

Unit of electrical power consumed over a period of time. KWH = watts/1000 x hours used.


An artificial source of light (also portable luminaire equipped with a cord and plug).

Lamp Efficacy

The ratio of lumens produced by a lamp to the watts consumed. Expressed as lumens per watt (LPW).

Low Pressure Sodium Lamp

A discharge lamp in which light is produced by radiation of sodium vapor at low pressure producing a single wavelength of visible energy, i.e. yellow.

Low Voltage Lamps

Incandescent lamps that operate at 6 to 12 volts.


The unit of luminous flux. It is the luminous flux emitted within a unit solid angle (one steradian) by a point source having a uniform luminous intensity of one candela.


A complete lighting unit consisting of a lamp or lamps together with the parts designed to distribute the light, to position and protect the lamps and to connect the lamps to the power supply.

Luminaire Efficiency

The ratio of luminous flux (lumens) emitted by a luminaire to that emitted by the lamp or lamps used.


The metric unit of illuminance. One lux is one lumen per square meter (lm/m2).

Mercury Lamp

A high intensity discharge (HID) lamp in which the major portion of the light is produced by radiation from mercury. Includes, clear, phosphor-coated and self-ballasted lamps.

Metal Halide Lamp

A high intensity discharge (HID) lamp in which the major portion of the light is produced by radiation of metal halides and their products of dissociation-possible in combination with metallic vapors such as mercury. Includes clear and phosphor coated lamps.

"PAR" Lamps

Parabolic aluminized reflector lamps which offer excellent beam control, come in a variety of beam patterns from very narrow spot to wide flood and can be used outdoors unprotected because they are made of "hard" glass that can withstand adverse weather.

Preheat Fluorescent Lamp

A fluorescent lamp designed for operation in a circuit requiring a manual or automatic starting switch to preheat the electrode in order to start the arc.

"R" Lamps

Reflectorized lamps available in spot (clear face) and flood (frosted face).

Rapid Start Fluorescent Lamp

A fluorescent lamp designed for operation with a ballast that provides a low-voltage winding for preheating the electrodes and initiating the arc without a starting switch or the application of high voltage.


The process by which the direction of a ray of light changes as it passes obliquely from one medium to another in which its speed is different.

Rough Service Lamps

Incandescent lamps designed with extra filament supports to withstand bumps, shocks and vibrations with some loss in lumen output.

Self-ballasted Mercury Lamps

Any mercury lamp of which the current-limiting device is an integral part.

Silver Bowl Lamps

Incandescent "A" lamps with a silver finish inside the bowl or portion of the bulb. Used for indirect lighting.

Surface Mounted Luminaire

A luminaire that is mounted directly on a ceiling.

Suspended (Pendant) Luminaire

A luminaire that is hung from a ceiling by supports.

Three-Way Lamps

Incandescent lamps that have two separately switched filaments permitting a choice of three levels or light such as 30/70/100, 50/100/150 or 100/200/300 watts. They can only be used in a base down position.


A device to raise or lower electric voltage.

Tungsten-Halogen Lamp

A gas filled tungsten incandescent lamp containing a certain proportion of halogens.