Lighting Research Center

Advancing the effective use of light for society and the environment

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One of the barriers to good lighting is that people who need good lighting do not know what to ask for! This glossary is provided to make asking easier.

A-lamp: Common incandescent "light bulb" used throughout most homes. An A-lamp can have a clear glass bulb or a white coating or an etched frost on the inside of the glass bulb.
Accent lighting: A technique that emphasizes a particular object or draws attention to a particular area. Accent lighting usually utilizes the tight beam control of PAR-lamps and MR-lamps. Also called highlighting.
Accent luminaire: A type of luminaire that includes ceiling-mounted track and directional luminaires and recessed accent luminaires. Accent luminaires provide directional lighting to accent an object or an area within a space.
Adjustable head: An adjustable luminaire that is surface-mounted, or that inserts into a linear track and provides directional lighting.
Ambient lighting: Lighting that is designed to provide a substantially uniform light level throughout an area, exclusive of any provision for special local requirements.
Baffle: A single opaque or translucent element that shields the direct view at certain angles of a light source, absorbs or block unwanted light, or reflects or redirects light.
Ballast: An electronic device that is used with a fluorescent or high-intensity discharge (e.g., metal halide) lamp to provide the necessary circuit conditions (voltage, current, and wave form) for starting and operating the lamp. See also electronic ballast and magnetic ballast.
Beam spread: The width of a light beam, expressed in degrees. The beam of light from a reflector-type lamp (PAR, R, ER, or MR) can be thought of as a cone. The beam spread is the angular width of the cone. Common beam spreads are known as spot, narrow, narrow flood and flood.
Brightness: Subjective impression of light intensity reaching the eye.
Bulb: The outer envelope of a light source, usually quartz glass or other varieties of glass.
Ceiling-mounted luminaire: See surface-mounted luminaire.
Color rendering index (CRI): A technique for describing the effect of a light source on the color appearance of objects being illuminated, with a CRI of 100 representing the reference condition (and thus the maximum CRI possible). In general, a lower CRI indicates that some colors may appear unnatural when illuminated by the lamp. CRIs of two or more lamps should only he compared if the lamps have the same correlated color temperature. See also correlated color temperature.
Compact fluorescent lamp: A small fluorescent lamp, usually with one or more bends in the tube.
Correlated color temperature (CCT): Describes the color appearance of the light that is produced, in terms of its warmth or coolness. A low color temperature (3000 K and lower) describes a warm source, such as a typical incandescent lamp and a warm fluorescent lamp. A high color temperature (4000 K and higher) describes a cool source, such as a cool white fluorescent lamp.
Cove luminaire: An architectural luminaire that directs light from sources that are mounted in a cove to the ceiling or upper wall. A cove is a ledge or shelf on the wall, or a recess in the wall.
Daylight: Light produced by solar radiation. Daylight includes direct sunlight, sunlight scattered by the atmosphere, and skylight reflected from clouds or other surfaces.
Diffuse lighting: Lighting provided on the work plane or on an object that does not come from any particular direction. Diffuse lighting produces less-distinct shadows than directional lighting.
Diffuser: A device to redirect or scatter the light from a source, primarily by the process of diffuse transmission.
Dimmer: A device used to control the intensity of light emitted by a luminaire by controlling the voltage or current available to it.
Direct glare: Glare resulting from very bright sources of light in the field of view. It usually is associated with bright light from luminaires and windows. A direct glare source may also affect performance by reducing the apparent contrast of objects in the field of view, especially those near the source of light.
Downlight: A directional luminaire that directs light downward.
Electronic ballast: A ballast that uses electronic circuitry to provide the voltage and current that are needed to start the lamp(s) and to maintain its operation. Electronic ballasts weigh less than magnetic ballasts and operate more quietly. Electronic ballasts operate lamps at a higher frequency than magnetic ballasts (20,000 to 60,000 hertz compared to 60 hertz), which eliminates flicker and increases efficacy. See also ballast.
ER-lamp: An ellipsoidal reflector lamp
Fixture: See luminaire.
Flood lamp: A lamp that produces a relatively wide beam of light.
Footcandle: See lux (lx)
Fluorescence: The ability of some materials, such as phosphors, to convert ultraviolet energy into visible light.
Fluorescent lamp: A lamp containing mercury under low pressure, relative to high-intensity discharge lamps. The mercury is ionized by an electric arc, producing ultraviolet energy, which in turn, excites phosphors coating the inside of the lamp to fluoresce or glow.
General lighting: See ambient lighting.
Glare: The loss of visibility and/or the sensation of discomfort associated with bright light within the field of view. See also direct glare and reflected glare.
Halogen incandescent lamp: An incandescent lamp whose filament is encapsulated; the capsule contains a halogen gas that reacts with tungsten evaporated from the filament to redeposit it on the filament. Halogen incandescent lamps have higher efficacies (i.e., more light per watt) than common incandescent lamps. They are sometimes referred to as quartz lamps because the capsule is made from quartz glass.
Highlighting: See accent lighting.
Incandescent lamp: A lamp producing visible radiant energy by electrical resistance heating of a filament. See A-lamp.
Indirect lighting: Light arriving at a point or surface after reflection from one or more surfaces (usually walls and/or ceilings) that are not part of the luminaire.
Infrared-reflecting lamp (IR-lamp): A halogen lamp with an infrared reflecting coating on the capsule that surrounds the filament. The coating redirects infrared energy onto the filament, which increases the temperature of the filament without additional input power, thereby increasing efficacy.
IR-lamp: See infrared-reflecting lamp.
IR PAR-lamp: An infrared-reflecting PAR-lamp. See infrared-reflecting lamp and PAR-lamp.
Kelvin (K): The standard unit of temperature that is used in the Systéme Intemationale d'Unités (SI) system of measurements. The Kelvin temperature scale is used to describe the correlated color temperature of a light source. See correlated color temperature.
Lamp: A manufactured light source. For electric lamps, it includes the bulb, the base, and the internal structure that produces light, either a filament or an arc tube. Lamps are often referred to as light bulbs. The term lamp also is commonly used to refer to plug-in luminaires (see desk, floor, and table lamps).
Lens: A glass or plastic element used in luminaires to refract, that is, to control, the distribution of light. Lenses can be fiat and fitted into the aperture, or cup-shaped or spherical to fit over a lamp.
Light: Radiant energy that is capable of producing a visual sensation. The visible portion of the electromagnetic spectrum extends from about 380 to 770 nanometers.
Light distribution: The pattern of light that is produced by a lamp or a luminaire, or the patterns of light created in a room.
Light output: Luminous flux, measured in lumens. The light output rating of a lamp is a measure of its total integrated light output. See also lumen.
Light source: The object that produces the light. For electric lighting, a lamp; for daylighting, the Sun
Linear fluorescent lamp: Any of the family of straight tubular fluorescent lamps. Lamps are available in 6-inch to 8-foot lengths, with the most-common length being 4 feet.
Louver: A series of baffles or reflectors that is used to shield a light source from view at certain angles, absorb unwanted light, or reflected light.
Low-voltage lamp: A lamp that nominally operates at 6, 12, or 24 volts. A transformer must be used to convert the 120-volt line voltage to the lower voltage.
Lumen: The unit of luminous flux. The lumen is the time rate of flow of light.
Luminaire: A complete lighting unit (fixture) 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. Also referred to as a light fixture, fitting, or unit.
Lux: Standard international unit of illuminance equal to 1 lumen per square meter. One lux equals 0.0929 footcandles.
Magnetic ballast: A ballast that uses a magnetic core and coil to provide the voltage and current that are needed to start the lamp(s) and to maintain its operation. Magnetic ballasts are heavier than electronic ballasts. See also ballast and electronic ballast.
Matte surface: A surface from which the reflection is predominantly diffuse.
Metal halide lamp: HID light source in which radiation from a mixture of metallic vapor and additives of halides (e.g., sodium, thallium, and indium) produces visible light.
Motion detector: Also called an occupancy sensor, a device that detects the movement of people, animals, and objects using a passive infrared and/or ultrasonic sensor. Motion detectors are used to control other devices, such as alarm systems and luminaires, so that these devices are activated when motion is detected. Some motion detectors offer manual on and or manual off override capabilities. See also passive infrared and ultrasonic.
MR-lamp: A multi-faceted reflector lamp.
Multi-faceted reflector lamp (MR-lamp): A low voltage halogen reflector lamp that is used in lighting applications where precise beam control is required, such as accent lighting. Some MR lamps, such as projection lamps, are designed for line-voltage operation.
Occupancy sensor: See motion detector.
PAR-lamp: A parabolic aluminized reflector lamp.
Parabolic aluminized reflector lamp (PAR- lamp): An incandescent or tungsten-halogen incandescent lamp with a hard glass bulb and an interior reflecting surface, a precisely placed filament, and a lens to control beam spread. The lens is hermetically sealed to the reflector. Metal halide PAR-lamps are also now available.
Pendant luminaire: See suspended luminaire.
Phosphors: Chemical compounds that are used to coat the inside of fluorescent and some high intensity discharge lamps. See also fluorescence.
Quartz-halogen lamp: See halogen incandescent lamp.
Recessed luminaire: A luminaire that is mounted above the ceiling (or behind a wall or other surface) with the opening of the luminaire flush with the surface.
Reflected glare: Glare resulting from bright reflections from polished or glossy surfaces in the field of view. Reflected glare usually is associated with reflections from when a visual task or areas in close proximity to the region being viewed.
Reflector lamps: A class of lamps that have reflecting material integrated into the lamp to direct the light. Types include common reflector (R), parabolic aluminized reflector (PAR), ellipsoidal reflector (ER), and multi-faceted reflector (MR) lamps.
Sconce: A decorative and/or functional wall-mounted luminaire.
Screw base compact fluorescent lamp: A compact fluorescent lamp with a ballast that has a medium screw base that fits into the standard incandescent lamp socket. A screw base compact fluorescent lamp may either be modular, in which the lamp and ballast are separate pieces, or self-ballasted, in which the lamp and ballast are inseparable. Both types are designed to replace incandescent lamps.
Self-ballasted compact fluorescent lamp: A one- piece screw base compact fluorescent lamp.
Single-pole switch: Single-location on-off switch that controls one luminaire, or group of luminaires.
Shade: A device on a luminaire that is used to prevent glare (by hiding the light source from direct view), control light distribution, and sometimes diffuse (and perhaps color) the light emitted.
Skylight: A clear or translucent panel set into a roof to admit daylight into a building.
Socket: The fitting on a luminaire that electrically connects the luminaire to the lamp.
Soffit luminaire: An architectural luminaire that directs light downward from the cornice or soffit between the wall and ceiling to light the wall surface below.
Specular surface: A surface from which the reflection is predominantly directional. Specular surfaces are mirror-like or shiny, as opposed to diffuse.
Spot lamp: A lamp that provides a relatively narrow beam of light.
Surface-mounted luminaire: A luminaire mounted directly on the ceiling or other surface.
Suspended luminaire: A luminaire hung from a ceiling by supports. Also called a pendant luminaire.
Switch: A device that turns a lamp or lamps on or off by completing or interrupting the power supplied to the lamp(s). See also single-pole switch, three-way switch, and four-way switch.
Task lighting: Lighting that is directed to a specific surface or area. Task lighting provides illumination for visual tasks.
Three-level lamp: Incandescent lamp having two filaments. Each can he operated separately or in combination with the other, which provides three different light outputs. A special socket is required to use the three levels of this lamp.
Torchiere: An indirect floor lamp sending all or nearly all of its light upward.
Track head: An adjustable luminaire that connects to the track in a track lighting system.
Track lighting: A lighting system with an electrically fed linear track that accepts one or more track heads. The track heads can be easily relocated along the track.
Tungsten-halogen lamp: See halogen incandescent lamp.
Under-cabinet lighting: Luminaires mounted on the underside of cabinets to provide task lighting, typically in a kitchen.
Universal Voltage Ballast: A ballast designed to operate over a wide range of input voltages. For example, they could be rated to operate from 120 volts to 277 volts, or from 108 volts (120 -10%) to 304 volts (277 +10%) considering the input tolerances. Universal Voltage Ballasts are almost always electronic ballasts.
Uplight: A luminaire that directs the light upward onto the ceiling and upper walls of a room.
Valance luminaire: An architectural luminaire with a longitudinal shielding member mounted across the top of a window or along a wall and usually parallel to the wall, to conceal light sources giving both upward and downward distributions.
Watt (W): Unit of active electric power, the rate at which electric energy is used.
Wattage: The active electrical power needed to operate by a device.

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