Glossary of Lighting Terms


Arc Tube: A tube in which a current traverses a gas between two electrodes.

Argon: Inert gas used in incandescent and fluorescent lamps.

Ballast: A device used to provide the starting voltage and regulate the current to the lamp.

Base: End (or ends) of the lamp that inserts into the lamp socket or holder.

Beam Spread: The angle of a beam from a light source to the main area lighted.

Burn Position: The position in which lamps are designed to be operated. Often designated by the position of the lamp base. BU = Base Up, H = Horizontal, Univ = Universal Burn.

Candela (cd): The measuring unit of luminous intensity of a light source in a given direction.

Candle Power (cp): Luminous intensity expressed in candelas.

Chromaticity of a Color: The quality of color that includes its dominant or complementary wavelength, purity, hue and saturation.

Color Rendering Index (CRI): An index from 1-100 measuring a light source’s ability to render color accurately.

Color Temperature: A scientific measurement of the balance of wavelengths making up any “white” light. The unit of measurement is in Kelvin(K) which determines the warm or cool appearance of a light source. The lower the color temperature, the warmer or more yellow – the higher the color temperature, the cooler or bluer in appearance.

Compact Fluorescent (CFL): A term given to small fluorescent bulb, some of which have a built-in ballasts and medium screw bases for replacement of incandescent bulbs.

Dichroic: A term meaning “two color”; often used as the coating applied to glass and reflectors to change or control the color of light passing through.

Efficacy: The measured effectiveness at which a lamp (bulb) converts power (watts) into light (lumens) or Lumens Per Watt.

Efficiency: A term used to measure the effectiveness of the entire lighting system in its given environment.

Electrode: a coated metal element which facilitates the emission of electrons to form an arc within a fluorescent bulb.

Energy Star: A registered trademark – was introduced by the EPA in 1992 to identify and promote energy-efficient products.

Filament: A tungsten wire that when heated electrically generates radiation in the visible, infrared and ultraviolet ranges.

Fluorescent: Introduced in 1938, a glass tube coated with fluorescent phosphors and electrodes at each end. When electrons flow from one electrode to the other collisions cause the emission of radiation. When this radiation strikes the phosphors, they “fluoresce”.

Footcandle (fc): A unit of illuminance. One footcandle is equal to one lumen per square foot (lm/ft2).

Glare: The effect of brightness (or difference of brightness) within the visual field, sufficiently enough to cause annoyance, discomfort or loss of visual performance.

Halogen: An incandescent bulb containing Halogen gas (see faq page for more details).

HID: An abbreviation for High Intensity Discharge lamps such as: Metal Halide (MH), High Pressure sodium (HPS) and Mercury Vapor (MV) or any other high intensity discharge source.

Incandescent: Perfected in 1879 by Thomas Edison was referred to as a “hot wire in a bottle”. a filament surrounded by inert gas – heated to incandescence by an electric current.

Infrared: Radiant energy with wavelengths between 770 and one million nanometers, i.e., longer than the wavelengths of visible light.

Kelvin Scale: A scale of temperature measured in degrees Celsius form absolute zero (see Color Temperature).

Krypton: A heavy inert gas used in incandescent bulbs. Lamp: Industry term for light bulb.

Lamp Life: Rated life of a lamp as established through laboratory testing. A sample group of lamps are burned, including being subjected to several starts per day – the length of time required for half of the lamps to fail is the rated life.

LED: An acronym for Light Emitting Diode (see faq page for more details).

Lens: A lens is used to change the direction and control the distribution of light. A diffuser is used to redirect or scatter the light.

Light: Radiant energy that excites the retina of the eye producing a visual sensation.

Light Center Length (LCL): The distance between the center of the filament or arc tube and the reference plane (usually the bottom of the lamp base).

Light Loss Factor: Used to calculate or project lighting system performance after a given period of time. Also called maintenance factor.

Low-Voltage Lamps: Typically a compact halogen bulb that operates at 12 or 24 volt and requires the use of a transformer.

Lumen: The international unit of measurement for light. Defined as the amount of light falling on a surface of one square foot, every point of which is one foot away from a one-candlepower source.

Lumen Depreciation: The decrease in lumen output of a light source over time.

Lumens Per Watt (LPW): A measure of the efficiency of a light source, equal to the lumens produced, divided by the power (watts).

Luminance: A photometric term that quantifies brightness of a light source or of a surface that emits light. It is expressed as footlamberts.

Luminaire: A complete lighting unit designed to distribute light, to position and protect the lamp and to connect to the power supply. Also called a fixture.

Lux (lx): A metric unit of measure for illuminance of a surface. One lux is equal to one lumen per square meter (1 lx = 1 lm/m2).

MR-16: A low-voltage mirrored reflector lamp with a 2-inch diameter. Smaller diameter lamps include MR-11 and MR-8.

Nanometer (nm): A unit of length equal to 10-9 meter. The preferred unit of measure for light in the visible and ultraviolet regions of the spectrum.

Neodymium: A rare earth metal used in reflector and glass coatings to help reduce the yellow light emitted by the lamp.

PAR Lamp (Parabolic Aluminized Reflector): A PAR lamp is used to redirect the light from the source using a parabolic reflector. Available with flood or spot distributions.

Parabolic Reflector: A smooth surfaced, curved reflector form in the shape of a parabola which focuses all the light at the focal point to create a parallel beam.

Photocell: A light-sensing device used to control luminaires and dimmers in response to detected light levels.

Polycarbonate: Material used in lenses that will not break or yellow.

Power Factor (PF): A measure of the effectiveness of which an electrical device converts amperes to watts. A high power factor means that an electrical system is utilizing power efficiently.

Quartz: An obsolete term for a tungsten-halogen lamp.

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

Refractor: A lens or diffuser that changes the direction of the light.

Specular: Mirrored or polished surface – used to describe the finish of the material used in some louvers and reflectors.

Socket: Electro-mechanical connection in the fixture for the lamp.

TCLP Test (Toxicity Characteristic Leaching Procedure): A test used to characterize fluorescent lamp waste as hazardous or non-hazardous.

Total Harmonic Distortion (THD): A measure of the distortion of an electrical wave form. THD is expressed in percentages.

Tungsten: A heavy metal used in wire filaments and electrodes. Also know as Wolfram.

Twin Tube: See compact fluorescent.

T8 Lamp: A fluorescent lamps that is 1-inch in diameter. Other sizes include T5 (5/8″), T10 (1.25″) and T12 (1.5″). Note that T12 lamps no longer meet current energy codes.

Ultraviolet: Radiant energy, beyond the violet end of the visible spectrum.

Underwriters Laboratories (UL): An independent organization whose responsibilities include rigorous testing of electrical products. UL tests for product safety only.

Xenon: A filament lamp in which Xenon gas is contained in a quartz capsule.