Lighting Glossary

Terms and Definitions to Help You Understand the Lighting World

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accent lighting – Directional lighting to emphasize a particular object or to draw attention to a part of the field of view. (see directional lighting)

adaptation – The process by which the retina of the eye becomes accustomed to more or less light than it was exposed to during an immediately proceeding period. It results in a change in the sensitivity of the eye to light.

ANSI (American National Standards Institute) – The organization that develops voluntary guidelines and produces standards for the electrical and other industries.

arc tube – A completely sealed quartz or ceramic tube where the electrical discharge (arc) occurs and light is generated.

average life – Value for life expectancy of a lamp.


baffle –  A single opaque or translucent element to shield a source from direct view at certain angles, or to absorb unwanted light.

ballast – A device used with an electric-discharge lamp to obtain the necessary circuit conditions (voltage, current and wave form) for starting and operating; all fluorescent and HID light sources require a ballast for proper operation. Dimming ballasts are special ballasts which when used together with a dimmer will vary the light output of a lamp.

ballast factor, BF –  The measured ability of a particular ballast to produce light from the lamp(s) it powers. Ballast factor is derived by dividing the lumen output of a particular lamp/ballast combination by the lumen output of the same lamp(s) on a reference ballast.

base – End of the lamp that inserts into the lamp socket.

beam angle – The angle between the two directions for which the intensity (candlepower) is 50% of the maximum intensity as measured in a plane through the nominal beam centerline (center beam candlepower).

beam spread –  (In any plane) the angle between the two directions in the plane in which the candlepower is equal to a stated percent (usually ten percent) of the maximum candlepower in the beam.

brightness – See luminance.

bulb – Outer jacket or envelope of a lamp.

burning position – Position that lamps are designed to operate.


candela, cd – The unit of measure indicating the luminous intensity (candle-power) of a light source in a specific direction; any given light source will have many different intensities, depending upon the direction considered.

candlepower distribution – A curve that represents the variation in luminous intensity (expressed in candelas) in a plane through the light center of a lamp or luminaire; each lamp or lamp/luminaire combination has a unique set of candlepower distributions that indicate how light will be spread.

candlepower distribution curve – A curve, generally polar, representing the variation of luminous intensity of a lamp or luminaire in a plane through the light center.

CCT rating – CCT rating is based on operation at rated watts in the designated operating position. For universal lamps, CCT is rated in the vertical position. All other operating positions will lead to higher values of CCT.

center beam candlepower, CBCP – The intensity of light produced at the center of a reflector lamp, expressed in candelas.

chromaticity – The aspect of color that includes consideration of its dominant wavelength and purity.

coefficient of utilization, CU – The ratio of the luminous flux (lumens) from a luminaire received on the work-plane to the lumens emitted by the luminaire’s lamps alone.

color rendering index, CRI – The measure of a light source’s ability to render the color of objects “correctly,” compared with a reference source with comparable color temperature; the scale peaks at 100.

color uniformity – Consistency of color from lamp to lamp.

correlated color temperature, CCT – A specification of the color appearance of a lamp relating its color to that of a reference source heated to a particular temperature, measured in degrees Kelvin (K); CCT generally measures the “warmth” or “coolness” of light source appearance.

current, I – A measure of the flow of electricity, expressed in amperes (A).

current crest factor – Ratio of peak to rms value of lamp current: mh values range from 1.4-1.8. Cannot go above 1.8 or it reduces lamp life.

cut-off angle – (Of a luminaire) the angle from the vertical at which a reflector, louver or other shielding device cuts off direct visibility of a light source. It is the complementary angle of the shielding angle.


diffuse lighting – Light that is not predominantly incident from any particular direction.

diffuser – A device to redirect or scatter the light from a source by the process of diffuse transmission.

direct glare – Glare resulting from high luminance or insufficiently shielded light sources in the field of view, or from reflecting areas of high luminance. It is usually associated with bright areas such as luminaires, ceilings and windows that are outside the visual task or region being viewed.

direct lighting – Lighting by luminaires distributing 90 to 100 percent of the emitted light in the general direction of the surface to be illuminated. The term usually refers to light emitted in a downward direction. (See accent lighting.)

directional lighting – Illumination on the work-plane or on an object predominantly from a single direction.

disability glare – Glare resulting in reduced visual performance and visibility. It is often accompanied by discomfort.

discharge lamp – Light-producing device that depends on electric arc, rather than filament, to create illumination.

discomfort glare – Glare producing discomfort. It does not necessarily interfere with visual performance or visibility.

double-ended – Lamps that have two bases opposite one another for series electrical connection, mounting connection and heat dissipation.


economic life – Number of hours a group of lamps will burn before it’s ergonomically advisable to group re-lamp (typically 60% – 75% of rated life).

efficacy – Efficiency of a light source expressed in lumens per watt (LPW or lm/W).

energy – A measure of work done by and electrical system over a given period of time, often expressed in kilowatt-hours (kWh).

enhancing reflections – Reflections which enhance appearance described in such terms as sparkling, glittering, etc.

ESCO – Energy Saving Service Company


fenestration – Any opening or arrangement of openings or windows (normally filled with media for light control) for the admission of daylight or for the transmission of electric lighting from one room to another room.

filament – A tungsten wire purposely positioned inside a lamp bulb that when heated, electrically generates radiation in the visible, infrared and ultraviolet ranges.

fixture – See luminaire.

floodlight – A reflector lamp with a relatively wide beam pattern.

fluorescent lamp – A low pressure mercury electric discharge lamp, tubular in shape in which a fluorescent coating (phosphor) transforms ultraviolet energy into visible light.

footcandle, FC – A unit of illuminance equal to 1 lumen per square foot.

formed body arc tube – Arc tube formed by quartz sculpting process, has 1/3 less quartz mass than standard arc tubes, 3rd starter electrode eliminated. Ballast requires external ignitor with 3-4 KV to start lamp.

frequency – The number of times per second that an alternating current system reverses from positive to negative and back to positive, expressed in cycles per second or hertz, Hz.


general lighting – Lighting designed to provide a substantially uniform illuminance throughout an area, exclusive of any provision for special and local requirements.
glare – Excessive brightness that may be caused by either direct or indirect viewing of a light source; any brightness or brightness relationship that annoys, distracts or reduces visibility.


harmonic – An electrical frequency that is an integer multiple of the fundamental frequency; for example, if 60 Hz is the fundamental frequency, then 120 Hz is the second harmonic and 180 Hz is the third harmonic; some electronic devices, such as ballasts or power supplies, can cause harmonic distortion, directly affecting power quality.
Hertz, Hz – A unit of frequency equal to one cycle per second. (See frequency)

high intensity discharge (HID) lamps – A general group of lamps consisting of mercury, metal halide and high pressure sodium lamps.


illuminance – Light arriving at a surface, expressed in lumens per unit area; 1 lumen per square foot equals 1 footcandle, while 1 lumen per square meter equals 1 lux.

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

indirect lighting – Lighting by luminaires distributing 90 to 100 percent of the emitted light upward.

infrared radiation, IR – Is a type of electromagnetic radiation given off by incandescent and fluorescent lighting.

initial lumens – The light output of a HID lamp at rated power on a reference ballast after 100 hours of operation.


joule – The derived unit of energy in the International System of Units, equal to the work done by a force of one newton when its point of application moves one meter in the direction of action of the force, equivalent to one 3600th of a watt-hour.


Kelvin, K – Is a unit of measurement used to describe the hue of a specific light source. This is not necessarily related to the heat output of the light source but rather the color of the light output. The higher the Kelvin rating, the more blue the light source is. The lower the Kelvin rating, the more yellow/amber the light source is.


lamp – Manufactured light source; the 3 broad categories of electric lamps are incandescent, fluorescent, and high intensity discharge (HID). Also, a generic term for a man-made source of light.

lamp life – An average rating, in hours, indicating when 50% of a large group of lamps have failed, when operated at nominal lamp voltage and current. Manufacturers use 3 hours per start for fluorescent lamps and 10 hours per start for HID lamps when performing lamp life testing procedures. Every lamp type has a unique mortality curve that depicts its average rated life.

lamp lumen depreciation factor, LLD – The multiplier to be used in illumination calculations to relate the initial rated output of light sources to the anticipated minimum rated output based on the re-lamping program to be used.

lamp voltage – Voltage which lamps operate when they are fully warmed up.

lens – A glass or plastic element used in luminaries to change the direction and control of the distribution of light rays.

level of illumination – See illuminance.

light – Radiant energy that is capable of producing a visual sensation.

light center length, LCL – The distance from the center of the visible arc discharge to the bottom of the contact of the base.

light emitting diode, LED – Semiconductor diodes, electronic devices that permit current flow in only one direction. The diode is formed by bringing two slightly different materials together, forming a junction. When a forward voltage is applied, current flow begins and energy is radiated in the form of light.

light loss factor, LLF – A factor used in calculating illuminance after a given period of time and under given conditions. It takes into account temperature and voltage variations, dirt accumulation on luminaire and room surfaces, lamp depreciation, maintenance procedures and atmosphere conditions. Formerly called maintenance factor.

light on centers, LOC – Distance between LED lights.

light source – See lamp.

lighting design lumens, LDL – Number of lumens at 2,000 hours of operation.

louver – A series of baffles used to shield a source from view at certain angles or to absorb unwanted light. The baffles are usually arranged in a geometric pattern.

lumen, lm – A unit of luminous flux; the overall light output of a luminous source is measured in lumens. Amount of light lamp emits once it’s started.

lumen depreciation – The decrease in lumen output of a light source over time; every lamp type has a unique lumen depreciation curve (sometimes called lumen maintenance curve) depicting the pattern of decreasing light output.

lumen maintenance – Lumen output provided by lamp at given point in, or percentage of, its life.

lumens per watt, LPW – Lumen (light) output divided by lamp watts consumed (i.E. Amount of electricity used); also known as efficacy.

luminaire – A light fixture; the complete lighting unit, including a lamp, reflector ballast, socket, wiring, diffuser, and housing.

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

luminance, L – Light reflected in a particular direction; the photometric quantity most closely associated with brightness perception, measured in units of luminous intensity (candelas) per unit area (square feet or square meters).

luminance contrast – The relationship between the luminance of and object and its immediate background.

luminance ratio – The ratio between the luminance of any two areas in the visual field.

luminous flux – The time rate of the flow of light.

lux, lx – A unit of illuminance equal to 1 lumen per square meter.


maximum overall length, MOL – The maximum allowable distance from the top of the glass bulb to the end of the content of the base.

matte surface – A non-glossy, dull surface as opposed to a shiny (specular) surface. Light reflected from a matte surface is diffused.

mean lumens – Lumen output of a light source after the source has been used. Mean lumen values for fluorescent and HID lamps are typically measured at 40% of rated life.

mili-candela, MCD – The measurement of light output of a LED in thousandths of a candela.


nanometer, nm – A unit of length equal to 10-9 meters; commonly used as a unit of wavelength.

non-shunted socket – electricity is received through two separate (not joined or connected) points of contact and current flows through multiple paths



open rated lamp, mogul base – Designed with an extended contact pin on the bottom of the base. Lamps should be used with an open fixture mogul socket (EX39) which prevents electrical contact if a non open rated lamp is used.

open rated lamp, medium base – Designed for open fixtures; has narrower neck than standard medium base lamps to fit into an exclusionary medium socket for open fixture use.

operating position – Metal halide lamps are specified and designed to be operated in certain positions such as vertical or base-up.


power – The rate at which energy is taken from an electrical system or dissipated by a load, expressed in watts (W); power that is generated by a utility is typically expressed in colt-amperes (V-A).

power factor – A measure of the effectiveness with which an electrical device converts volts-amperes to watts; devices with power factors >0.90 are “high power factor” devices.

pulse start lamp – Specially designed metal halide lamp that requires high voltage pulse for starting. Has improved lumen maintenance.


quality of lighting – Pertains to the distribution of luminance in a visual environment. The term is used in a positive sense and implies that all luminance  contribute favorably to visual performance, visual comfort, ease of seeing, safety and esthetics for the specific visual tasks involved.
quantity of light – The product of the luminous flux by the time it is maintained. It is the time integral of luminous flux.


rated life – The number of operating house at with 50% of most metal halide lamps or 70% of some pulse start lamps initially started will still be operating.

reference ballast – A ballast specifically constructed to have certain prescribed characteristics for use in testing electric-discgage lamps and other ballasts.

reflectance, rho – There percentage of light reflected back from a surface, the difference having been absorbed or transmitted by the surface.

reflected glare – Glare resulting from specular reflections of high luminance in polished or glassy surfaces in the field of view.

reflection – The process by which flux leaves a surface or medium from the incident side.

reflector – A device used to redirect the light by the process of reflection.

refraction – The process by which the direction of a ray of light changes as it passes obliquely from one medium to another.

refractor – A device used to redirect the luminous flux from a source, primarily by the process of refraction.

resistance, R – A measure of resistance to flow of the current, expressed in ohms.

restrict time – The time to re-ignite the arc of a HID lamp.


shielding – A general term to include all devices used to block, diffuse or redirect light rays, including baffles, louvers, shades, diffusers and lenses.

shielding angle – The complementary angle of the cut-off angle of a luminaire.

shroud – Glass cylinder surrounding the arc tube to protect against hot arc tube particles from breaking the glass bulb if an arc tube rupture occurs; reduces risk of using mh lamps in open fixtures.

shunted socket – electricity is received through a single point and is then spread to two points of contact


Special (SPC) – Items not stocked in TCP’s warehouse. These items have longer lead times than stocked (STK) items. Contact our customer service department at 800-324-1496 for additional details.

spectral power distribution, SPD – A curve illustrating the distribution of power produced by the lamp, at each wavelength across the spectrum.

specular surface – A shiny, highly polished surface which reflects hight at an angle equal to that of the incident light.

spotlight – A reflector lamp with a narrow mean pattern used to illuminate a specifically defined area.

Stocked (STK) – Item is in stock and applicable to standard shipping.

supplementary lighting – Lighting used to provide an additional quantity and quality of illumination that cannot be readily obtained by the general lighting system and that supplements the general lighting level usually for specific task requirements.

Special Order (SPO) – Items marked SPO require a lead time of 4-6 weeks, and utilize a premium delivery option. Contact our customer service department at 800-324-1496 for additional details.


transmission – The process by which incident flux leaves a surface or medium on a side other than the incident side, the characteristics of many materials such as glass, plastics and textiles.
transmittance – The ratio of the flux transmitted by a medium to the indigent flux.


ultraviolet radiation, UV – Is a type of electromagnetic radiation given off by incandescent and fluorescent lighting.


veiling reflections – Reflections which partially or totally obscure the details to be seen by reducing the contrast.

visual comfort probability, VCP – A discomfort glare calculation that predicts the percent of observers positioned in the least favorable part of the room who would be expected to judge a lighting condition to be comfortable. VCP rates the luminaire in its environment, taking into account such factors as illuminance level, room dimensions and reflectances, luminaire type, size and light distribution, number and location of luminaires, and observer location and line of sight. The higher the VCP the more comfortable the lighting environment.

visual field – The location of objects or points in a space where the head and eyes are kept fixed.

visual surround – All portions of the visual field except the visual task.

visual task – Those details and objects which must be seen for the performance of a given activity, including the immediate background of the details or objects.

voltage, V – A measure of the electrical potential, expressed in volts (V).


warm-up time – The amount of time from ignition of the lamp to 90% of full light output of a HID lamp.

watt, W – A unit of electrical power equal to 1 joule per second.

work plane – The plane at which work usually is done, and on which the illuminance is specified and measured. Unless otherwise indicated, this is assumed to be a horizontal plane 0.76 meters (30 inches) above the floor.
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