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Glossary of Lighting Terms


  • Absorption

    A process by which incident flux is converted to heat or other forms of energy.

  • Ambient lighting layer

    The layer of light that serves as general illumination for a space for basic visual recognition and movement.

  • Amp

    The unit for measuring rate of flow of electrical current: Current (Amps) = Power (Watts)/Voltage (Volts)

  • Amperes (Amps)

    A measurement of electrical current defined as: Amps = Watts (Power) / Volts (Voltage).

  • Architectural lighting

    Light from fixtures that are designed into the construction of the building or structure itself.


  • Back light

    A component of lighting that separates an object from the background and provides additional three-dimensional depth.

  • Baffle

    Translucent or opaque elements that shield the light source from view.

  • Ballast

    Used in light sources such as fluorescent and high intensity discharge (HID) lamps, this auxiliary piece of equipment is required to start and control the flow of current for the gas discharge.

  • Ballast factor

    The percentage lumen output from a lamp that can be expected when operated on a specific, commercially available ballast. Ballasts with a lower rating result in less light output but also typically consume less power.

  • Base

    The part of the lamp or light source that connects to the power supply. The most common base type used on incandescent lamps is the medium screw base, sometimes referred to as the Edison Screw Base.

  • Beam angle

    The angle between the two directions opposed to each other over the beam axis for which the luminous intensity is half that of the maximum luminous intensity.

  • Breakaway requirements

    Usually mandated by the Federal Highway Administration for federal interstate and state highways. They may also be required on certain county and local road systems. Common breakaway devices are slip bases, certain cast aluminum T-bases and breakaway couplings.

  • Brightness

    The perception elicited by the luminance of a visual target. This is a subjective attribute or property of an object being observed.

  • Brightness

    The extent to which an object is judged to emit more or less light. The brightness of an object can change depending on whether it is seen against a light or dark background.

  • Bulb

    The glass part of the lamp. Lamps are produced with a variety of bulb shapes. LED light fixtures do not utilize a bulb.

  • Burn position

    The allowed operating position of the lamp. Lamps (bulbs) restricted to a specific “burn position” (UP or DOWN) should not be operated in any other position for safety and/or performance.


  • Candela, cd

    The SI unit of luminous intensity. Formerly candle.

  • Catwalk

    A raised overhead platform used for mounting and accessing luminaires.

  • Cd (coefficient of drag)

    The ratio of the "apparent" wind area to the actual silhouette area of an object or luminaire. Streamlined objects have lower Cds than blunt or flat-sided objects.

  • Ceiling cavity

    The portion of the room that is above the light fixture.

  • Chromaticity

    The quality of a color regardless of its luminance as determined by its hue and saturation.

  • Chromaticity diagram

    Two-dimensional graph that plots one of the three chromacity coordinates against each other.

  • Chromaticity diagram

    A horseshoe-shaped line that connects the chromaticities of the spectrum of colors.

  • Coefficient of variation (CV)

    A measurement of illuminance uniformity. The standard deviation of a set of grid values divided by the average.

  • Color corrected

    A lamp with a special phosphor or coating to give it a color rendering profile similar to natural daylight.

  • Color rendering index (CRI)

    A measurement, on a scale of 1 to 100, used to describe the ability of a light source to render all of the colors in a space accurately.

  • Color temperature

    The absolute temp of a blackbody radiator having a chromacity equal to that of the light source.

  • Controller card

    A device that controls the output of light. Contains software components for configuring fixtures and hardware components for sending control data to fixtures.

  • Correlated color temperature

    Describes the overall color appearance of a lamp. Often referred to as CCT.

  • CSR (combined stress ratio)

    The ratio of the applied stresses imposed on a pole to the allowed stresses. These would include the bending stress (due to the OTM), shear and torsion stresses, and the axial stresses (from pole and luminaire weights). The combination of all these stresses shall not exceed a CSR value of 1.0.

  • Current

    A measure of the rate of flow of electricity, expressed in amperes (A).


  • Daylight harvesting

    Refers to systems that use daylight to offset the amount of electric lighting needed to properly light a space in order to reduce energy consumption.

  • DC (direct current)

    When electrical current and distribution flows in one direction through the conductor. Battery-operated systems are typical DC applications.

  • Delivered light

    The amount of light a luminaire delivers to a surface. It is measured in foot-candles (fc) or lux. LEDs are directional and deliver a greater proportion of light to where it is wanted.

  • Direct/Indirect

    Light emitted in a downward and upward direction. Often used to accent a lit space.

  • Discharge lamp

    The production of light by passing a current between electrodes through a vapor or gas. Examples include fluorescent and high intensity discharge (HID) lamps.

  • DMX

    A signal protocol for controlling dimming and color mixing in light fixtures.

  • Downlight

    A light fixture that emits light downward with no upward emission of light.

  • Driver

    A power supply that matches the electrical characteristics of the lamp. Similar to a ballast, it is used to power illumination sources.


  • Efficacy

    Expressed as LPW (lumens per watt), efficacy is how efficiently the lamp converts electrical energy into light.

  • Electromagnetic interference (EMI)

    Electromagnetic radiation from an external source that affects an electrical circuit. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit.

  • Electronic ballast

    High-frequency electronic ballast that uses solid-state electronic components and normally operates fluorescent lamps at frequencies in the range of 25-35 kHz. Advantages include increased lamp efficacy, reduced ballast losses and a lighter, smaller design compared to an electromagnetic ballast. May be used with high-intensity discharge lamps.

  • Energy Independence and Security Act (EISA) of 2007

    Signed into law on December 19, 2007. Builds on progress made by the Energy Policy Act of 2005 (EPAct) in setting out a comprehensive energy strategy for the 21st Century. A major step toward reducing the United States’ dependence on oil, thereby increasing the country’s energy security and making it cleaner for future generations.

  • Energy Star

    A joint program of the U.S. Environmental Protection Agency and the U.S. Department of Energy promoting money savings and the protection of the environment through the use of energy-efficient products and practices.

  • EPA (effective protected area)

    Light fixtures or luminaires are rated in EPA, referencing apparent wind profile of a fixture or object based on its geometric shape. A round fixture that is more streamlined has a lower EPA than a flat-sided fixture of the same silhouette. Poles are rated by EPA capacity at several wind speeds, and EPA equals Actual Wind Silhouette times Cd.


  • Field angle

    The angle between the two directions opposed to each other over the beam axis for which the luminous intensity is 10% that of the maximum luminous intensity.

  • Filament

    A wire used in incandescent lamps, normally made of tungsten and often coiled, that emits light when heated by an electrical current.

  • Flicker

    The (potentially visible) temporal variation of emitted light.

  • Footcandle, fc

    The unit of illuminance when the foot is taken as the unit of length. The illuminance on a surface one square foot in area on which there is a uniformly distributed flux of one lumen, or the illuminance produced on a surface, all points of which are at a distance of one foot from a directionally uniform point source of one candela.


  • Ghosting

    An effect that occurs when lighting fixtures in the off state faintly glow as a result of residual voltage in the circuit.

  • Glare

    An interference with visual perception caused by an uncomfortably bright light source or reflection within a person’s field of view; a form of visual noise.


  • Halogen

    A type of incandescent lighting in which the filament burns hotter and longer, producing a whiter light that typically lasts longer than standard incandescent.

  • Heat sink

    A part of the thermal system that conducts or convects heat away from sensitive components such as LEDs.

  • Height above grade

    Height above the surrounding terrain to the pole base (for example, a pole located on a parking deck or bridge). This is important because wind velocity increases with elevation (altitude).

  • HID

    A point source electric-discharge lamp in which the light-producing arc is contained within a secondary bulb. HID lamps include groups of lamps known as mercury, metal halide and high pressure sodium.

  • High lumen

    Lamps that operate with higher lumens (brightness) than a standard model.

  • Horizontal illuminance

    The quantity of light on a horizontal plane.



    International Engineering Society of North America. The IESNA is the recognized technical authority on illumination, communicating information on all aspects of good lighting practice to its members, to the lighting community, and to consumers through a variety of programs, publications, and services.

  • Illuminance, E

    The density of the luminous flux incident on a surface. It is measured in lux E v. Sometimes called “illumination.”

  • Indirect

    An upward distribution of light that produces illumination on the horizontal workplace via reflection from the ceiling and upper walls.

  • Initial light levels

    The average light levels when the luminaires are new. Measuring initial light levels assures that you receive a system that meets your requirements.

  • Instant start

    A type of fluorescent lamp-ballast circuit designed to start fluorescent lamps as soon as the power is applied. Initially, instant-start circuits were developed to eliminate separate mechanical starter devices.

  • Integrated circuit (IC)

    IC-based compact fluorescent lamps (CFLs) operate by controlling the voltage and current by adjusting the output frequency, which provides stable operation of the CFL. Controlling the current produces less stress on the cathode and the electronic components; this results in longer life, smoother dimming and less noise.

  • Intensity

    A shortening of the terms luminous intensity and radiant intensity.

  • IP ratings

    “IP” stands for Ingress Protection. IP ratings have two numbers: the first stands for the protection against solid objects, the second for protection against liquids. For example, an IP rating of 65 tells you that the protection against solid objects is a 6 and the protection against a liquid is a 5.


  • Kelvin

    The Kelvin unit is the basis of all temperature measurement. In lighting, Kelvin is the unit of measure for color temperature used to indicate the overall color of the light produced from a source. See “correlated color temperature.”

  • Kilowatt (kW)

    The measure of electrical power equal to 1,000 watts.

  • Kilowatt hour (kWh)

    The measure of electrical energy from which electricity billing is determined. For example, at the rate of $0.11 per kWh, a 100 watt lamp operating for 1,000 hours will cost $11.00 (100 x 1,000/1,000 = 100 kWh x .11 = $11.00).


  • L70 Hours

    Used to describe the LED’s expected light output over its stated life span. “L70” predicts when the LED reaches 70% of initial lumen output. Lumen maintenance is a prediction of the number of hours an LED will operate before it fades below a useful level of intensity.

  • Lamp

    A source that converts electricity into light. The official industry term for “light bulb” or “fluorescent tube.”

  • LED

    A light emitting diode (LED) is a solid state diode constructed to emit colored or white light. The two-lead semiconductor light source first appeared in practical electronic components in 1962.

  • LED Array

    An assembly of LEDs on a circuit board. Can include optical elements and additional thermal, mechanical and electrical interfaces that are intended to connect to the load side of an LED driver.

  • LED Chip

    The light-producing semiconductor device that may or may not be incorporated into an LED.

  • LED Driver

    An electronic circuit that inputs power into a current source — a source in which current remains constant despite fluctuations in voltage. An LED driver protects LEDs from normal voltage fluctuations, over voltages and voltage spikes.

  • LED Luminaire

    A complete lighting unit consisting of LED-based elements and all necessary components: driver, parts to distribute the light, and parts to position and protect the light-emitting elements and connect the unit to a circuit branch.

  • Light loss factor (LLF)

    A factor used in calculating luminance over a given period of time and under given conditions. It accounts for light loss due to temperature and voltage variations, dirt accumulation on luminaire, lamp depreciation, maintenance procedures and atmosphere conditions.

  • Light trespass

    Spill light that is either annoying or unwanted.

  • Low-voltage

    A group of incandescent sources that typically draw between five and 24 volts.

  • Lumen (lm)

    A measure of luminous flux or the total “amount” of visible light emitted by a source.

  • Lumen depreciation

    A reduction in output over time. Normally shown in graph form with the percentage reduction in hours.

  • Lumen efficiency

    The percentage of total lamp lumens that a luminaire or system emits, minus any blocked or wasted light.

  • Lumen maintenance

    The luminous flux at a given time in the life of an LED, expressed as a percentage of the initial luminous flux.

  • Lumen maintenance curve

    A graph illustrating the predicted average light output behavior over time of a single LED or solution.

  • Lumens per watt (LPW)

    A measure of the efficacy (efficiency) of a light source. The number is achieved by dividing lumens produced by watts consumed.

  • Luminaire

    Also called a fixture, a full lighting unit consisting of a lamp (or lamps) and ballast (or ballasts) to distribute light, place and guard the lamps and connect them to the power supply.

  • Luminaire efficiency

    The ratio of light produced by the luminaire to the light produced by its lamps.

  • Luminance

    A photometric calculation of a surface’s brightness as seen by the viewer, measured in candelas per square meter (cd/m2).

  • Luminous flux

    The measure of the perceived power of light, adjusted to reflect the varying sensitivity of the human eye to different wavelengths of light.

  • Lux (lx)

    A unit of illuminance or light hitting a surface. One lux equals one lumen per square meter. Ten lux is about one footcandle.


  • PAR lamp

    A parabolic aluminized reflector that may use an incandescent filament, halogen filament tube or HID arc tube. A PAR lamp is a precision pressed-glass reflector lamp that depends on the internal reflector and prisms in the lens for light beam control. When LED technology is incorporated, the lens is used to control the beam.

  • Photometry

    The measurement of light and associated quantities.

  • Pole geometry

    The dimensional and physical shape of the pole. Basic characteristics include height, shape (round and square cross sections), diameter (or square size), wall thickness, taper (if any), material and weight. When combined with different loadings of luminaires and brackets, the same pole will exhibit different vibration characteristics.

  • Power factor (PF)

    The measurement of the relationship between the AC source voltage and current. Power factors can range from 0 to 1.0, with 1.0 being ideal. Power factor is a measure of how efficiently electrical power is being used. The higher the power factor, the more efficient.