Lighting Terminology

Average Rated Life

The mean time it takes for a lamp to burn out. The time at which 50% of the test lamps have burned out and 50% are still working.


A device used with electrical discharge lamps such as fluorescent and high-intensity discharge (HID) lamps to provide the necessary voltage, current and waveform for starting and operating the lamp. These can be electromagnetic or electronic depending on the type of lamp. Electronic ballasts have been available since the early 1980s and have the advantage of being more energy-efficient and, because they operate at high frequencies (25-35 kHz), emit less flicker which is a factor with electromagnetic ballasts in cold weather.

Ballast Factor

Over the life of a ballast the efficiency decreases slightly. The ballast factor is a component of the Light Loss factor (see below).

Color Rendering Index (CRI)

An international system used to rate a lamp’s ability to render an object’s true color as compared to sunlight using a scale from 0 to 100 with a higher numerical value indicating a better color match. A value of 100 would be equivalent to sunlight and 0 would indicate that colors are not distinguishable from each other. Numerical comparison, using the CRI, of different lamps is only accurate if the Color Temperature Index values are similar. Differences of less than 5 points are usually not visible to the human eye. A CRI above 80 is required for color matching tasks. When selecting lighting for an area, it is important to consider the degree of color recognition that is required to complete tasks. A low CRI value makes it hard for humans to differentiate between colors.

Compact Fluorescent Lamp (CFL)

A device with small diameter fluorescent lamp(s), built-in ballast and a medium screw base for easy replacement of incandescent bulbs. Fluorescent tubes are usually 5/8″ in diameter and may be spiral or U-shaped.

Correlated Color Temperature (CCT)

(also called Chromaticity) A description of the color appearance of the light emitted by a lamp, relating its color to the color appearance of the light from a reference source when heated to a particular temperature expressed in degrees Kelvin (K). A lower numerical value indicates a lamp that emits a yellow or orange light while a higher numerical value indicates a bluer light source. Some examples of typical CRI values: high pressure sodium – 1900 K; incandescent – 2800 K; halogen – 3000 K; cool white fluorescent – 4100 K; daylight simulating fluorescent – 5000K.

Cutoff Distribution

A classification system for the type of reflector and refractor used on outdoor lighting. The typical yard light is classified as “non-cutoff” – light projects in all directions from the light fixture. A “full-cutoff” classification would cover the lamp and reflect light such that no light is projected above the horizontal plane of the fixture.

Fluorescent Lamp

An electrical discharge ionizes the gas inside the lamp, producing ultraviolet radiant energy which excites the phosphor coating on the inside of the glass. The phosphor coating then emits visible light.

Foot-candle (fc)

The unit of illumination on a surface one square foot in area on which is uniformly distributed a flux of one lumen. Equal to 10.76 Lux (ASAE EP344.2 Dec99)


The framework that holds the lamp(s) and may contain a ballast(s), reflector, diffuser, mounting hardware and protective shielding.

Fixture Efficiency

The ratio of light emitted from a fixture versus the light emitted by the lamp(s) contained in the fixture expressed as a percentage.

Halogen Lamp

A tungsten-halogen lamp which is a high pressure incandescent lamp containing halogen gases such as iodine or bromine. The halogen gases allow the filaments to operate at higher temperatures and higher efficiencies.

High-Intensity Discharge (HID) Lamps

Pressurized gas inside an arc tube is ionized by current flowing between electrodes, resulting in light being emitted. The three principle types of HID lamps include mercury vapor, metal halide and high pressure sodium. All HID lamps contain some mercury but the metal halide and high pressure sodium are considered more environmentally friendly because of the lower ratio of the mercury compared to the amount of light emitted and life of the lamp. All HID lamps must be disposed in accordance with local and state disposal laws.

High Pressure Sodium Lamp

This is a type of HID lamp. It has pressurized gas inside an arc tube is ionized by current flowing between electrodes, resulting in light being emitted. Contains sodium, mercury amalgam and xenon.


The density of light indicated on an area, measured in foot-candles or lux.

Incandescent Lamp

A device using a filament, usually coiled tungsten wire, which is heated by a flow of current to produce light.


A generic term for a device that creates optical radiation (light), sometimes called a bulb. There are three categories of electric lamps: incandescent, fluorescent and high-intensity discharge.

Lamp Efficiency

The efficiency of a lamp expressed in light output per unit of input energy or lumens per watt. Generally, lamp efficiency increases with lamp size.

Lamp Lumen Depreciation Factor

Over the life of a lamp or bulb the light output degrades from the initial installation until the end of its useful life. The LLD is a rating of the reduction of light output.

Light Loss Factor

A measure of the light output near the end of the lamp’s life compared to the initial lumen output expressed as percentage of the initial lumen output. It takes into account the decrease of lumen output as the lamp ages, the amount of dirt in the environment and how often the fixture is cleaned and the decrease in lamp output as the ballast ages. It takes into account the Lamp Lumen Depreciation Factor, the Luminaire Dirt Depreciation Factor and the ballast factor.


A standard international unit of luminous flux or quantity of light. The unit of the time rate of flow of light (luminous energy) equal to the energy emitted though a unit solid angle (one steradian) from a uniform point source of one candela (ASAE EP344.2 Dec99). Some examples: a dinner candle provides about 12 lumens while a 60-watt soft white incandescent bulb provides 840 lumens.

Luminaire Dirt Depreciation Factor

Dirt and dust collecting on the lamp or fixture will affect the usable light output. The LDD is a rating of how fast and the affect on light output.


An international unit of illuminance. One lux is equal to one lumen per square meter.

Mean Lumen

The average light output of a lamp over its rated life. The mean lumen ratings are measured at 40 or 50% of the rated lamp life, depending on the lamp type.

Mercury Vapor

This is a type of HID Lamp. It has a pressurized gas inside an arc tube ionized by current flowing between electrodes, resulting in light being emitted. Contains mercury plus small amounts of argon, neon and krypton gas.

Metal Halide

This is a type of HID Lamp. It has a pressurized gas inside an arc tube ionized by current flowing between electrodes, resulting in light being emitted. Contains mercury plus small amounts of argon, neon and krypton gas, and iodides of sodium and scandium or other rare-earth elements.


A built in reflective surface on a fixture.


A built in lens that bends the light rays to distribute the light over a wider area or to reduce glare.

Work Plane

The level at which the work is done and the level at which the design illumination is specified and measured. Typically the level is 2.5 feet above the floor for desk work, but for tie stall barn milking operations, one foot would more likely represent the work plane.

If you have questions about the information on this site, please contact
Scott Sanford, Distinguished Outreach Specialist, University of Wisconsin,