Lighting Glossary

Navigating lighting industry jargon can be a challenge, so we have put together a list of common lighting terms and definitions you may find helpful.


ANSI (American National Standards Institute)

The organization that develops voluntary guidelines and produces performance standards for the electrical and other industries.

Average Rated Life

An average rating, in hours, indicating when 50% of a large group of lamps has failed (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.



A device used with an electric-discharge lamp to obtain the necessary circuit conditions (voltage, current and waveform) for starting and operation. 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 Efficacy Factor (BEF)

Ballast factor divided by input power (watts). Used to compare ballast efficiency.

Ballast Factor

The measured ability of a particular ballast to produce light from the lamp(s) it powers. The number is calculated by dividing the lumen output of a particular lamp/ballast combination by the lumen output of the same lamps(s) on a reference ballast.

Beam Angle

The angle of light emitted from a lamp. The angle is measured between two directions for which the light intensity (candlepower) is 50% of the maximum intensity.

Beam Spread

The angle of light emitted from a lamp. The angle is measured between two directions for which the light intensity (candlepower) is 50% of the maximum intensity.


Candela (cd)

The unit of luminous intensity (candlepower) of a light source in a specific direction.

Center Beam Candlepower (CBCP)

The intensity of light at the center of a reflector lamp beam (expressed in candelas).

Color Rendering Index (CRI)

Measures the visual effect a light source has on the perceived color of objects it illuminates. High CRI light generally makes colors look natural and vibrant. Low CRI causes some colors to appear washed out or even to take on a completely different hue.

Color Temperature

Is measured in Kelvin and indicates whether a lamp has a warm, midrange or cool color appearance. Warm light sources, such as incandescent lamps, have a low color temperature (2000-3000K) and feature more light in the red/orange/yellow range. Cool light sources, such as some HID or fluorescent lamps, have a high color temperature (>5000K) and feature more light in the blue range.


A measure of the rate of flow of electricity expressed in amperes.


Double Ended

Lamps that have two bases opposite one another. These are often designed for series electrical connections, mechanical mounting and heat dissipation.



The rate at which a lamp is able to convert power (watts) into light (lumens), expressed in lumens per watt (LPW or lm/W).



A tungsten wire positioned inside an incandescent or halogen lamp, that when heated electrically generates radiation in the visible, infrared and ultraviolet ranges.

Fluorescent Lamp

A low pressure mercury vapor discharge light source. The electric discharge generates ultraviolet (UV) energy, which is absorbed by a phosphor and converted to visible light.

Footcandle (fc)

A unit of illuminance equal to 1 lumen per square foot.


Halogen Lamps

High-pressure tungsten filament lamps containing halogen gasses. The halogen gasses allow the filaments to operate at higher efficacies than incandescent lamps. Halogen lamps also provide brighter, whiter light with better color characteristics, longer life and improved energy efficiency.

High Intensity Discharge (HID)

Lamps in which an arc passing between two electrodes in a pressurized tube causes various metallic additives to vaporize and release large amounts of light. All HID lamps offer good energy efficiency and life.



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


Lamps that generate light by the application of electrical current to a tungsten filament.

Instant Start (IS)

Instant start ballasts apply high voltage across a lamp with no preheating of the cathode. This is the most energy-efficient starting method for fluorescent lamps. IS Ballasts generally use 1.5 to 2 watts less per lamp than rapid start ballasts. Other IS ballast benefits typically include parallel lamp circuitry, longer remote wiring distance, easier installation and capability to start lamps at colder temperatures than rapid start ballasts.


Light Emitting Diode (LED)

Semiconductor devices that produce visible light when an electrical currency
passes through them.

Light Center Length (LCL)

The distance from a specified reference point on a lamp base to its light center.

Lumen Depreciation

The decrease in lumen output of a light source over time. Every lamp type has a unique lumen depreciation curve depicting the pattern of decreasing light output.

Lumens (lm)

A unit of luminous flux. This measurement expresses the overall light output or quantity of light produced.


The complete lighting unit (“fixture”), including lamp, reflector, ballast, socket, wiring, diffuser and housing.

Luminaire Efficiency

The ratio of luminous flux emitted by a luminaire to that emitted by the lamp or lamps used within.


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


Maximum Overall length (MOL)

The overall length of a lamp, from the top of the bulb to the bottom of its base.

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 their rated lives. Most high-pressure sodium and mercury lamps are measured at 50% of their rated lives.


Nominal Watts

Wattage of a particular lamp.


Operating position

Some lamps are specified or designed to be operated in certain positions, such as horizontal or base-up.


Pressed Aluminized Reflector Lamp (PAR Lamp)

A lamp with the outer bulb formed from two pressed glass parts that are fused or sealed together. PAR lamps may be incandescent, halogen or HID types.


The rate at which energy is taken from an electrical system or dissipated by a load expressed in watts.

Power Factor

A measure of the effectiveness with which an electrical device converts volt amperes to watts. Devices with power factors greater than .90 are considered “high power factor” devices.


Rapid Start

Rapid start ballasts apply a low filament voltage to preheat the cathodes in a lamp. When the cathodes are hot enough, a starting voltage is applied, and the lamp will “strike”. Typically, RS ballast will not be able to start lamps reliably at cold temperatures.


Single Ended

Lamps having a single lamp base or point of electrical connection.



A measure of electrical potential expressed in volts. Voltage is the force that pushes electrical current through a conductor.


Watt (W)

A unit of electrical power equal to 1 joule per second. Lamps are rated in watts to indicate power consumption.

Start typing and press Enter to search