Here you'll find a list of terms related to the industrial lighting industry.

Color Rendering Index (CRI)

A method for describing the effect of a light source on the color appearance of objects being illuminated, with a CRI of 100 representing the reference condition (and the maximum CRI possible). In general, a lower CRI indicates that some colors may appear unnatural when illuminated by a lamp.

Color Temperature

See correlated color temperature.

Constant Wattage Autotransformer (CWA)

The most common type of ballast used for HID lamps, it maintains a constant power (wattage) supply to the lamp when system input voltage fluctuates.

Correlated Color Temperature (CCT)

A description of the color appearance of a light source in terms of warmth or coolness, as measured on the Kelvin scale (K). As the temperature rises, the color appearance shifts from yellow to blue. Thus, lamps with a low CCT (3000K or less) have a yellow-white color appearance and are described as "warm"; lamps with a high CCT (4000K and higher) have a blue-white color appearance and are described as "cool".

Ballast Factor (BF)

The ratio of light output of a lamp operated by a given ballast to the light output of the same lamp when operated by a reference ballast. Lamps operated by a ballast with BF of 0.90 will provide 90 percent of their rated light output (lumens). BFs between 0.85 and 1.0 are the most common.

Disability Glare

Light that is relatively bright compared to the background, making vision measurably worse. The inability to see clearly as a result of the brightness of headlights from an oncoming car at night is an example.

Discomfort Glare

The type of glare that is uncomfortable and distracting, yet less obvious than disability glare. A bright light source in an individual's peripheral vision is an example.


The ratio of light output (lumens) to input power (watts) expressed in lumens per watt (LPW).

High Pressure Sodium (HPS)

A high-intensity discharge lamp type that uses sodium under high pressure as the primary light-producing element.


The amount of light that reaches a surface. Illuminance is measured in footcandles (lumens/square foot) or lux (lumens/square meter). One footcandle equals 10.76 lux, although for convenience the IESNA uses 10 lux as the equivalent.


A method of starting fluorescent lamps. The voltage, which is applied across the electrodes to strike the electric arc, is up to twice as high as it is with other starting methods. The higher voltage is necessary because the electrodes are not heated prior to starting.

Lamp Life

The median life span of a very large number of lamps. Half of the lamps in a sample are likely to fail before the rated lamp life, and half are likely to survive beyond the rated lamp life.

Lamp Lumen Depreciation (LLD)

The reduction in lamp light output that progressively occurs during lamp life.

Lumen (lm)

A unit of measurement of the rate at which a lamp produces light. A lamp's light output rating expresses the total amount of light emitted in all directions per unit time. Ratings of initial light output provided by manufacturers express the total light output after 100 hours of operation.


The photometric quantity most closely associated with the perception of brightness, measured in units of luminous intensity (candelas) per unit area (feet squared or meters squared).

Luminaire Efficiency

The ratio of the light emitted by luminaire to the light emitted by the lamp or lamps within it. Components of a luminaire, such as reflectors and diffusers, absorb some of the light from the lamps(s). A highly efficient luminaire emits most of the light that the lamp(s) emits.

Mercury Vapor (MV)

A high-intensity discharge lamp type that uses mercury and several halide additives as light-producing elements.

Metal Halide (MH)

A high-intensity discharge lamp type that uses mercury and several halide additives as light-producing elements.

Open-Circuit Voltage

The voltage applied across the output terminals of a ballast when no load is connected. This is the voltage applied across a lamp circuit to start a lamp. After starting, the voltage rapidly decreases and stabilizes at the operating voltage.


A method of starting fluorescent lamps in which the electrodes are heated before a switch opens to allow a starting voltage to be applied across the lamp. With preheat starting, the lamp flashes on and off for a few seconds before staying lit.

Power Factor

A measure of how effectively a ballast converts current and voltage into usable power to operate the lamps. A power factor of 0.9 or greater indicates a high power-factor ballast.

Prismatic Lens

An optical component of a luminaire that is used to distribute the emitted light. It is usually a sheet of plastic with a pattern of pyramid-shaped refracting prisms on one side. Most ceiling mounted luminaries in commercial buildings use prismatic lenses.


A method of starting fluorescent lamps in which the ballast supplies voltage to heat the lamp electrodes for one to two seconds prior to starting and, in most cases, during lamp operation. A rapid-start system starts smoothly, without flashing.

Reflected Glare

Often called veiling reflections, glare that results from light shining off polished or glossy surfaces.

Restrike Time

The time required for a lamp to restrike, or start, after the lamp is extinguished. Normally, HID lamps need to cool before they can be restarted.

Visual Comfort Probability (VCP)

A system for estimating discomfort glare that predicts the percentage of people who are likely to find the lighting comfortable, against the percentage that find it uncomfortable.


A light-emitting diode (LED) is a semiconductor device that emits visible light when an electric current passes through it.

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