Common Terms

Average Rated 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.

Ballast factor

A measure of the actual lumen output for a specific lamp-ballast system relative to the rated lumen output measured with a reference ballast under ANSI test conditions (open air at 25 °C [77 °F]). An ANSI ballast for standard 40-watt F40T12 lamps requires a ballast factor of 0.95; the same ballast has a ballast factor of 0.87 for 34-watt energy saving F40T12 lamps. However, many ballasts are available with either high (conforming to the ANSI specifications) or low ballast factors (70 to 75%). It is important to note that the ballast factor value is not simply a characteristic of the ballast, but of the lamp-ballast system. Ballasts that can operate more than one type of lamp (e.g., the 40-watt F40 ballast can operate either 40-watt F40T12, 34-watt F40T12, or 40-watt F40T10 lamps) will generally have a different ballast factor for each combination (e.g., 95%,95%, respectively).

Candela (CD)

Describes the intensity of lighting in accordance with the internationally accepted standards (SI). This term is used to evaluate the intensity of other light sources based on a regular candle of a standard size.

Ceramic Metal Halide

Lamps are a relatively new source of light that is a variation of the mercury-vapor lamp. There is a ceramic tube inside the lamp that heats a mercury-argon mixture creating a bluish light that is close to daylight with a CRI (Color Rendering Index) of 96. They are five times brighter than comparable tungsten incandescent light bulbs.

Color Rendering Index (CRI)

A quantitative measure of the ability of a light source to reproduce the colors of various objects faithfully in comparison with an ideal or natural light source. Measured on a scale of 1-100. Light sources with a high CRI are desirable because they are closer to natural light. Most T8 fluorescents have a CRI of 85. Some HID lamps have a CRI as low as 25.

Daylight Sensor

A device which senses the amount of daylight in a room and controls the luminaire accordingly.

Demand Response (DR)

Manages customer consumption of electricity in response to supply conditions, for example, having electricity customers reduce their consumption at critical times or in response to market prices. Demand response can involve actually curtailing power used or by starting on site generation, which may or may not be connected in parallel with the grid. Current demand response schemes are implemented with large commercial customers, often through the use of dedicated control systems to shed loads in response to a request by a utility or market price conditions. Services (lights, machines, air conditioning) are reduced according to a preplanned load prioritization scheme during the critical timeframes.


Light which is cast downward from a fixture. The most common and direct form of lighting.


A measure expressed in lumens per watt representing the efficiency of a lamp/ballast system or luminaire.

Energy Service Company (ESCO)

A company dedicated to helping commercial and industrial clients reduce their energy consumption.

Foot Candle

The measurement of light reaching a specified location and the amount of light reflecting off a surface. Abbreviation (FC).


In the construction of some types of industrial facilities, a skeletal framework is used, which forms an interior subspace called a “bay,” which in turn marks the space as “high bay” or “low bay.” An older definition designated high-bay to mean >25 ft. off the floor, medium-bay to mean 15-25 ft., and low-bay to mean 25 ft.


High Intensity Discharge; It is a type of light source.


High Pressure Sodium (HPS) lamps are smaller and contain additional elements such as mercury, and produce a dark pink glow when first struck, and a pinkish orange light when warmed. Understanding the change in human color vision sensitivity is essential for proper planning when designing lighting with High Pressure Sodium.


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 1lux.

Initial Lumens

The lumens produced by a lamp after an initial burn in period (usually 100 hours).

Kelvin Temp

The measure of the color temperature of light sources. Higher Kelvin (5,000 – 6500) produces light closer to the blue part of the spectrum – light around 2,000K produces light in the yellow or orange part of the spectrum.


AKA Light bulb – the source from where light is produced within a luminaire.


Light Emitting Diode (LED) is a diode of semiconductor material, that emits light when a forward bias (a voltage used in the direction that produces the larger current) is applied. Each semiconductor comprises two regions of the opposite electrical charges. The electrons start moving as soon as voltage is applied, so certain amounts of energy are released as a result. As the energy disperses the wavelengths become visible and we may observe the light of a certain color.

LED Driver

An electronic devise which converts input power into a constant current source despite fluctuation in voltage. It protects LED from voltage fluctuations. In simple terms an electronic devise which feed input power to LED to produce light.


The measurement of total light emitting from a light source. A 100-watt incandescent lamp being rated at 1690 lumens and having an efficacy of 16.9 lumens per watt.

Lumen Maintenance

Lumen maintenance is simply the amount of light emitted from a source at any given time relative to the light output when the source was first measured. This is usually expressed as a percentage. If you’ve ever changed a light bulb and noticed how bright a new bulb is when compared with the older bulb, you’re seeing the effects of lumen depreciation. This steady decline over time is known as lumen maintenance.


A complete lighting unit, which contains a lamp, housing, ballast, sockets and any other necessary components.

Luminous Efficacy

Also efficacy; the measure of the luminous flux emitted by a source of radiation and the power it consumes, measured in lumens per watt (lm/W).

Lux (lx)

The derived SI unit of illumination equal to a luminous flux of 1 lumen per square meter.

Metal Halide (MH)

The metal halide lamp generates light by passing an electrical current from one electrode to another through a metal vapor, to form an arc between the electrodes. The lamp will last an average of 6,000 hours or 9,000 hours, if used with electronic control gear.

Occupancy Sensor

A device which activates a fixture upon sensing the presence of a person/machinery movement.

Photopic Lumens

A type of light measured in lumens that is generally detected by common light meters and accounts for part of the human eye’s perception of brightness.

Pulse Start Metal Halide (PSMH)

A change in the lamp and ballast construction allows pulse start metal halide lamps to start using a high voltage igniter in the ballast instead of a starting electrode (probe) in the lamp.


Refers to the addition of new technology or features to older systems. An example of this is upgrading T12 bulbs to T5, where older fixtures are fitted with new technologies.


Relative Light Output. The ratio of light output between a fixture’s potential light output at optimum ambient temperatures and actual light output at actual ambient temperatures. For example, if a fixture at its optimal temperature of 75°F produces 10,000 Lumens and 8,000 Lumens 50°F, that fixture’s RLO at 50°F is 8,000 Lumens ÷ 10,000 Lumens, or 80%.


A highly polished or mirrored surface.


A fluorescent lamp that is 5/8” in diameter. “T” stands for tubular, while the number “5” stands for the 5 in 5/8”.


A fluorescent lamp that is 1″ in diameter. “T” stands for tubular, while the number “8” stands for the 8 in 8/8”.


A fluorescent lamp that is 1 1/2″ in diameter.

Thermal Management

The process of adjusting the operating temperature of a luminaire through its design by using material that allows the fixture to ‘breathe’ and adding vents to improve airflow.


Fixtures have typically been designed to accommodate standard fluorescent lamps (T12, T8, or T5), but are now often designed with integral LED sources.


A source of light which is cast upwards to illuminate a ceiling cavity for aesthetic reasons. When combined with reflective ceiling materials, uplighting can function as a source of indirect lighting.