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Learn to Talk Like a Lighting Pro

Key Points
  • The words and phrases associated with lighting can be confusing to non-professionals.
  • These terms provide important insight into lighting technologies and performance measurement.
  • Understanding these terms can help you make more informed purchasing and design decisions.

Most areas of expertise have their own (often obscure) words and phrases, and the lighting industry is no exception. Terminology familiar to lighting professionals can sometimes leave others in the dark. Do you know what a foot-candle is, or the difference between T8 and T5 lamps? These and other terms provide important insight into specific lighting technologies and performance measurement. The following guide may not turn you into a lighting expert, but it will help you make more informed decisions regarding lighting purchases and design.

  • Source:
    Electronic ballast
    —equipment which regulates the starting and operating current for fluorescent and metal halide lamps by delivering high-frequency pulses.
  • Ballast factor—some loss in light output occurs when operating lamps with a ballast. Ballast factor is a percentage figure and measures the ability of a ballast to produce light from a lamp. Ballast factor is determined by dividing the light output of a lamp/ballast combination from the rated output of the lamp. 
  • Coloring rendering index (CRI)—an indication of a lamp's ability to emit individual colors and to distinguish between them. CRI is measured on a scale of 1 to 100. Conventional incandescent lamps have a CRI of 100, while optimal fluorescent lamp CRIs typically range from 80 to 85.
  • Correlated color temperature (CCT)—describes the color appearance of a white light source and is expressed in degrees Kelvin (K). Lamps with a CCT below 3200 K are considered warm or yellowish in appearance (in terms of different shades of white), while those above 4000 K have a cool or bluish appearance. 
  • Economic lamp life—the operating hours during which a lamp provides optimum light output and lighting quality. It is considered a more accurate description than rated lamp life (see below) because it takes into account performance depreciation and the need for replacement. Economic lamp life is generally between 60% and 75% of rated lamp life.
  • Efficacy—the amount of light output of a lamp compared to the power input, efficacy is indicated in lumens per watt (LPW). A lumen (see below) is a measure of the visible light produced by a light source. A 100-watt bulb with a light output of 1,600 lumens has an efficacy of 16 LPW.
  • Footcandle—a measure of the light intensity or illumination of a space. A footcandle is equal to one lumen per square foot of space. Organizations such as the Illuminating Engineering Society of North America (IESNA) provide lighting intensity recommendations for specific tasks and spaces in footcandles. For example, IESNA recommends that office space lighting range from 60 to 75 footcandles.
  • Initial light output—the light output of a lamp (measured in lumens) after 100 hours of operation at 25°C. This is often used as a baseline to measure lumen depreciation over time.
  • Lumen—a measure of the visible light produced by a lamp. Lumens are units of luminous flux, which is the range of frequencies the human eye perceives as light. For example, a candle typically produces 13 lumens of light, while a 100-watt bulb emits 1,600 lumens.
  • Lumen maintenance—the light output of lamps depreciates over time. Lumen maintenance is a percentage figure that measures the change in light output of a light source during its operational life, based on its initial measured light output.
  • Luminaire—also known as a fixture, a luminaire is a lighting unit consisting of lamps, as well as the components which hold the lamps, connect them to a power source, and distribute the light.
  • Lux—a measure of luminance in lumens per square meter. One lux equals 0.093 footcandles.
    Fluorescent tubes
  • Rated lamp life—the expected life of a specific type of lamp according to laboratory testing. The average life is the point at which 50% of lamps tested are still in operation.
  • T12, T8, and T5 lamps—specific types of commonly used tubular fluorescent lamps. The T stands for tubular, while the number refers to the diameter of the lamp in eighths of an inch. For example, a T8 lamp has a diameter of one inch or 8/8 of an inch. Most tubular fluorescent lamps use a bi-pin base that attaches to the fixture.
  • Watt—a unit of measure for electrical power. It describes the energy used by a lighting device while it is in operation, not its actual light output.
For information about specific lamp types and their advantages and disadvantages, see Lighting Choices: Measuring and Comparing Performance. You can access our Lighting Calculator and other resources that can help you save energy and reduce your operating costs by visiting our Tools You Can Use menu.     
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