This page provides a glossary of commonly used LED and lighting terminology, with a focus on terms relevant to LED tube lights.

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A technique that emphasizes a particular object or draws attention to a particular area. Accent lighting usually utilizes the tight beam control of PAR lamps or MR lamps. Also called highlighting.

Lighting that is designed to provide a substantially uniform light level throughout an area, exclusive of any provision for special local requirements.

A measure of electrical current.


Often referred to as the Beam Angle or even Beam Spread Angle, this is a measure of the width of a light beam, expressed in degrees. The beam of light from a reflector-type lamp (e.g., PAR, R, ER, or MR) can be thought of as a cone. The beam spread is the angular width of the cone. Common beam spreads are known as spot, narrow, narrow flood, and flood. LED bulbs typically produce highly directional spreads, so it is important to look for information about the beam spread of a bulb that you are considering to make sure that it meets your requirement. Even LED lights designed to replace non-reflector-type lights, such as tube lights and A-line lights, do not have the same omnidirectional spread of the the traditional equivalents.

A bipin or bi-pin, (sometimes referred to as 2-pin, bipin cap or bipin socket), is a standard from the IEC for lamp fittings. These are used on many small incandescent light bulbs (especially halogen lamps), and on most fluorescent lights as well, where the filaments are involved in starting the tube when it is turned on. LED bulbs designed as retrofits for these incandescent and fluorescent fixtures also incorporate bipin fittings.

When searching for compatible bulbs with bipin plugs, you will want to know the bipin code which begins with a G. The suffix after the G indicates the pin size. GU usually also indicates that each pin has two diameters, being larger near the ends, in order to twist-lock into position in the socket. (The transition between the two diameters is a right-angle step rather than a taper.) So, for example, an MR11 shape bulb might have a G4 bipin plug, meaning that the distance from center to center of the pins is 4mm. A T8 or T12 tube light typically has G13 bipin plugs.

Some lamps have pins placed closer together, preventing them from being interchanged with bulbs that are too high in wattage, which may cause excessive heat and possibly fire. These are sometimes called “mini-bipin”. Some of these, particularly in automotive lighting, have the pins bent back onto the sides of the base of the bulb, this is instead called a wedge base or socket.

BR standards for “bulged reflector.” BR lamps contain a small bulge near the bottom of the lamp neck. BR lamps are a type of flood that can be substituted for other R lamps. The BR shaped reflector focuses somewhat more light into the main beam, directing a little more light out of recessed down-lighting fixtures. BR lamps do not incorporate halogen technology, and thus operate at lower efficacies.

System created by IESNA based on TM-15-07 to rate the amount of light emitted from a luminaire in unwanted directions. The methodology represents a comprehensive system that limits lamp lumens to values appropriate for the lighting zone. The BUG rating system replaces the older IES cutoff classification system.


An acronym for the Commercially Available LED Product Evaluation and Reporting Program of the US Department of Energy (DOE). The program was established in 2006 to investigate the performance of luminaires and replacement lamps that use LEDs. To help users better compare LED products with conventional lighting technologies, CALiPER also has performed benchmark research and testing of traditional (i.e., non-LED) lamps and fixtures. Detailed information about the CALiPER Program and CALiPER reports may be found here.

Short for canister light. Another name for a recessed light or downlight, which is a lighting fixture that is installed into a hollow opening in a ceiling. When installed it appears to have light shining from a hole in the ceiling, concentrating the light in a downward direction as a broad floodlight or narrow spotlight.

There are two parts to recessed lights, the trim and housing. The trim is the visible portion of the light. It is the insert that is seen when looking up into the fixture, and also includes the thin lining around the edge of the light. The housing is the fixture itself that is installed inside the ceiling and contains the lamp holder.

Candela or candlepower is a measure of luminous intensity or power emitted by a light source in a particular direction. A common candle emits light with a luminous intensity of roughly one candela. So, it is a measure of how much light the bulb produces, measured at the bulb, rather than how much falls upon the thing you want to light up. A candela is the metric equivalent of the light output of that one candle. Candlepower is a rating of light output at the source, using English measurements. Foot-candles are a measurement of light at an illuminated object. Lumens are a metric equivalent to foot-candles in that they are measured at an object you want to illuminate.

A quantitative measure of the ability of a light source to reproduce the colors of illuminated objects accurately when compared to a reference light source, such as an incandescent lamp or daylight. A CRI of 100 is identical to the reference source, so the higher the CRI the better. You may recall the dull colors and washed-out faces resulting from old-style fluorescent tube lighting, which had a CRI ranging from 50 to 60. Phosphor-converted warm white LEDs are now being produced that are claimed to have a CRI of 80, a value most people find quite acceptable. Others exceeding 90 have also been reported. The industry is discovering that CRI is not the best metric for comparing LED light sources, especially at Mesopic (low light) levels. Originally developed in 1964, this index is based on outdated color models and assumes illumination sources with broad spectral distributions, whereas LEDs are narrow-band sources. Because of these issues and general concern about the accuracy of the CRI, a new measurement system is under development.

The measurement of color expressed in Kelvin (K). The reason this measurement is called a “temperature” is because it was derived from a theoretical object called a “black body radiator.” When the radiator is heated, it changes from black to red to yellow to white to blue. The lower the Kelvin rating, the “warmer” or more yellow the light. The higher the rating, the “cooler” or more blue the light.

Commonly referred to as CFLs, these are small fluorescent lamps with integrated ballasts that are increasingly being used as a more energy-efficient alternative to incandescent lighting. They are typically used in a standard medium-base (E26/E27) socket. The lamp life is about 1O times longer than incandescent lamps and is 3-4 times more efficacious. Also sometimes referred to as PLI DL or BIAX lamps.

The temperature of the surface of an ideal black body radiator measured in Kelvin (K), which ranges from warm (i.e., red to yellow, generally 3000K and below) to cool (i.e., blue, generally 5000K and above) tones.


A two-terminal device commonly used as a one-way switch allowing the flow of electric current in one direction (called the forward-biased condition), and blocking the flow of the current in the opposite direction (the reverse-biased condition).

Occurring in only a single direction. This quality of LEDs provides for high levels of efficiency since light is easily directed onto desired surfaces. Compare with omnidirectional.

A lamp, often a light bulb set in a metal cylinder, mounted on or recessed into the ceiling so that a beam of light is directed downward. See also can light.

An LED power supply that provides either a constant levels of current, or a constant level of voltage. Driver efficiency is a measure of luminaire efficiency that focuses on the performance of driver, or power supply, components. Specifically, driver efficiency is the ratio of the power delivered from the driver divided by the power required to operate the driver.


Edison screw fitting system designations for standard screw-in bulb bases in the US and Europe. Also referred to as “medium” or “standard” bases. The standard size for general-purpose lamps in the US is E26 in the US with 120 volt AC domestic power, while it is E27 in European countries with 220 to 240 volts AC domestic power. E27 and E26 base types are generally interchangeable.

This is a fitting is a system of light bulb connectors, developed by Thomas Edison and licensed starting in 1909 under the Mazda trademark. Most have a right-hand threading, so that it goes in when turned clockwise and comes out when turned counterclockwise, like a hardware screw. The designation Exx refers to the diameter in millimeters, even in the U.S., where the bulb glass is listed in eighths of an inch. (For example, E12 has a diameter of 12 mm.) There are four common sizes of screw-in sockets used for line-voltage lamps:* Candelabra: E12 North America, E11 in Europe

* Intermediate: E17 North America, E14 (SmallES) in Europe

* Medium or standard: E26 (MES) in North America, E27 (ES) in Europe

* Mogul: E39 North America, E40 (GoliathES) in Europe.

ER lamps (ellipsoidal reflector) have an ellipsoid reflector that is designed to move the focus of the lamp down several inches in order to minimize trapping of light in recessed fixtures. An ER lamp can be a good replacement choice for a reflector floodlamp in a deeply recessed ceiling downlight fixture, where 50 percent r more of the light may be trapped inside the fixture. The lower-powered ER lamp is designed to project more light out of the fixture than the floodlamp.


A lamp that provides a broad beam intended to light a general area. See Beam Spread.

A fluorescent lamp or fluorescent tube is a gas-discharge lamp that uses electricity to excite mercury vapor. The excited mercury atoms produce short-wave ultraviolet light that then causes a phosphor to fluoresce, producing visible light. Unlike incandescent lamps, fluorescent lamps always require a ballast to regulate the flow of power through the lamp.

A measure of illuminance or light intensity. A foot-candle reflects the amount of illumination the inside surface of a 1-foot radius sphere receives from a point source of one candela in the center of the sphere. A foot-candle is also equal to one lumen per square foot.

An achievement by a device or system that provides useful service for as long as the device or system manufacturer projects. For example, white LEDs are commonly marketed with 50,000 hour fully-rated lives, where the end of life is defined by the point at which the LED fails to deliver at least 70% of initial lumen output.


A code used to designate the size of the base on bipin plugs with straight pins. The suffix after the G indicates the pin size. So, for example, G4 might be the bipin type for a small MR11-type bulb, indicating that the distance from center to center of the two pins is 4 mm. T8 and T12 tube lights with bipin plugs ill have G13 plugs (although they actually measure only 12.7 mm center to center.

A type of semiconductor material used in manufacturing blue LEDs and other electronic devices. GaN semiconductors are the key to the LED lighting revolution.

Consists primarily of light emitted at high angles emitted from the front and back of the luminaire which may interfere with ground-level pedestrians and motorists. Glare is a component of the new TM-15-07 based BUG rating system, which replaces the older cutoff classification system.

GU is a bipin base code that usually indicates that each pin has two diameters, being larger near the ends, in order to twist-lock into position in the socket. So, for example, a typical GU5.3 bulb (perhaps an MR16 bulb type) might have bipin plugs that measure 5.33 mm center to center but taper from 1.45 mm at the base to 1.6 mm at the ends.


The intentional transition of thermal energy from a hotter object (such as a sensitive electronic device) to a cooler object. Heat dissipation is achieved with LEDs primarily by mounting them on heat sinks made on high-quality aluminum and other alloys.

Also sometimes spelled “heatsink.” An object that absorbs and dissipates heat from another object using thermal contact (either direct or radiant). Every light fixture is a heater, the brighter the light the more heat it produces, and this includes LEDs. Conventional light sockets are just bad sinks, period. They are designed for bulbs that release heat at the armature, not the base. One of the things that accounts for the unique appearance of many LED bulbs is that they have large integrated heat sinks. The quality of the heat sink and heat sink design is an important factor in ensuring the performance and life of an LED bulb.

A type of electrical lamp that produces light by striking an electric arc between tungsten electrodes, which is filled with certain gases and other compounds. HID lamps are a traditional light source that offers relatively high efficiency and long life, but sometimes with undesirable color effects. See correlated color temperature (CCT) and color rendering index (CRI).

LEDs designed to operate at several hundred milliamps and a watt or more of power. Since these devices create substantial heat, which would destroy the unit if special steps were not taken, specialized heat dissipation technologies (see Heat Sink) must be employed.

A popular type of high intensity discharge (HID) lamp offering a relatively warm correlated color temperature (CCT), high levels of efficiency, and long life, but only modest levels of color rendering index (CRI).

See Accent Lighting.


A non-profit organization that “seeks to improve the lighted environment by bringing together those with lighting knowledge and by translating that knowledge into actions that benefit the public.” (Source: IESNA web site) The IESNA periodically published a Lighting Handbook and other prescriptive publications that drive many lighting standards.

Term used to describe a light that uses incandescence, the emission of light (visible electromagnetic radiation) from a hot body due to its temperature. Incandescence occurs in incandescent light bulbs because the filament resists the flow of electrons, heating the filament to a temperature where part of the radiation falls in the visible spectrum.



A thermodynamic (absolute) temperature scale, based on a measure of absolute zero – “the absence of all thermal energy” at zero K. The Kelvin is described without reference to degrees and is written without a degree symbol.


An abbreviation for 70% of initial lumens levels delivered from an LED, which is a common definition of the useful life of an LED.

General term for a number of types of electric lighting components including incandescent, fluorescent and high intensity discharge (HID) lamps, which are commonly called light bulbs. Although LEDs are sometimes called lamps, the term more frequently refers to traditional lamps.

A predetermined pattern of a number of LEDs mounted on a printed circuit board (PCB) or other surface, which is capable of producing light when powered. LED light bulbs use an array of LEDs to produce light, and for this reason, when you look into a clear LED light bulb, you can see the individual LEDs mounted on the PCB. The number of LEDs is one indicator of the relative brightness of the bulb, and the number of LEDs incorporated in a particular bulb design is often used in product descriptions to describe and market the bulb.

A measure of the light output of an LED device, generally measured in lumens, divided by the power, generally measured in watts, required to operated the device. This relationship, lumens per watt, is a key measure of LED performance.
The Leadership in Energy and Environmental Design (LEED) Green Building Rating System is the nationally accepted benchmark for the design, construction, and operation of high performance green buildings.

An electronic light source derived from a semiconductor diode possessing unidirectional electric current properties – in other words, diodes are one-way switches. When a voltage is applied in the forward direction, electrons from one side of the p/n junction recombine with holes on the other side of the junction, and energy is released in the form of light.

Popular name for an LED measurement standard, the full title of which is IESNA LM-79-08, Approved Method for the Electrical and Photometric Measurements of Solid-State Lighting Products. This standard provides guidelines to photometric labs for the proper and repeatable measurement of LED lighting systems.

Popular name for an LED measurement standard, the full title of which is LM-80-08, Approved Method for Measuring Lumen Maintenance of LED Light Sources. This standard provides guidelines to LED manufacturers for measuring lumen maintenance, or the level of light output maintained over predetermined time periods.

A popular type of high intensity discharge (HID) lamp offering an extremely warm correlated color temperature (CCT), very high levels of efficiency, and long life, but very poor levels of color rendering index (CRI).

The lumen is the International Unit of luminous flux. It is a measure of the light taken at the object that it is targeted at as opposed to at the light source. It is defined in terms of candela steradians (cd multiplied by sr). One lumen is the amount of light emitted in a solid angle of 1 sr, from a source that radiates to an equal extent in all directions, and whose intensity is 1 cd. The luminous efficacy of a light source in lumens per watt is a key measure of LED performance. Lux is an abbreviation for lumens per square meter. See also Luminous Efficacy and Lumens Per Watt.

A measure of the declining level of lumen output produced by a lamp or luminaire over time.

A measure of the level of lumen output produced by a lamp or luminaire over time compared to the initial level of lumen output.

The amount of light produced by a luminaire at a given time. Compare to Lumen Maintenance.
The ratio of light produced by an LED device, measured in lumens, divided by the power, measured in watts, required to operated the device. LPW is a key measure of LED performance. See also Luminous Efficacy.

A lighting fixture complete with lamp, optical components used to direct light, housing, and power supply (e.g., a fluorescent ballast or LED driver).

A figure of merit for light sources. It is the ratio of luminous flux (in lumens) to power (usually measured in watts). The luminous efficacy of a source (LES) describes how well the source provides visible light from a given amount of electricity. See also Lumens Per Watt.
In photometry, luminous flux or luminous power is the measure of the perceived power of light. It is different from radiant flux, the measure of the total power of light emitted, because luminous flux is adjusted to reflect the varying sensitivity of the human eye to different wavelengths of light. The International System of Units (SI) unit of luminous flux is the lumen (lm). One lumen is defined as the luminous flux of light produced by a light source that emits one candela of luminous intensity over a solid angle of one steradian. In other systems of units, luminous flux may have units of power. See also Lumen and Candela.

Symbolized lx, it is a metric measure in the International System of Units (SI) of illuminance, or the apparent intensity of light hitting or passing through a surface. It is defined in terms of lumens per meter squared. Reduced to SI base units, one lux is equal to 0.00146 kilogram per second cubed.


MCD stand for “millicandela” or 1/1000 of a candela. This a unit of light energy of a single color, and in a single direction. For example: if you stand a few feet from a 100W light bulb, your eyes will get about 120 candelas of brightness reaching them, that’s 120,000 MCD.

Historically popular type of high intensity discharge (HID) lamp offering a relatively warm correlated color temperature (CCT), high levels of efficiency, and long life, but only modest levels of color rendering index (CRI). Due to environmental concerns, most mercury vapor luminaires are no longer available, but mercury vapor lamps remain available for existing luminaires.

A popular type of high intensity discharge (HID) lamp offering a relatively cool correlated color temperature (CCT) and long life, but only modest levels of efficiency and color rendering index (CRI).

MR refers to multifaceted reflector, indicating that this reflector used in this lamp is usually shaped with multiple small facets. This multifaceted reflector gives a soft edge to the area illuminated by the lamp. MR lamps are regularly used in place of compact fluorescent lamps or standard incandescent light bulbs for applications including residential lighting and retail lighting. See also MR16.

Sometimes referred to as MR-16. This is a standard format for halogen reflector lamps made by a variety of manufactures. MR16-compatible LED lamps are also available. MR16 lamps are regularly used in place of compact fluorescent lamps or standard incandescent light bulbs for applications including residential lighting and retail lighting. “MR” refers to multifaceted reflector, indicating that this reflector is usually shaped with multiple small facets. This multifaceted reflector gives a soft edge to the area illuminated by the lamp. MR16 lamps are also available with smooth reflectors, resulting in a sharper fall-off to the illuminated area. “16″ refers to the maximum diameter of the lamp in eighths of an inch, 16/8″ or 2 inches (5 cm).



Occurring in many or all directions, such as traditional HID and other lamps. This quality of traditional lamps limits their efficiency due to the challenges inherent in directing the light produced onto desired surfaces. See Directional and Beam Spread.

A measure of luminaire efficiency that focuses on the performance of optical components. Specifically, optical efficiency is the ratio of lighting delivered by a luminaire to the light produced by the lamps.


PAR is an acronym for “parabolic aluminized reflector.” A PAR lamp is typically an incandescent or tungsten-halogen incandescent lamp with a hard glass bulb and an interior reflecting surface, a precisely placed filament, and a lens to control beam spread. The lens is hermetically sealed to the reflector. PAR lamps rely on both the internal reflector and prisms in the lens for the control of the light beam. Metal halide and LED PAR-lamps are also now available.PAR flood light bulbs are available in diameters from 1.75″ to 4.75″. As a general rule of thumb, if the glass covering the PAR lamp is dimpled then the bulb has a wider (flood) beam spread. PAR lamps with a narrow beam (spots) have smooth glass covering the reflector.

Soft glass floods look identical to standard floods (generally called either “R shape” or “BR shape”. These light bulbs should be used either indoors or in fixtures protected from direct weather exposure. Whether the bulb is a PAR, R, or BR the diameter of the bulb may be found in the item number directly following the PAR, R, or BR designation. These bulbs are measure in 1/8″ increments. For example a PAR30 light bulb is 30/8″ diameter, or 3.75″ diameter. The most common LED PAR lights on the market today are PAR38 lights, meaning that they are 4.75″ in diameter.

A substance that exhibits phosphorescence, which is the process of glowing occurring after exposure to energized particles. Phosphors coat the inside of fluorescent bulbs. Many white LEDs are produced by combining GaN or InGaN LEDs, which produce blue light, with YAG (yttrium aluminum garnet) phosphors.

A generic term for any device that supplies electrical or other types of energy to a load. In lighting, common power supplies include fluorescent and HID ballasts, various types of transformers, and LED drivers. The latter are available as devices providing either constant current or constant voltage.

A material used to mechanically support and electrically connect electronic components using conductive pathways, or traces, etched from copper sheets laminated onto a non-conductive substrate. Common types include FR-4 (Flame Retardant 4) and metal-core PCBs. The latter are commonly used with LEDs to aid in thermal management. In an LED bulb, LED arrays are mounted on a PCB.



A common reflector lamp. Typically, an incandescent filament or electric discharge lamp in which the sides of the outer blown-glass bulb are coated with a reflecting material so as to direct the light. The light-transmitting region may be clear, frosted, or patterned.


Electronic components and systems based on the use of semiconductors rather than vacuum tubes. Common types of solid state components include integrated circuits, liquid-crystal displays, and LEDs. For this reason, LED lighting is often referred to as solid state lighting or SSL.

A measure of luminaire efficiency that focuses on the performance of lamps. The source efficiency of LEDs is currently about equal with many fluorescent and HID sources, but the directional nature of light produced from LEDs delivers substantially higher fixture (or system) efficiency than traditional sources.


See Tube Light.

See Tube Light.

A measure of luminaire efficiency that focuses on the ability of the luminaire system to transfer heat from sensitive components, such as LEDs, to the outside environment. High levels of thermal efficiency are made possibly through the use of high-quality components (such as LEDs with low thermal resistance) and materials (such as aluminum alloys with low thermal resistance), as well as intelligent product designs.

Rating system developed by IESNA which replaces the older IES cutoff classification system for controlling light emitted in unwanted regions surrounding a luminaire.

A lighting system with an electrically fed linear track that accepts one or more track heads. The track heads can be easily relocated along the track.

Light sources commonly developed and used before or during the twentieth century, including incandescent, fluorescent, HID and other sources. Typically used to distinguish a lighting source from LED lighting, which we would consider an alternative to traditional lamps.

A recessed luminaire that is installed in the plenum with the opening flush with the ceiling. Typically rectangular or square in shape, as in a 2-foot by 4-foot luminaire. Fluorescent and LED tube lights are often installed in troffers.

A term often used to describe a fluorescent lamp or fluorescent tube, which in traditional lighting is a gas-discharge lamp that uses electricity to excite mercury vapor. Many vendors are now introducing LED tube lights, many of them designed as retrofits for fluorescent tubes. Tube lights are typically identified by a code such as F##T##, where F is for fluorescent, the first number indicates the power in watts (or where lamps can be operated at different power levels, the length in inches), the T indicates that the shape of the bulb is tubular, and the last number is the diameter in eighths of an inch (sometimes in millimeters, rounded to the nearest millimeter). Typical diameters are T12 or T38 (11/2″ or 38.1 mm) for residential bulbs with old magnetic ballasts, T8 or T26 (1″ or 25.4 mm) for commercial energy-saving lamps with electronic ballasts, and T5 or T16 (5/8″ or 15.875 mm) for very small lamps, which may operate from a battery-powered device. When purchasing an LED tube light for retrofit, the key descriptive items to look for are the length and the diameter. For bipin tube lights (there are single pin types, but they are rare), T4 and T5 tubes use the same G5 bipin socket types and T8 and T12 tubes use the same G13 socket types.


Light emitted from the luminaire directly into the sky which causes artificial sky glow and generally represents wasted energy. Uplight is a component of the new TM-15-07-based BUG rating system, which replaces the older cutoff classification system.

The length of time an asset or device can be used without major refurbishing. In lighting, the useful life of lamps is typically stated at 70% of initial lumens since it is difficult to perceive this reduction in light output.


Electrical potential difference, which is commonly expressed as (VA minus VB). Voltage is conceptualized as the electrical driving force that drives a conventional electric current in the direction A to B. LED voltage is determined by the physical structure of the semiconductor material, and the level of light produced by the LED is determined largely by the level of current flowing to the LEDs.


A lighting technique that lights a wall or any vertical surface fairly evenly from top to bottom without spilling or wasting light away from the wall into the room. A luminaire that illuminates a vertical surface is called a “wall washer” or “wallwasher.”

A measure of power, or the rate of energy conversion. The watts consumed by an LED are typically derived from a relatively fixed voltage level and varying levels of current.




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Standard LED Lighting 48 Inch T8 Tube Light Replacement | LED Tube Lights
February 26, 2010 at 1:52 pm
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