LED GLOSSARY

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What’s up with all of the LED terminologies?

With the emergence of LED lighting, you may have experienced several new, unfamiliar lighting terms – not to mention the too-good-to-be-true claims. Making informed decisions about whether LEDs are the correct choice for your facility or application has become increasingly more difficult and costly if the wrong decision is made. We hope the following glossary of LED terminology helps to demystify this technology.

Jump to any A – Z Glossary Term

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W

A

Amp
A measure of electrical current.

B

Backlight
Light emitted in the direction of the luminaire OPPOSITE from the area intended to be lighted which may create unwanted trespass, usually towards the house side of a street light pole. Backlight is a component of the new TM-15-07 based BUG rating system, which replaces the older cutoff classification system.

BUG
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.

C

Candela
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.

Chromaticity
A measure of color quality as reflected in the CIE (International Commission on Illumination) 1931 color chart, which provides x/y coordinates of reds, greens and blues (RGB). The color of white LEDs, as measured in Kelvin (K), is reflected in a narrow strip at the center of the CIE chart where RGB combinations create various shades of white. See correlated color temperature (CCT).

Cones
Photoreceptor cells in the retina of the eye that function best in relatively bright light. See rods.

Correlated Color Temperature (CCT) 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.

Color Rendering Index (CRI)
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.

Cutoff Luminaire One of the IESNA luminaire classifications that prescribe light distribution metrics intended to avoid light pollution. A cutoff luminaire is one in which the luminous intensity (in candelas) at or above an angle of 90° above nadir does not numerically exceed 2.5% of the luminous flux (in lumens) of the lamp or lamps in the luminaire, and the luminous intensity (in candelas) at or above a vertical angle of 80° above nadir does not numerically exceed 10% of the luminous flux (in lumens) of the lamp or lamps in the luminaire. See full-cutoff luminaire.

D

Die
A small block of semiconductor material, which is cut from a larger silicon wafer.

Diode
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).

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

Driver
An LED power supply that provides either a constant levels of current, or a constant level of voltage.

Driver Efficiency
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.

E

Efficiency
A general term for the amount of useful work per quantity of energy. See driver efficiency, fixture efficiency, LED efficiency, optical efficiency, and source efficiency.

F

Fixture (Or System) Efficiency
A measure of luminaire efficiency that focuses on the performance of the combined luminaire, including source efficiency, driver efficiency, and optical efficiency. Fixture efficiency reflects the amount of useful light delivered by the luminaire divided by the power required to operate it.

Fitted Target Efficiency (FTE)
A system created under the draft Energy Star program for outdoor SSL luminaires used to measure how efficiently a luminaire illuminates its targeted area. The FTE calculator uses a selected LM-79-08 absolute photometry IES file for a desired luminaire, and projects the luminaire output onto a grid along the area to be illuminated, such as a street or parking garage floor.

Foot-Candle
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.

Full-Cutoff Luminaire
One of the IESNA luminaire classifications that prescribe light distribution metrics intended to avoid light pollution. A full-cutoff luminaire is one in which the luminous intensity (in candelas) at or above an angle of 90° above nadir is zero, and the luminous intensity (in candelas) at or above a vertical angle of 80° above nadir does not numerically exceed 10% of the luminous flux (in lumens) of the lamp or lamps in the luminaire. See cutoff luminaire.

Fully-Rated Life
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.

G

Gallium Nitride (GaN)
A type of semiconductor material used in manufacturing blue LEDs and other electronic devices.

Glare
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.

H

Heat Dissipation
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.

Heat Sink
An object that absorbs and dissipates heat from another object using thermal contact (either direct or radiant).

High Intensity Discharge (HID)
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).

High Power LEDs
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 must be employed.

High Pressure Sodium (HPS)
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).

I

Illuminating Engineering Society of North America (IESNA)
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: www.iesna.org) The IESNA periodically published a Lighting Handbook and other prescriptive publications that drive many lighting standards.

Indium Gallium Nitride (InGaN)
A type of semiconductor material used in manufacturing blue LEDs and other electronic devices.

K

Kelvin (K)
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.

L

L70
An abbreviation for 70% of initial lumens levels delivered from an LED, which is a common definition of the useful life of an LED.

Lamp Sources
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 totraditional lamps.

LED
Acronym for light emitting diode.

LED Array
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 Efficiency
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.

LED Junction Temperature (TJ)
The temperature of the p/n junction inside the LED. Although TJ cannot be directly measured, there are a number of widely-used, reliable methods of accurately estimating TJ .

Light Emitting Diode (LED)
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.

LM-79
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.

LM-80
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.

Low Pressure Sodium (LPS)
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).

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

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

Lumen Output
The amount of light produced by a luminaire at a given time.

Lumens Per Watt (LPW)
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.

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

Lux
A metric measure of illuminance, or the apparent intensity of light hitting or passing through a surface.

M

Mercury Vapor
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.

Mesopic Vision
A combination of photopic vision and scotopic vision in environments with low, but not completely dark, light levels.

Metal Halide (MH)
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).

Milliamp (mA)
A measure of electrical current, the milliamp is one thousandth of an amp. Common LED drive currents include 350mA, 525mA and 700mA.

N

Nakamura, Shuji
A noted LED researcher whose credits include the development of technologies critical to the mass production of white phosphor LEDs. Specifically, Nakamura perfected technologies enabling the production of high brightness GaN (i.e., gallium nitride) LEDs, which, when combined with yellow phosphors, create white light.

O

Omnidirectional
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.

Optical Efficiency
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.

P

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

Photopic Vision
Vision under well-lit conditions, mediated primarily by cone cells, which allows color perception. Photopic vision is the basis for most commonly-accepted lighting metrics, including those published by the IESNA.

P/N Junction
The border region in a semiconductor device formed by placing P-type (or materials carrying a positive charge) and N-type (or materials carrying a positive charge) semiconductor materials together in close proximity. It is the location in an LED where light is created, as well as the location inside the LED where heat is created.

PoE
Power over Ethernet or PoE technology describes a system to pass electrical power safely, along with data, on Ethernet cabling.

Power Factor
A measure of the ratio of the real power flowing to a load divided by the apparent power. A load with low power factor draws more current than a load with a high power factor (which is generally defined as higher than 0.9) for the same amount of useful power transferred.

Power Supply
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.

Printed Circuit Board (PCB)
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.

R

Rods
Photoreceptor cells in the retina of the eye that can function in low light levels, and which are primarily responsible for night vision.

S

Scotopic Vision
Monochromatic vision in low light levels, which is produced by rod cells.

Solid State
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.

Source Efficiency
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.

System Efficiency
See fixture efficiency.

T

Thermal Efficiency
A measure of luminaire efficiency that focuses on the ability of the luminaire system to transfer heat from sensitive components, such asLEDs, 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.

TM-15-07
Rating system developed by IESNA which replaces the older IES cutoff classification system for controlling light emitted in unwanted regions surrounding a luminaire.

Traditional Lamps
Light sources developed and used during the last century including incandescent, fluorescent, HID and other sources.

T

Uplight
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-15-07based BUG rating system, which replaces the older cutoff classification system.

Useful Life
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.

V

Voltage
Electrical potential difference, which is commonly expressed as (VA – 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.

W

Watts
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.

White Phosphor LEDs
Combinations of InGaN (indium gallium nitride) semiconductor material—which produces blue light—and YAG (yttrium aluminum garnet) phosphors create white light. Other LED systems deliver white light by combining red, green and blue (RGB) LEDs.