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An LED lamp (or LED light bulb) is a solid-state lamp that uses light-emitting diodes (LEDs) as the source of light. LED lamps offer long service life and high energy efficiency, but initial costs are higher than those of fluorescent and incandescent lamps. Chemical decomposition of LED chips reduces luminous flux over life cycle as with conventional lamps.
Commercial LED lighting products use semiconductor light-emitting diodes. Research into organic LEDs (OLED), or polymer light-emitting diodes (PLED) is aimed at reducing the production cost of lighting products. Diode technology currently improves at an exponential rate.
LED lamps can be made interchangeable with other types of lamps. Assemblies of high power light-emitting diodes can be used to replace incandescent or fluorescent lamps. Some LED lamps are made with bases directly interchangeable with those of incandescent bulbs. Since the luminous efficacy (amount of visible light produced per unit of electrical power input) varies widely between LED and incandescent lamps, lamps are usefully marked with their lumen output to allow comparison with other types of lamps. LED lamps are sometimes marked to show the watt rating of an incandescent lamp with approximately the same lumen output, for consumer reference in purchasing a lamp that will provide a similar level of illumination.
Efficacy of LED devices continues to improve, with some chips able to emit more than 100 lumens per watt. LEDs do not emit light in all directions, and their directional characteristics affect the design of lamps. The efficacy of conversion from electric power to light is generally higher than for incandescent lamps. Since the light output of many types of light-emitting diodes is small compared to incandescent and compact fluorescent lamps, in most applications multiple diodes are assembled.
Light-emitting diodes use direct current (DC) electrical power. To use them on AC power they are operated with internal or external rectifier circuits that provide a regulated current output at low voltage. LEDs are degraded or damaged by operating at high temperatures, so LED lamps typically include heat dissipation elements such as heat sinks and cooling fins.
Technology overview 
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General-purpose lighting needs white light. LEDs emit light in a very small band of wavelengths, emitting light of a color characteristic of the energy bandgap of the semiconductor material used to make the LED. To emit white light from LEDs requires either mixing light from red, green, and blue LEDs, or using a phosphor to convert some of the light to other colors.
One method is (RGB- or trichromatic white LEDs) uses multiple LED chips, each emitting a different wavelength, in close proximity to generate white light. This arrangement allows for the adjustment of the intensity of each LED to "tune" the apparent color of the final color.
The second method uses LEDs in conjunction with a phosphor. The CRI (color rendering index) value can range from less than 70 to over 90, and color temperatures in the range of 2700 K (matching incandescent lamps) up to 7000 K are available.
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The main difference to other light sources is the directed light. LED lamps are used for both general and special-purpose lighting. Where colored light is needed, LEDs that inherently emit single colored light require no energy-absorbing filters. This improves the energy efficiency over a white light source that generates all colors of light then discards some of the visible energy in a filter.
Compared to fluorescent bulbs, introduced at the 1939 World's Fair, advantages claimed for LED light bulbs are that they contain no mercury (unlike a Compact fluorescent lamp or CFL), that they turn on instantly, and that lifetime is unaffected by cycling on and off, so that they are well suited for light fixtures where bulbs are often turned on and off. LED light bulbs are also mechanically robust; most other artificial light sources are fragile.
White-light light-emitting diode lamps have longer life expectancy and higher efficiency (the same light for less electricity) than most other lighting. LED sources are compact, which gives flexibility in designing lighting fixtures and good control over the distribution of light with small reflectors or lenses. Because of the small size of LEDs, control of the spatial distribution of illumination is extremely flexible, and the light output and spatial distribution of a LED array can be controlled with no efficiency loss.
LED lamps have no glass tubes to break (some models have a decorative glass bulb, however), and their internal parts are rigidly supported, making them resistant to vibration and impact. With proper driver electronics design, an LED lamp can be made dimmable over a wide range; there is no minimum current needed to sustain lamp operation.
LEDs using the color-mixing principle can emit a wide range of colors by changing the proportions of light generated in each primary color. This allows full color mixing in lamps with LEDs of different colors. In contrast to other lighting technologies, LED emission tends to be directional (or at least lambertian). This can be either an advantage or a disadvantage, depending on requirements. For applications where non-directional light is required, either a diffuser is used, or multiple individual LED emitters are used to emit in different directions.
Household LED lamps 
Lamp sizes and bases 
LED lamps intended to be interchangeable with incandescent lamps are made in standard light bulb shapes, such as an Edison screw base, an MR16 shape with a bi-pin base, or a GU5.3 (Bipin cap) or GU10 (bayonet socket) and are made compatible with the voltage supplied to the sockets. LED lamps include circuitry to rectify the AC power and to convert the voltage to a level usable by the LED.
LED light bulbs 
LED lamps are made that replace screw-in incandescent or compact fluorescent light bulbs. Most LED lamps replace incandescent bulbs rated from 5 to 60 watts. As of 2010[update], some LED lamps replace higher wattage bulbs; for example, one manufacturer claims a 16-watt LED bulb as bright as a 150W halogen lamp. A standard general-purpose incandescent bulb emits light at an efficiency of about 14 to 17 lumens/W depending on its size and voltage. According to the European Union standard, an energy-efficient bulb that claims to be the equivalent of a 60W tungsten bulb must have a minimum light output of 806 lumens.
Some models of LED bulbs work with dimmers as used for incandescent lamps. LED lamps often have directional light characteristics. The lamps have declined in cost to between US$10 to $50 each as of 2012[update]. These bulbs are more power-efficient than compact fluorescent bulbs and offer lifespans of 30,000 or more hours, reduced if operated at a higher temperature than specified. Incandescent bulbs have a typical life of 1,000 hours, compact fluorescents about 8,000 hours. The bulbs maintain output light intensity well over their life-times. Energy Star specifications require the bulbs to typically drop less than 10% after 6000 or more hours of operation, and in the worst case not more than 15%. They are also mercury-free, unlike fluorescent lamps. LED lamps are available with a variety of color properties. The higher purchase cost than other types may be more than offset by savings in energy and maintenance.
Several companies offer LED lamps for general lighting purposes. The technology is improving rapidly and new energy-efficient consumer LED lamps are available.
LED tubes 
LED tubes are made that are pin-compatible with, and designed to replace, standard fluorescent tubes. Most LED tubes available can be used in place of T8, T10, or T12 tube designations, in lengths of 2, 4, and 8 feet. Some LED tubes are designed to drop directly into an existing fixture, while others require rewiring of the fixture to remove the ballast. An LED tube generally uses many individual LEDs to point light downward toward the subject. This differs from most fluorescent tubes that create light 360 degrees around the center axis of the tube, which the fixture then reflects downward.
In 2013, Philips announced they had developed the world's most energy-efficient LED tube, producing 200 lumens per watt (200 lm/W) compared with a standard 100 lm/W achieved with traditional strip lighting. The LED tube light offers significant improvements on current designs.
Specialty uses 
White LED lamps have achieved market dominance in applications where high efficiency is important at low power levels. Some of these applications include flashlights, solar-powered garden or walkway lights, and bicycle lights. Monochromatic (colored) LED lamps are now commercially used for traffic signal lamps, where the ability to emit bright monochromatic light is a desired feature, and in strings of holiday lights.
LED lights have also become very popular in gardening and agriculture by 2010. First used by NASA to grow plants in space, LEDs came into use for home and commercial applications for indoor horticulture (aka grow lights). The wavelengths of light emitted from LED lamps have been specifically tailored to supply light in the spectral range needed for chlorophyll absorption in plants, promoting growth while reducing wastage of energy by emitting minimal light at wavelengths that plants do not require. The red and blue wavelengths of the visible light spectrum are used for photosynthesis, so these are the colors almost always used in LED grow light panels.
Pioneering mass use 
Japan quickly embraced LED lighting to replace home incandescent lighting. By 2009, LED direct socket-replacement lightbulbs from five competing companies eclipsed incandescent bulbs in stores. One of them, Toshiba, had discontinued their incandescent bulbs by 2010.
In 2008 Sentry Equipment Corporation in Oconomowoc, Wisconsin, USA, was able to light its new factory interior and exterior almost solely with LEDs. Initial cost was three times more than a traditional mix of incandescent and fluorescent lamps, but the extra cost will be repaid within two years via electricity savings, and the lamps should not need replacing for 20 years. In 2009, the Manapakkam, Chennai office of the Indian IT company iGate spent ₨ 3,700,000 (US$80,000) to light 57,000 sq ft (5,300 m2) of office space with LEDs. The firm expects the new lighting to pay for itself fully within 5 years.
In 2009 the exceptionally large Christmas tree standing in front of the Turku Cathedral in Finland was hung with 710 LED bulbs, each using 2 watts. It has been calculated that these LED lamps will pay for themselves in three and a half years, even though the lights run for only 48 days per year.
By 2010 mass installations of LED lighting for commercial and public uses were becoming common. LED lamps have also been used for a number of demonstration projects for outdoor lighting and LED street lights. The United States Department of Energy has available several reports on the results of many pilot projects for municipal outdoor lighting. Many additional streetlight and municipal outdoor lighting projects have been announced.
Comparison to other lighting technologies 
See luminous efficacy for an efficiency chart comparing various technologies.
- Incandescent lamps (light bulbs) generate light by passing electric current through a resistive filament, thereby heating the filament to a very high temperature so that it glows and emits visible light over a broad range of wavelengths. Incandescent sources yield a "warm" yellow or white color quality depending on the filament operating temperature. Incandescent lamps emit 98% of the energy input as heat. A 100 W light bulb for 120 V operation emits about 1,180 lumens, about 11.8 lumens/W; for 230 V bulbs the figures are 1340 lm and 13.4 lm/W. Incandescent lamps are relatively inexpensive to make. The typical lifespan of an AC incandescent lamp is 750 to 1,000 hours. They work well with dimmers. Most older light fixtures are designed for the size and shape of these traditional bulbs. In the U.S. the regular sockets are E26 and E11, like E27 and E14 in some European countries.
- Fluorescent lamps work by passing electricity through mercury vapor, which in turn emits ultraviolet light. The ultraviolet light is then absorbed by a phosphor coating inside the lamp, causing it to glow, or fluoresce. Conventional linear fluorescent lamps have life spans around 20,000 and 30,000 hours based on 3 hours per cycle according to lamps NLPIP reviewed in 2006. Induction fluorescent relies on electromagnetism rather than the cathodes used to start conventional linear fluorescent. The newer rare earth triphosphor blend linear fluorescent lamps made by Osram, Philips, Crompton and others have a life expectancy greater than 40,000 hours, if coupled with a warm-start electronic ballast. The life expectancy depends on the number of on/off cycles, and is lower if the light is cycled often. The ballast-lamp combined system efficacy for then current linear fluorescent systems in 1998 as tested by NLPIP ranged from 80 to 90 lm/W. For comparison, general household LED bulbs available in 2011 emit 64 lumens/W, with the best LED bulbs coming in at about 100 lumens/W.
|Incandescent||Halogen||CFL||LED (Generic)||LED (Philips)||LED (Philips L-Prize)|
|Electricity usage||60 W||42 W||13 W||13.5 W||12.5 W||10 W|
|Color Temperature Kelvin||2700||3000||2700||3000||2700||2700|
|Bulb lifetime in years @ 6 hours/day||0.5||1.6||3.7||>11.4||>11.4||>13.7|
|Energy cost over 10 years @ 15 cents/kWh||$197||$138||$43||$44||$41||$33|
|Comparison based on 6 hours use per day (21,900 hours over 10 yrs)|
Research and development 
US Department of Energy 
In May 2008, the U.S. Department of Energy (DOE) announced details of the Bright Tomorrow Lighting Prize competition, known as the "L Prize". This is the first government-sponsored technology competition designed to spur lighting producers to develop high quality, high efficiency solid-state lighting products to replace the common light bulb. The competition will award cash prizes, and may also lead to opportunities for federal purchasing agreements, utility programs, and other incentives for winning products.
The Energy Independence and Security Act (EISA) of 2007 authorized the DOE to establish the Bright Tomorrow Lighting Prize competition. The legislation challenges industry to develop replacement technologies for the most commonly used and inefficient products, 60 W incandescent lamps and PAR 38 halogen lamps. The L Prize specifies technical requirements for these two competition categories. Lighting products meeting the competition requirements would use just 17% of the energy used by most incandescent lamps in use today. A future L Prize program announcement will call for developing a new “21st Century Lamp”, as authorized in the legislation.
The EISA legislation establishes basic requirements and prize amounts for each category. The legislation authorizes up to $20 million in cash prizes. On September 24, 2009 the DOE announced that Philips Lighting North America was the first to submit lamps in the category to replace the standard 60 W A-19 "Edison screw fixture" light bulb, with a design based on their earlier "AmbientLED" consumer product. On August 3, 2011, DOE awarded the prize in the 60 W replacement category to Philips' bulb after 18 months of extensive testing.
National Institute of Standards and Technology 
In June 2008, NIST announced the first two standards for solid-state lighting in the United States. These standards detail performance specifications for LED light sources and prescribe test methods for solid-state lighting products.
The Illuminating Engineering Society of North America (IESNA) published a documentary standard LM-79, which describes the methods for testing solid-state lighting products for their light output (lumens), efficacy (lumens per watt) and chromaticity.
The solid-state lights being studied are intended for general illumination, but white lights used today vary greatly in chromaticity. ANSI C78.377-2008 specifies the recommended color ranges for solid-state lighting products using cool to warm white LEDs with various correlated color temperatures.
DOE launched the Energy Star program for solid-state lighting products in 2008.
Environmental Protection Agency 
In the United States and Canada, the Energy Star program since 2008 labels lamps that meet a set of standards for starting time, life expectancy, color, and consistency of performance. The intent of the program is to reduce consumer concerns due to variable quality of products, by providing transparency and standards for the labeling and usability of products available in the market. Energy Star Light Bulbs for Consumers is a resource for finding and comparing Energy Star qualified lamps.
Other venues 
Philips Lighting has ceased research on compact fluorescents, and is devoting the bulk of its research and development budget, 5 percent of the company's global lighting revenue, to solid-state lighting.
In January 2009, it was reported that researchers at Cambridge University had developed an LED bulb that costs £2 (about $3 U.S.), is 12 times as energy efficient as a tungsten bulb, and lasts for 100,000 hours. Honeywell Electrical Devices and Systems (ED&S) recommend world wide usage of LED lighting as it is energy efficient and can help save the Climate.
Color rendition is not identical to incandescent lamps. A measurement unit called CRI is used to express how the light source's ability to render the eight color sample chips compare to a reference on a scale from 0 to 100. LEDs with CRI below 75 are not recommended for use in indoor lighting.
LEDs are also sensitive to heat and increase in temperature causes efficacy drop. This limits the practical power that can be used in lamps that physically replace existing filament and compact fluorescent types. Thermal management of high-power LEDs is a significant factor in design of solid state lighting equipment.
The long life of solid-state lighting products, expected to be about 50 times the most common incandescent bulbs, poses a problem for bulb makers, whose current customers buy frequent replacements.
According to research by Spanish scientists, like other artificial lighting, prolonged direct exposure to LED lights can cause serious damage to retina of the eye, which cannot regenerate. This is caused by high levels of radiation in the blue band. Experts have suggested using built in filter to remove blue glare to solve the problem. Also usage of uv filter glasses and vitamin A rich foods can help. 
See also 
- List of emerging technologies
- List of light sources
- Photometry (optics)
- Radiation angle
- Solar lamp
- "Warsaw Top 10". Warsaw tour Edition nr 5, 2012. p. 20. Retrieved 2013-03-01. "The National Museum in Warsaw is also one of the most modern in Europe. (...) The LED system allows to adjust the light to every painting so that its unique qualities are enhanced."
- Ivan Moreno, Maximino Avendaño-Alejo, and Rumen I. Tzonchev (2006). "Designing light-emitting diode arrays for uniform near-field irradiance". Applied Optics 45 (10): 2265–2272. doi:10.1364/AO.45.002265. PMID 16607994.
- Ivan Moreno, Ulises Contreras (2007). "Color distribution from multicolor LED arrays". Optics Express 15 (6): 3607–18. doi:10.1364/OE.15.003607. PMID 19532605.
- Commercially available 150W Halogen Equivalent PAR38 (240V). Ledlightingsupplier.co.uk. Retrieved 2012-06-02.
- Lonsdale, Sarah (July 7, 2010). "Green property: energy-efficient bulbs". The Daily Telegraph (London). Retrieved 2011-06-08.
- Elisabeth Rosenthal and Felicity Barringer, "Green Promise Seen in Switch to LED Lighting", The New York Times, May 29, 2009
- "Integral LED Lamps Criteria Development".
- Taub, Eric; leora Broydo Vestel (2009-09-24). "Build a Better Bulb for a $10 Million Prize". New York Times. Retrieved 2010-02-06.
- Eric A. Taub, "LED Bulbs for the Home Near the Marketplace", The New York Times, May 16, 2010; see also Matthew L. Wald, New York Times Green Blog, "An LED That Mimics an Old Standby", June 24, 2010,
- Sterling, Toby (April 12, 2013). "Philips says its LED bests fluorescent tube lighting". Pittsburgh Post-Gazette. Retrieved 2013-04-26.
- Fans of L.E.D.'s Say This Bulb's Time Has Come By Eric A. Taub. Published: July 28, 2008 – NY Times
- Led'ing the way, Nitya Varadarajan, October 5, 2009
- "Of the top six in Turku, led a move – HS.fi – Domestic". November 19, 2009. Retrieved 2012-01-09.
- New highway connecting Lisbon to Oporto includes first European LED based lighting in a highway, Aveiro September 11, 2009
- U. S. Department of Energy, Solid-State Lighting GATEWAY Demonstration Results (Retrieved 2010-07-16)
- for example, Seattle: "Seattle Picked to Lead National Effort on LED Street Lights" (Retrieved 2010-07-16); Scottsdale: "LED Streetlight Installation" (Retrieved 2010-07-16); Ann Arbor: LED street lights (Retrieved 2010-07-16)
- Keefe, T.J. (2007). "The Nature of Light". Retrieved 2009-09-10.
- Vergleich für Osram CLAS A 100 E27 klar, Osram CLAS A FR 100 E27, Philips Standard 100W E27 klar. idealo.de
- Raatma, Lucia (2010). Green Living: No Action Too Small. Compass Point Books. p. 22. ISBN 978-0756542931.
- A Short History of Electric Light, The Incandescent Lamp, 1900 to 1920
- Guide to Selecting Frequently Switched T8 Fluorescent Lamp-Ballast Systems. RPI NLPIP, April 1998
- "Philips wins Energy Star Award for 800 lumen, 12.5 Watt LED bulb". February 11, 2011. Retrieved 2011-12-21.
- "HomeDepot.com: Philips 60-Watt Household Incandescent Light Bulb". Retrieved 2012-07-26.
- Progress Alerts – 2010, US Department of Energy
- Department of Energy Announces Philips Lighting North America as Winner of L Prize Competition | Department of Energy. Energy.gov (2011-08-03). Retrieved 2012-06-02.
- American National Standard for Specifications for the Chromaticity of Solid-State Lighting (SSL) Products. Nema.org. Retrieved 2012-06-02.
- Energy Star Program Requirements for CFLS Partner Commitments, 4th edition, dated 03/07/08, retrieved 2008-06-25.
- Energy saving lighting. Energysavingtrust.org.uk. Retrieved 2013-01-18.
- Great bright hope to end battle of the light bulbs, The Daily Mail, January 29, 2009
- "Switching to LED is a global task: Honeywell". 7 June 2012.
- ENERGY STAR Program Requirements for Solid State Lighting Luminaires. (PDF) . Retrieved 2012-06-02.
Further reading 
- Light Emitting Diodes, Second edition by E. F. Schubert (Cambridge University Press, 2006) ISBN 0-521-86538-7
|Wikimedia Commons has media related to: LED lamps|
- e-lumen.eu – a website from the European Commission about the second generation of energy-saving lightbulbs
- Notes on LEDs, gizmology.net
- U. S. Department of Energy, Using Light Emitting Diodes
- U. S. Department of Energy, Solid-State Lighting GATEWAY Demonstration Results
- Detailed spectra plots, of hundreds of LEDs etc, ledmuseum.home.att.net
- Efficient LED lighting in conjunction with low-voltage domestic solar PV and mains, earth.org.uk