LED lamp

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An assortment of LED lamps commercially available as of 2010 as replacements for screw-in bulbs, including floodlight fixtures (left), reading light (center), household lamps (center right and bottom), and low-power accent light (right) applications.
A 17W tube of LEDs which has the same intensity as a 45W fluorescent tube
LED spotlight using 60 individual diodes for mains voltage power

An LED lamp is a light-emitting diode (LED) product that is assembled into a lamp (or light bulb) for use in lighting fixtures. LED lamps have a lifespan and electrical efficiency that is several times better than incandescent lamps, and significantly better than most fluorescent lamps, with some chips able to emit more than 100 lumens per watt. The LED lamp market is projected to grow more than 12-fold over the next decade, from $2 billion today to $25 billion in 2023, which is a compound annual growth rate (CAGR) of 25%.[1]

Like incandescent lamps and unlike most fluorescent lamps (e.g. tubes and CFL), LED lights come to full brightness without need for a warm-up time; the life of fluorescent lighting is also reduced by frequent switching on and off.[citation needed] Initial cost of LED is usually higher. Degradation of LED dye and packaging materials reduces light output to some extent over time.

With research into organic LEDs (OLED) and polymer LEDs (PLED), cost per lumen and output per device have been improving rapidly according to what has been called Haitz's law, analogous to Moore's law for semiconductor devices.[citation needed]

Some LED lamps are made to be a directly compatible drop-in replacement for incandescent or fluorescent lamps. An LED lamp packaging may show the lumen output, power consumption in watts, color temperature in kelvins or description (e.g. "warm white") and sometimes the equivalent wattage of an incandescent lamp of similar luminous output.

LEDs do not emit light in all directions, and their directional characteristics affect the design of lamps. The light output of single LEDs is less than that of incandescent and compact fluorescent lamps; in most applications multiple LEDs are used to form a lamp, although high-power versions (see below) are becoming available.

LED chips need controlled direct current (DC) electrical power; an appropriate power supply is needed. LEDs are adversely affected by high temperature, so LED lamps typically include heat dissipation elements such as heat sinks and cooling fins.

Technology overview[edit]

Dropped ceiling with LED lamps

General-purpose lighting needs white light. LEDs emit light in a very narrow 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 (RGB or trichromatic white LEDs) uses multiple LED chips, each emitting a different wavelength, in close proximity to generate white light. This allows the intensity of each LED to be adjusted to change the overall 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.[citation needed]

Application[edit]

A significant difference from other light sources is that the light is more directional, i.e., emitted as a narrower beam. LED lamps are used for both general and special-purpose lighting. Where colored light is needed, LEDs that inherently emit light of a single color require no energy-absorbing filters.

BAPS Shri Swaminarayan Mandir Atlanta Illumination with color mixing LED fixtures.
Computer-led LED lighting allows enhancement of unique qualities of paintings in the National Museum in Warsaw.[2]

White-light LED lamps have longer life expectancy and higher efficiency (more light for the same 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,[3] and the light output and spatial distribution of a LED array can be controlled with no efficiency loss.

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.[4] Unlike other lighting technologies, LED emission tends to be directional (or at least lambertian), which can be either advantageous or disadvantageous, 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[edit]

Replacement for existing lighting[edit]

Lamp sizes and bases[edit]

LED lamps are made that replace screw-in incandescent or compact fluorescent light bulbs, mostly replacing incandescent bulbs rated from 5 to 60 watts. Such lamps are made with standard light bulb connections and shapes, such as an Edison screw base, an MR16 shape with a bi-pin base, or a GU5.3 (bi-pin cap) or GU10 (bayonet fitting) and are made compatible with the voltage supplied to the sockets. They include circuitry to rectify the AC power and convert the voltage to an appropriate value.

As of 2010 some LED lamps replaced higher wattage bulbs; for example, one manufacturer claimed a 16-watt LED bulb was as bright as a 150 W halogen lamp.[5] 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 60 W tungsten bulb must have a minimum light output of 806 lumens.[6]

A selection of consumer LED bulbs available in 2012 as drop-in replacements for incandescent bulbs in screw-type sockets

Some models of LED bulbs are compatible 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. These bulbs are more power-efficient than compact fluorescent bulbs[7] 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, and compact fluorescents about 8,000 hours.[citation needed] The bulbs maintain output light intensity well over their lifetimes. Energy Star specifications require the bulbs to typically drop less than 10% after 6,000 or more hours of operation, and in the worst case not more than 15%.[8] LED lamps are available with a variety of color properties. The purchase price is higher than most other, but the higher efficiency may make total cost of ownership (purchase price plus cost of electricity and changing bulbs) lower.[9]

Several companies offer LED lamps for general lighting purposes. The technology is improving rapidly and new energy-efficient consumer LED lamps are available.[10]

LED lamps are close to being adopted as the mainstream light source because of the falling prices and because 40 and 60 watt incandescent bulbs are being phased out.[11] In the U.S. the Energy Independence and Security Act of 2007 effectively bans the manufacturing and importing[when?] of most current incandescent light bulbs.[citation needed]

LED tube lamps[edit]

LED tube lights are designed to physically fit in fixtures intended for fluorescent tubes. Some LED tube lamps are intended to be a drop-in replacement into existing fixtures. Others require rewiring of the fixtures to remove the ballast. An LED tube lamp generally uses many individual LEDs and may be directional. Fluorescent lamps emit light all the way around the lamp. Most LED tube lights available can be used in place of T8, T10, or T12 tube designations, in lengths of 2, 4, and 8 feet.

Lighting designed for LEDs[edit]

Newer light fittings designed for LED lamps, or indeed with long-lived LEDs built-in, have been coming into use as the need for compatibility with existing fittings diminishes. Such lighting does not require each bulb to contain circuitry to operate from mains voltage.

Specialty uses[edit]

LED Flashlight replacement bulb (left), with tungsten equivalent (right)

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.

Commercial and industrial use[edit]

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 was recovered within two years via electricity savings, and the lamps should not need replacing for 20 years.[12] 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 expected the new lighting to pay for itself fully within 5 years.[13]

LEDs as Christmas illumination in Viborg, Denmark.

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 paid for themselves in three and a half years, even though the lights run for only 48 days per year.[14]

In 2009 a new highway (A29) was inaugurated in Aveiro, Portugal, it included the first European public LED-based lighting highway.[15]

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.[16] Many additional streetlight and municipal outdoor lighting projects have been announced.[17]

Comparison to other lighting technologies[edit]

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.[18] 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.[19] Incandescent lamps are relatively inexpensive to make. The typical lifespan of an AC incandescent lamp is 750 to 1,000 hours.[20][21] 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, and 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.[22] For comparison, general household LED bulbs available in 2011 emit 64 lumens/W,[23]
  • Electricity prices vary state to state and are customer dependent. Generally commercial (10.3 cent/kWh) and industrial (6.8 cent/kWh) electricity prices are lower than residential (12.3 cent/kWh) due to fewer transmission losses.[24]
Cost Comparison for 60 watt incandescent equivalent lightbulb (U.S. residential electricity prices)
Incandescent[25] Halogen[26] CFL[27] LED (Generic)[28] LED (Philips)[29] LED (LEDNovation)
Purchase price $0.36 $1.50 $1.75 $8 $16 $17
Power used (watts) 60 43 13 10 9.5 9
lumens (mean) 860 750 775[30] 800 806 810
lumens/watt 14.3 17.4 59.6 80 84.8 94
Color Temperature kelvin 2700 2900 2700 2700 2700 2700
CRI 100 100 82 82 >80 94
Lifespan (hours) 1,000 1,000 10,000 25,000 15,000 50,000
Bulb lifetime in years @ 6 hours/day 0.5 0.5 4.6 >11.4 6.8 >22.8
Energy cost over 20 years @ 13 cents/kWh $342 $245 $74 $57 $54 $51
Total cost $360 $311 $83 $73 $102 $68
Total cost per 860 lumens $333 $335 $86 $71 $105 $68
Comparison based on 6 hours use per day (43,800 hours over 20 yrs)

In keeping with the long life claimed for LED lamps, long warranties are offered. One manufacturer warrants lamps for professional use, depending upon type, for periods of (defined) "normal use" ranging from 1 year or 2,000 hours (whichever comes first) to 5 years or 20,000 hours.[31] A typical domestic lamp is stated to have an "average life" of 15,000 hours (15 years at 3 hours/day), and to support 50,000 switch cycles.[29]

Research and development[edit]

US Department of Energy[edit]

The Energy Independence and Security Act (EISA) of 2007 authorized the Department of Energy (DOE) to establish the Bright Tomorrow Lighting Prize competition, known as the "L Prize", the first government-sponsored technology competition designed to challenge industry to develop replacement for the most commonly used and inefficient products, the 60 W incandescent lamps and PAR 38 halogen lamps.

The EISA legislation establishes basic requirements and prize amounts for each of the two competition categories, and authorizes up to $20 million in cash prizes.[32] The competition may also lead to opportunities for federal purchasing agreements, utility programs, and other incentives for winning products.

In May 2008, DOE announced details of the competition and technical requirements for each category. 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.

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,[9] 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.[33]

National Institute of Standards and Technology[edit]

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.[34]

DOE launched the Energy Star program for solid-state lighting products in 2008.

Environmental Protection Agency[edit]

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.[35] Energy Star Light Bulbs for Consumers is a resource for finding and comparing Energy Star qualified lamps.

Other venues[edit]

In the United Kingdom a program is run by the Energy Saving Trust to identify lighting products that meet energy conservation and performance guidelines.[36]

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.[12]

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.[37] 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.[38]

Energy Star qualification[edit]

Energy Star is an international standard for energy efficient consumer products.[39][40] Devices carrying the Energy Star service mark generally use 20–30% less energy than required by federal standards.[41]

Energy Star LED qualifications:

  • Reduces energy costs — uses at least 75% less energy than incandescent lighting, saving on operating expenses.
  • Reduces maintenance costs — lasts 35 to 50 times longer than incandescent lighting and about 2 to 5 times longer than fluorescent lighting. No bulb-replacements, no ladders, no ongoing disposal program.
  • Reduces cooling costs — LEDs produce very little heat.
  • Is guaranteed — comes with a minimum three-year warranty — far beyond the industry standard.
  • Offers convenient features — available with dimming on some indoor models and automatic daylight shut-off and motion sensors on some outdoor models.
  • Is durable — won’t break like a bulb.

To qualify for Energy Star certification, LED lighting products must pass a variety of tests to prove that the products will display the following characteristics:

  • Brightness is equal to or greater than existing lighting technologies (incandescent or fluorescent) and light is well distributed over the area lighted by the fixture.
  • Light output remains constant over time, only decreasing towards the end of the rated lifetime (at least 35,000 hours or 12 years based on use of 8 hours per day).
  • Excellent color quality. The shade of white light appears clear and consistent over time.
  • Efficiency is as good as or better than fluorescent lighting.
  • Light comes on instantly when turned on.
  • No flicker when dimmed.
  • No off-state power draw. The fixture does not use power when it is turned off, with the exception of external controls, whose power should not exceed 0.5 watts in the off state.

Limitations[edit]

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.[42] LEDs with CRI below 75 are not recommended for use in indoor lighting.[43]

LED efficiency and life span drop at higher temperatures, which limits the 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.

LED lamps are sensitive to excessive heat, like most solid state electronic components. LED lamps should be checked for compatibility for use in totally enclosed fixtures before installation since heat build-up could cause lamp failure and/or fire.

LED lamps may flicker. The extent of flicker is based on the quality of the DC power supply built into the lamp structure, usually located in the lamp base.

Depending on the design of the lamp, the LED lamp may be sensitive to electrical surges. This is generally not an issue with incandescents, but can be an issue with LED and compact fluorescent bulbs. Power circuits that supply LED lamps should be protected from electrical surges through the use of surge protection devices.

The long life of LEDs, expected to be about 50 times that of the most common incandescent bulbs and significantly longer than fluorescent types, is advantageous for users but will affect manufacturers as it reduces the market for replacements in the distant future.[12]

Efficiency droop[edit]

The term "efficiency droop" refers to the decrease in luminous efficacy of LEDs as the electrical current increases above tens of milliamps(mA). Instead of increasing current levels, luminance is usually increased by combining multiple LEDs in one bulb. Solving the problem of efficiency droop would mean that household LED light bulbs would need fewer LEDs, which would significantly reduce costs.

In addition to being less efficient, operating LEDs at higher electrical currents create higher heat levels which compromise the lifetime of the LED. Because of this increased heating at higher currents, high-brightness LEDs have an industry standard of operating at only 350 mA. 350 mA is a good compromise between light output, efficiency, and longevity.[44][45][46][47]

Early suspicions were that the LED droop was caused by elevated temperatures. Scientists proved the opposite to be true that, although the life of the LED would be shortened, elevated temperatures actually improved the efficiency of the LED.[48] The mechanism causing efficiency droop was identified in 2007 as Auger recombination, which was taken with mixed reaction.[47] In 2013, a study conclusively identified Auger recombination as the cause of efficiency droop.[49]

Possible hazard to vision[edit]

Tests (in vitro) performed at the Complutense University of Madrid indicate that prolonged exposure to the shorter blue band spectrum LED lights may permanently damage the pigment epithelial cells of the retina. The test conditions were the equivalent of staring at a 100 watt blue incandescent source from 20 cm (7.9 in) for 12 hours; researchers say additional testing is required to ascertain what intensities, wavelengths, and exposure times of LED lighting devices are lethal and non-lethal for retinal tissue.[50][51]

See also[edit]

References[edit]

  1. ^ LED Lighting Market to Grow Over 12-Fold to $25 Billion in 2023, Lux Research, 28 January 2014, Carole Jacques
  2. ^ "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." 
  3. ^ 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. 
  4. ^ 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. 
  5. ^ Commercially available 150W Halogen Equivalent PAR38 (240V). Ledlightingsupplier.co.uk. Retrieved 2012-06-02.
  6. ^ Lonsdale, Sarah (July 7, 2010). "Green property: energy-efficient bulbs". The Daily Telegraph (London). Retrieved 2011-06-08. 
  7. ^ Elisabeth Rosenthal and Felicity Barringer, "Green Promise Seen in Switch to LED Lighting", The New York Times, May 29, 2009
  8. ^ "Integral LED Lamps Criteria Development". 
  9. ^ a b Taub, Eric; leora Broydo Vestel (2009-09-24). "Build a Better Bulb for a $10 Million Prize". New York Times. Retrieved 2010-02-06. 
  10. ^ 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,
  11. ^ Philips Flattens the Light Bulb, Mashable, Pete Pachal, 16 December 2013
  12. ^ a b c Fans of L.E.D.'s Say This Bulb's Time Has Come By Eric A. Taub. Published: July 28, 2008 – NY Times
  13. ^ Led'ing the way, Nitya Varadarajan, October 5, 2009
  14. ^ "Of the top six in Turku, led a move – HS.fi – Domestic". November 19, 2009. Retrieved 2012-01-09. 
  15. ^ New highway connecting Lisbon to Oporto includes first European LED based lighting in a highway, Aveiro September 11, 2009
  16. ^ U. S. Department of Energy, Solid-State Lighting GATEWAY Demonstration Results (Retrieved 2010-07-16)
  17. ^ 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)
  18. ^ Keefe, T.J. (2007). "The Nature of Light". Retrieved 2009-09-10. 
  19. ^ Vergleich für Osram CLAS A 100 E27 klar, Osram CLAS A FR 100 E27, Philips Standard 100W E27 klar. idealo.de
  20. ^ a b Raatma, Lucia (2010). Green Living: No Action Too Small. Compass Point Books. p. 22. ISBN 978-0756542931. 
  21. ^ A Short History of Electric Light, The Incandescent Lamp, 1900 to 1920
  22. ^ Guide to Selecting Frequently Switched T8 Fluorescent Lamp-Ballast Systems. RPI NLPIP, April 1998
  23. ^ "Philips wins Energy Star Award for 800 lumen, 12.5 Watt LED bulb". February 11, 2011. Retrieved 2011-12-21. 
  24. ^ "Table 5.6.A. Average Retail Price of Electricity to Ultimate Customers by End-Use Sector (Oct 2013)". .S. Energy Information Administration. Retrieved 30 December 2013. 
  25. ^ "HomeDepot.com: Philips 60-Watt Household Incandescent Light Bulb". Retrieved 2012-07-26. 
  26. ^ "Philips EcoVantage 43-Watt (60-Watt) A19 Soft White Dimmable Light Bulb (4-Pack)". Home Depot. Retrieved 5 November 2013. 
  27. ^ "EcoSmart 60W Equivalent Soft White (2700K) Twister CFL Light Bulb (4-pack)". Retrieved 20 January 2014. 
  28. ^ "TCP 60W Equivalent Soft White (2700K) A19 LED Light Bulb (3-Pack) (E)*". Home Depot. Retrieved 20 January 2014. 
  29. ^ a b Specification of a typical domestic 9.5W LED lamp as of November 2013
  30. ^ "Lightbulbs - LEDs and CFLs offer more choices and savings". ConsumerReports. 2011. Retrieved 21 January 2014. 
  31. ^ One manufacturer's warranty terms for professional LED lamps
  32. ^ Progress Alerts – 2010, US Department of Energy
  33. ^ 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.
  34. ^ American National Standard for Specifications for the Chromaticity of Solid-State Lighting (SSL) Products. Nema.org. Retrieved 2012-06-02.
  35. ^ Energy Star Program Requirements for CFLS Partner Commitments, 4th edition, dated 03/07/08, retrieved 2008-06-25.
  36. ^ Energy saving lighting. Energysavingtrust.org.uk. Retrieved 2013-01-18.
  37. ^ Great bright hope to end battle of the light bulbs, The Daily Mail, January 29, 2009
  38. ^ "Switching to LED is a global task: Honeywell". 7 June 2012. 
  39. ^ "The Clinton Presidency: Protecting Our Environment and Public Health". Retrieved 26 March 2012. 
  40. ^ "History of Energy Star". Retrieved 27 March 2012. 
  41. ^ Alena Tugend (10 May 2008). "If Your Appliances Are Avocado, They're Probably not Green". New York Times. Retrieved 29 June 2008. 
  42. ^ http://www.lrc.rpi.edu/programs/nlpip/lightinganswers/lightsources/appendixB.asp
  43. ^ ENERGY STAR Program Requirements for Solid State Lighting Luminaires. (PDF) . Retrieved 2012-06-02.
  44. ^ The LED's dark secret. EnergyDaily. Retrieved on March 16, 2012.
  45. ^ Efremov, A. A.; Bochkareva, N. I.; Gorbunov, R. I.; Lavrinovich, D. A.; Rebane, Y. T.; Tarkhin, D. V.; Shreter, Y. G. (2006). "Effect of the joule heating on the quantum efficiency and choice of thermal conditions for high-power blue InGaN/GaN LEDs". Semiconductors 40 (5): 605. doi:10.1134/S1063782606050162.  edit
  46. ^ Smart Lighting: New LED Drops The 'Droop'. Sciencedaily.com (January 13, 2009). Retrieved on March 16, 2012.
  47. ^ a b Stevenson, Richard (August 2009) The LED’s Dark Secret: Solid-state lighting won't supplant the lightbulb until it can overcome the mysterious malady known as droop. IEEE Spectrum
  48. ^ Identifying the Causes of LED Efficiency Droop, By Steven Keeping, Digi-Key Corporation Tech Zone
  49. ^ Justin Iveland, Lucio Martinelli, Jacques Peretti, James S. Speck, Claude Weisbuch. "Cause of LED Efficiency Droop Finally Revealed". Physical Review Letters, 2013. Science Daily. Retrieved 23 April 2013. 
  50. ^ http://www.thinkspain.com/news-spain/22749/led-lighting-damages-eyes-says-spanish-investigator
  51. ^ http://www.lighting.co.uk/news/latest-news/specialists-question-validity-of-led-eye-damage-study/8648323.article

Further reading[edit]

External links[edit]