LED lamp

A 230-volt LED filament light bulb, with a E27 base. The filament is visible as the eight yellow vertical lines.
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
LED spotlight using 60 individual diodes for mains voltage usage
LED light bulb to replace G24 compact fluorescent lamp
An 80W Chip-On-Board COB LED Module from an industrial light luminaire, thermally bonded to the heat sink

An LED lamp is a light-emitting diode (LED) product which is assembled into a lamp (or light bulb) for use in lighting fixtures. LED lamps have a lifespan and electrical efficiency which are several times longer than incandescent lamps, and significantly more efficient than most fluorescent lamps, with some chips able to emit more than 300 lumens per watt (as claimed by Cree and some other LED manufacturers). The LED lamp market is projected to grow by more than twelve-fold over the next decade, from $2 billion in the beginning of 2014 to $25 billion in 2023, a compound annual growth rate (CAGR) of 25%.[1]

Like incandescent lamps and unlike most fluorescent lamps (e.g. tubes and compact fluorescent lamps or CFLs), LEDs 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.[2] The initial cost of LED is usually higher. Degradation of LED dye and packaging materials reduces light output to some extent over time.

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"), operating temperature range, and sometimes the equivalent wattage of an incandescent lamp of similar luminous output.

Most LEDs do not emit light in all directions, and their directional characteristics affect the design of lamps, although omnidirectional lamps which radiate light over a 360° angle are becoming more common. The light output of single LED 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 circuit is required to convert alternating current from the supply to the regulated low voltage direct current used by the LEDs. LEDs are adversely affected by high temperature, so LED lamps typically include heat dissipation elements such as heat sinks and cooling fins.

Technology overview

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.

Application

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[3]

White-light LED lamps have longer life expectancy and higher efficiency (more light for the same electricity) than most other lighting when used at the proper temperature. 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,[4] and the light output and spatial distribution of an 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.[5] 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

Replacement for existing lighting

Disassembled LED-light bulb with driver circuit board and Edison screw

Lamp sizes and bases

LED lamps are made of arrays of SMD modules that replace 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 driver circuitry to rectify the AC power and convert the voltage to an appropriate value, usually Switched-mode power supplies.

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. 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$2.49 to $33.98 each as of 2015. 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,[8] and compact fluorescents about 8,000 hours.[9] 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%.[10] 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.[11]

High-power LED "corn cob" light bulb

Several companies offer LED lamps for general lighting purposes. The technology is improving rapidly and new energy-efficient consumer LED lamps are available.[12]

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.[13] In the U.S. the Energy Independence and Security Act of 2007 effectively bans the manufacturing and importing of most current incandescent light bulbs. LED bulbs have decreased substantially in pricing and many varieties are sold with subsidized prices from local utilities.[14]

A 17 W tube of LEDs which has the same intensity as a 45 W fluorescent tube

LED tube lamps

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 if appropriate ballast is used. Others require rewiring of the fixtures to remove the ballast. An LED tube lamp generally uses many individual Surface-Mounted LEDs which are directional and require proper orientation during installation as opposed to Fluorescent tube lamps emit light in all directions around the tube. Most LED tube lights available can be used in place of T8, T10, or T12 tube designations, T8 is D26mm, T10 is D30mm, in lengths of 590 mm (23 in), 1,200 mm (47 in) and 1,500 mm (59 in).

Lighting designed for LEDs

LED-wall lamp

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

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. LED automotive lamps are widely used for their long life and small size (allowing for multiple bulbs), improving road safety. LED lamps are also becoming popular in homes, especially for bathroom and medicine cabinet lighting.

Comparison to other lighting technologies

See luminous efficacy for an efficiency chart comparing various technologies.

Comparison table

Cost Comparison for 60 watt incandescent equivalent lightbulb (U.S. residential electricity prices)
Incandescent[22] Halogen[23] CFL[24] LED (Cree)[25] LED (Philips)[26] LED (LEDNovation)[27] LED (Nanoleaf NL02-1200)[28]
Purchase price $0.41 $1.50 $0.99 $9.97 $4.35 $31.50 $24.99
PF 1 1 0.5 0.75 0.5 0.5 0.5
Real power used (watts) 60 35 14 9.5 8.5 9.4 10
lumens (mean) 860 860 775[29] 800 800 810 1200
lumens/watt 14.3 24.6 55.4 84 94.1 86.2 120
Color Temperature kelvin 2700 2900 2700 2700 2700 2700 3000
CRI 100 100 82 80 80 94 80
Lifespan (hours) 1,000 4,000 10,000 25,000 10,000 50,000 30,000
Bulb lifetime in years @ 6 hours/day 0.46 1.83 4.6 11.4 4.6 22.8 13.7
Energy cost over 20 years @ 13 cents/kWh $342 $199 $159 $72 $97 $107 $114
Total cost over 20 years $360 $216 $164 $90 $116 $135 $150
Total cost per 860 lumens $360 $216 $182 $96 $125 $143 $108
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.[30] A typical domestic LED 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.[31]

Incandescent and Halogen have a natural Power factor of 1, but Compact fluorescent and LED lamps are using input rectifier and this causes high harmonics content in input current and also reactive power consumption. This causes extra loss (harmonics) and power transfer cost (copper usage) toward the power plant and energy cost will be distributed to all customers by rising energy bills. Future developments may implement PFC-circuits to bring the PF up to 1, but higher material cost and volume of electronics will result. Dimmable LED-Lamps typical have higher PF by using so called Valley-fill circuits, non-dimmable uses cheaper bridge rectifiers. The EU-Standard requires a PF better than 0.5 for power up to 25 Watt.[32]

Energy Star qualification

Energy Star is an international standard for energy efficient consumer products.[33][34] Devices carrying the Energy Star service mark generally use 20–30% less energy than required by US standards.[35]

Energy Star LED qualifications:

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:

Limitations

Camera of mobile phone can detect flickering of LED light bulb

Color rendering is not identical to incandescent lamps which emit close to perfect Black-body radiation as that from the sun and what eyes have evolved for. 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.[36] LEDs with CRI below 75 are not recommended for use in indoor lighting.[37]

LED lamps may flicker. The effect can be seen on a slow motion video of such a lamp. 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. Longer exposures to flickering light contribute to headaches and eye strain.

LED lamps are high intensity point sources of light. As such looking directly at them is damaging for the eye. The reason for this is the same as for looking at the sun on a solar eclipse. At daytime the bright light causes the pupil to contract and activates a reflex to blink or look away. With a point light source this reflex does not activate but the damage to the retina is the same just to a smaller area of it.

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 or partially enclosed fixtures before installation since heat build-up could cause lamp failure and/or fire.

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 can 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.[38]

Efficiency droop

The term "efficiency droop" refers to the decrease in luminous efficacy of LEDs as the electric 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 electric currents creates 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.[39][40][41][42]

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.[43] The mechanism causing efficiency droop was identified in 2007 as Auger recombination, which was taken with mixed reaction.[42] In 2013, a study conclusively identified Auger recombination as the cause of efficiency droop.[44]

Development and adoption history

The first LEDs were developed in the early 1960s, however, they were low-powered and only produced light in the low, red frequencies of the spectrum. The first high-brightness blue LED was demonstrated by Shuji Nakamura of Nichia Corporation in 1994.[45] The existence of blue LEDs and high-efficiency LEDs led to the development of the first 'white LED', which employed a phosphor coating to partially convert the emitted blue light to red and green frequencies creating a light that appears white.[46] Isamu Akasaki, Hiroshi Amano and Nakamura were later awarded the 2014 Nobel prize in physics for the invention of the blue LED.[47]

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 replacements for 60 W incandescent lamps and PAR 38 halogen lamps. The EISA legislation established basic requirements and prize amounts for each of the two competition categories, and authorized up to $20 million in cash prizes.[48] The competition also included the possibility for winners to obtain federal purchasing agreements, utility programs, and other incentives. In May 2008, they announced details of the competition and technical requirements for each category. Lighting products meeting the competition requirements could use just 17% of the energy used by most incandescent lamps in use today. That same year the DOE also launched the Energy Star program for solid-state lighting products. The EISA legislation also authorized an additional L Prize program for developing a new "21st Century Lamp".

Philips Lighting ceased research on compact fluorescents in 2008 and began devoting the bulk of its research and development budget to solid-state lighting.[38] On 24 September 2009, Philips Lighting North America became the first to submit lamps in the category to replace the standard 60 W A-19 "Edison screw fixture" light bulb,[11] with a design based on their earlier "AmbientLED" consumer product. On 3 August 2011, DOE awarded the prize in the 60 W replacement category to a Philips' LED lamp after 18 months of extensive testing.[49]

Early LED lamps varied greatly in chromaticity from the incandescent lamps they were replacing. A standard was developed, ANSI C78.377-2008, that specified the recommended color ranges for solid-state lighting products using cool to warm white LEDs with various correlated color temperatures.[50] 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.

Also in 2008 in the United States and Canada, the Energy Star program began to label 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.[51] Energy Star Light Bulbs for Consumers is a resource for finding and comparing Energy Star qualified lamps. A similar program in the United Kingdom (run by the Energy Saving Trust) was launched to identify lighting products that meet energy conservation and performance guidelines.[52]

The Illuminating Engineering Society of North America (IESNA) in 2008 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.

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.[53] Honeywell Electrical Devices and Systems (ED&S) recommend worldwide usage of LED lighting as it is energy efficient and can help save the climate.[54]

As of 2016, in the opinion of Noah Horowitz of the Natural Resources Defense Council, new standards proposed by the United States Department of Energy would likely mean most light bulbs used in the future would be LED.[55]

Examples of early adoption

LEDs as Christmas illumination in Viborg, Denmark

In 2008 Sentry Equipment Corporation in Oconomowoc, Wisconsin, US, was able to light its new factory interior and exterior almost solely with LEDs. Initial cost was three times that of 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.[38] 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.[56]

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.[57]

In 2009 a new highway (A29) was inaugurated in Aveiro, Portugal, it included the first European public LED-based lighting highway.[58]

By 2010 mass installations of LED lighting for commercial and public uses were becoming common. LED lamps were used for a number of demonstration projects for outdoor lighting and LED street lights. The United States Department of Energy made several reports available on the results of many pilot projects for municipal outdoor lighting,[59] and many additional streetlight and municipal outdoor lighting projects soon followed.[60]

See also

References

  1. ↑ Jacques, Carole (28 January 2014) LED Lighting Market to Grow Over 12-Fold to $25 Billion in 2023, Lux Research
  2. ↑ Damir, B (2012). "Longevity of light bulbs and how to make them last longer". RobAid. Retrieved 10 August 2015.
  3. ↑ "Warsaw Top 10" (PDF). Warsaw tour Edition nr 5, 2012. p. 20. Retrieved 1 March 2013. 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.
  4. ↑ Moreno, Ivan; Avendaño-Alejo, Maximino; Tzonchev, Rumen I. (2006). "Designing light-emitting diode arrays for uniform near-field irradiance" (PDF). Applied Optics 45 (10): 2265–2272. Bibcode:2006ApOpt..45.2265M. doi:10.1364/AO.45.002265. PMID 16607994.
  5. ↑ Moreno, Ivan; Contreras, Ulises (2007). "Color distribution from multicolor LED arrays". Optics Express 15 (6): 3607–18. Bibcode:2007OExpr..15.3607M. doi:10.1364/OE.15.003607. PMID 19532605.
  6. ↑ Lonsdale, Sarah (7 July 2010). "Green property: energy-efficient bulbs". The Daily Telegraph (London). Retrieved 8 June 2011.
  7. ↑ Elisabeth Rosenthal and Felicity Barringer, "Green Promise Seen in Switch to LED Lighting", The New York Times, 29 May 2009
  8. ↑ Taub, Eric (11 February 2009). "How Long Did You Say That Bulb Would Last". New York Times. Retrieved 9 March 2016.
  9. ↑ "Q&A: How much can I save by replacing incandescent bulbs with CFLs?". Consumer Reports. 29 March 2010. Retrieved 9 March 2016.
  10. ↑ "Integral LED Lamps Criteria Development" (PDF).
  11. 1 2 Taub, Eric; leora Broydo Vestel (24 September 2009). "Build a Better Bulb for a $10 Million Prize". New York Times. Retrieved 6 February 2010.
  12. ↑ Eric A. Taub, "LED Bulbs for the Home Near the Marketplace", The New York Times, 16 May 2010; see also Matthew L. Wald, New York Times Green Blog, "An LED That Mimics an Old Standby", 24 June 2010,
  13. ↑ Philips Flattens the Light Bulb, Mashable, Pete Pachal, 16 December 2013
  14. ↑ led grow lights guide
  15. ↑ Keefe, T.J. (2007). "The Nature of Light". Retrieved 10 September 2009.
  16. ↑ "Smart Grid Home – Quentin Wells – Google Books". Books.google.com.au. Retrieved 2015-12-24.
  17. ↑ Vergleich für Osram CLAS A 100 E27 klar, Osram CLAS A FR 100 E27, Philips Standard 100W E27 klar. idealo.de
  18. 1 2 Raatma, Lucia (2010). Green Living: No Action Too Small. Compass Point Books. p. 22. ISBN 978-0756542931.
  19. ↑ A Short History of Electric Light, The Incandescent Lamp, 1900 to 1920
  20. ↑ Guide to Selecting Frequently Switched T8 Fluorescent Lamp-Ballast Systems. RPI NLPIP, April 1998
  21. ↑ "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.
  22. ↑ "HomeDepot.com: Philips 60-Watt Household Incandescent Light Bulb". Retrieved 26 July 2012.
  23. ↑ "Eco 35-Watt Soft White Dimmable Light Bulb (4-Pack) 2015".
  24. ↑ "EcoSmart 60W Equivalent Soft White (2700K) Twister CFL Light Bulb (4-pack)". Retrieved 20 January 2014.
  25. ↑ "60W Equivalent Soft White (2700K) A19 Dimmable LED Light Bulb". Home Depot. Retrieved 8 November 2014.
  26. ↑ "Philips 455576 60W Equivalent 2700K A19 LED Light Bulb, Soft White (2-Pack)". Amazon. Retrieved 8 June 2015.
  27. ↑ "LEDnovation – 9.4 Watt – 60 Watt Equal". Retrieved 6 November 2014.
  28. ↑ "Nanoleaf One – 10 Watt – 75 Watt Equivalent". earthled.com. Retrieved 15 June 2015.
  29. ↑ "Lightbulbs – LEDs and CFLs offer more choices and savings" (PDF). ConsumerReports. 2011. Retrieved 21 January 2014.
  30. ↑ One manufacturer's warranty terms for professional LED lamps
  31. ↑ Specification of a typical domestic 9.5W LED lamp as of November 2013
  32. ↑ PF vs. Power in EU
  33. ↑ "The Clinton Presidency: Protecting Our Environment and Public Health". Retrieved 26 March 2012.
  34. ↑ "History of Energy Star". Retrieved 27 March 2012.
  35. ↑ Alena Tugend (10 May 2008). "If Your Appliances Are Avocado, They're Probably not Green". New York Times. Retrieved 29 June 2008.
  36. ↑ Appendix B: Calculating color rendering metrics. lrc.rpi.edu
  37. ↑ ENERGY STAR Program Requirements for Solid State Lighting Luminaires. (PDF) . Retrieved 2 June 2012.
  38. 1 2 3 Fans of L.E.D.'s Say This Bulb's Time Has Come By Eric A. Taub. Published: 28 July 2008 – NY Times
  39. ↑ The LED's dark secret. EnergyDaily. Retrieved on 16 March 2012.
  40. ↑ 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.
  41. ↑ Smart Lighting: New LED Drops The 'Droop'. Sciencedaily.com (13 January 2009). Retrieved on 16 March 2012.
  42. 1 2 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
  43. ↑ Identifying the Causes of LED Efficiency Droop, By Steven Keeping, Digi-Key Corporation Tech Zone
  44. ↑ Iveland, Justin; et al. (23 April 2013). "Cause of LED Efficiency Droop Finally Revealed". Physical Review Letters, 2013. Science Daily.
  45. ↑ Nakamura, S.; Mukai, T. and Senoh, M. (1994). "Candela-Class High-Brightness InGaN/AlGaN Double-Heterostructure Blue-Light-Emitting-Diodes". Appl. Phys. Lett. 64 (13): 1687. Bibcode:1994ApPhL..64.1687N. doi:10.1063/1.111832.
  46. ↑ 2006 Millennium technology prize awarded to UCSB's Shuji Nakamura. Ia.ucsb.edu (15 June 2006). Retrieved on 16 March 2012.
  47. ↑ "The Nobel Prize in Physics 2014 – Press release". www.nobelprize.org. Retrieved 7 October 2014.
  48. ↑ Progress Alerts – 2010, US Department of Energy
  49. ↑ Department of Energy Announces Philips Lighting North America as Winner of L Prize Competition | Department of Energy. Energy.gov (3 August 2011). Retrieved 2012-06-02.
  50. ↑ American National Standard for Specifications for the Chromaticity of Solid-State Lighting (SSL) Products. Nema.org. Retrieved 2 June 2012.
  51. ↑ Energy Star Program Requirements for CFLS Partner Commitments, 4th edition, dated 03/07/08, retrieved 25 June 2008.
  52. ↑ Energy saving lighting. Energysavingtrust.org.uk. Retrieved 18 January 2013.
  53. ↑ Great bright hope to end battle of the light bulbs, The Daily Mail, 29 January 2009
  54. ↑ "Switching to LED is a global task: Honeywell". 7 June 2012.
  55. ↑ Wolverton, Troy (2016-03-12). "Be prepared to say goodbye to the lightbulbs you've loved". The Charlotte Observer. San Jose Mercury News. p. 1C.
  56. ↑ Led'ing the way, Nitya Varadarajan, 5 October 2009
  57. ↑ "Of the top six in Turku, led a move – HS.fi – Domestic". 19 November 2009. Retrieved 9 January 2012.
  58. ↑ New highway connecting Lisbon to Oporto includes first European LED based lighting in a highway, Aveiro 11 September 2009
  59. ↑ U. S. Department of Energy, Solid-State Lighting GATEWAY Demonstration Results (Retrieved 16 July 2010)
  60. ↑ for example, Seattle: "Seattle Picked to Lead National Effort on LED Street Lights" (Retrieved 16 July 2010); Scottsdale: "LED Streetlight Installation" (Retrieved 2010-07-16); Ann Arbor: LED street lights (Retrieved 2010-07-16)

Further reading

External links

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