Light-emitting diode: Difference between revisions

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{{Redirect|LED}}
{{Infobox electronic component
|component = Light-emitting diode
|photo = [[File:RBG-LED.jpg|235px]]
|photo_caption = Red, pure green and blue LEDs of the 5mm diffused type
|type = [[Passive component|Passive]], [[optoelectronic]]
|working_principle = [[Electroluminescence]]
|invented = [[Oleg Losev]] (1927)<ref name="100-YEAR HISTORY">{{cite news | url=http://holly.orc.soton.ac.uk/fileadmin/downloads/100_years_of_optoelectronics__2_.pdf| title=The life and times of the LED — a 100-year history | date=April 2007| agency=The Optoelectronics Research Centre, University of Southampton| accessdate=September 4, 2012}}</ref><br> [[Nick Holonyak Jr.]] (1962)<ref name="LEMELSON-MIT">{{cite news | url=http://web.mit.edu/invent/n-pressreleases/n-press-04LMP.html | title=Inventor of Long-Lasting, Low-Heat Light Source Awarded $500,000 Lemelson-MIT Prize for Invention| date=April 21, 2004 | agency=Massachusetts Institute of Technology | accessdate=December 21, 2011 | location=Washington, D.C.}}</ref>
|first_produced =1968<ref name="Schubert" />
|symbol = [[File:LED symbol.svg]]
|pins = [[anode]] and [[cathode]]
}}
[[File:LED Labelled.svg|thumb|Parts of an LED. Although not directly labeled, the flat bottom surfaces of the anvil and post embedded inside the epoxy act as anchors, to prevent the conductors from being forcefully pulled out from mechanical strain or vibration.]]
[[File:Br20 1.jpg|thumb|alt=Modern LED [[Green retrofit|retrofit]] with E27 screw in base|A modern LED retrofit "bulb" with aluminium [[heatsink]], a light [[Diffuser (optics)|diffusing]] dome and [[E27 screw]] base, using a built-in power supply working on [[mains voltage]]]]
A '''light-emitting diode''' ('''LED''') is a [[semiconductor]] light source.<ref>
{{Cite dictionary
|title=LED
|encyclopedia=The American heritage science dictionary
|year=2005
|publisher=[[Houghton Mifflin Company]]
}} Via http://dictionary.reference.com/browse/led and http://www.thefreedictionary.com/LED, 2011-06-22.</ref> LEDs are used as indicator lamps in many devices and are increasingly used for other [[lighting]]. Appearing as practical electronic components in 1962,<ref name=LemelsonMIT/> early LEDs emitted low-intensity red light, but modern versions are available across the [[visible spectrum|visible]], [[ultraviolet]], and [[infrared]] [[wavelength]]s, with very high brightness.

When a light-emitting [[semiconductor diode|diode]] is forward-[[voltage bias|biased]] (switched on), [[electrons]] are able to [[carrier generation and recombination|recombine]] with [[electron hole]]s within the device, releasing energy in the form of [[photon]]s. This effect is called [[electroluminescence]] and the [[color]] of the light (corresponding to the energy of the photon) is determined by the [[energy gap]] of the semiconductor. An LED is often small in area (less than 1&nbsp;mm<sup>2</sup>), and integrated optical components may be used to shape its radiation pattern.<ref>{{Cite journal|doi=10.1364/OE.16.001808|title=Modeling the radiation pattern of LEDs|year=2008|last1=Moreno|first1=Ivan|first2=Ching-Cherng|journal=Optics Express|volume=16|page=1808|pmid=18542260|issue=3|last2=Sun|bibcode = 2008OExpr..16.1808M|pages=1808–1819 }}</ref> LEDs present many [[#Advantages|advantages]] over incandescent light sources including [[energy conservation|lower energy consumption]], longer [[Service life|lifetime]], improved physical robustness, smaller size, and faster switching. LEDs powerful enough for room lighting are relatively expensive and require more precise current and [[thermal management of high-power LEDs|heat management]] than compact [[fluorescent lamp]] sources of comparable output.

Light-emitting diodes are used in applications as diverse as [[navigation light#Aviation navigation lights|aviation lighting]], [[automotive lighting#Light emitting diodes (LED)|automotive lighting]], advertising, general lighting, and [[traffic signal]]s. LEDs have allowed new text, video displays, and sensors to be developed, while their high switching rates are also useful in advanced communications technology. Infrared LEDs are also used in the [[remote control]] units of many commercial products including televisions, DVD players, and other domestic appliances.

==History==

===Discoveries and early devices===
[[File:SiC LED historic.jpg|thumb|Green electroluminescence from a point contact on a crystal of [[silicon carbide|SiC]] recreates [[H. J. Round]]'s original experiment from 1907.]]
[[Electroluminescence]] as a phenomenon was discovered in 1907 by the British experimenter [[H. J. Round]] of [[Marconi Company|Marconi Labs]], using a crystal of [[silicon carbide]] and a [[cat's-whisker detector]].<ref>{{Cite journal
|author=H. J. Round
|year=1907
|title=A Note on Carborundum
|journal=Electrical World
|volume=19
|page=309}}
</ref><ref>{{cite web
|url=http://www.jmargolin.com/history/trans.htm
|author=Margolin J
|title=''The Road to the Transistor''}}
</ref>
Russian [[Oleg Vladimirovich Losev]] reported creation of the first LED in 1927.<ref name="Losev1927">{{cite journal | author=Losev, O. V. | journal=Telegrafiya i Telefoniya bez Provodov | year=1927 | volume=44 | pages=485–494}}</ref><ref>{{Cite patent|SU|12191}}</ref>
His research was distributed in Russian, German and British scientific journals, but no practical use was made of the discovery for several decades.<ref name="Zheludev_100yearhistory">{{Cite journal
|author=Zheludev, N.
|year=2007
|title=The life and times of the LED&nbsp;— a 100-year history
|journal=Nature Photonics
|volume=1
|issue=4
|pages=189–192
|url=http://www.nanophotonics.org.uk/niz/publications/zheludev-2007-ltl.pdf
|format=free-download PDF
|doi = 10.1038/nphoton.2007.34
|bibcode=2007NaPho...1..189Z}}
</ref><ref>{{cite book|author=Thomas H. Lee, |title=The design of CMOS radio-frequency integrated circuits|publisher=[[Cambridge University Press]]|year=2004|isbn=0-521-83539-9|page=20|note=visible as a Google Books preview}}</ref> Rubin Braunstein<ref>[http://personnel.physics.ucla.edu/directory/faculty/braunstein/ Rubin Braunstein]</ref> of the [[Radio Corporation of America]] reported on infrared emission from [[gallium arsenide]] (GaAs) and other semiconductor alloys in 1955.<ref>{{Cite journal|author=Braunstein, Rubin
|year=1955|title=Radiative Transitions in Semiconductors|journal=Physical Review
|volume=99
|page=1892|doi=10.1103/PhysRev.99.1892|issue=6|bibcode = 1955PhRv...99.1892B }}
</ref> Braunstein observed infrared emission generated by simple diode structures using [[gallium antimonide]] (GaSb), GaAs, [[indium phosphide]] (InP), and [[silicon-germanium]] (SiGe) alloys at room temperature and at 77&nbsp;kelvin.

In 1961 American experimenters Robert Biard and Gary Pittman, working at [[Texas Instruments]],<ref>{{cite web|url=http://invention.smithsonian.org/centerpieces/quartz/inventors/biard.html
|title=The first LEDs were infrared (invisible)
|work=The Quartz Watch |publisher=The Lemelson Center
|accessdate=2007-08-13}}
</ref> found that GaAs emitted infrared radiation when electric current was applied and received the patent for the infrared LED.

The first practical visible-spectrum (red) LED was developed in 1962 by [[Nick Holonyak|Nick Holonyak, Jr.]], while working at [[General Electric Company]].<ref name=LemelsonMIT>{{cite web
|url=http://web.mit.edu/invent/a-winners/a-holonyak.html
|title=Nick Holonyak, Jr. 2004 Lemelson-MIT Prize Winner
|publisher=Lemenson-MIT Program |accessdate=2007-08-13}}
</ref> Holonyak first reported this breakthrough in the journal Applied Physics Letters on the 1st December 1962. <ref>{{cite journal | url=http://apl.aip.org/resource/1/applab/v1/i4 | title=Coherent (Visible) Light Emission from Ga(As1−x Px) Junctions | author=Holonyak Nick | journal=Applied Physics Letters | year=1962 | month=December | issue=4}}</ref> Holonyak is seen as the "father of the light-emitting diode".<ref name="chisuntimes">{{Cite news
| title = U. of I.'s Holonyak out to take some of Edison's luster
| author = Wolinsky, Howard
| date = February 5, 2005
| url = http://findarticles.com/p/articles/mi_qn4155/is_20050202/ai_n9504926
| publisher = ''Chicago Sun-Times''
| accessdate = 2007-07-29 |archiveurl = http://web.archive.org/web/20080228071504/http://findarticles.com/p/articles/mi_qn4155/is_20050202/ai_n9504926 |archivedate = 2008-02-28}}</ref>
[[M. George Craford]],<ref>{{Cite journal|year=1995
|volume=32|pages= 52–55|doi=10.1109/6.343989|title=M. George Craford [biography]|last1=Perry|first1=T.S.|journal=IEEE Spectrum|issue=2}}</ref> a former graduate student of Holonyak, invented the first yellow LED and improved the brightness of red and red-orange LEDs by a factor of ten in 1972.<ref>{{cite web
|url=http://www.technology.gov/Medal/2002/bios/Holonyak_Craford_Dupuis.pdf
|title=Brief Biography&nbsp;— Holonyak, Craford, Dupuis
|publisher=Technology Administration
|format=PDF
|accessdate=2007-05-30}}
</ref> In 1976, T. P. Pearsall created the first high-brightness, high-efficiency LEDs for optical fiber telecommunications by inventing new semiconductor materials specifically adapted to optical fiber transmission wavelengths.<ref>{{Cite journal|title=Efficient, Lattice-matched, Double Heterostructure LEDs at 1.1 mm from GaxIn1-xAsyP1-y by Liquid-phase Epitaxy|journal=Appl. Phys. Lett.|volume=28|page=499|year=1976|doi=10.1063/1.88831|last1=Pearsall|first1=T. P.|last2=Miller|first2=B. I.|last3=Capik|first3=R. J.|last4=Bachmann|first4=K. J.|issue=9|bibcode = 1976ApPhL..28..499P }}</ref>

===Commercial development===
The first commercial LEDs were commonly used as replacements for [[incandescence|incandescent]] and [[neon lamp|neon]] indicator lamps, and in [[seven-segment display]]s,<ref>{{cite journal | url=http://www.datamath.org/Display/Monsanto.htm | title=LEDs cast Monsanto in Unfamiliar Role | author=Rostky, George | journal=Electronic Engineering Times (EETimes) | year=1997 | month=March | issue=944}}</ref> first in expensive equipment such as laboratory and electronics test equipment, then later in such appliances as TVs, radios, telephones, calculators, and even watches (see list of [[#Indicators and signs|signal uses]]).
Until 1968, visible and infrared LEDs were extremely costly, on the order of [[United States dollar|US$]]200 per unit, and so had little practical use.<ref name="Schubert" />
The [[Monsanto Company]] was the first organization to mass-produce visible LEDs, using gallium arsenide phosphide (GaAsP) in 1968 to produce red LEDs suitable for indicators.<ref name="Schubert">{{Cite book|author=E. Fred Schubert|title=Light-Emitting Diodes|publisher=Cambridge University Press|year=2003|chapter=1|isbn=0-8194-3956-8}}</ref> [[Hewlett Packard]] (HP) introduced LEDs in 1968, initially using GaAsP supplied by Monsanto. These red LEDs were bright enough only for use as indicators, as the light output was not enough to illuminate an area. Readouts in calculators were so small that plastic lenses were built over each digit to make them legible. Later, other colors grew widely available and also appeared in appliances and equipment. In the 1970s commercially successful LED devices at less than five cents each were produced by Fairchild Optoelectronics. These devices employed compound semiconductor chips fabricated with the [[planar process]] invented by Dr. Jean Hoerni at [[Fairchild Semiconductor]].<ref>{{cite patent | title=Method of Manufacturing Semiconductor Devices, US patent 3025589 | author=Hoerni, J.A. | url=http://v3.espacenet.com/textdoc?DB=EPODOC&IDX=US3025589 | year=1959 | month=May}}</ref> The combination of planar processing for chip fabrication and innovative packaging methods enabled the team at Fairchild led by optoelectronics pioneer Thomas Brandt to achieve the needed cost reductions{{fact|date=October 2012}}. These methods continue to be used by LED producers.<ref>{{Cite journal|year=2009 |journal=Science|volume=325|doi=10.1126/science.1175690|title=Printed Assemblies of Inorganic Light-Emitting Diodes for Deformable and Semitransparent Displays|last1=Park|first1=S.-I.|last2=Xiong|first2=Y.|last3=Kim|first3=R.-H.|last4=Elvikis|first4=P.|last5=Meitl|first5=M.|last6=Kim|first6=D.-H.|last7=Wu|first7=J.|last8=Yoon|first8=J.|last9=Yu|first9=C.-J.|page=977|pmid=19696346|issue=5943|bibcode = 2009Sci...325..977P|pages=977–81 }}</ref>

[[Image:TI-30-LED-Display-3682e1.jpg|thumb|right|250px|LED display of a [[TI-30]] scientific calculator (ca. 1978), which uses plastic lenses to increase the visible digit size]] As LED materials technology grew more advanced, light output rose, while maintaining efficiency and reliability at acceptable levels. The invention and development of the high-power white-light LED led to use for illumination, which is fast replacing incandescent and fluorescent lighting<ref>[http://www.electrooptics.com/features/junjul06/junjul06leds.html LED there be light]. Electrooptics.com. Retrieved on 2012-03-16.</ref><ref>{{Cite news
|url=http://www.forbes.com/2008/02/27/incandescent-led-cfl-pf-guru_in_mm_0227energy_inl.html
|title= The LED Illumination Revolution
|accessdate = 2009-03-04
| work=Forbes
|date=2008-02-27}}</ref>
(see list of [[#Lighting|illumination applications]]).
Most LEDs were made in the very common 5&nbsp;mm T1¾ and 3&nbsp;mm T1 packages, but with rising power output, it has grown increasingly necessary to shed excess heat to maintain reliability,<ref>[http://www.lunaraccents.com/educational-LED-thermal-management.html LED Thermal Management]. Lunaraccents.com. Retrieved on 2012-03-16.</ref> so more complex packages have been adapted for efficient heat dissipation. Packages for state-of-the-art [[#High-power|high-power LEDs]] bear little resemblance to early LEDs.

===The blue and white LED===
[[File:Haitz law.svg|thumb|300px|Illustration of [[Haitz's law]]. Light output per LED as a function of production year; note the logarithmic scale on the vertical axis]]
The first high-brightness blue LED was demonstrated by [[Shuji Nakamura]] of [[Nichia Corporation]] in 1994 and was based on [[indium gallium nitride|InGaN]].<ref name="Nakamura">{{cite journal | title=Candela-Class High-Brightness InGaN/AlGaN Double-Heterostructure Blue-Light-Emitting-Diodes | author=S. Nakamura, T. Mukai, M. Senoh | journal=Appl. Phys. Lett. | year=1994 | volume=64 | page=1687|bibcode = 1994ApPhL..64.1687N |doi = 10.1063/1.111832 }}</ref> Its development built on critical developments in [[gallium nitride|GaN]] nucleation on sapphire substrates and the demonstration of [[P-type semiconductor|p-type doping]] of GaN, developed by [[Isamu Akasaki]] and H. Amano in [[Nagoya]].{{Citation needed|date=January 2012}} In 1995, [[Alberto Barbieri]] at the [[Cardiff University]] Laboratory (GB) investigated the efficiency and reliability of high-brightness LEDs and demonstrated a "transparent contact" LED using [[indium tin oxide]] (ITO) on (AlGaInP/GaAs). The existence of blue LEDs and high-efficiency LEDs quickly led to the development of the first [[#Phosphor-based LEDs|white LED]], which employed a {{chem|Y|3|Al|5|O|12}}:Ce, or "[[YAG]]", phosphor coating to mix ([[Optical downconverter|down-converted]] yellow light with blue to produce light that appears white. Nakamura was awarded the 2006 [[Millennium Technology Prize]] for his invention.<ref>[http://www.ia.ucsb.edu/pa/display.aspx?pkey=1475 2006 Millennium technology prize awarded to UCSB's Shuji Nakamura]. Ia.ucsb.edu (2006-06-15). Retrieved on 2012-03-16.</ref>

The development of LED technology has caused their efficiency and light output to [[exponential growth|rise exponentially]], with a doubling occurring approximately every 36 months since the 1960s, in a way similar to [[Moore's law]]. This trend is generally attributed to the parallel development of other semiconductor technologies and advances in optics and material science, and has been called [[Haitz's law]] after Dr. Roland Haitz.<ref>{{Cite journal|doi=10.1038/nphoton.2006.78|title=Haitz's law|year=2007|journal=Nature Photonics|volume=1|page=23|issue=1|bibcode = 2007NaPho...1...23. }}</ref>

In 2001<ref>
{{Cite journal |doi=10.1002/1521-396X(200111)188:1<155::AID-PSSA155>3.0.CO;2-P |journal=Physica Status Solidi (a) |volume=188 |page=155 |title=Crack-Free InGaN/GaN Light Emitters on Si(111) |year=2001
|issue=1 |last1=Dadgar|first1=A.|last2=Alam |first2=A. |last3=Riemann |first3=T>. |last4=Blaesing |first4=J. |last5=Diez |first5=A. |last6=Poschenrieder |first6=M. |last7=Strassburg |first7=M. |last8=Heuken |first8=A. |last9=Christen |first9=J. |last10=Krost |first10=A. }}</ref> and 2002,<ref>
{{Cite journal |doi=10.1063/1.1479455 |journal=Appl. Phys. Lett. |volume=80 |page=3670 |title=Thick, crack-free blue light-emitting diodes on Si(111) using low-temperature AlN interlayers and in-situ SixNy masking |year=2002 |issue=20 |last1=Dadgar |first1=A.|last2=Poschenrieder |first2=M. |last3=Bläsing |first3=J. |last4=Fehse |first4=K. |last5=Diez |first5=A. |last6=Krost |first6=A. |bibcode = 2002ApPhL..80.3670D }}</ref> processes for growing [[gallium nitride]] (GaN) LEDs on [[silicon]] were successfully demonstrated. In January 2012, high-power LEDs such "GaN on Si" LEDs were demonstrated commercially.<ref>↑ http://www.osram-os.de/osram_os/EN/Press/Press_Releases/Company_Information/2012/_documents/OSRAM_PI_Production_GaNonSi_e.pdf, In: www.osram.de. 12. January 2012, access date 12th of January 2012 (PDF)</ref> It has been speculated that the use of six-inch silicon wafers instead of two-inch [[sapphire]] wafers and [[epitaxy]] manufacturing processes could reduce production costs by up to 90%.<ref>[http://www.rsc.org/chemistryworld/News/2009/January/30010901.asp Colin J. Humphreys' cheap LED production method]. Rsc.org (2009-01-30). Retrieved on 2012-03-16.</ref>

==Technology==
[[File:PnJunction-LED-E.svg|thumb|300px|right|The inner workings of an LED]]
[[File:Diode-IV-Curve.svg|thumb|300px|right|I-V diagram for a [[diode]]. An LED will begin to emit light when the on-[[voltage]] is exceeded. Typical on voltages are 2–3 [[volt]]s.]]

===Physics===
The LED consists of a chip of semiconducting material [[Doping (semiconductor)|doped]] with impurities to create a ''[[p-n junction]]''. As in other diodes, current flows easily from the p-side, or [[anode]], to the n-side, or [[cathode]], but not in the reverse direction. Charge-carriers&nbsp;— [[electron]]s and [[electron hole|holes]]&nbsp;— flow into the junction from [[electrode]]s with different voltages. When an electron meets a hole, it falls into a lower [[energy level]], and releases [[energy]] in the form of a [[photon]].

The [[wavelength]] of the light emitted, and thus its color depends on the [[band gap]] energy of the materials forming the ''p-n junction''. In [[silicon]] or [[germanium]] diodes, the electrons and holes recombine by a ''non-radiative transition'', which produces no optical emission, because these are [[indirect band gap]] materials. The materials used for the LED have a [[direct band gap]] with energies corresponding to near-infrared, visible, or near-ultraviolet light.

LED development began with infrared and red devices made with [[gallium arsenide]]. Advances in [[materials science]] have enabled making devices with ever-shorter wavelengths, emitting light in a variety of colors.

LEDs are usually built on an n-type substrate, with an electrode attached to the p-type layer deposited on its surface. P-type substrates, while less common, occur as well. Many commercial LEDs, especially GaN/InGaN, also use [[sapphire]] substrate.

Most materials used for LED production have very high [[refractive index|refractive indices]]. This means that much light will be reflected back into the material at the material/air surface interface. Thus, [[light extraction in LEDs]] is an important aspect of LED production, subject to much research and development.

===Refractive index===
[[File:LED-chip-20-deg-crti-angle - both types - crop.png|thumb|300px|Idealized example of light emission cones in a semiconductor, for a single point-source emission zone. The left illustration is for a fully translucent wafer, while the right illustration shows the half-cones formed when the bottom layer is fully opaque. The light is actually emitted equally in all directions from the point-source, so the areas between the cones shows the large amount of trapped light energy that is wasted as heat.<ref name="Electroluminescence">{{cite book | title=Electroluminescence I | publisher=Academic Press | author=Mueller, Gerd | year=2000 | page=67 | isbn=0-12-752173-9 | url=http://books.google.com/books?id=2plxAU3tPj4C&lpg=PA67}}</ref>]]
[[File:LED light emission cones from 2D plane emission zone.png|thumb|300px|

The light emission cones of a real LED wafer are far more complex than a single point-source light emission. The light emission zone is typically a two-dimensional plane between the wafers. Every atom across this plane has an individual set of emission cones.





Drawing the billions of overlapping cones is impossible, so this is a simplified diagram showing the extents of all the emission cones combined. The larger side cones are clipped to show the interior features and reduce image complexity; they would extend to the opposite edges of the two-dimensional emission plane.

]]

Bare uncoated semiconductors such as [[silicon]] exhibit a very high [[refractive index]] relative to open air, which prevents passage of photons at sharp angles relative to the air-contacting surface of the semiconductor. This property affects both the light-emission efficiency of LEDs as well as the light-absorption efficiency of [[photovoltaic cell]]s. The refractive index of silicon is 3.96 (590&nbsp;nm),<ref name="si_website">{{cite web|url=http://pvcdrom.pveducation.org/APPEND/OPTICAL.HTM |title=Optical Properties of Silicon |accessdate=2009-05-31}}</ref> while air is 1.0002926.<ref>[http://interactagram.com/physics/optics/refraction/ Refraction&nbsp;— Snell's Law]. Interactagram.com. Retrieved on 2012-03-16.</ref>

In general, a flat-surface uncoated LED semiconductor chip will emit light only perpendicular to the semiconductor's surface, and a few degrees to the side, in a cone shape referred to as the ''light cone'', ''cone of light'',<ref>Bela G. Liptak, Béla G. Lipták [http://books.google.com/books?id=TxKynbyaIAMC&lpg=PA537 Instrument Engineers' Handbook: Process control and optimization], CRC Press, 2005, ISBN 0-8493-1081-4 p. 537, "cone of light" in context of optical fibers</ref> or the ''escape cone''.<ref name="critical">Gerd Mueller [http://books.google.com/books?id=2plxAU3tPj4C&lpg=PA67 Electroluminescence I], Academic Press, 2000, ISBN 0-12-752173-9, p. 67, "escape cone of light" from semiconductor, illustrations of light cones on p. 69</ref> The maximum [[angle of incidence]] is referred to as the [[critical angle (optics)|critical angle]]. When this angle is exceeded, photons no longer penetrate the semiconductor but are instead reflected both internally inside the semiconductor crystal and externally off the surface of the crystal as if it were a [[mirror]].<ref name="critical" />

[[Internal reflection]]s can escape through other crystalline faces, if the incidence angle is low enough and the crystal is sufficiently transparent to not re-absorb the photon emission. But for a simple square LED with 90-degree angled surfaces on all sides, the faces all act as equal angle mirrors. In this case the light can not escape and is lost as waste heat in the crystal.<ref name="critical" />

A convoluted chip surface with angled [[facet]]s similar to a jewel or [[fresnel lens]] can increase light output by allowing light to be emitted perpendicular to the chip surface while far to the sides of the photon emission point.<ref>{{cite book|author=Peter Capper, Michael Mauk |url=http://books.google.com/books?id=IfLGPRJDfqgC&lpg=PA389 |title=Liquid phase epitaxy of electronic, optical, and optoelectronic materials|publisher= Wiley|year=2007|isbn=0-470-85290-9| page=389|note="faceted structures are of interest for solar cells, LEDs, thermophotovoltaic devices, and detectors in that nonplanar surfaces and facets can enhance optical coupling and light-trapping effects", with example microphotograph of a faceted crystal substrate.}}</ref>

The ideal shape of a semiconductor with maximum light output would be a [[microsphere]] with the photon emission occurring at the exact center, with electrodes penetrating to the center to contact at the emission point. All light rays emanating from the center would be perpendicular to the entire surface of the sphere, resulting in no internal reflections. A hemispherical semiconductor would also work, with the flat back-surface serving as a mirror to back-scattered photons.<ref>John Dakin, Robert G. W. Brown (ed.) [http://books.google.com/books?id=3GmcgL7Z-6YC&lpg=PA356 Handbook of optoelectronics, Volume 2], Taylor & Francis, 2006 ISBN 0-7503-0646-7 p. 356, "Die shaping is a step towards the ideal solution, that of a point light source at the center of a spherical semiconductor die."</ref>

====Transition coatings====
Many LED semiconductor chips are [[Potting (electronics)|potted]] in clear or colored molded plastic shells. The plastic shell has three purposes:
# Mounting the semiconductor chip in devices is easier to accomplish.
# The tiny fragile electrical wiring is physically supported and protected from damage.
# The plastic acts as a refractive intermediary between the relatively high-index semiconductor and low-index open air.<ref>E. Fred Schubert [http://books.google.com/books?id=0H4bWIpaXb0C&lpg=PA97 Light-emitting diodes], Cambridge University Press, 2006, ISBN 0-521-86538-7 p. 97, "Epoxy Encapsulants", "The light extraction efficiency can be enhanced by using dome-shaped encapsulants with a large refractive index."</ref>
The third feature helps to boost the light emission from the semiconductor by acting as a diffusing lens, allowing light to be emitted at a much higher angle of incidence from the light cone than the bare chip is able to emit alone.

===Efficiency and operational parameters===
Typical indicator LEDs are designed to operate with no more than 30–60 [[milliwatt]]s (mW) of electrical power. Around 1999, [[Philips Lumileds Lighting Company|Philips Lumileds]] introduced power LEDs capable of continuous use at one [[watt]]. These LEDs used much larger semiconductor die sizes to handle the large power inputs. Also, the semiconductor dies were mounted onto metal slugs to allow for heat removal from the LED die.

One of the key advantages of LED-based lighting sources is high [[luminous efficiency]]. White LEDs quickly matched and overtook the efficacy of standard incandescent lighting systems. In 2002, Lumileds made five-watt LEDs available with a [[luminous efficacy]] of 18–22 lumens per watt (lm/W). For comparison, a conventional [[incandescent light bulb]] of 60–100 watts emits around 15 lm/W, and standard [[fluorescent light]]s emit up to 100 lm/W. A recurring problem is that efficacy falls sharply with rising current. This effect is known as [[droop (LED)|droop]] and effectively limits the light output of a given LED, raising heating more than light output for higher current.<ref>{{Cite journal
|doi=10.1134/S1063782606050162
|journal=Semiconductors
|volume=40
|page=605
|title=Effect of the joule heating on the quantum efficiency and choice of thermal conditions for high-power blue InGaN/GaN LEDs
|author=Efremov
|year=2006
|issue=5|bibcode = 2006Semic..40..605E
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|first6=D. V.
|last7=Shreter
|first7=Yu. G. }}</ref><ref name="sciencedaily1">[http://www.sciencedaily.com/releases/2009/01/090113123718.htm Smart Lighting: New LED Drops The 'Droop']. Sciencedaily.com (2009-01-13). Retrieved on 2012-03-16.</ref><ref>Richard Stevenson [http://www.spectrum.ieee.org/semiconductors/optoelectronics/the-leds-dark-secret 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, August 2009</ref>

In September 2003, a new type of blue LED was demonstrated by the company [[Cree Inc.]] to provide 24&nbsp;mW at 20 [[milliampere]]s (mA). This produced a commercially packaged white light giving 65 lm/W at 20 mA, becoming the brightest white LED commercially available at the time, and more than four times as efficient as standard incandescents. In 2006, they demonstrated a prototype with a record white LED luminous efficacy of 131 lm/W at 20 mA. [[Nichia Corporation]] has developed a white LED with luminous efficacy of 150 lm/W at a forward current of 20 mA.<ref>{{Cite news|url=http://techon.nikkeibp.co.jp/english/NEWS_EN/20061221/125713/ |title=Nichia Unveils White LED with 150 lm/W Luminous Efficiency |publisher=Tech-On! |date=December 21, 2006 |accessdate=2007-08-13}}</ref> Cree's XLamp XM-L LEDs, commercially available in 2011, produce 100 lumens per watt at their full power of 10 watts, and up to 160 lumens/watt at around 2 watts input power.

Practical general lighting needs high-power LEDs, of one watt or more. Typical operating currents for such devices begin at 350 mA.

Note that these efficiencies are for the LED chip only, held at low temperature in a lab. Lighting works at higher temperature and with drive circuit losses, so efficiencies are much lower. [[United States Department of Energy]] (DOE) testing of commercial LED lamps designed to replace incandescent lamps or [[Compact fluorescent lamp|CFLs]] showed that average efficacy was still about 46 lm/W in 2009 (tested performance ranged from 17&nbsp;lm/W to 79&nbsp;lm/W).<ref name="doe">
{{Cite book| title=DOE Solid-State Lighting CALiPER Program Summary of Results: Round 9 of Product Testing.| publisher=U.S. Department of Energy
| date=October 2009
| url=http://apps1.eere.energy.gov/buildings/publications/pdfs/ssl/caliper_round-9_summary.pdf
| format=PDF}}</ref>

Cree issued a press release on February 3, 2010 about a laboratory prototype LED achieving 208 lumens per watt at room temperature. The correlated [[color temperature]] was reported to be 4579&nbsp;K.<ref>{{cite press release|http://www.cree.com/press/press_detail.asp?i=1265232091259 |title=Cree Breaks 200 Lumen Per Watt Efficacy Barrier |publisher=CREE |location=Durham, N.C |date=February 3, 2010 |accessdate=20109-03-22}}</ref>

===Lifetime and failure===
{{Main|List of LED failure modes}}
Solid-state devices such as LEDs are subject to very limited [[wear and tear]] if operated at low currents and at low temperatures. Many of the LEDs made in the 1970s and 1980s are still in service today. Typical lifetimes quoted are 25,000 to 100,000 hours, but heat and current settings can extend or shorten this time significantly.
<ref>[http://www1.eere.energy.gov/buildings/ssl/lifetime.html Department of Energy] Lifetime of White LEDs, accessdate 2009-02-20</ref>

The most common symptom of LED (and [[diode laser]]) failure is the gradual lowering of light output and loss of efficiency. Sudden failures, although rare, can occur as well. Early red LEDs were notable for their short service life. With the development of high-power LEDs the devices are subjected to higher [[junction temperature]]s and higher current densities than traditional devices. This causes stress on the material and may cause early light-output degradation. To quantitatively classify useful lifetime in a standardized manner it has been suggested to use the terms L70 and L50, which is the time it will take a given LED to reach 70% and 50% light output respectively.<ref>Narendran, N. and Y. Gu. 2005. Life of LED-based white light sources. IEEE/OSA Journal of Display Technology 1(1): 167-171.</ref>

Like other lighting devices, LED performance is temperature dependent. Most manufacturers' published ratings of LEDs are for an operating temperature of 25&nbsp;°C. LEDs used outdoors, such as traffic signals or in-pavement signal lights, and that are utilized in climates where the temperature within the luminaire gets very hot, could result in low signal intensities or even failure.<ref name="RPI.edu">Conway, K. M. and J. D. Bullough. 1999. [http://www.lrc.rpi.edu/resources/pdf/57-1999.pdf Will LEDs transform traffic signals as they did exit signs?] Proceedings of the Illuminating Engineering Society of North America Annual Conference (pp. 1–9), New Orleans, Louisiana, August 9–11. New York, NY: Illuminating Engineering Society of North America.</ref>

LED light output rises at lower temperatures, leveling off, depending on type, at around -30&nbsp;°C.{{Citation needed|date=September 2010}} Thus, LED technology may be a good replacement in uses such as supermarket freezer lighting<ref>Narendran, N., J. Brons, and J. Taylor. 2006. [http://www.lrc.rpi.edu/programs/solidstate/cr_freezers.asp Energy-efficient Alternative for Commercial Refrigeration]. Project report prepared for the New York State Energy Research and Development Authority.</ref><ref>ASSIST. 2008. [http://www.lrc.rpi.edu/programs/solidstate/assist/pdf/AR-FreezerCaseTesting-Nov2008.pdf Recommendations for Testing and Evaluating Luminaires for Refrigerated and Freezer Display Cases]. Vol. 5, Issue 1. Troy, N.Y.: Lighting Research Center.</ref><ref>Narendran, N. 2006. [http://www.lrc.rpi.edu/programs/delta/pdf/FTDeltaFreezer.pdf Field Test DELTA Snapshots: LED Lighting In Freezer Cases]. Troy, N.Y.: Lighting Research Center.</ref> and will last longer than other technologies. Because LEDs emit less heat than incandescent bulbs, they are an energy-efficient technology for uses such as in freezers and refrigerators. However, because they emit little heat, ice and snow may build up on the LED luminaire in colder climates.<ref name="RPI.edu"/> Similarly, this lack of waste heat generation has been observed to sometimes cause significant problems with street traffic signals and airport runway lighting in snow-prone areas. In response to this problem, some LED lighting systems have been designed with an added heating circuit at the expense of reduced overall electrical efficiency of the system; additionally, research has been done to develop heat sink technologies that will transfer heat produced within the junction to appropriate areas of the luminaire.<ref>Gu, Y., A. Baker, and N. Narendran. 2007. [http://www.lrc.rpi.edu/programs/solidstate/cr_blueTaxiway.asp Investigation of thermal management technique in blue LED airport taxiway fixtures]. Seventh International Conference on Solid State Lighting, Proceedings of SPIE 6669: 66690U.</ref>

==Colors and materials==
Conventional LEDs are made from a variety of inorganic [[semiconductor materials]]. The following table shows the available colors with wavelength range, voltage drop and material:

{| class="wikitable"
!
!Color
![[Wavelength]] [nm]
![[Voltage drop]] [ΔV]
!Semiconductor material
|-
| style="background:#200000;"|
|[[Infrared]] ||[[Wavelength|''λ'']] > 760 ||[[Delta (letter)|Δ]]''V'' < 1.63 || [[Gallium arsenide]] (GaAs)<br /> [[Aluminium gallium arsenide]] (AlGaAs)
|-
| style="background:red;"|
|[[Red]] ||610 < ''λ'' < 760 ||1.63 < Δ''V'' < 2.03 || [[Aluminium gallium arsenide]] (AlGaAs)<br />[[Gallium arsenide phosphide]] (GaAsP)<br />[[Aluminium gallium indium phosphide]] (AlGaInP) <br /> [[Gallium(III) phosphide]] (GaP)
|-
| style="background:#ff7f00;"|
|[[Orange (colour)|Orange]] ||590 < ''λ'' < 610 ||2.03 < Δ''V'' < 2.10 || [[Gallium arsenide phosphide]] (GaAsP)<br />[[Aluminium gallium indium phosphide]] (AlGaInP) <br />[[Gallium(III) phosphide]] (GaP)
|-
| style="background:yellow;"|
|[[Yellow]] ||570 < ''λ'' < 590 ||2.10 < Δ''V'' < 2.18 || [[Gallium arsenide phosphide]] (GaAsP)<br />[[Aluminium gallium indium phosphide]] (AlGaInP) <br /> [[Gallium(III) phosphide]] (GaP)
|-
| style="background:#0f0;"|
|[[Green]] ||500 < ''λ'' < 570 ||1.9<ref>[http://catalog.osram-os.com/media/_en/Graphics/00041987_0.pdf OSRAM: green LED]. (PDF) . Retrieved on 2012-03-16.</ref> < Δ''V'' < 4.0 || [[Indium gallium nitride]] (InGaN) / [[Gallium(III) nitride]] (GaN)<br />[[Gallium(III) phosphide]] (GaP)<br />[[Aluminium gallium indium phosphide]] (AlGaInP)<br />[[Aluminium gallium phosphide]] (AlGaP)
|-
| style="background:blue;"|
|[[Blue]] ||450 < ''λ'' < 500 ||2.48 < Δ''V'' < 3.7 || [[Zinc selenide]] (ZnSe)<br />[[Indium gallium nitride]] (InGaN)<br />[[Silicon carbide]] (SiC) as substrate<br />[[Silicon]] (Si) as substrate&nbsp;— under development
|-
| style="background:#8b00ff;"|
|[[Violet (color)|Violet]] ||400 < ''λ'' < 450 ||2.76 < Δ''V'' < 4.0 || [[Indium gallium nitride]] (InGaN)
|-
| style="background:#bf00ff;"|
|[[Purple]] ||multiple types ||2.48 < Δ''V'' < 3.7 || Dual blue/red LEDs,<br /> blue with red phosphor,<br /> or white with purple plastic
|-
| style="background:#200020;"|
|[[Ultraviolet]] ||''λ'' < 400 ||3.1 < Δ''V'' < 4.4 || [[Diamond]] (235&nbsp;nm)<ref name=dia>{{Cite journal|doi=10.1126/science.1060258|title=Ultraviolet Emission from a Diamond pn Junction|year=2001|last1=Koizumi|first1=S.|journal=Science|volume=292|page=1899|pmid=11397942|first2=K|first3=M|first4=H|issue=5523|last2=Watanabe|last3=Hasegawa|last4=Kanda|bibcode = 2001Sci...292.1899K|pages=1899–1901 }}</ref><br />[[Boron nitride]] (215&nbsp;nm)<ref name=BN>{{Cite journal|doi=10.1126/science.1144216|title=Deep Ultraviolet Light-Emitting Hexagonal Boron Nitride Synthesized at Atmospheric Pressure|year=2007|last1=Kubota|first1=Y.|first2=K.|first3=O.|first4=T.|journal=Science|volume=317|page=932|pmid=17702939|issue=5840|last2=Watanabe|last3=Tsuda|last4=Taniguchi|bibcode = 2007Sci...317..932K|pages=932–934 }}</ref><ref name=bn2>{{Cite journal|doi=10.1038/nmat1134|title=Direct-bandgap properties and evidence for ultraviolet lasing of hexagonal boron nitride single crystal|year=2004|last1=Watanabe|first1=Kenji|first2=Takashi|first3=Hisao|journal=Nature Materials|volume=3|page=404|pmid=15156198|issue=6|last2=Taniguchi|last3=Kanda|bibcode = 2004NatMa...3..404W|pages=404–409 }}</ref><br /> [[Aluminium nitride]] (AlN) (210&nbsp;nm)<ref name=aln>{{Cite journal|doi=10.1038/nature04760|title=An aluminium nitride light-emitting diode with a wavelength of 210 nanometres|year=2006|last1=Taniyasu|first1=Yoshitaka|first2=Makoto|first3=Toshiki|journal=Nature|volume=441|page=325|pmid=16710416|issue=7091|last2=Kasu|last3=Makimoto|bibcode = 2006Natur.441..325T|pages=325–328 }}</ref><br /> [[Aluminium gallium nitride]] (AlGaN)<br /> [[Aluminium gallium indium nitride]] (AlGaInN)&nbsp;— down to 210&nbsp;nm<ref>{{Cite news|url=http://physicsworld.com/cws/article/news/24926 |title=LEDs move into the ultraviolet |date=May 17, 2006 |publisher=physicsworld.com |accessdate=2007-08-13}}</ref>
|-
| style="background:#ff80C0;"|
|[[Pink]] ||multiple types ||Δ''V'' ~ 3.3<ref>[http://www.llamma.com/xbox360/mods/How%20to%20use%20an%20LED.htm How to Wire/Connect LEDs]. Llamma.com. Retrieved on 2012-03-16.</ref> || Blue with one or two phosphor layers: <br /> yellow with red, orange or pink phosphor added afterwards,<br /> or white with pink pigment or dye.<ref>[http://donklipstein.com/ledc.html LED types by Color, Brightness, and Chemistry]. Donklipstein.com. Retrieved on 2012-03-16.</ref>
|-
| style="background:white;"|
|White || Broad spectrum ||Δ''V'' = 3.5 || Blue/UV diode with yellow phosphor &nbsp;
|}

===Ultraviolet and blue LEDs===
[[File:Blue light emitting diodes over a proto-board.jpg|thumb|[[Blue]] LEDs]]

Current bright blue LEDs are based on the wide [[band gap]] semiconductors GaN ([[gallium nitride]]) and [[Indium gallium nitride|InGaN]] (indium gallium nitride). They can be added to existing red and green LEDs to produce the impression of white light, though white LEDs today rarely use this principle.

The first blue LEDs using gallium nitride were made in 1971 by Jacques Pankove at [[RCA|RCA Laboratories]].<ref>E. Fred Schubert ''Light-emitting diodes 2nd ed.'', Cambridge University Press, 2006 ISBN 0-521-86538-7 page 16-17</ref> These devices had too little light output to be of practical use and research into gallium nitride devices slowed. In August 1989, Cree Inc. introduced the first commercially available blue LED based on the [[Direct and indirect band gaps|indirect bandgap]] semiconductor, silicon carbide.<ref>[http://www.cree.com/about/milestones.asp Major Business and Product Milestones]. Cree.com. Retrieved on 2012-03-16.</ref> SiC LEDs had very low efficiency, no more than about 0.03%, but did emit in the blue portion of the visible light spectrum.

In the late 1980s, key breakthroughs in GaN [[epitaxial]] growth and [[P-type semiconductor|p-type]] doping<ref>{{cite web
|title = GaN-based blue light emitting device development by Akasaki and Amano
|work = Takeda Award 2002 Achievement Facts Sheet
|publisher = The Takeda Foundation
|date = 2002-04-05
|url = http://www.takeda-foundation.jp/en/award/takeda/2002/fact/pdf/fact01.pdf
|format = PDF
|accessdate = 2007-11-28}}</ref> ushered in the modern era of GaN-based optoelectronic devices. Building upon this foundation, in 1993 high-brightness blue LEDs were demonstrated. Efficiency (light energy produced vs. electrical energy used) reached 10%.<ref>{{US patent|5578839}} "Light-emitting gallium nitride-based compound semiconductor device" Nakamura et al., Issue date: November 26, 1996</ref>
High-brightness blue LEDs invented by [[Shuji Nakamura]] of [[Nichia Corporation]] using gallium nitride revolutionized LED lighting, making high-power light sources practical.

By the late 1990s, blue LEDs had become widely available. They have an active region consisting of one or more InGaN [[quantum well]]s sandwiched between thicker layers of GaN, called cladding layers. By varying the relative InN-GaN fraction in the InGaN quantum wells, the light emission can be varied from violet to amber. AlGaN [[aluminium gallium nitride]] of varying AlN fraction can be used to manufacture the cladding and quantum well layers for ultraviolet LEDs, but these devices have not yet reached the level of efficiency and technological maturity of the InGaN-GaN blue/green devices. If the active quantum well layers are GaN, instead of alloyed InGaN or AlGaN, the device will emit near-ultraviolet light with wavelengths around 350–370&nbsp;nm. Green LEDs manufactured from the InGaN-GaN system are far more efficient and brighter than green LEDs produced with non-nitride material systems.

With nitrides containing aluminium, most often [[Aluminium gallium nitride|AlGaN]] and [[aluminium gallium indium nitride|AlGaInN]], even shorter wavelengths are achievable. Ultraviolet LEDs in a range of wavelengths are becoming available on the market. Near-UV emitters at wavelengths around 375–395&nbsp;nm are already cheap and often encountered, for example, as [[black light]] lamp replacements for inspection of anti-[[counterfeiting]] UV watermarks in some documents and paper currencies. Shorter-wavelength diodes, while substantially more expensive, are commercially available for wavelengths down to 247&nbsp;nm.<ref>[http://www.s-et.com/products.htm Sensor Electronic Technology, Inc.: Nitride Products Manufacturer]{{dead link|date=March 2012}}</ref> As the photosensitivity of microorganisms approximately matches the absorption spectrum of [[DNA]], with a peak at about 260&nbsp;nm, UV LED emitting at 250–270&nbsp;nm are to be expected in prospective disinfection and sterilization devices. Recent research has shown that commercially available UVA LEDs (365&nbsp;nm) are already effective disinfection and sterilization devices.<ref name="water sterilization">{{Cite journal|doi=10.1007/s11517-007-0263-1|title=Development of a new water sterilization device with a 365 nm UV-LED|year=2007|last1=Mori|first1=Mirei|first2=Akiko|first3=Akira|first4=Masayuki|first5=Noriko|first6=Satoko|first7=Toshitaka|first8=Yutaka|first9=Masatake|journal=Medical & Biological Engineering & Computing|volume=45|page=1237|last2=Hamamoto|last3=Takahashi|last4=Nakano|last5=Wakikawa|last6=Tachibana|last7=Ikehara|last8=Nakaya|last9=Akutagawa|pmid=17978842|issue=12|pages=1237–1241}}</ref>

Deep-UV wavelengths were obtained in laboratories using [[aluminium nitride]] (210&nbsp;nm),<ref name=aln/> [[boron nitride]] (215&nbsp;nm)<ref name=BN/><ref name=bn2/> and [[diamond]] (235&nbsp;nm).<ref name=dia/>

In 2011, Zhong Lin Wang from the [[Georgia Institute of Technology]] discovered that zinc oxide nanowires can increase the energy efficiency of [[Piezoelectric]] UV LEDs by a factor of 4, from 2% to 8% .<ref>[http://gtresearchnews.gatech.edu/zinc-oxide-led-efficiency/ Increasing energy efficiency of LEDs by 400%]. Gtresearchnews.gatech.edu (2011-10-31). Retrieved on 2012-03-16.</ref>

===White light===
There are two primary ways of producing white light-emitting diodes (WLEDs), LEDs that generate high-intensity [[white light]]. One is to use individual LEDs that emit three [[primary color]]s<ref>{{Cite journal|doi=10.1002/1520-6378(2001)26:1+<::AID-COL47>3.0.CO;2-4|year = 2000|title=The derivation of XYZ tristimulus spaces: A comparison of two alternative methods|author=J. H. Wold and A. Valberg|journal=Color Research & Application|volume=26|issue=S1|pages=S222}}</ref>&nbsp;— red, green, and blue&nbsp;— and then mix all the colors to form white light. The other is to use a phosphor material to convert monochromatic light from a blue or UV LED to broad-spectrum white light, much in the same way a fluorescent light bulb works.

Due to [[Metamerism (color)|metamerism]], it is possible to have quite different spectra that appear white.

====RGB systems====
[[File:Red-YellowGreen-Blue LED spectra.png|thumb|right|350px|Combined spectral curves for blue, yellow-green, and high-brightness red solid-state semiconductor LEDs. [[Full width at half maximum|FWHM]] spectral bandwidth is approximately 24–27 nm for all three colors.]]

White light can be formed by mixing differently colored lights; the most common method is to use [[rgb|red, green, and blue]] (RGB). Hence the method is called multi-color white LEDs (sometimes referred to as RGB LEDs). Because these need electronic circuits to control the blending and [[diffusion]] of different colors, and because the individual color LEDs typically have slightly different emission patterns (leading to variation of the color depending on direction) even if they are made as a single unit, these are seldom used to produce white lighting. Nevertheless, this method is particularly interesting in many uses because of the flexibility of mixing different colors,<ref>{{Cite journal|title=Color distribution from multicolor LED arrays|journal=Optics Express |year=2007|author=Ivan Moreno, Ulises Contreras|volume=15|issue=6|page=3607|doi=10.1364/OE.15.003607|pmid=19532605|bibcode = 2007OExpr..15.3607M|pages=3607–3618 }}</ref> and, in principle, this mechanism also has higher quantum efficiency in producing white light.

There are several types of multi-color white LEDs: [[:wiktionary:dichromatic|di-]], [[trichromatic|tri-]], and [[tetrachromatic]] white LEDs. Several key factors that play among these different methods, include color stability, [[color rendering index|color rendering]] capability, and [[luminous efficacy]]. Often, higher efficiency will mean lower color rendering, presenting a trade-off between the luminous efficiency and color rendering. For example, the dichromatic white LEDs have the best luminous efficacy (120 lm/W), but the lowest color rendering capability. However, although [[tetrachromatic]] white LEDs have excellent color rendering capability, they often have poor luminous efficiency. Trichromatic white LEDs are in between, having both good luminous efficacy (>70 lm/W) and fair color rendering capability.

Multi-color LEDs offer not merely another means to form white light but a new means to form light of different colors. Most [[color#Perception|perceivable colors]] can be formed by mixing different amounts of three primary colors. This allows precise dynamic color control. As more effort is devoted to investigating this method, multi-color LEDs should have profound influence on the fundamental method that we use to produce and control light color. However, before this type of LED can play a role on the market, several technical problems must be solved. These include that this type of LED's emission power [[exponential decay|decays exponentially]] with rising temperature,<ref>{{Cite journal|author= E. Fred Schubert and Jong Kyu Kim| journal = Science|volume= 308|doi=10.1126/science.1108712|page=1274| year = 2005|title= Solid-State Light Sources Getting Smart|issue= 5726|bibcode = 2005Sci...308.1274S }}</ref>
resulting in a substantial change in color stability. Such problems inhibit and may preclude industrial use. Thus, many new package designs aimed at solving this problem have been proposed and their results are now being reproduced by researchers and scientists.

====Phosphor-based LEDs====
[[File:White LED.png|thumb|right|350px|Spectrum of a “white” LED clearly showing blue light directly emitted by the GaN-based LED (peak at about 465 nm) and the more broadband [[Stokes shift|Stokes-shifted]] light emitted by the Ce<sup>3+</sup>:YAG phosphor, which emits at roughly 500–700 nm.]]

This method involves [[coating]] LEDs of one color (mostly blue LEDs made of InGaN) with [[phosphor]] of different colors to form white light; the resultant LEDs are called '''phosphor-based white LEDs'''.<ref>{{Cite journal
|title=YAG glass-ceramic phosphor for white LED (II): luminescence characteristics
|author=Tanabe, S. and Fujita, S. and Yoshihara, S. and Sakamoto, A. and Yamamoto, S.
|journal=Proc. Of SPIE|url=http://lib.semi.ac.cn:8080/tsh/dzzy/wsqk/SPIE/vol5941/594112.pdf
|volume=5941|doi=10.1117/12.614681
|page=594112
|year=2005}}</ref> A fraction of the blue light undergoes the [[Stokes shift]] being transformed from shorter wavelengths to longer. Depending on the color of the original LED, phosphors of different colors can be employed. If several phosphor layers of distinct colors are applied, the emitted spectrum is broadened, effectively raising the [[Color Rendering Index|color rendering index]] (CRI) value of a given LED.<ref>{{Cite journal
|title=Color rendering and luminous efficacy of white LED spectra
|author=Ohno, Y.
|journal=Proc. Of SPIE|doi=10.1117/12.565757|url=http://lib.semi.ac.cn:8080/tsh/dzzy/wsqk/SPIE/vol5530/5530-88.pdf
|volume=5530
|page=89
|year=2004}}</ref>

Phosphor-based LEDs efficiency losses are due to the heat loss from the Stokes shift and also other phosphor-related degradation issues. Its efficiencies compared to normal LEDs are dependent on the spectral distribution of the resultant light output and the original wavelength of the LED itself. Due to the simplicity of manufacturing the phosphor method is still the most popular method for making high-intensity white LEDs. The design and production of a light source or light fixture using a monochrome emitter with phosphor conversion is simpler and cheaper than a complex [[#RGB systems|RGB]] system, and the majority of high-intensity white LEDs presently on the market are manufactured using phosphor light conversion.

Among the challenges being faced to improve the efficiency of LED-based white light sources are the development of more efficient phosphors as well as the development of more efficient green LEDs. The theoretical maximum for green LEDs is at 683 lumens per watt but today few green LEDs exceed even 100 lumens per watt. Today the most efficient yellow phosphor is still the YAG phosphor, with less than 10% Stoke shift loss. Losses attributable to internal optical losses due to re-absorption in the LED chip and in the LED packaging itself account typically for another 10% to 30% of efficiency loss. Currently, in the area of phosphor LED development, much effort is being spent on optimizing these devices to higher light output and higher operation temperatures. For instance, the efficiency can be raised by adapting better package design or by using a more suitable type of phosphor. Conformal coating process is frequently used to address the issue of varying phosphor thickness.

The phosphor-based white LEDs encapsulate InGaN blue LEDs inside phosphor coated epoxy. A common yellow phosphor material is [[cerium]]-[[Doping (Semiconductors)|doped]] [[yttrium aluminium garnet]] (Ce<sup>3+</sup>:YAG).

White LEDs can also be made by [[coating]] near-[[ultraviolet]] (NUV) LEDs with a mixture of high-efficiency [[europium]]-based phosphors that emit red and blue, plus copper and aluminium-doped zinc sulfide (ZnS:Cu, Al) that emits green. This is a method analogous to the way [[fluorescent lamp]]s work. This method is less efficient than blue LEDs with YAG:Ce phosphor, as the [[Stokes shift]] is larger, so more energy is converted to heat, but yields light with better spectral characteristics, which render color better. Due to the higher radiative output of the ultraviolet LEDs than of the blue ones, both methods offer comparable brightness. A concern is that UV light may leak from a malfunctioning light source and cause harm to human eyes or skin.

====Other white LEDs====
Another method used to produce experimental white light LEDs used no phosphors at all and was based on [[epitaxy|homoepitaxially]] grown [[zinc selenide]] (ZnSe) on a ZnSe substrate that simultaneously emitted blue light from its active region and yellow light from the substrate.<ref>{{cite web| title = Joint venture to make ZnSe white LEDs
| date = December 6, 2002
| author = Tim Whitaker
| url = http://optics.org/cws/article/research/16534
| accessdate = 3 January 2009
}}</ref>

===Organic light-emitting diodes (OLEDs)===
{{Main|Organic light-emitting diode}}
[[File:OLED EarlyProduct.JPG|thumb|Demonstration of a [[flexible organic light-emitting diode|flexible OLED]] device]]
In an organic light-emitting diode ([[organic light-emitting diode|OLED]]), the [[electroluminescence|electroluminescent]] material comprising the emissive layer of the diode is an [[organic compound]]. The organic material is electrically conductive due to the [[Delocalized electron|delocalization]] of pi electrons caused by [[conjugated system|conjugation]] over all or part of the molecule, and the material therefore functions as an [[organic semiconductor]].<ref>{{Cite journal|title=Light-emitting diodes based on conjugated polymers|last1=Burroughes|first1=J H|last2=Bradley|first2=D D C|last3=Brown|first3=A R|last4=Marks|first4=R N|last5=Friend|first5=R H|last6=Burns|first6=P L|last7=Holmes|first7=A B|journal=Nature|volume=347|pages=539–541|year=1990|doi=10.1038/347539a0|issue=6293|last8=Holmes|first8=A. B.|bibcode = 1990Natur.347..539B }}</ref>
The organic materials can be small organic [[molecule]]s in a [[crystal]]line [[phase (matter)|phase]], or [[polymer]]s.

The potential advantages of OLEDs include thin, low-cost displays with a low driving voltage, wide viewing angle, and high contrast and color gamut.<ref name=bardsley>{{cite journal|title=International OLED technology roadmap|last1=Bardsley|first1=J N|journal=IEEE Journal of Selected Topics in Quantum Electronics|volume=10|pages=3–9|year=2004|issue=1|doi=10.1109/JSTQE.2004.824077}}</ref> Polymer LEDs have the added benefit of printable<ref>{{cite journal|doi=10.1063/1.120807|title=Ink-jet printing of doped polymers for organic light emitting devices|year=1998|last1=Hebner|first1=T. R.|last2=Wu|first2=C. C.|last3=Marcy|first3=D.|last4=Lu|first4=M. H.|last5=Sturm|first5=J. C.|journal=Applied Physics Letters|volume=72|page=519|issue=5|bibcode = 1998ApPhL..72..519H }}</ref><ref>{{cite journal|doi=10.1063/1.121090|title=Polymer electroluminescent devices processed by inkjet printing: I. Polymer light-emitting logo|year=1998|last1=Bharathan|first1=Jayesh|last2=Yang|first2=Yang|journal=Applied Physics Letters|volume=72|page=2660|issue=21|bibcode = 1998ApPhL..72.2660B }}</ref> and [[flexible organic light-emitting diode|flexible]]<ref>{{cite journal|doi=10.1038/357477a0|title=Flexible light-emitting diodes made from soluble conducting polymers|year=1992|last1=Gustafsson|first1=G.|last2=Cao|first2=Y.|last3=Treacy|first3=G. M.|last4=Klavetter|first4=F.|last5=Colaneri|first5=N.|last6=Heeger|first6=A. J.|journal=Nature|volume=357|page=477|issue=6378|bibcode = 1992Natur.357..477G }}</ref> displays. OLEDs have been used to make visual displays for portable electronic devices such as cellphones, digital cameras, and MP3 players while possible future uses include lighting and televisions.<ref name=bardsley />

===Quantum dot LEDs (experimental)===
[[Quantum dots]] (QD) are semiconductor [[nanocrystals]] that possess unique optical properties.<ref name=MITqdot2002>[http://web.mit.edu/newsoffice/2002/dot.html Quantum-dot LED may be screen of choice for future electronics] [[Massachusetts Institute of Technology]] News Office, December 18, 2002</ref> Their emission color can be tuned from the visible throughout the infrared spectrum. This allows quantum dot LEDs to create almost any color on the [[International Commission on Illumination|CIE]] diagram. This provides more color options and better color rendering than white LEDs.{{Citation needed|date=August 2011}} Quantum dot LEDs are available in the same package types as traditional [[phosphor]]-based LEDs.{{Citation needed|date=August 2011}}There are two types of schemes for QD excitation.
One uses photo excitation with a primary light source LED (typically blue or UV LEDs are used). The other is direct electrical excitation first demonstrated by Alivisatos et al.<ref>{{cite journal|last=V.L. Colvin|coauthors=M.C. Schlamp, and A.P. Alivisatos|journal=Nature|year=1994|volume=370|issue=354}}</ref>

One example of the photo-excitation scheme is a method developed by Michael Bowers, at [[Vanderbilt University]] in Nashville, involving coating a blue LED with quantum dots that glow white in response to the blue light from the LED. This method emits a warm, yellowish-white light similar to that made by [[incandescent bulb]]s.<ref>{{Cite news|title = Accidental Invention Points to End of Light Bulbs|publisher =LiveScience.com|date = October 21, 2005|url = http://www.livescience.com/technology/051021_nano_light.html|accessdate = 2007-01-24}}</ref> Quantum dots are also being considered for use in white light-emitting diodes in liquid crystal display (LCD) televisions.<ref>[http://www.nanocogroup.com/content/Library/NewsandEvents/articles/Nanoco_Signs_Agreement_with_Major_Japanese_Electronics_Company/136.aspx Nanoco Signs Agreement with Major Japanese Electronics Company], 23 September 2009</ref>

The major difficulty in using quantum dots-based LEDs is the insufficient stability of QDs under prolonged irradiation.{{Citation needed|date=August 2011}} In February 2011 scientists at PlasmaChem GmbH could synthesize quantum dots for LED applications and build a light converter on their basis, which could efficiently convert light from blue to any other color for many hundred hours.<ref>Nanotechnologie Aktuell, pp. 98-99, v.4, 2011, ISSN 1866-4997</ref> Such QDs can be used to emit visible or near infrared light of any wavelength being excited by light with a shorter wavelength.

The structure of QD-LEDs used for the electrical-excitation scheme is similar to basic design of [[OLED]]. A layer of quantum dots is sandwiched between layers of electron-transporting and hole-transporting materials. An applied electric field causes electrons and holes to move into the quantum dot layer and recombine forming an [[exciton]]
that excites a QD. This scheme is commonly studied for [[quantum dot display]]. The tunability of emission wavelengths and narrow bandwidth is also beneficial as excitation sources for fluorescence imaging. Fluorescence near-field scanning optical microscopy
([[NSOM]]) utilizing an integrated QD-LED has been demonstrated.<ref>{{cite journal|last=K.Hoshino|coauthors=A. Gopal, M. Glaz, D.Vanden Bout, X.J. Zhang|journal=Applied Physics Letters|year=2012|volume=101|issue=4|doi=10.1063/1.4739235|url=http://link.aip.org/link/?APL/101/043118|bibcode = 2012ApPhL.101d3118H }}</ref>

In February 2008, a [[luminous efficacy]] of 300 [[lumen (unit)|lumens]] of visible light per watt of [[radiant flux|radiation]] (not per electrical watt) and warm-light emission was achieved by using [[nanocrystal]]s.<ref>{{cite news | url=http://technology.newscientist.com/channel/tech/dn13266-crystal-coat-warms-up-led-light.html?feedId=online-news_rss20 | title=Crystal coat warms up LED light | date=1. feb 2008 | agency=newscientist.com | accessdate=January 30, 2012 | author=Inman, Mason}}</ref>
==Types==
[[File:Verschiedene LEDs.jpg|thumb|center|750px|LEDs are produced in a variety of shapes and sizes. The color of the plastic lens is often the same as the actual color of light emitted, but not always. For instance, purple plastic is often used for [[infrared]] LEDs, and most blue devices have clear housings. Modern high power LEDs such as those used for lighting and backlighting are generally found in [[surface-mount technology]] (SMT) packages, (not shown).]]
The main types of LEDs are miniature, high power devices and custom designs such as alphanumeric or multi-color.<ref>[http://www.flexfireleds.com/pages/Comparison-between-3528-LEDs-and-5050-LEDs.html What is the difference between 3528 LEDs and 5050 LEDs |SMD 5050 SMD 3528]. Flexfireleds.com. Retrieved on 2012-03-16.</ref>

===Miniature===
[[File:LEDs 8 5 3mm.JPG|thumb|Different sized LEDs. 8 mm, 5 mm and 3 mm, with a wooden match-stick for scale]]
These are mostly single-die LEDs used as indicators, and they come in various sizes from 2&nbsp;mm to 8&nbsp;mm, [[through-hole]] and [[surface mount]] packages. They usually do not use a separate [[heat sink]].<ref>[http://www.elektor.com/magazines/2008/february/power-to-the-leds.350167.lynkx LED-design]. Elektor.com. Retrieved on 2012-03-16.</ref> Typical current ratings ranges from around 1 mA to above 20 mA. The small size sets a natural upper boundary on power consumption due to heat caused by the high current density and need for a heat sink.
[[File:Arduino led-4.jpg|thumb|A green [[surface-mount technology|surface-mount]] LED mounted on an [[Arduino]] circuit board]]

Common package shapes include round, with a domed or flat top, rectangular with a flat top (as used in bar-graph displays), and triangular or square with a flat top.
The encapsulation may also be clear or tinted to improve contrast and viewing angle.

There are three main categories of miniature single die LEDs:
* Low-current&nbsp;— typically rated for 2 mA at around 2 V (approximately 4&nbsp;mW consumption).
* Standard&nbsp;— 20 mA LEDs at around 2 V (approximately 40&nbsp;mW) for red, orange, yellow, and green, and 20 mA at 4–5 V (approximately 100&nbsp;mW) for blue, violet, and white.
* Ultra-high-output&nbsp;— 20 mA at approximately 2 V or 4–5 V, designed for viewing in direct sunlight.

Five- and twelve-volt LEDs are ordinary miniature LEDs that incorporate a suitable series [[resistor]] for direct connection to a 5 V or 12 V supply.

===Mid-range===
Medium-power LEDs are often through-hole-mounted and used when an output of a few lumen is needed. They sometimes have the diode mounted to four leads (two cathode leads, two anode leads) for better heat conduction and carry an integrated lens. An example of this is the Superflux package, from Philips Lumileds. These LEDs are most commonly used in light panels, emergency lighting, and automotive tail-lights. Due to the larger amount of metal in the LED, they are able to handle higher currents (around 100 mA). The higher current allows for the higher light output required for tail-lights and emergency lighting.

===High-power===
{{See also|Solid-state lighting|LED lamp}}
[[File:2007-07-24 High-power light emiting diodes (Luxeon, Lumiled).jpg|thumb|right|High-power light-emitting diodes ([[Luxeon]], [[Philips Lumileds Lighting Company|Lumileds]])]]

High-power LEDs (HPLED) can be driven at currents from hundreds of mA to more than an ampere, compared with the tens of mA for other LEDs. Some can emit over a thousand lumens.<ref>{{cite web|url=http://www.luminus.com/content1044|title=Luminus Products |publisher=Luminus Devices, Inc. |accessdate=2009-10-21}}</ref><ref>{{cite web|url=http://www.luminus.com/stuff/contentmgr/files/0/7c8547b3575bcecc577525b80d210ac7/misc/pds_001314_rev_03__cst_90_w_product_datasheet_illumination.pdf|title=Luminus Products CST-90 Series Datasheet |publisher=Luminus Devices, Inc. |accessdate=2009-10-25}}</ref> Since overheating is destructive, the HPLEDs must be mounted on a heat sink to allow for heat dissipation. If the heat from a HPLED is not removed, the device will fail in seconds. One HPLED can often replace an incandescent bulb in a [[flashlight]], or be set in an array to form a powerful [[LED lamp]].

Some well-known HPLEDs in this category are the Lumileds Rebel Led, Osram Opto Semiconductors Golden Dragon, and Cree X-lamp. As of September 2009, some HPLEDs manufactured by [[Cree Inc.]] now exceed 105 lm/W
<ref>[http://www.cree.com/products/xlamp_xpg.asp Xlamp Xp-G Led]. Cree.com. Retrieved on 2012-03-16.</ref> (e.g. the XLamp XP-G LED chip emitting Cool White light) and are being sold in lamps intended to replace incandescent, halogen, and even fluorescent lights, as LEDs grow more cost competitive.

LEDs have been developed by Seoul Semiconductor that can operate on AC power without the need for a DC converter. For each half-cycle, part of the LED emits light and part is dark, and this is reversed during the next half-cycle. The efficacy of this type of HPLED is typically 40 lm/W.<ref>{{cite web|url=http://www.ledsmagazine.com/news/3/11/14|title=Seoul Semiconductor launches AC LED lighting source Acriche |publisher=LEDS Magazine |accessdate=2008-02-17|date=17 November 2006}}</ref> A large number of LED elements in series may be able to operate directly from line voltage. In 2009, Seoul Semiconductor released a high DC voltage LED capable of being driven from AC power with a simple controlling circuit. The low-power dissipation of these LEDs affords them more flexibility than the original AC LED design.<ref>{{Cite book| title=Visibility, Environmental, and Astronomical Issues Associated with Blue-Rich White Outdoor Lighting| publisher=International Dark-Sky Association| date=May 4, 2010| url=http://docs.darksky.org/Reports/IDA-Blue-Rich-Light-White-Paper.pdf| format=PDF}}</ref>

===Application-specific variations===
* ''Flashing LEDs'' are used as attention seeking indicators without requiring external electronics. Flashing LEDs resemble standard LEDs but they contain an integrated [[multivibrator]] circuit that causes the LED to flash with a typical period of one second. In diffused lens LEDs this is visible as a small black dot. Most flashing LEDs emit light of one color, but more sophisticated devices can flash between multiple colors and even fade through a color sequence using RGB color mixing.

* ''Bi-color LEDs'' are two different LED emitters in one case. There are two types of these. One type consists of two dies connected to the same two leads [[antiparallel (electronics)|antiparallel]] to each other. Current flow in one direction emits one color, and current in the opposite direction emits the other color. The other type consists of two dies with separate leads for both dies and another lead for common anode or cathode, so that they can be controlled independently.

* ''Tri-color LEDs'' are three different LED emitters in one case. Each emitter is connected to a separate lead so they can be controlled independently. A four-lead arrangement is typical with one common lead (anode or cathode) and an additional lead for each color.

* ''RGB LEDs'' are Tri-color LEDs with red, green, and blue emitters, in general using a four-wire connection with one common lead (anode or cathode). These LEDs can have either common positive or common negative leads. Others however, have only two leads (positive and negative) and have a built in tiny [[electronic control unit]].

[[File:LED DISP.JPG|thumb|right|[[Calculator]] LED display, 1970s]]
* ''Alphanumeric LED displays'' are available in [[seven-segment display|seven-segment]] and [[Starburst display|starburst]] format. Seven-segment displays handle all numbers and a limited set of letters. Starburst displays can display all letters. Seven-segment LED displays were in widespread use in the 1970s and 1980s, but rising use of [[liquid crystal display]]s, with their lower power needs and greater display flexibility, has reduced the popularity of numeric and alphanumeric LED displays.

==Considerations for use==

===Power sources===
{{Main|LED power sources}}
The current/voltage characteristic of an LED is similar to other diodes, in that the current is dependent exponentially on the voltage (see [[Shockley diode equation]]). This means that a small change in voltage can cause a large change in current. If the maximum voltage rating is exceeded by a small amount, the current rating may be exceeded by a large amount, potentially damaging or destroying the LED. The typical solution is to use [[constant current|constant-current]] power supplies, or driving the LED at a voltage much below the maximum rating. Since most common power sources (batteries, mains) are not constant-current sources, most LED fixtures must include a power converter. However, the ''I''/''V'' curve of nitride-based LEDs is quite steep above the knee and gives an ''I''<sub>''f''</sub> of a few milliamperes at a ''V''<sub>''f''</sub> of 3.2 V, making it possible to power a nitride-based LED from a 3 V battery such as a [[coin cell]] without the need for a current-limiting resistor.

===Electrical polarity===
{{Main|Electrical polarity of LEDs}}
As with all diodes, current flows easily from p-type to n-type material.<ref>{{Cite book|author=E. Fred Schubert|title=Light-Emitting Diodes|publisher= Cambridge University Press|year= 2005|chapter =Chapter 4|isbn=0-8194-3956-8}}</ref>
However, no current flows and no light is emitted if a small voltage is applied in the reverse direction. If the reverse voltage grows large enough to exceed the [[breakdown voltage]], a large current flows and the LED may be damaged. If the reverse current is sufficiently limited to avoid damage, the reverse-conducting LED is a useful [[Hardware random number generator#Physical phenomena without quantum-random properties|noise diode]].

===Safety and health===
The vast majority of devices containing LEDs are "safe under all conditions of normal use", and so are classified as "Class 1 LED product"/"LED Klasse 1".
At present, only a few LEDs&nbsp;— extremely bright LEDs that also have a tightly focused viewing angle of 8° or less&nbsp;— could, in theory, cause temporary blindness, and so are classified as "Class 2".<ref>[http://www.datasheetarchive.com/datasheet-pdf/054/DSA0017490.html "Visible LED Device Classifications"]. Datasheetarchive.com. Retrieved on 2012-03-16.</ref>
In general, [[laser safety]] regulations&nbsp;— and the "Class 1", "Class 2", etc. system&nbsp;— also apply to LEDs.<ref>
[http://www.beadlight.com/pages/health_and_safety.asp "Eye Safety and LED (Light Emitting Diode) diffusion"]: "The relevant standard for LED lighting is EN 60825-1:2001 (Safety of laser products) ... The standard states that throughout the standard ”light emitting diodes (LED) are included whenever the word “laser” is used."
</ref>

While LEDs have the advantage over [[fluorescent lamp]]s that they do not contain [[mercury (element)|mercury]], they may contain other hazardous metals such as [[lead]] and [[arsenic]]. A study published in 2011 states: "According to federal standards, LEDs are not hazardous except for low-intensity red LEDs, which leached Pb [lead] at levels exceeding regulatory limits (186 mg/L; regulatory limit: 5). However, according to California regulations, excessive levels of copper (up to 3892 mg/kg; limit: 2500), lead (up to 8103 mg/kg; limit: 1000), [[nickel]] (up to 4797 mg/kg; limit: 2000), or [[silver]] (up to 721 mg/kg; limit: 500) render all except low-intensity yellow LEDs hazardous."<ref name=Limetal2011>{{cite journal| author=Lim SR, Kang D, Ogunseitan OA, Schoenung JM| title=Potential environmental impacts of light-emitting diodes (LEDs): metallic resources, toxicity, and hazardous waste classification | journal=Environ Sci Technol | year= 2011 | volume= 45 | issue= 1 | pages= 320–327 | pmid=21138290 | doi=10.1021/es101052q | pmc= | url= }} [http://pubs.acs.org/doi/full/10.1021/es101052q Free full-text].</ref>

===Advantages===
* '''Efficiency:''' LEDs emit more light per watt than [[incandescent light bulb]]s.<ref>{{cite web|url=http://www1.eere.energy.gov/buildings/ssl/comparing.html|title=Solid-State Lighting: Comparing LEDs to Traditional Light Sources}}</ref> Their efficiency is not affected by shape and size, unlike fluorescent light bulbs or tubes.
* '''Color:''' LEDs can emit light of an intended color without using any color filters as traditional lighting methods need. This is more efficient and can lower initial costs.
* '''Size:''' LEDs can be very small (smaller than 2&nbsp;mm<sup>2</sup><ref>{{cite web|url=http://www.dialight.com/Assets/Brochures_And_Catalogs/Indication/MDEI5980603.pdf|format=PDF|title=Dialight Micro LED SMD LED "598 SERIES" Datasheet}}
</ref>) and are easily attached to printed circuit boards.
* '''On/Off time:''' LEDs light up very quickly. A typical red indicator LED will achieve full brightness in under a [[microsecond]].<ref>{{cite web|url=http://www.avagotech.com/docs/AV02-1555EN|title=Data Sheet&nbsp;— HLMP-1301, T-1 (3 mm) Diffused LED Lamps |publisher=Avago Technologies, Inc. |accessdate=2010-05-30}}</ref> LEDs used in communications devices can have even faster response times.
* '''Cycling:''' LEDs are ideal for uses subject to frequent on-off cycling, unlike fluorescent lamps that fail faster when cycled often, or [[HID lamp]]s that require a long time before restarting.
* '''Dimming:''' LEDs can very easily be [[Dimmer|dimmed]] either by [[pulse-width modulation]] or lowering the forward current.<ref>{{cite journal |author=Prathyusha Narra and Zinger, D.S. |journal=Industry Applications Conference, 2004. 39th IAS Annual Meeting. Conference Record of the 2004 IEEE|title=An effective LED dimming approach |year=2004 |month=Oct |volume=3 |issue= |pages= 1671–1676 |doi=10.1109/IAS.2004.1348695 |issn=0197-2618 |isbn=0-7803-8486-5 }}</ref>
* '''Cool light:''' In contrast to most light sources, LEDs radiate very little heat in the form of [[Infrared|IR]] that can cause damage to sensitive objects or fabrics. Wasted energy is dispersed as heat through the base of the LED.
* '''Slow failure:''' LEDs mostly fail by dimming over time, rather than the abrupt failure of incandescent bulbs.<ref>{{cite web|url=http://www1.eere.energy.gov/buildings/ssl/lifetime.html|title=Solid-State Lighting: Lifetime of White LEDs}}</ref>
* '''Lifetime:''' LEDs can have a relatively long useful life. One report estimates 35,000 to 50,000 hours of useful life, though time to complete failure may be longer.<ref>[http://apps1.eere.energy.gov/buildings/publications/pdfs/ssl/lifetime_white_leds_aug16_r1.pdf Department of Energy]. (PDF) . Retrieved on 2012-03-16.</ref> Fluorescent tubes typically are rated at about 10,000 to 15,000 hours, depending partly on the conditions of use, and incandescent light bulbs at 1,000 to 2,000 hours. Several DOE demonstrations have shown that reduced maintenance costs from this extended lifetime, rather than energy savings, is the primary factor in determining the payback period for an LED product.<ref>{{cite web|url=http://energy.ltgovernors.com/in-depth-advantages-of-led-lighting.html|title=In depth: Advantages of LED Lighting}}</ref>
* '''Shock resistance:''' LEDs, being solid-state components, are difficult to damage with external shock, unlike fluorescent and incandescent bulbs, which are fragile.
* '''Focus:''' The solid package of the LED can be designed to [[focus (optics)|focus]] its light. Incandescent and fluorescent sources often require an external reflector to collect light and direct it in a usable manner. For larger LED packages [[total internal reflection]] (TIR) lenses are often used to the same effect. However, when large quantities of light is needed many light sources are usually deployed, which are difficult to focus or [[collimate]] towards the same target.

===Disadvantages===
* '''High initial price:''' LEDs are currently more expensive, price per lumen, on an initial capital cost basis, than most conventional lighting technologies. As of 2010, the cost per thousand lumens (kilolumen) was about $18. The price is expected to reach $2/kilolumen by 2015.<ref>{{cite web |url=http://energy.ltgovernors.com/led-lighting-explained-questions-and-answers.html |title=LED Lighting Explained: Questions and Answers}}</ref> The additional expense partially stems from the relatively low lumen output and the drive circuitry and power supplies needed.
* '''Temperature dependence:''' LED performance largely depends on the ambient temperature of the operating environment - or "thermal management" properties. Over-driving an LED in high ambient temperatures may result in overheating the LED package, eventually leading to device failure. An adequate [[heat sink]] is needed to maintain long life. This is especially important in automotive, medical, and military uses where devices must operate over a wide range of temperatures, which require low failure rates.
* '''Voltage sensitivity:''' LEDs must be supplied with the voltage above the threshold and a current below the rating. This can involve series resistors or current-regulated power supplies.<ref>[http://www.ledmuseum.org/ The Led Museum]. The Led Museum. Retrieved on 2012-03-16.</ref>
* '''Light quality:''' Most cool-[[#Other white LEDs|white LEDs]] have spectra that differ significantly from a [[black body]] radiator like the sun or an incandescent light. The spike at 460&nbsp;nm and dip at 500&nbsp;nm can cause the color of objects to be [[color vision|perceived differently]] under cool-white LED illumination than sunlight or incandescent sources, due to [[metamerism (color)|metamerism]],<ref>{{cite web|url = http://www.jimworthey.com/jimtalk2006feb.html|title = How White Light Works|author = James A. Worthey|work = LRO Lighting Research Symposium, Light and Color|accessdate = 2007-10-06}}</ref> red surfaces being rendered particularly badly by typical phosphor-based cool-white LEDs. However, the color rendering properties of common fluorescent lamps are often inferior to what is now available in state-of-art white LEDs.
* '''Area light source:''' Single LEDs do not approximate a [[point source]] of light giving a spherical light distribution, but rather a [[Lambert's cosine law|lambertian]] distribution. So LEDs are difficult to apply to uses needing a spherical light field, however different fields of light can be manipulated by the application of different optics or "lenses". LEDs cannot provide divergence below a few degrees. In contrast, lasers can emit beams with divergences of 0.2 degrees or less.<ref>{{Cite book|author=E. Hecht|title=Optics|edition=4|page=591|publisher=Addison Wesley|year= 2002|isbn=0-19-510818-3}}</ref>
* '''[[Electrical polarity]]:''' Unlike [[incandescent]] light bulbs, which illuminate regardless of the electrical [[Electrical polarity|polarity]], LEDs will only light with correct electrical polarity. To automatically match source polarity to LED devices, [[rectifier]]s can be used.
* '''Blue hazard:''' There is a concern that [[#Ultraviolet and blue LEDs|blue LEDs]] and cool-white LEDs are now capable of exceeding safe limits of the so-called [[blue-light hazard]] as defined in eye safety specifications such as ANSI/IESNA RP-27.1–05: Recommended Practice for Photobiological Safety for Lamp and Lamp Systems.<ref name="BlueLEDAhealthHazard">{{cite news | url=http://texyt.com/bright+blue+leds+annoyance+health+risks | title=Blue LEDs: A health hazard? | publisher=texyt.com | date=January 15, 2007 | accessdate=2007-09-03}}</ref><ref>{{cite web|title=Light Impacts: Science News|date=May 27, 2006|url=http://www.sciencenews.org/articles/20060527/bob9.asp|publisher=Sciencenews.org}}</ref>
* '''Blue pollution:''' Because cool-[[#White light|white LEDs]] with high [[color temperature]] emit proportionally more blue light than conventional outdoor light sources such as high-pressure [[sodium vapor lamp]]s, the strong wavelength dependence of [[Rayleigh scattering]] means that cool-white LEDs can cause more [[light pollution]] than other light sources. The [[International Dark-Sky Association]] discourages using white light sources with correlated color temperature above 3,000 K.<ref name="IDA">{{Cite book| title=Visibility, Environmental, and Astronomical Issues Associated with Blue-Rich White Outdoor Lighting| publisher=International Dark-Sky Association| date=May 4, 2010| url=http://docs.darksky.org/Reports/IDA-Blue-Rich-Light-White-Paper.pdf| format=PDF}}</ref>{{failed verification|date=August 2011}}
* '''Droop''': The [[Efficient energy use|efficiency]] of LEDs tends to decrease as one increases [[electrical current|current]].<ref name="sciencedaily1"/><ref>{{cite journal|author=A. A. Efremov, N. I. Bochkareva, R. I. Gorbunov, D. A. Larinovich, Yu. T. Rebane, D. V. Tarkhin, and Yu. G. Shreter, |journal=Semiconductors|volume=40|doi=10.1134/S1063782606050162|page=605|year=2006|title=Effect of the joule heating on the quantum efficiency and choice of thermal conditions for high-power blue InGaN/GaN LEDs|issue=5|bibcode = 2006Semic..40..605E }}</ref><ref>{{cite web |url=http://www.spectrum.ieee.org/semiconductors/optoelectronics/the-leds-dark-secret|title=The LED’s Dark Secret: Solid-state lighting won't supplant the lightbulb until it can overcome the mysterious malady known as droop," by Richard Stevenson, IEEE Spectrum, August 2009}}</ref><ref>[http://www.energy-daily.com/reports/The_LED_Dark_Secret_999.html The LED's dark secret]. EnergyDaily. Retrieved on 2012-03-16.</ref>

===Common misconceptions===
{{Unreferenced section|date=October 2012}}
One of the most common misunderstandings about LED lighting is that energy consumption can be used to measure light output. Unlike conventional incandescent light bulbs, of which the light output is relative to energy consumed, the light output of an LED light is measured in lumens and is given regardless of energy consumption.

LED lights can consume a variable amount of energy, however any given LED system will have, depending on whether phosphor has been used to coat the diodes (and the amount) an optimal lumens/watt output that maximises the lifespan of that particular LED system. The lifespan of the LED system is also dependent upon the thermal management properties employed by the manufacturer.

Other misunderstandings of LED lighting is that there is a limited colour temperature. The application of phosphorous coating to a diode will give a warmer colour temperature. The more phosphor applied to a diode, the warmer the colour temperature of the LED. The colour temperature output of LED lighting systems should be given particular consideration when employing LED lighting in commercial retail display.

One example of this in practice is the increasing use of "warmer" yellow light to display meat in butchers of supermarkets. A warmer light will emphasise the red in the meat, while reducing the visibility of the whiter (fatty) parts, thus making the meat more appealing. The opposite technique is commonly used in jewellery lighting, whereby a "colder" colour temperature is employed to increase the brightness of jewels, silver or metallic objects and other precious stones.

It should be noted that phosphorous coating has direct implications for energy consumption and thermal management considerations, particularly when calculating the lifespan of the LED system and potential failure rates.

==Applications==
In general, all the LED products can be divided into two major parts, the public lighting and indoor lighting. LED uses fall into four major categories:

* Visual signals where light goes more or less directly from the source to the human eye, to convey a message or meaning.
* [[Lighting|Illumination]] where light is reflected from objects to give visual response of these objects.
* Measuring and interacting with processes involving no human vision.<ref>[[European Photonics Industry Consortium]] (EPIC).</ref>
* Narrow band light sensors where LEDs operate in a reverse-bias mode and respond to incident light, instead of emitting light.{{citation needed|date=June 2012}}
[[File:LEDClose6545.jpg|right|thumb|250px|Automotive applications for LEDs continue to grow yearly]]
For more than 70 years, until the LED, practically all lighting was incandescent and fluorescent with the first fluorescent light only being commercially available after the [[1939 World's Fair]].

===Indicators and signs===
[[File:Red and green traffic signals, Stamford Road, Singapore - 20111210.jpg|thumb|Red and green traffic signals]]
The [[energy conservation|low energy consumption]], low maintenance and small size of modern LEDs has led to uses as status indicators and displays on a variety of equipment and installations. Large-area [[LED display]]s are used as stadium displays and as dynamic decorative displays. Thin, lightweight message displays are used at airports and railway stations, and as [[Destination sign|destination displays]] for trains, buses, trams, and ferries.

One-color light is well suited for [[traffic light]]s and signals, [[exit sign]]s, [[emergency vehicle lighting]], ships' navigation lights or [[lantern]]s (chromacity and luminance standards being set under the Convention on the International Regulations for Preventing Collisions at Sea 1972, Annex I and the CIE) and [[Christmas lighting technology#LEDs|LED-based Christmas lights]]. In cold climates, LED traffic lights may remain snow covered.<ref>[http://www.ledsmagazine.com/news/7/1/4 LED advantages outweigh potential snow hazards in traffic signals], LEDs magazine 7 January 2010</ref> Red or yellow LEDs are used in indicator and alphanumeric displays in environments where night vision must be retained: aircraft cockpits, submarine and ship bridges, astronomy observatories, and in the field, e.g. night time animal watching and military field use.

Because of their long life and fast switching times, LEDs have been used in brake lights for cars' [[center high-mounted stop lamp|high-mounted brake lights]], trucks, and buses, and in turn signals for some time, but many vehicles now use LEDs for their rear light clusters. The use in brakes improves safety, due to a great reduction in the time needed to light fully, or faster rise time, up to 0.5 second faster than an incandescent bulb. This gives drivers behind more time to react. It is reported that at normal highway speeds, this equals one car length equivalent in increased time to react. In a dual intensity circuit (rear markers and brakes) if the LEDs are not pulsed at a fast enough frequency, they can create a [[flicker fusion threshold#Visual phenomena|phantom array]], where ghost images of the LED will appear if the eyes quickly scan across the array. White LED headlamps are starting to be used. Using LEDs has styling advantages because LEDs can form much thinner lights than incandescent lamps with [[parabolic reflector]]s.

Due to the relative cheapness of low output LEDs, they are also used in many temporary uses such as [[glowstick]]s, [[throwies]], and the photonic [[textile]] [[Lumalive]]. Artists have also used LEDs for [[LED art]].

[[weatheradio|Weather/all-hazards radio]] receivers with [[Specific Area Message Encoding]] (SAME) have three LEDs: red for warnings, orange for watches, and yellow for advisories & statements whenever issued.

===Lighting===
{{See also|LED lamp|LED-backlit LCD display}}
With the development of high-efficiency and high-power LEDs, it has become possible to use LEDs in lighting and illumination. Replacement [[light bulb]]s have been made, as well as dedicated fixtures and [[LED lamp]]s. To encourage the shift to very high efficiency lighting, the [[US Department of Energy]] has created the [[L Prize]] competition. The [[Philips]] Lighting North America LED bulb won the first competition on August 3, 2011 after successfully completing 18 months of intensive field, lab, and product testing.<ref>[http://www.lightingprize.org "L-Prize U.S. Department of Energy"], L-Prize Website, August 3, 2011</ref>

LEDs are used as [[street light]]s and in other [[architectural lighting design|architectural lighting]] where color changing is used. The mechanical robustness and long lifetime is used in [[automotive lighting]] on cars, motorcycles, and [[Bicycle lighting#LEDs|bicycle lights]].

[[LED street light]]s are employed on poles and in parking garages. In 2007, the Italian village [[Torraca]] was the first place to convert its entire illumination system to LEDs.<ref>[http://www.scientificamerican.com/article.cfm?id=led-there-be-light LED There Be Light], Scientific American, March 18, 2009</ref>

LEDs are used in aviation lighting. [[Airbus]] has used LED lighting in their [[Airbus A320 Enhanced]] since 2007, and Boeing plans its use in the [[Boeing 787 Dreamliner|787]]. LEDs are also being used now in airport and heliport lighting. LED airport fixtures currently include medium-intensity runway lights, runway centerline lights, taxiway centerline and edge lights, guidance signs, and obstruction lighting.

LEDs are also suitable for [[backlight]]ing for [[Liquid crystal display|LCD]] televisions and lightweight [[laptop]] displays and light source for [[Digital Light Processing|DLP]] projectors (See [[LED TV]]). RGB LEDs raise the color [[gamut]] by as much as 45%. Screens for TV and computer displays can be made thinner using LEDs for backlighting.<ref>{{Cite news|url=http://www.nytimes.com/2007/06/24/business/yourmoney/24novel.html|publisher=New York Times|title=In Pursuit of Perfect TV Color, With L.E.D.’s and Lasers|date=June 24, 2007|first=Anne|last=Eisenberg|accessdate=2010-04-04}}</ref>

LEDs are used increasingly in aquarium lights. In particular for reef aquariums, LED lights provide an efficient light source with less heat output to help maintain optimal aquarium temperatures. LED-based aquarium fixtures also have the advantage of being manually adjustable to emit a specific color-spectrum for ideal coloration of corals, fish, and invertebrates while optimizing photosynthetically active radiation (PAR), which raises growth and sustainability of photosynthetic life such as corals, anemones, clams, and macroalgae. These fixtures can be electronically programmed to simulate various lighting conditions throughout the day, reflecting phases of the sun and moon for a dynamic reef experience. LED fixtures typically cost up to five times as much as similarly rated fluorescent or high-intensity discharge lighting designed for reef aquariums and are not as high output to date.

The lack of IR or heat radiation makes LEDs ideal for [[stage light]]s using banks of RGB LEDs that can easily change color and decrease heating from traditional stage lighting, as well as medical lighting where IR-radiation can be harmful. In energy conservation, the lower heat output of LEDs also means air conditioning (cooling) systems have less heat to dispose of, reducing carbon dioxide emissions.

LEDs are small, durable and need little power, so they are used in hand held devices such as [[flashlight]]s. LED [[strobe light]]s or [[camera flash]]es operate at a safe, low voltage, instead of the 250+ volts commonly found in [[xenon]] flashlamp-based lighting. This is especially useful in cameras on [[mobile phone]]s, where space is at a premium and bulky voltage-raising circuitry is undesirable.

LEDs are used for infrared illumination in [[night vision]] uses including [[security camera]]s. A ring of LEDs around a [[video camera]], aimed forward into a [[retroreflective]] [[projection screen|background]], allows [[chroma keying]] in [[video production]]s.

LEDs are now used commonly in all market areas from commercial to home use: standard lighting, AV, stage, theatrical, architectural, and public installations, and wherever artificial light is used.

LEDs are increasingly finding uses in medical and educational applications, for example as mood enhancement {{Citation needed|date=December 2011}}, and new technologies such as [[AmBX]], exploiting LED versatility. [[NASA]] has even sponsored research for the use of LEDs to promote health for astronauts.<ref>{{cite press_release | url=http://www.sti.nasa.gov/tto/Spinoff2008/hm_3.html | title=LED Device Illuminates New Path to Healing | publisher=nasa.gov | accessdate=January 30, 2012}}</ref>

===Smart lighting===
Light can be used to transmit [[broadband]] data, which is already implemented in [[IrDA]] standards using infrared LEDs. Because LEDs can [[frequency|cycle on and off]] millions of times per second, they can be [[wireless]] transmitters and [[wireless access point|access points]] for [[data]] transport.<ref>{{Cite news|first=Hank |last=Green |url=http://www.ecogeek.org/content/view/2194/74/ |title=Transmitting Data Through LED Light Bulbs |publisher=EcoGeek |date=2008-10-09 |accessdate=2009-02-15}}</ref> [[Laser]]s can also be [[modulation|modulated]] in this manner.

===Sustainable lighting===
Efficient lighting is needed for [[sustainable architecture]]. In 2009, a typical 13-watt LED lamp emitted 450 to 650 lumens,<ref name="doe2009">{{Cite book| title=DOE Solid-State Lighting CALiPER Program Summary of Results: Round 7 of Product Testing| publisher=U.S. Department of Energy| date=February 2009| url=http://apps1.eere.energy.gov/buildings/publications/pdfs/ssl/caliper_round_7_summary_final.pdf| format=PDF}}</ref> which is equivalent to a standard 40-watt incandescent bulb. In 2011, LEDs have become more efficient, so that a 6-watt LED can easily achieve the same results.<ref>[http://genet.gelighting.com/LightProducts/Dispatcher?REQUEST=RESULTPAGE&CHANNEL=Consumer&BREADCRUMP=General+Purpose_Standard%230 GE lighting catalog]. Genet.gelighting.com (2005-04-13). Retrieved on 2012-03-16.</ref> A standard 40-watt incandescent bulb has an expected lifespan of 1,000 hours, whereas an LED can continue to operate with reduced efficiency for more than 50,000 hours, 50 times longer than the incandescent bulb.

====Energy consumption====
In the US, one kilowatt-hour of electricity will cause {{convert|1.34|lb|g}} of {{chem|CO|2}} emission.<ref>[http://www.eia.doe.gov/oiaf/1605/emission_factors.html US DOE EIA: Electricity Emission Factors]. Eia.doe.gov. Retrieved on 2012-03-16.</ref> Assuming the average light bulb is on for 10 hours a day, one 40-watt incandescent bulb will cause {{convert|196|lb}} of {{chem|CO|2}} emission per year. The 6-watt LED equivalent will only cause {{convert|30|lb}} of {{chem|CO|2}} over the same time span. A building’s carbon footprint from lighting can be reduced by 85% by exchanging all incandescent bulbs for new LEDs.

====Economically sustainable====
LED light bulbs could be a cost-effective option for lighting a home or office space because of their very long lifetimes. Consumer use of LEDs as a replacement for conventional lighting system is currently hampered by the high cost and low efficiency of available products. 2009 DOE testing results showed an average efficacy of 35 lm/W, below that of typical [[Compact fluorescent lamp|CFLs]], and as low as 9 lm/W, worse than standard incandescents.<ref name="doe2009"/> However, as of 2011, there are LED bulbs available as efficient as 150 lm/W and even inexpensive low-end models typically exceed 50 lm/W. The high initial cost of commercial LED bulbs is due to the expensive [[sapphire]] [[Substrate (materials science)|substrate]], which is key to the production process. The sapphire apparatus must be coupled with a mirror-like collector to reflect light that would otherwise be wasted.

===Other applications===
The light from LEDs can be modulated very quickly so they are used extensively in [[optical fiber]] and [[free space optics]] communications. This include [[remote control]]s, such as for TVs, VCRs, and LED Computers, where infrared LEDs are often used. [[Opto-isolator]]s use an LED combined with a [[photodiode]] or [[phototransistor]] to provide a signal path with electrical isolation between two circuits. This is especially useful in medical equipment where the signals from a low-voltage [[sensor]] circuit (usually battery-powered) in contact with a living organism must be electrically isolated from any possible electrical failure in a recording or monitoring device operating at potentially dangerous voltages. An optoisolator also allows information to be transferred between circuits not sharing a common ground potential.

Many sensor systems rely on light as the signal source. LEDs are often ideal as a light source due to the requirements of the sensors. LEDs are used as [[movement sensors]], for example in [[optical mouse|optical computer mice]]. The Nintendo [[Wii]]'s sensor bar uses infrared LEDs. [[Pulse oximeter]]s use them for measuring [[oxygen saturation]]. Some flatbed scanners use arrays of RGB LEDs rather than the typical [[cold-cathode fluorescent lamp]] as the light source. Having independent control of three illuminated colors allows the scanner to calibrate itself for more accurate color balance, and there is no need for warm-up. Further, its sensors only need be monochromatic, since at any one time the page being scanned is only lit by one color of light. [[Touchscreen|Touch sensing]]: Since LEDs can also be used as [[photodiode]]s, they can be used for both photo emission and detection. This could be used, for example, in a touch-sensing screen that registers reflected light from a finger or [[stylus]].<ref>{{Cite journal|author=Dietz, Yerazunis, and Leigh|title=Very Low-Cost Sensing and Communication Using Bidirectional LEDs|year=2004|url=http://www.merl.com/publications/TR2003-035/}}</ref>

Many materials and biological systems are sensitive to or dependent on light. [[Grow lights]] use LEDs to increase [[photosynthesis]] in [[plant]]s<ref>{{Cite journal|author = Goins, GD and Yorio, NC and Sanwo, MM and Brown, CS
|title = Photomorphogenesis, photosynthesis, and seed yield of wheat plants grown under red light-emitting diodes (LEDs) with and without supplemental blue lighting|journal = Journal of Experimental Botany|year = 1997|volume = 48|page = 1407|doi = 10.1093/jxb/48.7.1407|issue = 7}}</ref> and bacteria and viruses can be removed from water and other substances using [[Ultraviolet|UV]] LEDs for [[Sterilization (microbiology)|sterilization]].<ref name="water sterilization"/> Other uses are as [[UV curing]] devices for some ink and coating methods, and in [[LED printer]]s.

Plant growers are interested in LEDs because they are more energy-efficient, emit less heat (can damage plants close to hot lamps), and can provide the optimum light frequency for plant growth and bloom periods compared to currently used grow lights: [[sodium vapor lamp|HPS]] (high-pressure sodium), [[metal-halide lamp|metal-halide]] (MH) or [[compact fluorescent lamp|CFL]]/low-energy. However, LEDs have not replaced these grow lights due to higher price. As mass production and LED kits develop, the LED products will become cheaper.

LEDs have also been used as a medium-quality [[voltage reference]] in electronic circuits. The forward voltage drop (e.g., about 1.7 V for a normal red LED) can be used instead of a [[Zener diode]] in low-voltage regulators. Red LEDs have the flattest ''I''/''V'' curve above the knee. Nitride-based LEDs have a fairly steep ''I''/''V'' curve and are useless for this purpose. Although LED forward voltage is far more current-dependent than a good Zener, Zener diodes are not widely available below voltages of about 3 V.

===Light sources for machine vision systems===
[[Machine vision]] systems often require bright and homogeneous illumination, so features of interest are easier to process.
LEDs are often used for this purpose, and this is likely to remain one of their major uses until price drops low enough to make signaling and illumination uses more widespread. [[Barcode scanner]]s are the most common example of machine vision, and many low cost ones use red LEDs instead of lasers. Optical computer mice are also another example of LEDs in machine vision, as it is used to provide an even light source on the surface for the miniature camera within the mouse. LEDs constitute a nearly ideal light source for [[machine vision]] systems for several reasons:

The size of the illuminated field is usually comparatively small and machine vision systems are often quite expensive, so the cost of the light source is usually a minor concern. However, it might not be easy to replace a broken light source placed within complex machinery, and here the long service life of LEDs is a benefit.

LED elements tend to be small and can be placed with high density over flat or even-shaped substrates (PCBs etc.) so that bright and homogeneous sources that direct light from tightly controlled directions on inspected parts can be designed. This can often be obtained with small, low-cost lenses and diffusers, helping to achieve high light densities with control over lighting levels and homogeneity. LED sources can be shaped in several configurations (spot lights for reflective illumination; ring lights for coaxial illumination; back lights for contour illumination; linear assemblies; flat, large format panels; dome sources for diffused, omnidirectional illumination).

LEDs can be easily strobed (in the microsecond range and below) and synchronized with imaging. High-power LEDs are available allowing well-lit images even with very short light pulses. This is often used to obtain crisp and sharp “still” images of quickly moving parts.

LEDs come in several different colors and wavelengths, allowing easy use of the best color for each need, where different color may provide better visibility of features of interest. Having a precisely known spectrum allows tightly matched filters to be used to separate informative bandwidth or to reduce disturbing effects of ambient light. LEDs usually operate at comparatively low working temperatures, simplifying heat management and dissipation. This allows using plastic lenses, filters, and diffusers. Waterproof units can also easily be designed, allowing use in harsh or wet environments (food, beverage, oil industries).

<gallery>
File:A_set_end_carriage_vestibule_cityrail.jpg|LEDs used on a train for both overhead lighting and destination signage.
File:LED screen behind Tsach Zimroni in Tel Aviv Israel.jpg|A large LED display behind a [[disc jockey]]
File:LED bus destination displays.jpg|LED [[destination sign]]s on buses, one with a colored route number
File:LED Digital Display.jpg|LED digital display that can display four digits and points
File:LED traffic light on red.jpg|[[Traffic light]] using LED
File:WAPOL TE204 rear.jpg|[[Western Australia Police]] car with LEDs used in its high-mounted brake light, its [[Police car#Active visual warnings|rear window and roof-mounted]] [[Emergency vehicle lighting|flashing Police vehicle lights]] and roof-mounted road user information display
File:LED DaytimeRunningLights.jpg|LED [[daytime running light]]s of Audi A4
File:LED panel and plants.jpg|LED panel light source used in an experiment on [[plant]] growth. The findings of such experiments may be used to grow food in space on long duration missions.
File:LED KOC.JPG|LED illumination
File:AmBXBlue.jpg|LED lights reacting dynamically to video feed via [[AmBX]]
</gallery>

==See also==
{{col-begin}}
{{col-break|width=33%}}
* [[Display examples]]
* [[Laser diode]]
* [[LED circuit]]
* [[LED lamp]]
* [[LEDs as light sensors|LED as light sensor]]
* [[Luminous efficacy]]
{{col-break}}
{{Portal|Electronics|Energy}}<!--placed here for proper rendering in most browsers-->
* [[Nixie tube]]
* [[OLED]]
* [[Photovoltaics]]
* [[Seven-segment display]]
* [[Solar lamp]]
* [[Solid-state lighting]]
{{col-end}}

==References==
{{Reflist|colwidth=35em}}

==Further reading==
<div class="references-small">
* {{Cite book|author=Shuji Nakamura, Gerhard Fasol, Stephen J Pearton |title=The Blue Laser Diode: The Complete Story|publisher=Springer Verlag|year=2000|isbn=3-540-66505-6|url=http://books.google.com/?id=AHyMBJ_LMykC&printsec=frontcover}}
</div>

==External links==
{{Wiktionary|light-emitting diode}}
{{Commons|LED|Light-emitting diode}}
* {{dmoz|Business/Electronics_and_Electrical/Optoelectronics_and_Fiber/Vendors/}}
* [http://berkeley.academia.edu/OzzieZehner/Papers/911577/Promises_and_Limitations_of_Light-Emitting_Diodes/ Promises and Limitations of Light-Emitting Diodes] A concise University of California, Berkeley summary of the history, operation, benefits and limitations of LEDs
* [http://www.ecse.rpi.edu/~schubert/Light-Emitting-Diodes-dot-org/ Rensselaer Electrical Engineering Department] LED information arranged in textbook form, aimed at introductory to advanced audience

{{Artificial light sources}}
{{Display technology}}
{{Electronic components}}

[[Category:Optical diodes]]
[[Category:Lighting]]
[[Category:Signage]]
[[Category:Semiconductor devices]]
[[Category:1907 introductions]]
[[Category:Light-emitting diodes| ]]
[[Category:American inventions]]

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Revision as of 22:23, 7 November 2012