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A light-emitting diode (LED) is a two-lead semiconductor light source. It is a p–n junction diode that emits light when activated.[5] When a suitable current is applied to the leads,[6][7] electrons are able to recombine with electron holes within the device, releasing energy in the form of photons. This effect is called electroluminescence, and the color of the light (corresponding to the energy of the photon) is determined by the energy band gap of the semiconductor. LEDs
LEDs
are typically small (less than 1 mm2) and integrated optical components may be used to shape the radiation pattern.[8] Appearing as practical electronic components in 1962, the earliest LEDs
LEDs
emitted low-intensity infrared light.[9] Infrared
Infrared
LEDs
LEDs
are still frequently used as transmitting elements in remote-control circuits, such as those in remote controls for a wide variety of consumer electronics. The first visible-light LEDs
LEDs
were of low intensity and limited to red. Modern LEDs
LEDs
are available across the visible, ultraviolet, and infrared wavelengths, with very high brightness. Early LEDs
LEDs
were often used as indicator lamps for electronic devices, replacing small incandescent bulbs. They were soon packaged into numeric readouts in the form of seven-segment displays and were commonly seen in digital clocks. Recent developments have produced LEDs
LEDs
suitable for environmental and task lighting. LEDs
LEDs
have led to new displays and sensors, while their high switching rates are useful in advanced communications technology. LEDs
LEDs
have many advantages over incandescent light sources, including lower energy consumption, longer lifetime, improved physical robustness, smaller size, and faster switching. Light-emitting diodes are used in applications as diverse as aviation lighting, automotive headlamps, advertising, general lighting, traffic signals, camera flashes, lighted wallpaper and medical devices.[10] They are also significantly more energy efficient and, arguably, have fewer environmental concerns linked to their disposal.[11][12]

A 4x zoom photo of an ultra-violet LED
LED
has been shown in picture.

Unlike a laser, the color of light emitted from an LED
LED
is neither coherent nor monochromatic, but the spectrum is narrow with respect to human vision, and for most purposes the light from a simple diode element can be regarded as functionally monochromatic.[13][better source needed]

Contents

1 History

1.1 Discoveries and early devices 1.2 Initial commercial development 1.3 Blue
Blue
LED 1.4 White
White
LEDs
LEDs
and the illumination breakthrough

2 Working principle 3 Technology

3.1 Physics 3.2 Refractive index

3.2.1 Transition coatings

3.3 Efficiency and operational parameters

3.3.1 Efficiency droop

3.3.1.1 Possible solutions

3.4 Lifetime and failure

4 Colors and materials

4.1 Blue
Blue
and ultraviolet 4.2 RGB 4.3 White

4.3.1 RGB systems 4.3.2 Phosphor-based LEDs 4.3.3 Other white LEDs

4.4 Organic light-emitting diodes (OLEDs) 4.5 Quantum-dot LEDs

5 Types

5.1 Miniature 5.2 High-power 5.3 AC-driven 5.4 Application-specific variations

5.4.1 Flashing 5.4.2 Bi-color 5.4.3 Tri-color 5.4.4 RGB 5.4.5 Decorative-multicolor 5.4.6 Alphanumeric 5.4.7 Digital-RGB 5.4.8 Filament

6 Considerations for use

6.1 Power sources 6.2 Electrical polarity 6.3 Safety and health 6.4 Advantages 6.5 Disadvantages

7 Applications

7.1 Indicators and signs 7.2 Lighting 7.3 Data communication and other signalling 7.4 Sustainable lighting

7.4.1 Energy
Energy
consumption

7.5 Light sources for machine vision systems 7.6 Other applications

8 See also 9 References 10 Further reading 11 External links

History[edit] Discoveries and early devices[edit]

Green
Green
electroluminescence from a point contact on a crystal of SiC recreates Round's original experiment from 1907.

Electroluminescence
Electroluminescence
as a phenomenon was discovered in 1907 by the British experimenter H. J. Round
H. J. Round
of Marconi Labs, using a crystal of silicon carbide and a cat's-whisker detector.[14][15] Russian inventor Oleg Losev
Oleg Losev
reported creation of the first LED
LED
in 1927.[16] His research was distributed in Soviet, German and British scientific journals, but no practical use was made of the discovery for several decades.[17][18] Kurt Lehovec, Carl Accardo, and Edward Jamgochian explained these first light-emitting diodes in 1951 using an apparatus employing SiC crystals with a current source of battery or pulse generator and with a comparison to a variant, pure, crystal in 1953.[19][20] Rubin Braunstein[21] of the Radio Corporation of America
Radio Corporation of America
reported on infrared emission from gallium arsenide (GaAs) and other semiconductor alloys in 1955.[22] 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 Kelvin. In 1957, Braunstein further demonstrated that the rudimentary devices could be used for non-radio communication across a short distance. As noted by Kroemer[23] Braunstein "…had set up a simple optical communications link: Music emerging from a record player was used via suitable electronics to modulate the forward current of a GaAs diode. The emitted light was detected by a PbS diode some distance away. This signal was fed into an audio amplifier and played back by a loudspeaker. Intercepting the beam stopped the music. We had a great deal of fun playing with this setup." This setup presaged the use of LEDs
LEDs
for optical communication applications.

A Texas Instruments
Texas Instruments
SNX-100 GaAs LED
LED
contained in a TO-18 transistor metal case.

In September 1961, while working at Texas Instruments
Texas Instruments
in Dallas, Texas, James R. Biard
James R. Biard
and Gary Pittman discovered near-infrared (900 nm) light emission from a tunnel diode they had constructed on a GaAs substrate.[9] By October 1961, they had demonstrated efficient light emission and signal coupling between a GaAs p-n junction light emitter and an electrically-isolated semiconductor photodetector.[24] On August 8, 1962, Biard and Pittman filed a patent titled " Semiconductor
Semiconductor
Radiant Diode" based on their findings, which described a zinc diffused p–n junction LED
LED
with a spaced cathode contact to allow for efficient emission of infrared light under forward bias. After establishing the priority of their work based on engineering notebooks predating submissions from G.E.
G.E.
Labs, RCA Research Labs, IBM
IBM
Research Labs, Bell Labs, and Lincoln Lab
Lincoln Lab
at MIT, the U.S. patent office issued the two inventors the patent for the GaAs infrared (IR) light-emitting diode (U.S. Patent US3293513), the first practical LED.[9] Immediately after filing the patent, Texas Instruments (TI) began a project to manufacture infrared diodes. In October 1962, TI announced the first commercial LED
LED
product (the SNX-100), which employed a pure GaAs crystal to emit a 890 nm light output.[9] In October 1963, TI announced the first commercial hemispherical LED, the SNX-110.[25] The first visible-spectrum (red) LED
LED
was developed in 1962 by Nick Holonyak, Jr. while working at General Electric. Holonyak first reported his LED
LED
in the journal Applied Physics Letters on December 1, 1962.[26][27] M. George Craford,[28] a former graduate student of Holonyak, invented the first yellow LED
LED
and improved the brightness of red and red-orange LEDs
LEDs
by a factor of ten in 1972.[29] In 1976, T. P. Pearsall created the first high-brightness, high-efficiency LEDs
LEDs
for optical fiber telecommunications by inventing new semiconductor materials specifically adapted to optical fiber transmission wavelengths.[30] Initial commercial development[edit] The first commercial LEDs
LEDs
were commonly used as replacements for incandescent and neon indicator lamps, and in seven-segment displays,[31] first in expensive equipment such as laboratory and electronics test equipment, then later in such appliances as TVs, radios, telephones, calculators, as well as watches (see list of signal uses). Until 1968, visible and infrared LEDs
LEDs
were extremely costly, in the order of US$200 per unit, and so had little practical use.[32] The Monsanto Company
Monsanto Company
was the first organization to mass-produce visible LEDs, using gallium arsenide phosphide (GaAsP) in 1968 to produce red LEDs
LEDs
suitable for indicators.[32] Hewlett-Packard (HP) introduced LEDs
LEDs
in 1968, initially using GaAsP supplied by Monsanto. These red LEDs
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 became widely available and appeared in appliances and equipment. In the 1970s commercially successful LED
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.[33][34] 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.[35] LED producers continue to use these methods.[36]

LED display
LED display
of a TI-30
TI-30
scientific calculator (ca. 1978), which uses plastic lenses to increase the visible digit size

Most LEDs
LEDs
were made in the very common 5 mm T1¾ and 3 mm T1 packages, but with rising power output, it has grown increasingly necessary to shed excess heat to maintain reliability,[37] so more complex packages have been adapted for efficient heat dissipation. Packages for state-of-the-art high-power LEDs
LEDs
bear little resemblance to early LEDs. Blue
Blue
LED[edit] Blue
Blue
LEDs
LEDs
were first developed by Herbert Paul Maruska at RCA
RCA
in 1972 using gallium nitride (GaN) on a sapphire substrate.[38] SiC-types were first commercially sold in the United States
United States
by Cree in 1989.[39] However, neither of these initial blue LEDs
LEDs
were very bright. The first high-brightness blue LED
LED
was demonstrated by Shuji Nakamura of Nichia Corporation
Nichia Corporation
in 1994 and was based on InGaN.[40][41] In parallel, Isamu Akasaki
Isamu Akasaki
and Hiroshi Amano
Hiroshi Amano
in Nagoya
Nagoya
were working on developing the important GaN nucleation on sapphire substrates and the demonstration of p-type doping of GaN. Nakamura, Akasaki, and Amano were awarded the 2014 Nobel prize in physics for their work.[42] In 1995, Alberto Barbieri at the Cardiff University
Cardiff University
Laboratory (GB) investigated the efficiency and reliability of high-brightness LEDs and demonstrated a "transparent contact" LED
LED
using indium tin oxide (ITO) on (AlGaInP/GaAs). In 2001[43] and 2002,[44] processes for growing gallium nitride (GaN) LEDs
LEDs
on silicon were successfully demonstrated. In January 2012, Osram demonstrated high-power InGaN
InGaN
LEDs
LEDs
grown on silicon substrates commercially,[45] and GaN-on-silicon LEDs
LEDs
are in production at Plessey Semiconductors. As of 2017, some manufacturers are using SiC as the substrate for LED
LED
production, but sapphire is more common. White
White
LEDs
LEDs
and the illumination breakthrough[edit] The attainment of high efficiency in blue LEDs
LEDs
was quickly followed by the development of the first white LED. In this device a Y 3Al 5O 12:Ce (known as "YAG") phosphor coating on the emitter absorbs some of the blue emission and produces yellow light through fluorescence. The combination of that yellow with remaining blue light appears white to the eye. However, using different phosphors (fluorescent materials) it also became possible to instead produce green and red light through fluorescence. The resulting mixture of red, green and blue is not only perceived by humans as white light but is superior for illumination in terms of color rendering, whereas one cannot appreciate the color of red or green objects illuminated only by the yellow (and remaining blue) wavelengths from the YAG
YAG
phosphor.

Illustration of Haitz's law, showing improvement in light output per LED
LED
over time, with a logarithmic scale on the vertical axis

The first white LEDs
LEDs
were expensive and inefficient. However, the light output of LEDs
LEDs
has increased exponentially, with a doubling occurring approximately every 36 months since the 1960s (similar to Moore's law). The latest research and development has been propagated by Japanese manufacturers such as Panasonic, Nichia, etc. and later by Korean and Chinese factories and investment such as: Samsung, Solstice, Kingsun, and countless others.[46] This trend is generally attributed to the parallel development of other semiconductor technologies and advances in optics[citation needed] and materials science and has been called Haitz's law
Haitz's law
after Dr. Roland Haitz.[47] Light output and efficiency of blue and near-ultraviolet LEDs
LEDs
rose as the cost of reliable devices fell. This led to relatively high-power white-light LEDs
LEDs
for illumination, which are replacing incandescent and fluorescent lighting.[48][49] Experimental white LEDs
LEDs
have been demonstrated to produce over 300 lumens per watt of electricity; some can last up to 100,000 hours.[50] Compared to incandescent bulbs, this is not only a huge increase in electrical efficiency but – over time – a similar or lower cost per bulb.[51] Working principle[edit]

The inner workings of an LED, showing circuit (top) and band diagram (bottom)

A P-N junction can convert absorbed light energy into a proportional electric current. The same process is reversed here (i.e. the P-N junction emits light when electrical energy is applied to it). This phenomenon is generally called electroluminescence, which can be defined as the emission of light from a semiconductor under the influence of an electric field. The charge carriers recombine in a forward-biased P-N junction as the electrons cross from the N-region and recombine with the holes existing in the P-region. Free electrons are in the conduction band of energy levels, while holes are in the valence energy band. Thus the energy level of the holes is less than the energy levels of the electrons. Some portion of the energy must be dissipated to recombine the electrons and the holes. This energy is emitted in the form of heat and light. The electrons dissipate energy in the form of heat for silicon and germanium diodes but in gallium arsenide phosphide (GaAsP) and gallium phosphide (GaP) semiconductors, the electrons dissipate energy by emitting photons. If the semiconductor is translucent, the junction becomes the source of light as it is emitted, thus becoming a light-emitting diode. However, when the junction is reverse biased, the LED
LED
produces no light and—if the potential is great enough, the device is damaged. Technology[edit]

I-V diagram for a diode. An LED
LED
begins to emit light when more than 2 or 3 volts is applied. The reverse bias region uses a different vertical scale from the forward bias region to show that the leakage current is nearly constant with voltage until breakdown occurs. In forward bias, the current is small but increases exponentially with voltage.

Physics[edit] The LED
LED
consists of a chip of semiconducting material 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—electrons and holes—flow into the junction from electrodes 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 usually recombine by a non-radiative transition, which produces no optical emission, because these are indirect band gap materials. The materials used for the LED
LED
have a direct band gap with energies corresponding to near-infrared, visible, or near-ultraviolet light. LED
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
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. Refractive index[edit]

Idealized example of light emission cones in a simple square semiconductor, for a single point-source emission zone. The left illustration is for a translucent wafer, while the right illustration shows the half-cones formed when the bottom layer is opaque. The light is actually emitted equally in all directions from the point-source, but can only escape perpendicular to the semiconductor's surface and some degrees to the side, which is illustrated by the cone shapes. When the critical angle is exceeded, photons are reflected internally. The areas between the cones represent the trapped light energy wasted as heat.[52]

Most materials used for LED
LED
production have very high refractive indices. This means that much of the light is reflected back into the material at the material/air surface interface. Thus, light extraction in LEDs
LEDs
is an important aspect of LED
LED
production, subject to much research and development. The light emission cones of a real LED
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 arriving at sharp angles relative to the air-contacting surface of the semiconductor due to total internal reflection. This property affects both the light-emission efficiency of LEDs
LEDs
as well as the light-absorption efficiency of photovoltaic cells. The refractive index of silicon is set at 3.96 (at 590 nm),[53] while air's refractive index is set at 1.0002926.[54] In general, a flat-surface uncoated LED
LED
semiconductor chip emits 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,[55] or the escape cone.[52] The maximum angle of incidence is referred to as the critical angle. When the critical angle is exceeded, photons no longer escape the semiconductor but are, instead, reflected internally inside the semiconductor crystal as if it were a mirror.[52] Internal reflections 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
LED
with 90-degree angled surfaces on all sides, the faces all act as equal angle mirrors. In this case, most of the light can not escape and is lost as waste heat in the crystal.[52] A convoluted chip surface with angled facets similar to a jewel or fresnel lens can increase light output by distributing light perpendicular to the chip surface and far to the sides of the photon emission point.[56] 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.[57] Transition coatings[edit] After the doping of the wafer, it is usually cut apart into individual dies. Each die is commonly called a chip. Many LED
LED
semiconductor chips are encapsulated or 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.[58]

The third feature helps to boost the light emission from the semiconductor by acting as a diffusing lens, emitting light at a much higher angle of incidence from the light cone than the bare chip would alone. Efficiency and operational parameters[edit] Typical indicator LEDs
LEDs
are designed to operate with no more than 30–60 milliwatts (mW) of electrical power. Around 1999, Philips Lumileds introduced power LEDs
LEDs
capable of continuous use at one watt. These LEDs
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 greater heat dissipation from the LED
LED
die. One of the key advantages of LED-based lighting sources is high luminous efficacy. White
White
LEDs
LEDs
quickly matched and overtook the efficacy of standard incandescent lighting systems. In 2002, Lumileds made five-watt LEDs
LEDs
available with 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 lights emit up to 100 lm/W. As of 2012[update], Philips
Philips
had achieved the following efficacies for each color.[59] The efficiency values show the physics – light power out per electrical power in. The lumen-per-watt efficacy value includes characteristics of the human eye and is derived using the luminosity function.

Color Wavelength
Wavelength
range (nm) Typical efficiency coefficient Typical efficacy (lm/W)

Red 620 < λ < 645 0.39 72

Red-orange 610 < λ < 620 0.29 98

Green 520 < λ < 550 0.15 93

Cyan 490 < λ < 520 0.26 75

Blue 460 < λ < 490 0.35 37

In September 2003, a new type of blue LED
LED
was demonstrated by Cree. This produced a commercially packaged white light giving 65 lm/W at 20 mA, becoming the brightest white LED
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
LED
luminous efficacy of 131 lm/W at 20 mA. Nichia Corporation has developed a white LED
LED
with luminous efficacy of 150 lm/W at a forward current of 20 mA.[60] Cree's XLamp XM-L LEDs, commercially available in 2011, produce 100 lm/W at their full power of 10 W, and up to 160 lm/W at around 2 W input power. In 2012, Cree announced a white LED
LED
giving 254 lm/W,[61] and 303 lm/W in March 2014.[62] Practical general lighting needs high-power LEDs, of one watt or more. Typical operating currents for such devices begin at 350 mA. These efficiencies are for the light-emitting diode only, held at low temperature in a lab. Since LEDs
LEDs
installed in real fixtures operate at higher temperature and with driver losses, real-world efficiencies are much lower. United States
United States
Department of Energy
Energy
(DOE) testing of commercial LED
LED
lamps designed to replace incandescent lamps or CFLs showed that average efficacy was still about 46 lm/W in 2009 (tested performance ranged from 17 lm/W to 79 lm/W).[63] Efficiency droop[edit] Efficiency droop is the decrease in luminous efficiency of LEDs
LEDs
as the electric current increases above tens of milliamperes. This effect was initially thought related to elevated temperatures. Scientists proved the opposite is true: though the life of an LED
LED
is shortened, the efficiency droop is less severe at elevated temperatures.[64] The mechanism causing efficiency droop was identified in 2007 as Auger recombination, which was taken with mixed reaction.[65] In 2013, a study confirmed Auger recombination
Auger recombination
as the cause of efficiency droop.[66] In addition to being less efficient, operating LEDs
LEDs
at higher electric currents creates higher heat levels, which can compromise LED lifetime. Because of this increased heat at higher currents, high-brightness LEDs
LEDs
have an industry standard of operating at only 350 mA, which is a compromise between light output, efficiency, and longevity.[65][67][68][69] Possible solutions[edit] Instead of increasing current levels, luminance is usually increased by combining multiple LEDs
LEDs
in one bulb. Solving the problem of efficiency droop would mean that household LED
LED
light bulbs would need fewer LEDs, which would significantly reduce costs. Researchers at the U.S. Naval Research Laboratory
U.S. Naval Research Laboratory
have found a way to lessen the efficiency droop. They found that the droop arises from non-radiative Auger recombination
Auger recombination
of the injected carriers. They created quantum wells with a soft confinement potential to lessen the non-radiative Auger processes.[70] Researchers at Taiwan National Central University
Taiwan National Central University
and Epistar Corp are developing a way to lessen the efficiency droop by using ceramic aluminium nitride (AlN) substrates, which are more thermally conductive than the commercially used sapphire. The higher thermal conductivity reduces self-heating effects.[71] Lifetime and failure[edit] Main article: List of LED
LED
failure modes Solid-state devices such as LEDs
LEDs
are subject to very limited wear and tear if operated at low currents and at low temperatures. Typical lifetimes quoted are 25,000 to 100,000 hours, but heat and current settings can extend or shorten this time significantly.[72] It is important to note that these projections are based on a standard test that may not accelerate all the potential mechanisms that can induce failures in LEDs.[73] The most common symptom of LED
LED
(and diode laser) failure is the gradual lowering of light output and loss of efficiency. Sudden failures, although rare, can also occur. Early red LEDs
LEDs
were notable for their short service life. With the development of high-power LEDs, the devices are subjected to higher junction temperatures and higher current densities than traditional devices. This causes stress on the material and may cause early light-output degradation. To quantify useful lifetime in a standardized manner, some suggest using L70 or L50, which are runtimes (typically in thousands of hours) at which a given LED
LED
reaches 70% and 50% of initial light output, respectively.[74] Whereas in most previous sources of light (incandescent lamps, discharge lamps, and those that burn combustible fuel, e.g. candles and oil lamps) the light results from heat, LEDs
LEDs
only operate if they are kept cool enough. The manufacturer commonly specifies a maximum junction temperature of 125 or 150 °C, and lower temperatures are advisable in the interests of long life. At these temperatures, relatively little heat is lost by radiation, which means that the light beam generated by an LED
LED
is cool. The waste heat in a high-power LED
LED
(which as of 2015 can be less than half the power that it consumes) is conveyed by conduction through the substrate and package of the LED
LED
to a heat sink, which gives up the heat to the ambient air by convection. Careful thermal design is, therefore, essential, taking into account the thermal resistances of the LED’s package, the heat sink and the interface between the two. Medium-power LEDs
LEDs
are often designed to solder directly to a printed circuit board that contains a thermally conductive metal layer. High-power LEDs
LEDs
are packaged in large-area ceramic packages that attach to a metal heat sink—the interface being a material with high thermal conductivity (thermal grease, phase-change material, thermally conductive pad, or thermal adhesive). If an LED-based lamp is installed in an unventilated luminaire, or a luminaire is located in an environment that does not have free air circulation, the LED
LED
is likely to overheat, resulting in reduced life or early catastrophic failure. Thermal design is often based on an ambient temperature of 25 °C (77 °F). LEDs
LEDs
used in outdoor applications, such as traffic signals or in-pavement signal lights, and in climates where the temperature within the light fixture becomes very high, could experience reduced output or even failure.[75] Since LED
LED
efficacy is higher at low temperatures, LED
LED
technology is well suited for supermarket freezer lighting.[76][77][78] Because LEDs produce less waste heat than incandescent lamps, freezer tube lighting[79] use can save on refrigeration costs as well. However, they may be more susceptible to frost and snow buildup than incandescent lamps,[75] so some LED
LED
lighting systems have been designed with an added heating circuit. Additionally, research has developed heat sink technologies that transfer heat produced within the junction to appropriate areas of the light fixture.[80] Colors and materials[edit] Conventional LEDs
LEDs
are made from a variety of inorganic semiconductor materials. The following table shows the available colors with wavelength range, voltage drop, and material:

Color Wavelength
Wavelength
[nm] Voltage drop [ΔV] Semiconductor
Semiconductor
material

Infrared λ > 760 ΔV < 1.63 Gallium arsenide
Gallium arsenide
(GaAs) Aluminium gallium arsenide
Aluminium gallium arsenide
(AlGaAs)

Red 610 < λ < 760 1.63 < ΔV < 2.03 Aluminium gallium arsenide
Aluminium gallium arsenide
(AlGaAs) Gallium arsenide
Gallium arsenide
phosphide (GaAsP) Aluminium gallium indium phosphide
Aluminium gallium indium phosphide
(AlGaInP) Gallium(III) phosphide
Gallium(III) phosphide
(GaP)

Orange 590 < λ < 610 2.03 < ΔV < 2.10 Gallium arsenide
Gallium arsenide
phosphide (GaAsP) Aluminium gallium indium phosphide
Aluminium gallium indium phosphide
(AlGaInP) Gallium(III) phosphide
Gallium(III) phosphide
(GaP)

Yellow 570 < λ < 590 2.10 < ΔV < 2.18 Gallium arsenide
Gallium arsenide
phosphide (GaAsP) Aluminium gallium indium phosphide
Aluminium gallium indium phosphide
(AlGaInP) Gallium(III) phosphide
Gallium(III) phosphide
(GaP)

Green 500 < λ < 570 1.9[81] < ΔV < 4.0 Traditional green: Gallium(III) phosphide
Gallium(III) phosphide
(GaP) Aluminium gallium indium phosphide
Aluminium gallium indium phosphide
(AlGaInP) Aluminium gallium phosphide (AlGaP) Pure green: Indium gallium nitride
Indium gallium nitride
(InGaN) / Gallium(III) nitride
Gallium(III) nitride
(GaN)

Blue 450 < λ < 500 2.48 < ΔV < 3.7 Zinc selenide
Zinc selenide
(ZnSe) Indium gallium nitride
Indium gallium nitride
(InGaN) Silicon carbide
Silicon carbide
(SiC) as substrate Silicon
Silicon
(Si) as substrate—under development

Violet 400 < λ < 450 2.76 < ΔV < 4.0 Indium gallium nitride
Indium gallium nitride
(InGaN)

Ultraviolet λ < 400 3 < ΔV < 4.1 Indium gallium nitride
Indium gallium nitride
(InGaN) (385-400 nm) Diamond
Diamond
(235 nm)[82] Boron nitride
Boron nitride
(215 nm)[83][84] Aluminium nitride
Aluminium nitride
(AlN) (210 nm)[85] Aluminium gallium nitride (AlGaN) Aluminium gallium indium nitride (AlGaInN)—down to 210 nm[86]

Pink Multiple types ΔV ≈3.3[87] Blue
Blue
with one or two phosphor layers, yellow with red, orange or pink phosphor added afterwards, white with pink plastic, or white phosphors with pink pigment or dye over top.[88]

Purple Multiple types 2.48 < ΔV < 3.7 Dual blue/red LEDs, blue with red phosphor, or white with purple plastic

White Broad spectrum 2.8 < ΔV < 4.2 Cool / Pure White: Blue/UV diode with yellow phosphor Warm White: Blue
Blue
diode with orange phosphor

Blue
Blue
and ultraviolet[edit]

Blue
Blue
LEDs

External video

“The Original Blue
Blue
LED”, Science History Institute

The first blue-violet LED
LED
using magnesium-doped gallium nitride was made at Stanford University
Stanford University
in 1972 by Herb Maruska and Wally Rhines, doctoral students in materials science and engineering.[89][90] At the time Maruska was on leave from RCA
RCA
Laboratories, where he collaborated with Jacques Pankove on related work. In 1971, the year after Maruska left for Stanford, his RCA
RCA
colleagues Pankove and Ed Miller demonstrated the first blue electroluminescence from zinc-doped gallium nitride, though the subsequent device Pankove and Miller built, the first actual gallium nitride light-emitting diode, emitted green light.[91][92] In 1974 the U.S. Patent Office
U.S. Patent Office
awarded Maruska, Rhines and Stanford professor David Stevenson a patent for their work in 1972 (U.S. Patent US3819974 A) and today, magnesium-doping of gallium nitride remains the basis for all commercial blue LEDs
LEDs
and laser diodes. In the early 1970s, these devices were too dim for practical use, and research into gallium nitride devices slowed. In August 1989, Cree introduced the first commercially available blue LED based on the indirect bandgap semiconductor, silicon carbide (SiC).[93] SiC LEDs
LEDs
had very low efficiency, no more than about 0.03%, but did emit in the blue portion of the visible light spectrum.[citation needed] In the late 1980s, key breakthroughs in GaN epitaxial growth and p-type doping[94] ushered in the modern era of GaN-based optoelectronic devices. Building upon this foundation, Theodore Moustakas at Boston University patented a method for producing high-brightness blue LEDs
LEDs
using a new two-step process.[95] Two years later, in 1993, high-brightness blue LEDs
LEDs
were demonstrated again by Shuji Nakamura
Shuji Nakamura
of Nichia Corporation
Nichia Corporation
using a gallium nitride growth process similar to Moustakas's.[96] Both Moustakas and Nakamura were issued separate patents, which confused the issue of who was the original inventor (partly because although Moustakas invented his first, Nakamura filed first).[citation needed] This new development revolutionized LED
LED
lighting, making high-power blue light sources practical, leading to the development of technologies like Blu-ray, as well as allowing the bright high-resolution screens of modern tablets and phones.[citation needed] Nakamura was awarded the 2006 Millennium Technology Prize
Millennium Technology Prize
for his invention.[97] Nakamura, Hiroshi Amano
Hiroshi Amano
and Isamu Akasaki
Isamu Akasaki
were awarded the Nobel Prize in Physics
Nobel Prize in Physics
in 2014 for the invention of the blue LED.[98][99][100] In 2015, a US court ruled that three companies (i.e. the litigants who had not previously settled out of court) that had licensed Nakamura's patents for production in the United States
United States
had infringed Moustakas's prior patent, and ordered them to pay licensing fees of not less than 13 million USD.[101] By the late 1990s, blue LEDs
LEDs
became widely available. They have an active region consisting of one or more InGaN
InGaN
quantum wells sandwiched between thicker layers of GaN, called cladding layers. By varying the relative In/Ga fraction in the InGaN
InGaN
quantum wells, the light emission can in theory be varied from violet to amber. Aluminium gallium nitride (AlGaN) of varying Al/Ga 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 InGaN/GaN blue/green devices. If un-alloyed GaN is used in this case to form the active quantum well layers, the device emits near-ultraviolet light with a peak wavelength centred around 365 nm. Green
Green
LEDs
LEDs
manufactured from the InGaN/GaN system are far more efficient and brighter than green LEDs
LEDs
produced with non-nitride material systems, but practical devices still exhibit efficiency too low for high-brightness applications.[citation needed] With nitrides containing aluminium, most often AlGaN and AlGaInN, even shorter wavelengths are achievable. Ultraviolet
Ultraviolet
LEDs
LEDs
in a range of wavelengths are becoming available on the market. Near-UV emitters at wavelengths around 375–395 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 240 nm.[102] As the photosensitivity of microorganisms approximately matches the absorption spectrum of DNA, with a peak at about 260 nm, UV LED
LED
emitting at 250–270 nm are expected in prospective disinfection and sterilization devices. Recent research has shown that commercially available UVA LEDs
LEDs
(365 nm) are already effective disinfection and sterilization devices.[103] UV-C wavelengths were obtained in laboratories using aluminium nitride (210 nm),[85] boron nitride (215 nm)[83][84] and diamond (235 nm).[82] RGB[edit]

RGB-SMD-LED

RGB LEDs
LEDs
consist of one red, one green, and one blue LED.[104] By independently adjusting each of the three, RGB LEDs
LEDs
are capable of producing a wide color gamut. Unlike dedicated-color LEDs, however, these obviously do not produce pure wavelengths. Moreover, such modules as commercially available are often not optimized for smooth color mixing. White[edit] There are two primary ways of producing white light-emitting diodes (WLEDs), LEDs
LEDs
that generate high-intensity white light. One is to use individual LEDs
LEDs
that emit three primary colors[105]—red, green, and blue—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
LED
to broad-spectrum white light, much in the same way a fluorescent light bulb works. The 'whiteness' of the light produced is essentially engineered to suit the human eye. There are three main methods of mixing colors to produce white light from an LED:

blue LED
LED
+ green LED
LED
+ red LED
LED
(color mixing; can be used as backlighting for displays, extremely poor for illumination due to gaps in spectrum) near-UV or UV LED
LED
+ RGB phosphor (an LED
LED
producing light with a wavelength shorter than blue's is used to excite an RGB phosphor) blue LED
LED
+ yellow phosphor (two complementary colors combine to form white light; more efficient than first two methods and more commonly used)[106]

Because of metamerism, it is possible to have quite different spectra that appear white. However, the appearance of objects illuminated by that light may vary as the spectrum varies, this is the issue of color rendition, quite separate from color temperature, where a really orange or cyan object could appear with the wrong color and much darker as the LED
LED
or phosphor does not emit the wavelength it reflects. The best color rendition CFL and LEDs
LEDs
use a mix of phosphors, resulting in less efficiency but better quality of light. Though incandescent halogen lamps have a more orange color temperature, they are still the best easily available artificial light sources in terms of color rendition. RGB systems[edit]

Combined spectral curves for blue, yellow-green, and high-brightness red solid-state semiconductor LEDs. FWHM spectral bandwidth is approximately 24–27 nm for all three colors.

RGB LED

White
White
light can be formed by mixing differently colored lights; the most common method is to use red, green, and blue (RGB). Hence the method is called multicolor white LEDs
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
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. Nonetheless, this method has many applications because of the flexibility of mixing different colors,[107] and in principle, this mechanism also has higher quantum efficiency in producing white light.[citation needed] There are several types of multicolor white LEDs: di-, tri-, and tetrachromatic white LEDs. Several key factors that play among these different methods include color stability, color rendering capability, and luminous efficacy. Often, higher efficiency means lower color rendering, presenting a trade-off between the luminous efficacy and color rendering. For example, the dichromatic white LEDs
LEDs
have the best luminous efficacy (120 lm/W), but the lowest color rendering capability. However, although tetrachromatic white LEDs
LEDs
have excellent color rendering capability, they often have poor luminous efficacy. Trichromatic
Trichromatic
white LEDs
LEDs
are in between, having both good luminous efficacy (>70 lm/W) and fair color rendering capability. One of the challenges is the development of more efficient green LEDs. The theoretical maximum for green LEDs
LEDs
is 683 lumens per watt but as of 2010 few green LEDs
LEDs
exceed even 100 lumens per watt. The blue and red LEDs
LEDs
approach their theoretical limits. Multicolor LEDs
LEDs
offer not merely another means to form white light but a new means to form light of different colors. Most 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, multicolor LEDs
LEDs
should have profound influence on the fundamental method that we use to produce and control light color. However, before this type of LED
LED
can play a role on the market, several technical problems must be solved. These include that this type of LED's emission power decays exponentially with rising temperature,[108] 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. However multicolor LEDs
LEDs
without phosphors can never provide good quality lighting because each LED
LED
is a narrow band source (see graph). LEDs
LEDs
without phosphor while a poorer solution for general lighting are the best solution for displays, either backlight of LCD, or direct LED
LED
based pixels. Correlated color temperature
Correlated color temperature
(CCT) dimming for LED
LED
technology is regarded as a difficult task since LED
LED
binning,[109] age and temperature drift effects of LEDs
LEDs
change the actual color value output. Feedback loop systems are used for example with color sensors, to actively monitor and control the color output of multiple color mixing LEDs.[110] Phosphor-based LEDs[edit]

Spectrum of a white LED
LED
showing blue light directly emitted by the GaN-based LED
LED
(peak at about 465 nm) and the more broadband Stokes-shifted light emitted by the Ce3+: YAG
YAG
phosphor, which emits at roughly 500–700 nm

This method involves coating LEDs
LEDs
of one color (mostly blue LEDs
LEDs
made of InGaN) with phosphors of different colors to form white light; the resultant LEDs
LEDs
are called phosphor-based or phosphor-converted white LEDs
LEDs
(pcLEDs).[111] A fraction of the blue light undergoes the Stokes shift, which transforms it from shorter wavelengths to longer. Depending on the original LED
LED
color, various color phosphors are used. Several phosphor layers of distinct colors broadens the emitted spectrum, effectively raising the color rendering index (CRI).[112] Phosphor-based LED
LED
have efficiency losses due to heat loss from the Stokes shift
Stokes shift
and also other phosphor-related issues. Their luminous efficacies compared to normal LEDs
LEDs
depend on the spectral distribution of the resultant light output and the original wavelength of the LED itself. For example, the luminous efficacy of a typical YAG
YAG
yellow phosphor based white LED
LED
ranges from 3 to 5 times the luminous efficacy of the original blue LED
LED
because of the human eye's greater sensitivity to yellow than to blue (as modeled in the luminosity function). 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 system, and the majority of high-intensity white LEDs
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 is the development of more efficient phosphors. As of 2010, the most efficient yellow phosphor is still the YAG
YAG
phosphor, with less than 10% Stokes shift
Stokes shift
loss. Losses attributable to internal optical losses due to re-absorption in the LED
LED
chip and in the LED
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. Some phosphor-based white LEDs
LEDs
encapsulate InGaN
InGaN
blue LEDs
LEDs
inside phosphor-coated epoxy. Alternatively, the LED
LED
might be paired with a remote phosphor, a preformed polycarbonate piece coated with the phosphor material. Remote phosphors provide more diffuse light, which is desirable for many applications. Remote phosphor designs are also more tolerant of variations in the LED
LED
emissions spectrum. A common yellow phosphor material is cerium-doped yttrium aluminium garnet (Ce3+:YAG). White
White
LEDs
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 lamps work. This method is less efficient than blue LEDs with YAG:Ce phosphor, as the Stokes shift
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
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[edit] Another method used to produce experimental white light LEDs
LEDs
used no phosphors at all and was based on homoepitaxially grown zinc selenide (ZnSe) on a ZnSe substrate that simultaneously emitted blue light from its active region and yellow light from the substrate.[113] A new style of wafers composed of gallium-nitride-on-silicon (GaN-on-Si) is being used to produce white LEDs
LEDs
using 200-mm silicon wafers. This avoids the typical costly sapphire substrate in relatively small 100- or 150-mm wafer sizes.[114] The sapphire apparatus must be coupled with a mirror-like collector to reflect light that would otherwise be wasted. It is predicted that by 2020, 40% of all GaN LEDs
LEDs
will be made with GaN-on-Si. Manufacturing large sapphire material is difficult, while large silicon material is cheaper and more abundant. LED
LED
companies shifting from using sapphire to silicon should be a minimal investment.[115] Organic light-emitting diodes (OLEDs)[edit] Main article: Organic light-emitting diode

Orange light-emitting diode

In an organic light-emitting diode (OLED), the electroluminescent material comprising the emissive layer of the diode is an organic compound. The organic material is electrically conductive due to the delocalization of pi electrons caused by conjugation over all or part of the molecule, and the material therefore functions as an organic semiconductor.[116] The organic materials can be small organic molecules in a crystalline phase, or polymers.[117] The potential advantages of O LEDs
LEDs
include thin, low-cost displays with a low driving voltage, wide viewing angle, and high contrast and color gamut.[118] Polymer
Polymer
LEDs
LEDs
have the added benefit of printable and flexible displays.[119][120][121] O LEDs
LEDs
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.[117][118] Quantum-dot LEDs[edit] See also: quantum dot display Quantum dots (QD) are semiconductor nanocrystals with optical properties that let their emission color be tuned from the visible into the infrared spectrum.[122][123] This allows quantum dot LEDs
LEDs
to create almost any color on the CIE diagram. This provides more color options and better color rendering than white LEDs
LEDs
since the emission spectrum is much narrower, characteristic of quantum confined states. There are two types of schemes for QD excitation. One uses photo excitation with a primary light source LED
LED
(typically blue or UV LEDs are used). The other is direct electrical excitation first demonstrated by Alivisatos et al.[124] One example of the photo-excitation scheme is a method developed by Michael Bowers, at Vanderbilt University
Vanderbilt University
in Nashville, involving coating a blue LED
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 light bulbs.[125] Quantum dots are also being considered for use in white light-emitting diodes in liquid crystal display (LCD) televisions.[126] In February 2011 scientists at PlasmaChem GmbH were able to synthesize quantum dots for LED
LED
applications and build a light converter on their basis, which was able to efficiently convert light from blue to any other color for many hundred hours.[127] 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
LEDs
used for the electrical-excitation scheme is similar to basic design of OLEDs. 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
Fluorescence
near-field scanning optical microscopy (NSOM) utilizing an integrated QD- LED
LED
has been demonstrated.[128] In February 2008, a luminous efficacy of 300 lumens of visible light per watt of radiation (not per electrical watt) and warm-light emission was achieved by using nanocrystals.[129] Types[edit]

LEDs
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 colorless housings. Modern high-power LEDs
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
LEDs
are miniature, high-power devices, and custom designs such as alphanumeric or multicolor.[130] Miniature[edit]

Photo of miniature surface mount LEDs
LEDs
in most common sizes. They can be much smaller than a traditional 5 mm lamp type LED, shown on the upper left corner.

Very small (1.6x1.6x0.35 mm) red, green, and blue surface mount miniature LED
LED
package with gold wire bonding details.

These are mostly single-die LEDs
LEDs
used as indicators, and they come in various sizes from 2 mm to 8 mm, through-hole and surface mount packages. They usually do not use a separate heat sink.[131] Typical current ratings range 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. Often daisy chained as used in LED
LED
tapes. 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. Researchers at the University of Washington
University of Washington
have invented the thinnest LED. It is made of two-dimensional (2-D) flexible materials. It is three atoms thick, which is 10 to 20 times thinner than three-dimensional (3-D) LEDs
LEDs
and is also 10,000 times smaller than the thickness of a human hair. These 2-D LEDs
LEDs
are going to make it possible to create smaller, more energy-efficient lighting, optical communication and nano lasers.[132][133] There are three main categories of miniature single die LEDs:

Low-current Typically rated for 2 mA at around 2 V (approximately 4 mW consumption) Standard 20 mA LEDs
LEDs
(ranging from approximately 40 mW to 90 mW) at around:

1.9 to 2.1 V for red, orange, yellow, and traditional green 3.0 to 3.4 V for pure green and blue 2.9 to 4.2 V for violet, pink, purple and white

Ultra-high-output 20 mA at approximately 2 or 4–5 V, designed for viewing in direct sunlight

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

High-power light-emitting diodes attached to an LED
LED
star base (Luxeon, Lumileds)

See also: Solid-state lighting, LED
LED
lamp, and Thermal management of high-power LEDs High-power LEDs
LEDs
(HP-LEDs) or high-output LEDs
LEDs
(HO-LEDs) 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.[134][135] LED
LED
power densities up to 300 W/cm2 have been achieved. Since overheating is destructive, the HP- LEDs
LEDs
must be mounted on a heat sink to allow for heat dissipation. If the heat from an HP- LED
LED
is not removed, the device fails in seconds. One HP- LED
LED
can often replace an incandescent bulb in a flashlight, or be set in an array to form a powerful LED
LED
lamp. Some well-known HP- LEDs
LEDs
in this category are the Nichia
Nichia
19 series, Lumileds Rebel Led, Osram
Osram
Opto Semiconductors Golden Dragon, and Cree X-lamp. As of September 2009, some HP- LEDs
LEDs
manufactured by Cree now exceed 105 lm/W.[136] Examples for Haitz's law—which predicts an exponential rise in light output and efficacy of LEDs
LEDs
over time—are the CREE XP-G series LED, which achieved 105 lm/W in 2009[136] and the Nichia
Nichia
19 series with a typical efficacy of 140 lm/W, released in 2010.[137] AC-driven[edit] LEDs
LEDs
developed by Seoul Semiconductor
Semiconductor
can operate on AC power without a DC converter. For each half-cycle, part of the LED
LED
emits light and part is dark, and this is reversed during the next half-cycle. The efficacy of this type of HP- LED
LED
is typically 40 lm/W.[138] A large number of LED
LED
elements in series may be able to operate directly from line voltage. In 2009, Seoul Semiconductor
Semiconductor
released a high DC voltage LED, named as 'Acrich MJT', 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
LED
design.[139] Application-specific variations[edit] Flashing[edit] Flashing LEDs
LEDs
are used as attention seeking indicators without requiring external electronics. Flashing LEDs
LEDs
resemble standard LEDs but they contain an integrated multivibrator circuit that causes the LED
LED
to flash with a typical period of one second. In diffused lens LEDs, this circuit 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[edit] Bi-color LEDs
LEDs
contain two different LED
LED
emitters in one case. There are two types of these. One type consists of two dies connected to the same two leads 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. The most common bi-color combination is red/traditional green, however, other available combinations include amber/traditional green, red/pure green, red/blue, and blue/pure green. Tri-color[edit] Tri-color LEDs
LEDs
contain three different LED
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[edit] RGB LEDs
LEDs
are tri-color LEDs
LEDs
with red, green, and blue emitters, in general using a four-wire connection with one common lead (anode or cathode). These LEDs
LEDs
can have either common positive leads in the case of a common anode LED, or common negative leads in the case of a common cathode LED. Others, however, have only two leads (positive and negative) and have a built-in electronic control unit. Decorative-multicolor[edit] Decorative-multicolor LEDs
LEDs
incorporate several emitters of different colors supplied by only two lead-out wires. Colors are switched internally by varying the supply voltage. Alphanumeric[edit] Alphanumeric LEDs
LEDs
are available in seven-segment, starburst, and dot-matrix format. Seven-segment displays handle all numbers and a limited set of letters. Starburst displays can display all letters. Dot-matrix displays typically use 5x7 pixels per character. Seven-segment LED
LED
displays were in widespread use in the 1970s and 1980s, but rising use of liquid crystal displays, with their lower power needs and greater display flexibility, has reduced the popularity of numeric and alphanumeric LED
LED
displays. Digital-RGB[edit] Digital RGB Addressable LEDs
LEDs
are RGB LEDs
LEDs
that contain their own "smart" control electronics. In addition to power and ground, these provide connections for data-in, data-out, and sometimes a clock or strobe signal. These are connected in a daisy chain, with the data in of the first LED
LED
sourced by a microprocessor, which can control the brightness and color of each LED
LED
independently of the others. They are used where a combination of maximum control and minimum visible electronics are needed such as strings for Christmas and LED
LED
matrices. Some even have refresh rates in the kHz range, allowing for basic video applications. These devices are also known by their part number (WS2812 being the most common) or a brand name such as NeoPixel Filament[edit] An LED filament
LED filament
consists of multiple LED
LED
chips connected in series on a common longitudinal substrate that forms a thin rod reminiscent of a traditional incandescent filament.[140] These are being used as a low-cost decorative alternative for traditional light bulbs that are being phased out in many countries. The filaments require a rather high voltage to light to nominal brightness, allowing them to work efficiently and simply with mains voltages. Often a simple rectifier and capacitive current limiting are employed to create a low-cost replacement for a traditional light bulb without the complexity of the low voltage, high current converter that single die LEDs
LEDs
need.[141] Usually, they are packaged in a sealed enclosure with a shape similar to lamps they were designed to replace (e.g. a bulb) and filled with inert nitrogen or carbon dioxide gas to remove heat efficiently. Considerations for use[edit] Power sources[edit] Main article: LED
LED
power sources

Simple LED
LED
circuit with resistor for current limiting

The current–voltage characteristic of an LED
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.[142] If the applied voltage exceeds the LED's forward voltage drop 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 power supplies to keep the current below the LED's maximum current rating. Since most common power sources (batteries, mains) are constant-voltage sources, most LED
LED
fixtures must include a power converter, at least a current-limiting resistor. However, the high resistance of three-volt coin cells combined with the high differential resistance of nitride-based LEDs
LEDs
makes it possible to power such an LED
LED
from such a coin cell without an external resistor. Electrical polarity[edit] Main article: Electrical polarity of LEDs As with all diodes, current flows easily from p-type to n-type material.[143] 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
LED
may be damaged. If the reverse current is sufficiently limited to avoid damage, the reverse-conducting LED
LED
is a useful noise diode. Safety and health[edit] The vast majority of devices containing LEDs
LEDs
are "safe under all conditions of normal use", and so are classified as "Class 1 LED product"/" LED
LED
Klasse 1". At present, only a few LEDs—extremely bright LEDs
LEDs
that also have a tightly focused viewing angle of 8° or less—could, in theory, cause temporary blindness, and so are classified as "Class 2".[144] The opinion of the French Agency for Food, Environmental and Occupational Health & Safety (ANSES) of 2010, on the health issues concerning LEDs, suggested banning public use of lamps in the moderate Risk Group 2, especially those with a high blue component, in places frequented by children.[145] [146] In general, laser safety regulations—and the "Class 1", "Class 2", etc. system—also apply to LEDs.[147] While LEDs
LEDs
have the advantage over fluorescent lamps that they do not contain mercury, they may contain other hazardous metals such as lead and arsenic. Regarding the toxicity of LEDs
LEDs
when treated as waste, a study published in 2011 stated: "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
LEDs
hazardous."[148] In 2016 a statement of the American Medical Association
American Medical Association
(AMA) concerning the possible influence of blueish street lighting on the sleep-wake cycle of city-dwellers led to some controversy. So far high-pressure sodium lamps (HPS) with an orange light spectrum were the most efficient light sources commonly used in street-lighting. Now many modern street lamps are equipped with Indium gallium nitride
Indium gallium nitride
LEDs (InGaN). These are even more efficient and mostly emit blue-rich light with a higher correlated color temperature (CCT). Since light with a high CCT resembles daylight it is thought that this might have an effect on the normal circadian physiology by suppressing melatonin production in the human body. There have been no relevant studies as yet and critics claim exposure levels are not high enough to have a noticeable effect. [149] Advantages[edit]

Efficiency: LEDs
LEDs
emit more lumens per watt than incandescent light bulbs.[150] The efficiency of LED
LED
lighting fixtures is not affected by shape and size, unlike fluorescent light bulbs or tubes. Color: LEDs
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
LEDs
can be very small (smaller than 2 mm2[151]) and are easily attached to printed circuit boards. Warmup time: LEDs
LEDs
light up very quickly. A typical red indicator LED achieves full brightness in under a microsecond.[152] LEDs
LEDs
used in communications devices can have even faster response times. Cycling: LEDs
LEDs
are ideal for uses subject to frequent on-off cycling, unlike incandescent and fluorescent lamps that fail faster when cycled often, or high-intensity discharge lamps (HID lamps) that require a long time before restarting. Dimming: LEDs
LEDs
can very easily be dimmed either by pulse-width modulation or lowering the forward current.[153] This pulse-width modulation is why LED
LED
lights, particularly headlights on cars, when viewed on camera or by some people, seem to flash or flicker. This is a type of stroboscopic effect. Cool light: In contrast to most light sources, LEDs
LEDs
radiate very little heat in the form of 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
LEDs
mostly fail by dimming over time, rather than the abrupt failure of incandescent bulbs.[72] Lifetime: LEDs
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.[154] 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.[155] 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
LED
can be designed to focus its light. Incandescent
Incandescent
and fluorescent sources often require an external reflector to collect light and direct it in a usable manner. For larger LED
LED
packages total internal reflection (TIR) lenses are often used to the same effect. However, when large quantities of light are needed many light sources are usually deployed, which are difficult to focus or collimate towards the same target.

Disadvantages[edit]

Initial price: LEDs
LEDs
are currently slightly more expensive (price per lumen) on an initial capital cost basis, than other lighting technologies. As of March 2014[update], at least one manufacturer claims to have reached $1 per kilolumen.[156] The additional expense partially stems from the relatively low lumen output and the drive circuitry and power supplies needed. Temperature dependence: LED
LED
performance largely depends on the ambient temperature of the operating environment – or thermal management properties. Overdriving an LED
LED
in high ambient temperatures may result in overheating the LED
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. Toshiba has produced LEDs
LEDs
with an operating temperature range of −40 to 100 °C, which suits the LEDs
LEDs
for both indoor and outdoor use in applications such as lamps, ceiling lighting, street lights, and floodlights.[114] Voltage sensitivity: LEDs
LEDs
must be supplied with a voltage above their threshold voltage and a current below their rating. Current and lifetime change greatly with a small change in applied voltage. They thus require a current-regulated supply (usually just a series resistor for indicator LEDs).[157] Color rendition: Most cool-white LEDs
LEDs
have spectra that differ significantly from a black body radiator like the sun or an incandescent light. The spike at 460 nm and dip at 500 nm can make the color of objects appear differently under cool-white LED illumination than sunlight or incandescent sources, due to metamerism,[158] red surfaces being rendered particularly poorly by typical phosphor-based cool-white LEDs. Area light source: Single LEDs
LEDs
do not approximate a point source of light giving a spherical light distribution, but rather a lambertian distribution. So LEDs
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.[159] Electrical polarity: Unlike incandescent light bulbs, which illuminate regardless of the electrical polarity, LEDs
LEDs
only light with correct electrical polarity. To automatically match source polarity to LED devices, rectifiers can be used. Blue
Blue
hazard: There is a concern that blue LEDs
LEDs
and cool-white LEDs
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".[160][161] However, a later paper showed that there is no risk in normal use at domestic illuminance for the general population.[162][163] Light pollution: Because white LEDs, especially those with high color temperature, emit much more short wavelength light than conventional outdoor light sources such as high-pressure sodium vapor lamps, the increased blue and green sensitivity of scotopic vision means that white LEDs
LEDs
used in outdoor lighting cause substantially more sky glow.[139][164][165][166][167] The American Medical Association
American Medical Association
warned on the use of high blue content white LEDs
LEDs
in street lighting, due to their higher impact on human health and environment, compared to low blue content light sources (e.g. High-Pressure Sodium, PC amber LEDs, and low CCT LEDs).[168] Efficiency droop: The efficiency of LEDs
LEDs
decreases as the electric current increases. Heating also increases with higher currents, which compromises LED
LED
lifetime. These effects put practical limits on the current through an LED
LED
in high power applications.[65][67][68][169] Impact on insects: LEDs
LEDs
are much more attractive to insects than sodium-vapor lights, so much so that there has been speculative concern about the possibility of disruption to food webs.[170][171] Use in winter conditions: Since they do not give off much heat in comparison to incandescent lights, LED
LED
lights used for traffic control can have snow obscuring them, leading to accidents.[172][173]

Applications[edit] LED
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 Illumination where light is reflected from objects to give visual response of these objects Measuring and interacting with processes involving no human vision[174] Narrow band light sensors where LEDs
LEDs
operate in a reverse-bias mode and respond to incident light, instead of emitting light[175][176][177][178]

Indicators and signs[edit] The low energy consumption, low maintenance and small size of LEDs
LEDs
has led to uses as status indicators and displays on a variety of equipment and installations. Large-area LED
LED
displays are used as stadium displays, dynamic decorative displays, and dynamic message signs on freeways. Thin, lightweight message displays are used at airports and railway stations, and as destination displays for trains, buses, trams, and ferries.

Red
Red
and green LED
LED
traffic signals

One-color light is well suited for traffic lights and signals, exit signs, emergency vehicle lighting, ships' navigation lights or lanterns (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 LED-based Christmas lights. In cold climates, LED
LED
traffic lights may remain snow-covered.[179] Red
Red
or yellow LEDs
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.

Automotive applications for LEDs
LEDs
continue to grow.

Because of their long life, fast switching times, and visibility in broad daylight due to their high output and focus, LEDs
LEDs
have been used in brake lights for cars' high-mounted brake lights, trucks, and buses, and in turn signals for some time. However, many vehicles now use LEDs
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[citation needed] than an incandescent bulb. This gives drivers behind more time to react. In a dual intensity circuit (rear markers and brakes) if the LEDs
LEDs
are not pulsed at a fast enough frequency, they can create a phantom array, where ghost images of the LED
LED
appear if the eyes quickly scan across the array. White
White
LED
LED
headlamps are beginning to appear. Using LEDs
LEDs
has styling advantages because LEDs
LEDs
can form much thinner lights than incandescent lamps with parabolic reflectors. Due to the relative cheapness of low output LEDs, they are also used in many temporary uses such as glowsticks, throwies, and the photonic textile Lumalive. Artists have also used LEDs
LEDs
for LED
LED
art. Weather and all-hazards radio receivers with Specific Area Message Encoding (SAME) have three LEDs: red for warnings, orange for watches, and yellow for advisories and statements whenever issued. Lighting[edit] With the development of high-efficiency and high-power LEDs, it has become possible to use LEDs
LEDs
in lighting and illumination. To encourage the shift to LED
LED
lamps and other high-efficiency lighting, the US Department of Energy
Energy
has created the L Prize
L Prize
competition. The Philips Lighting
Lighting
North America LED
LED
bulb won the first competition on August 3, 2011, after successfully completing 18 months of intensive field, lab, and product testing.[180] LEDs
LEDs
are used as street lights and in other architectural lighting. The mechanical robustness and long lifetime are used in automotive lighting on cars, motorcycles, and bicycle lights. LED
LED
light emission may be efficiently controlled by using nonimaging optics principles. LED
LED
street lights are employed on poles and in parking garages. In 2007, the Italian village of Torraca
Torraca
was the first place to convert its entire illumination system to LEDs.[181] LEDs
LEDs
are used in aviation lighting. Airbus
Airbus
has used LED
LED
lighting in its Airbus
Airbus
A320 Enhanced since 2007, and Boeing uses LED
LED
lighting in the 787 and 737 Sky Interior. LEDs
LEDs
are also being used now in airport and heliport lighting. LED
LED
airport fixtures currently include medium-intensity runway lights, runway centerline lights, taxiway centerline and edge lights, guidance signs, and obstruction lighting. LEDs
LEDs
are also used as a light source for DLP projectors, and to backlight LCD
LCD
televisions (referred to as LED
LED
TVs) and laptop displays. RGB LEDs
LEDs
raise the color gamut by as much as 45%. Screens for TV and computer displays can be made thinner using LEDs
LEDs
for backlighting.[182] The lack of IR or heat radiation makes LEDs
LEDs
ideal for stage lights using banks of RGB LEDs
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
LEDs
also means air conditioning (cooling) systems have less heat in need of disposal. LEDs
LEDs
are small, durable and need little power, so they are used in handheld devices such as flashlights. LED
LED
strobe lights or camera flashes 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 phones, where space is at a premium and bulky voltage-raising circuitry is undesirable. LEDs
LEDs
are used for infrared illumination in night vision uses including security cameras. A ring of LEDs
LEDs
around a video camera, aimed forward into a retroreflective background, allows chroma keying in video productions.

LED
LED
for miners, to increase visibility inside mines

Los Angeles Vincent Thomas Bridge illuminated with blue LEDs

LEDs
LEDs
are used in mining operations, as cap lamps to provide light for miners. Research has been done to improve LEDs
LEDs
for mining, to reduce glare and to increase illumination, reducing risk of injury to the miners.[183] LEDs
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
LEDs
are increasingly finding uses in medical and educational applications, for example as mood enhancement,[citation needed] and new technologies such as AmBX, exploiting LED
LED
versatility. NASA
NASA
has even sponsored research for the use of LEDs
LEDs
to promote health for astronauts.[184] Data communication and other signalling[edit] See also: Li-Fi See also: fibre optics Light can be used to transmit data and analog signals. For example, lighting white LEDs
LEDs
can be used in systems assisting people to navigate in closed spaces while searching necessary rooms or objects.[185] Assistive listening devices in many theaters and similar spaces use arrays of infrared LEDs
LEDs
to send sound to listeners' receivers. Light-emitting diodes (as well as semiconductor lasers) are used to send data over many types of fiber optic cable, from digital audio over TOSLINK
TOSLINK
cables to the very high bandwidth fiber links that form the Internet backbone. For some time, computers were commonly equipped with IrDA
IrDA
interfaces, which allowed them to send and receive data to nearby machines via infrared. Because LEDs
LEDs
can cycle on and off millions of times per second, very high data bandwidth can be achieved.[186] Sustainable lighting[edit] Efficient lighting is needed for sustainable architecture. In 2009, US Department of Energy
Energy
testing results on LED
LED
lamps showed an average efficacy of 35 lm/W, below that of typical CFLs, and as low as 9 lm/W, worse than standard incandescent bulbs. A typical 13-watt LED lamp
LED lamp
emitted 450 to 650 lumens,[187] which is equivalent to a standard 40-watt incandescent bulb. However, as of 2011, there are LED
LED
bulbs available as efficient as 150 lm/W and even inexpensive low-end models typically exceed 50 lm/W, so that a 6-watt LED
LED
could achieve the same results as a standard 40-watt incandescent bulb. The latter has an expected lifespan of 1,000 hours, whereas an LED
LED
can continue to operate with reduced efficiency for more than 50,000 hours.

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See the chart below for a comparison of common light types:

LED CFL Incandescent

Lightbulb Projected Lifespan 50,000 hours 10,000 hours 1,200 hours

Watts Per Bulb (equiv. 60 watts) 10 14 60

Cost Per Bulb $2.00 $2.00 $1.25

kWh of Electricity Used Over 50,000 Hours 500 700 3000

Cost of Electricity (@ 0.10 per kWh) $50 $70 $300

Bulbs Needed for 50,000 Hours of Use 1 5 42

Equivalent 50,000 Hours Bulb Expense $7.00 $10.00 $52.50

TOTAL Cost for 50,000 Hours $57.00 $80.00 $352.50

Energy
Energy
consumption[edit] In the US, one kilowatt-hour (3.6 MJ) of electricity currently causes an average 1.34 pounds (610 g) of CO 2 emission.[188] Assuming the average light bulb is on for 10 hours a day, a 40-watt bulb causes 196 pounds (89 kg) of CO 2 emission per year. The 6-watt LED
LED
equivalent only causes 30 pounds (14 kg) of CO 2 over the same time span. A building’s carbon footprint from lighting can, therefore, be reduced by 85% by exchanging all incandescent bulbs for new LEDs—if a building previously used only incandescent bulbs. In practice, most buildings that use a lot of lighting use fluorescent lighting, which has 22% luminous efficiency compared with 5% for filaments, so changing to LED
LED
lighting would still give a 34% reduction in electrical power use and carbon emissions. The reduction in carbon emissions depends on the source of electricity. Nuclear power in the United States
United States
produced 19.2% of electricity in 2011, so reducing electricity consumption in the U.S. reduces carbon emissions more than in France (75% nuclear electricity) or Norway (almost entirely hydroelectric). Replacing lights that spend the most time lit results in the most savings, so LED
LED
lights in infrequently used locations bring a smaller return on investment. Light sources for machine vision systems[edit] Machine vision
Machine vision
systems often require bright and homogeneous illumination, so features of interest are easier to process. LEDs
LEDs
are often used for this purpose, and this is likely to remain one of their major uses until the price drops low enough to make signaling and illumination uses more widespread. Barcode scanners are the most common example of machine vision, and many low-cost products use red LEDs
LEDs
instead of lasers.[189] Optical computer mice are an example of LEDs
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
LEDs
is a benefit. LED
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
LED
sources can be shaped in several configurations (spot lights for reflective illumination; ring lights for coaxial illumination; backlights for contour illumination; linear assemblies; flat, large format panels; dome sources for diffused, omnidirectional illumination). LEDs
LEDs
can be easily strobed (in the microsecond range and below) and synchronized with imaging. High-power LEDs
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
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 lets tightly matched filters 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).[189]

A large LED display
LED display
behind a disc jockey

LED
LED
digital display that can display four digits and points

Traffic light
Traffic light
using LED

LED
LED
daytime running lights of Audi A4

LED
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.

LED
LED
lights reacting dynamically to video feed via AmBX

Different sized LEDs. 8 mm, 5 mm and 3 mm, with a wooden match-stick for scale.

A green surface-mount colored LED
LED
mounted on an Arduino
Arduino
circuit board

Other applications[edit]

LED
LED
costume for stage performers

LED
LED
wallpaper by Meystyle

USB
USB
powered 90 mm diameter mini-fan with LED
LED
during function.

The light from LEDs
LEDs
can be modulated very quickly so they are used extensively in optical fiber and free space optics communications. This includes remote controls, such as for TVs, VCRs, and LED Computers, where infrared LEDs
LEDs
are often used. Opto-isolators use an LED
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 lets information be transferred between circuits that don't share a common ground potential. Many sensor systems rely on light as the signal source. LEDs
LEDs
are often ideal as a light source due to the requirements of the sensors. LEDs are used as motion sensors, for example in optical computer mice. The Nintendo Wii's sensor bar uses infrared LEDs. Pulse oximeters use them for measuring oxygen saturation. Some flatbed scanners use arrays of RGB LEDs
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. Since LEDs
LEDs
can also be used as photodiodes, they can be used for both photo emission and detection. This could be used, for example, in a touchscreen that registers reflected light from a finger or stylus.[190] Many materials and biological systems are sensitive to, or dependent on, light. Grow lights use LEDs
LEDs
to increase photosynthesis in plants,[191] and bacteria and viruses can be removed from water and other substances using UV LEDs
LEDs
for sterilization.[103] LEDs
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
Zener diode
in low-voltage regulators. Red
Red
LEDs
LEDs
have the flattest I/V curve above the knee. Nitride-based LEDs
LEDs
have a fairly steep I/V curve and are useless for this purpose. Although LED
LED
forward voltage is far more current-dependent than a Zener diode, Zener diodes with breakdown voltages below 3 V are not widely available. The progressive miniaturization of low-voltage lighting technology, such as LEDs
LEDs
and OLEDs, suitable to incorporate into low-thickness materials has fostered experimentation in combining light sources and wall covering surfaces for interior walls.[192] The new possibilities offered by these developments have prompted some designers and companies, such as Meystyle,[193] Ingo Maurer,[194] Lomox[195] and Philips,[196] to research and develop proprietary LED
LED
wallpaper technologies, some of which are currently available for commercial purchase. Other solutions mainly exist as prototypes or are in the process of being further refined. See also[edit]

Electronics portal Energy
Energy
portal

History of display technology Laser
Laser
diode LEAD (diode), a light-emitting and absorbing diode LED
LED
circuit LED
LED
lamp LED
LED
tattoo Li-Fi Light-emitting electrochemical cell List of LED
LED
failure modes Nixie tube OLED Photovoltaics Seven-segment display SMD LED
LED
Module Solar lamp Solid-state lighting Thermal management of high-power LEDs UV curing

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Further reading[edit]

Shuji Nakamura; Gerhard Fasol; Stephen J Pearton (2000). The Blue Laser
Laser
Diode: The Complete Story. Springer Verlag. ISBN 3-540-66505-6. 

External links[edit]

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Lamps Lighting

Concepts

Accent lighting Color temperature Efficiency Glare Lamp Light fixture Light pollution Lightbulb socket Task lighting

Methods of generation

Incandescent

Regular Halogen Nernst Parabolic aluminized reflector (PAR)

Luminescent

Fluorescent

Fluorescent lamp (compact) Fluorescent induction

Photoluminescent

laser lamp

Chemiluminescent Solid-state

LED
LED
bulb

Cathodoluminescent

Electron-stimulated

Electroluminescent

field-induced polymer

Combustion

Acetylene/Carbide Argand Campfire Candle Carcel Diya Flare Gas Kerosene Lantern Limelight Oil Rushlight Safety Tilley Torch

Electric arc

Carbon arc Klieg light Yablochkov candle

Gas discharge

Deuterium arc Neon Plasma Sulfur Xenon
Xenon
arc Xenon
Xenon
flash

High-intensity discharge (HID)

Hydrargyrum medium-arc iodide (HMI) Hydrargyrum quartz iodide (HQI) Mercury-vapor Metal-halide

ceramic

Sodium vapor

Theatrical Cinematic

Floodlight Footlight Gobo Scoop Spotlight

ellipsoidal reflector

Stage lighting
Stage lighting
instrument

Stationary

Aircraft warning Balanced-arm lamp Chandelier Emergency light Gas lighting Gooseneck lamp Intelligent street lighting Light tube Nightlight Neon lighting Pendant light Recessed light Sconce Street light Torchère Track lighting Troffer

Mobile

Flashlight

tactical

Glow stick Headlamp (outdoor) Lantern Laser
Laser
pointer Navigation light Searchlight Solar lamp

Industrial Scientific

Germicidal Grow light Infrared
Infrared
lamp Stroboscope Tanning

Display Decorative

Aroma lamp Black light Bubble light Christmas lights Crackle tube DJ lighting Electroluminescent wire Lava lamp Marquee Plasma globe Strobe light

Related topics

Bioluminescence Chemiluminescence Electroluminescence Laser Photoluminescence Radioluminescence

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Display technology

Video displays

Current generation

Eidophor Electroluminescent display (ELD) Electronic paper

E Ink Gyricon

Light emitting diode display (LED) Cathode
Cathode
ray tube (CRT) (Monoscope) Liquid-crystal display
Liquid-crystal display
(LCD)

TFT TN LED Blue
Blue
Phase IPS

Plasma display
Plasma display
panel (PDP)

ALiS

Digital Light Processing
Digital Light Processing
(DLP) Liquid crystal on silicon
Liquid crystal on silicon
(LCoS)

Next generation

Organic light-emitting diode
Organic light-emitting diode
(OLED)

AMOLED

Organic light-emitting transistor (OLET) Surface-conduction electron-emitter display
Surface-conduction electron-emitter display
(SED) Field emission display (FED) Laser
Laser
TV

Quantum dot Liquid crystal

MEMS display

IMoD TMOS DMS

Quantum dot display
Quantum dot display
(QD-LED) Ferro liquid crystal display (FLCD) Thick-film dielectric electroluminescent technology (TDEL) Telescopic pixel display (TPD) Laser-powered phosphor display (LPD)

Non-video

Electromechanical

Flip-dot Split-flap Vane

Eggcrate Fiber optic Nixie tube Vacuum fluorescent display
Vacuum fluorescent display
(VFD) Light-emitting electrochemical cell (LEC) Lightguide display Dot-matrix display Seven-segment display
Seven-segment display
(SSD) Nine-segment display Fourteen-segment display
Fourteen-segment display
(FSD) Sixteen-segment display
Sixteen-segment display
(SISD)

3D display

Stereoscopic Autostereoscopic Multiscopic Hologram

Holographic display Computer-generated holography

Volumetric Musion Eyeliner Fog display

Static media

Movie projector Neon sign Destination sign Slide projector Transparency Laser
Laser
beam

Display capabilities

EDID

CEA-861

DisplayID DisplayHDR

Related articles

History of display technology Large-screen television technology Optimum HDTV viewing distance High-dynamic-range imaging
High-dynamic-range imaging
(HDRI) Color Light Output Flexible display

Comparison of display technology

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Electronic components

Semiconductor
Semiconductor
devices

Avalanche diode Transistor Tetrode
Tetrode
transistor Pentode
Pentode
transistor Memistor Memristor Bipolar junction transistor
Bipolar junction transistor
(BJT) FinFET CMOS MOSFET JFET Field-effect transistor
Field-effect transistor
(FET) Quantum circuit Constant-current diode
Constant-current diode
(CLD, CRD) Darlington transistor DIAC Diode Heterostructure barrier varactor Insulated-gate bipolar transistor
Insulated-gate bipolar transistor
(IGBT) Integrated circuit
Integrated circuit
(IC) Light-emitting diode
Light-emitting diode
(LED) Photodetector Photodiode PIN diode Schottky diode Silicon
Silicon
controlled rectifier (SCR) Thyristor TRIAC Unijunction transistor
Unijunction transistor
(UJT) Varicap Zener diode

Voltage regulators

Linear regulator Low-dropout regulator Switching regulator Buck Boost Buck–boost Split-pi Ćuk SEPIC Charge pump Switched capacitor

Vacuum tubes

Acorn tube Audion Beam tetrode Barretter Compactron Diode Fleming valve Nonode Nuvistor Pentagrid (Hexode, Heptode, Octode) Pentode Photomultiplier Phototube Tetrode Triode

Vacuum tubes (RF)

Backward-wave oscillator
Backward-wave oscillator
(BWO) Cavity magnetron Crossed-field amplifier
Crossed-field amplifier
(CFA) Gyrotron Inductive output tube
Inductive output tube
(IOT) Klystron Maser Sutton tube Traveling-wave tube
Traveling-wave tube
(TWT)

Cathode
Cathode
ray tubes

Beam deflection tube Charactron Iconoscope Magic eye tube Monoscope Selectron tube Storage tube Trochotron Video camera
Video camera
tube Williams tube

Gas-filled tubes

Cold cathode Crossatron Dekatron Ignitron Krytron Mercury-arc valve Neon lamp Nixie tube Thyratron Trigatron Voltage-regulator tube

Adjustable

Potentiometer

digital

Variable capacitor Varicap

Passive

Connector

audio and video electrical power RF

Electrolytic detector Ferrite Fuse

resettable

Resistor Switch Thermistor Transformer Varistor Wire

Wollaston wire

Reactive

Capacitor

types

Ceramic resonator Crystal
Crystal
oscillator Inductor Parametron Relay

reed relay mercury switch

Authority control

GND: 41251

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