Light-emitting diode

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This article is about the basics of light emitting diodes. Light-emitting diode_sentence_0

For application to area lighting, see LED lamp. Light-emitting diode_sentence_1

"LED" redirects here. Light-emitting diode_sentence_2

For other uses, see LED (disambiguation). Light-emitting diode_sentence_3

Not to be confused with LCD. Light-emitting diode_sentence_4

Light-emitting diode_table_infobox_0

Light-emitting diode (LED)Light-emitting diode_table_caption_0
Working principleLight-emitting diode_header_cell_0_0_0 ElectroluminescenceLight-emitting diode_cell_0_0_1
InventedLight-emitting diode_header_cell_0_1_0 H. J. Round (1907)

Oleg Losev (1927) James R. Biard (1961) Nick Holonyak (1962)Light-emitting diode_cell_0_1_1

First productionLight-emitting diode_header_cell_0_2_0 October 1962Light-emitting diode_cell_0_2_1
Pin configurationLight-emitting diode_header_cell_0_3_0 Anode and cathodeLight-emitting diode_cell_0_3_1
Electronic symbolLight-emitting diode_header_cell_0_4_0

A light-emitting diode (LED) is a semiconductor light source that emits light when current flows through it. Light-emitting diode_sentence_5

Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. Light-emitting diode_sentence_6

The color of the light (corresponding to the energy of the photons) is determined by the energy required for electrons to cross the band gap of the semiconductor. Light-emitting diode_sentence_7

White light is obtained by using multiple semiconductors or a layer of light-emitting phosphor on the semiconductor device. Light-emitting diode_sentence_8

Appearing as practical electronic components in 1962, the earliest LEDs emitted low-intensity infrared (IR) light. Light-emitting diode_sentence_9

Infrared LEDs are used in remote-control circuits, such as those used with a wide variety of consumer electronics. Light-emitting diode_sentence_10

The first visible-light LEDs were of low intensity and limited to red. Light-emitting diode_sentence_11

Modern LEDs are available across the visible, ultraviolet (UV), and infrared wavelengths, with high light output. Light-emitting diode_sentence_12

Early LEDs were often used as indicator lamps, replacing small incandescent bulbs, and in seven-segment displays. Light-emitting diode_sentence_13

Recent developments have produced high-output white light LEDs suitable for room and outdoor area lighting. Light-emitting diode_sentence_14

LEDs have led to new displays and sensors, while their high switching rates are useful in advanced communications technology. Light-emitting diode_sentence_15

LEDs have many advantages over incandescent light sources, including lower energy consumption, longer lifetime, improved physical robustness, smaller size, and faster switching. Light-emitting diode_sentence_16

LEDs are used in applications as diverse as aviation lighting, automotive headlamps, advertising, general lighting, traffic signals, camera flashes, lighted wallpaper, horticultural grow lights, and medical devices. Light-emitting diode_sentence_17

Unlike a laser, the light emitted from an LED is neither spectrally coherent nor even highly monochromatic. Light-emitting diode_sentence_18

However, its spectrum is sufficiently narrow that it appears to the human eye as a pure (saturated) color. Light-emitting diode_sentence_19

Also unlike most lasers, its radiation is not spatially coherent, so it cannot approach the very high brightnesses characteristic of lasers. Light-emitting diode_sentence_20

History Light-emitting diode_section_0

Discoveries and early devices Light-emitting diode_section_1

Electroluminescence as a phenomenon was discovered in 1907 by the British experimenter H. Light-emitting diode_sentence_21 J. Round of Marconi Labs, using a crystal of silicon carbide and a cat's-whisker detector. Light-emitting diode_sentence_22

Russian inventor Oleg Losev reported creation of the first LED in 1927. Light-emitting diode_sentence_23

His research was distributed in Soviet, German and British scientific journals, but no practical use was made of the discovery for several decades. Light-emitting diode_sentence_24

In 1936, Georges Destriau observed that electroluminescence could be produced when zinc sulphide (ZnS) powder is suspended in an insulator and an alternating electrical field is applied to it. Light-emitting diode_sentence_25

In his publications, Destriau often referred to luminescence as Losev-Light. Light-emitting diode_sentence_26

Destriau worked in the laboratories of Madame Marie Curie, also an early pioneer in the field of luminescence with research on radium. Light-emitting diode_sentence_27

Hungarian Zoltán Bay together with György Szigeti pre-empted LED lighting in Hungary in 1939 by patenting a lighting device based on SiC, with an option on boron carbide, that emitted white, yellowish white, or greenish white depending on impurities present. Light-emitting diode_sentence_28

Kurt Lehovec, Carl Accardo, and Edward Jamgochian explained these first LEDs in 1951 using an apparatus employing SiC crystals with a current source of a battery or a pulse generator and with a comparison to a variant, pure, crystal in 1953. Light-emitting diode_sentence_29

Rubin Braunstein of the Radio Corporation of America reported on infrared emission from gallium arsenide (GaAs) and other semiconductor alloys in 1955. Light-emitting diode_sentence_30

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 kelvins. Light-emitting diode_sentence_31

In 1957, Braunstein further demonstrated that the rudimentary devices could be used for non-radio communication across a short distance. Light-emitting diode_sentence_32

As noted by Kroemer 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. Light-emitting diode_sentence_33

The emitted light was detected by a PbS diode some distance away. Light-emitting diode_sentence_34

This signal was fed into an audio amplifier and played back by a loudspeaker. Light-emitting diode_sentence_35

Intercepting the beam stopped the music. Light-emitting diode_sentence_36

We had a great deal of fun playing with this setup." Light-emitting diode_sentence_37

This setup presaged the use of LEDs for optical communication applications. Light-emitting diode_sentence_38

In September 1961, while working at Texas Instruments in Dallas, Texas, James R. Biard and Gary Pittman discovered near-infrared (900 nm) light emission from a tunnel diode they had constructed on a GaAs substrate. Light-emitting diode_sentence_39

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. Light-emitting diode_sentence_40

On August 8, 1962, Biard and Pittman filed a patent titled "Semiconductor Radiant Diode" based on their findings, which described a zinc-diffused p–n junction LED with a spaced cathode contact to allow for efficient emission of infrared light under forward bias. Light-emitting diode_sentence_41

After establishing the priority of their work based on engineering notebooks predating submissions from G.E. Light-emitting diode_sentence_42

Labs, RCA Research Labs, IBM Research Labs, Bell Labs, and Lincoln Lab at MIT, the U.S. Light-emitting diode_sentence_43 patent office issued the two inventors the patent for the GaAs infrared light-emitting diode (U.S. Patent ), the first practical LED. Light-emitting diode_sentence_44

Immediately after filing the patent, Texas Instruments (TI) began a project to manufacture infrared diodes. Light-emitting diode_sentence_45

In October 1962, TI announced the first commercial LED product (the SNX-100), which employed a pure GaAs crystal to emit an 890 nm light output. Light-emitting diode_sentence_46

In October 1963, TI announced the first commercial hemispherical LED, the SNX-110. Light-emitting diode_sentence_47

The first visible-spectrum (red) LED was demonstrated by Nick Holonyak, Jr. on October 9, 1962 while he was working for General Electric in Syracuse, New York. Light-emitting diode_sentence_48

Holonyak and Bevacqua reported this LED in the journal Applied Physics Letters on December 1, 1962. Light-emitting diode_sentence_49

M. Light-emitting diode_sentence_50 George Craford, a former graduate student of Holonyak, invented the first yellow LED and improved the brightness of red and red-orange LEDs by a factor of ten in 1972. Light-emitting diode_sentence_51

In 1976, T. P. Pearsall designed the first high-brightness, high-efficiency LEDs for optical fiber telecommunications by inventing new semiconductor materials specifically adapted to optical fiber transmission wavelengths. Light-emitting diode_sentence_52

Initial commercial development Light-emitting diode_section_2

The first commercial visible-wavelength LEDs were commonly used as replacements for incandescent and neon indicator lamps, and in seven-segment displays, first in expensive equipment such as laboratory and electronics test equipment, then later in such appliances as calculators, TVs, radios, telephones, as well as watches (see list of signal uses). Light-emitting diode_sentence_53

Until 1968, visible and infrared LEDs were extremely costly, in the order of US$200 per unit, and so had little practical use. Light-emitting diode_sentence_54

Hewlett-Packard (HP) was engaged in research and development (R&D) on practical LEDs between 1962 and 1968, by a research team under Howard C. Borden, Gerald P. Pighini and Mohamed M. Atalla at HP Associates and HP Labs. Light-emitting diode_sentence_55

During this time, Atalla launched a material science investigation program on gallium arsenide (GaAs), gallium arsenide phosphide (GaAsP) and indium arsenide (InAs) devices at HP, and they collaborated with Monsanto Company on developing the first usable LED products. Light-emitting diode_sentence_56

The first usable LED products were HP's LED display and Monsanto's LED indicator lamp, both launched in 1968. Light-emitting diode_sentence_57

Monsanto was the first organization to mass-produce visible LEDs, using GaAsP in 1968 to produce red LEDs suitable for indicators. Light-emitting diode_sentence_58

Monsanto had previously offered to supply HP with GaAsP, but HP decided to grow its own GaAsP. Light-emitting diode_sentence_59

In February 1969, Hewlett-Packard introduced the HP Model 5082-7000 Numeric Indicator, the first LED device to use integrated circuit (integrated LED circuit) technology. Light-emitting diode_sentence_60

It was the first intelligent LED display, and was a revolution in digital display technology, replacing the Nixie tube and becoming the basis for later LED displays. Light-emitting diode_sentence_61

Atalla left HP and joined Fairchild Semiconductor in 1969. Light-emitting diode_sentence_62

He was the vice president and general manager of the Microwave & Optoelectronics division, from its inception in May 1969 up until November 1971. Light-emitting diode_sentence_63

He continued his work on LEDs, proposing they could be used for indicator lights and optical readers in 1971. Light-emitting diode_sentence_64

In the 1970s, commercially successful LED devices at less than five cents each were produced by Fairchild Optoelectronics. Light-emitting diode_sentence_65

These devices employed compound semiconductor chips fabricated with the planar process (developed by Jean Hoerni, based on Atalla's surface passivation method). Light-emitting diode_sentence_66

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. Light-emitting diode_sentence_67

LED producers continue to use these methods. Light-emitting diode_sentence_68

The early red LEDs were bright enough only for use as indicators, as the light output was not enough to illuminate an area. Light-emitting diode_sentence_69

Readouts in calculators were so small that plastic lenses were built over each digit to make them legible. Light-emitting diode_sentence_70

Later, other colors became widely available and appeared in appliances and equipment. Light-emitting diode_sentence_71

Early LEDs were packaged in metal cases similar to those of transistors, with a glass window or lens to let the light out. Light-emitting diode_sentence_72

Modern indicator LEDs are packed in transparent molded plastic cases, tubular or rectangular in shape, and often tinted to match the device color. Light-emitting diode_sentence_73

Infrared devices may be dyed, to block visible light. Light-emitting diode_sentence_74

More complex packages have been adapted for efficient heat dissipation in high-power LEDs. Light-emitting diode_sentence_75

Surface-mounted LEDs further reduce the package size. Light-emitting diode_sentence_76

LEDs intended for use with fiber optics cables may be provided with an optical connector. Light-emitting diode_sentence_77

Blue LED Light-emitting diode_section_3

The first blue-violet LED using magnesium-doped gallium nitride was made at Stanford University in 1972 by Herb Maruska and Wally Rhines, doctoral students in materials science and engineering. Light-emitting diode_sentence_78

At the time Maruska was on leave from RCA Laboratories, where he collaborated with Jacques Pankove on related work. Light-emitting diode_sentence_79

In 1971, the year after Maruska left for Stanford, his 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. Light-emitting diode_sentence_80

In 1974 the U.S. Light-emitting diode_sentence_81 Patent Office awarded Maruska, Rhines and Stanford professor David Stevenson a patent for their work in 1972 (U.S. Patent ). Light-emitting diode_sentence_82

Today, magnesium-doping of gallium nitride remains the basis for all commercial blue LEDs and laser diodes. Light-emitting diode_sentence_83

In the early 1970s, these devices were too dim for practical use, and research into gallium nitride devices slowed. Light-emitting diode_sentence_84

In August 1989, Cree introduced the first commercially available blue LED based on the indirect bandgap semiconductor, silicon carbide (SiC). Light-emitting diode_sentence_85

SiC LEDs had very low efficiency, no more than about 0.03%, but did emit in the blue portion of the visible light spectrum. Light-emitting diode_sentence_86

In the late 1980s, key breakthroughs in GaN epitaxial growth and p-type doping ushered in the modern era of GaN-based optoelectronic devices. Light-emitting diode_sentence_87

Building upon this foundation, Theodore Moustakas at Boston University patented a method for producing high-brightness blue LEDs using a new two-step process in 1991. Light-emitting diode_sentence_88

Two years later, in 1993, high-brightness blue LEDs were demonstrated by Shuji Nakamura of Nichia Corporation using a gallium nitride growth process. Light-emitting diode_sentence_89

In parallel, Isamu Akasaki and Hiroshi Amano in Nagoya were working on developing the important GaN deposition on sapphire substrates and the demonstration of p-type doping of GaN. Light-emitting diode_sentence_90

This new development revolutionized LED lighting, making high-power blue light sources practical, leading to the development of technologies like Blu-ray. Light-emitting diode_sentence_91

Nakamura was awarded the 2006 Millennium Technology Prize for his invention. Light-emitting diode_sentence_92

Nakamura, Hiroshi Amano and Isamu Akasaki were awarded the Nobel Prize in Physics in 2014 for the invention of the blue LED. Light-emitting diode_sentence_93

In 2015, a US court ruled that three companies had infringed Moustakas's prior patent, and ordered them to pay licensing fees of not less than US$13 million. Light-emitting diode_sentence_94

In 1995, Alberto Barbieri at the Cardiff University Laboratory (GB) investigated the efficiency and reliability of high-brightness LEDs and demonstrated a "transparent contact" LED using indium tin oxide (ITO) on (AlGaInP/GaAs). Light-emitting diode_sentence_95

In 2001 and 2002, processes for growing gallium nitride (GaN) LEDs on silicon were successfully demonstrated. Light-emitting diode_sentence_96

In January 2012, Osram demonstrated high-power InGaN LEDs grown on silicon substrates commercially, and GaN-on-silicon LEDs are in production at Plessey Semiconductors. Light-emitting diode_sentence_97

As of 2017, some manufacturers are using SiC as the substrate for LED production, but sapphire is more common, as it has the most similar properties to that of gallium nitride, reducing the need for patterning the sapphire wafer (patterned wafers are known as epi wafers). Light-emitting diode_sentence_98

Samsung, the University of Cambridge, and Toshiba are performing research into GaN on Si LEDs. Light-emitting diode_sentence_99

Toshiba has stopped research, possibly due to low yields. Light-emitting diode_sentence_100

Some opt towards epitaxy, which is difficult on silicon, while others, like the University of Cambridge, opt towards a multi-layer structure, in order to reduce (crystal) lattice mismatch and different thermal expansion ratios, in order to avoid cracking of the LED chip at high temperatures (e.g. during manufacturing), reduce heat generation and increase luminous efficiency. Light-emitting diode_sentence_101

Epitaxy (or patterned sapphire) can be carried out with nanoimprint lithography. Light-emitting diode_sentence_102

GaN-on-Si is desirable since it takes advantage of existing semiconductor manufacturing infrastructure, however, it is difficult to achieve. Light-emitting diode_sentence_103

It also allows for the wafer-level packaging of LED dies resulting in extremely small LED chips. Light-emitting diode_sentence_104

GaN is often deposited using Metalorganic vapour-phase epitaxy (MOCVD), and it also utilizes Lift-off. Light-emitting diode_sentence_105

White LEDs and the illumination breakthrough Light-emitting diode_section_4

Even though white light can be created using individual red, green and blue LEDs, this results in poor color rendering, since only three narrow bands of wavelengths of light are being emitted. Light-emitting diode_sentence_106

The attainment of high efficiency blue LEDs was quickly followed by the development of the first white LED. Light-emitting diode_sentence_107

In this device a Y 3Al 5O 12:Ce (known as "YAG" or Ce:YAG phosphor) cerium doped phosphor coating produces yellow light through fluorescence. Light-emitting diode_sentence_108

The combination of that yellow with remaining blue light appears white to the eye. Light-emitting diode_sentence_109

Using different phosphors produces green and red light through fluorescence. Light-emitting diode_sentence_110

The resulting mixture of red, green and blue is perceived as white light, with improved color rendering compared to wavelengths from the blue LED/YAG phosphor combination. Light-emitting diode_sentence_111

The first white LEDs were expensive and inefficient. Light-emitting diode_sentence_112

However, the light output of LEDs has increased exponentially. Light-emitting diode_sentence_113

The latest research and development has been propagated by Japanese manufacturers such as Panasonic, and Nichia, and by Korean and Chinese manufacturers such as Samsung, Kingsun, and others. Light-emitting diode_sentence_114

This trend in increased output has been called Haitz's law after Dr. Roland Haitz. Light-emitting diode_sentence_115

Light output and efficiency of blue and near-ultraviolet LEDs rose and the cost of reliable devices fell. Light-emitting diode_sentence_116

This led to relatively high-power white-light LEDs for illumination, which are replacing incandescent and fluorescent lighting. Light-emitting diode_sentence_117

Experimental white LEDs were demonstrated in 2014 to produce 303 lumens per watt of electricity (lm/w); some can last up to 100,000 hours. Light-emitting diode_sentence_118

However, commercially available LEDs have an efficiency of up to 223 lm/w as of 2018. Light-emitting diode_sentence_119

A previous record of 135lm/w was achieved by Nichia in 2010. Light-emitting diode_sentence_120

Compared to incandescent bulbs, this is a huge increase in electrical efficiency, and even though LEDs are more expensive to purchase, overall cost is significantly cheaper than that of incandescent bulbs. Light-emitting diode_sentence_121

The LED chip is encapsulated inside a small, plastic, white mold. Light-emitting diode_sentence_122

It can be encapsulated using resin (polyurethane-based), silicone, or epoxy containing (powdered) Cerium doped YAG phosphor. Light-emitting diode_sentence_123

After allowing the solvents to evaporate, the LEDs are often tested, and placed on tapes for SMT placement equipment for use in LED light bulb production. Light-emitting diode_sentence_124

Encapsulation is performed after probing, dicing, die transfer from wafer to package, and wire bonding or flip chip mounting, perhaps using Indium tin oxide, a transparent electrical conductor. Light-emitting diode_sentence_125

In this case, the bond wire(s) are attached to the ITO film that has been deposited in the LEDs. Light-emitting diode_sentence_126

Some "remote phosphor" LED light bulbs use a single plastic cover with YAG phosphor for several blue LEDs, instead of using phosphor coatings on single chip white LEDs. Light-emitting diode_sentence_127

The temperature of the phosphor during operation and how it is applied limits the size of an LED die. Light-emitting diode_sentence_128

Wafer-level packaged white LEDs allow for extremely small LEDs. Light-emitting diode_sentence_129

Physics of light production and emission Light-emitting diode_section_5

Main article: Light-emitting diode physics Light-emitting diode_sentence_130

In a light emitting diode, the recombination of electrons and electron holes in a semiconductor produces light (be it infrared, visible or UV), a process called "electroluminescence". Light-emitting diode_sentence_131

The wavelength of the light depends on the energy band gap of the semiconductors used. Light-emitting diode_sentence_132

Since these materials have a high index of refraction, design features of the devices such as special optical coatings and die shape are required to efficiently emit light. Light-emitting diode_sentence_133

Colors Light-emitting diode_section_6

By selection of different semiconductor materials, single-color LEDs can be made that emit light in a narrow band of wavelengths from near-infrared through the visible spectrum and into the ultraviolet range. Light-emitting diode_sentence_134

As the wavelengths become shorter, because of the larger band gap of these semiconductors, the operating voltage of the LED increases. Light-emitting diode_sentence_135

Blue and ultraviolet Light-emitting diode_section_7

Light-emitting diode_table_infobox_1

External videoLight-emitting diode_header_cell_1_0_0

Blue LEDs have an active region consisting of one or more InGaN quantum wells sandwiched between thicker layers of GaN, called cladding layers. Light-emitting diode_sentence_136

By varying the relative In/Ga fraction in the InGaN quantum wells, the light emission can in theory be varied from violet to amber. Light-emitting diode_sentence_137

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. Light-emitting diode_sentence_138

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. Light-emitting diode_sentence_139

Green LEDs manufactured from the InGaN/GaN system are far more efficient and brighter than green LEDs produced with non-nitride material systems, but practical devices still exhibit efficiency too low for high-brightness applications. Light-emitting diode_sentence_140

With AlGaN and AlGaInN, even shorter wavelengths are achievable. Light-emitting diode_sentence_141

Near-UV emitters at wavelengths around 360–395 nm are already cheap and often encountered, for example, as black light lamp replacements for inspection of anti-counterfeiting UV watermarks in documents and bank notes, and for UV curing. Light-emitting diode_sentence_142

While substantially more expensive, shorter-wavelength diodes are commercially available for wavelengths down to 240 nm. Light-emitting diode_sentence_143

As the photosensitivity of microorganisms approximately matches the absorption spectrum of DNA, with a peak at about 260 nm, UV LED emitting at 250–270 nm are expected in prospective disinfection and sterilization devices. Light-emitting diode_sentence_144

Recent research has shown that commercially available UVA LEDs (365 nm) are already effective disinfection and sterilization devices. Light-emitting diode_sentence_145

UV-C wavelengths were obtained in laboratories using aluminium nitride (210 nm), boron nitride (215 nm) and diamond (235 nm). Light-emitting diode_sentence_146

White Light-emitting diode_section_8

There are two primary ways of producing white light-emitting diodes. Light-emitting diode_sentence_147

One is to use individual LEDs that emit three primary colors—red, green and blue—and then mix all the colors to form white light. Light-emitting diode_sentence_148

The other is to use a phosphor material to convert monochromatic light from a blue or UV LED to broad-spectrum white light, similar to a fluorescent lamp. Light-emitting diode_sentence_149

The yellow phosphor is cerium-doped YAG crystals suspended in the package or coated on the LED. Light-emitting diode_sentence_150

This YAG phosphor causes white LEDs to appear yellow when off, and the space between the crystals allow some blue light to pass through. Light-emitting diode_sentence_151

Alternatively, white LEDs may use other phosphors like manganese(IV)-doped potassium fluorosilicate (PFS) or other engineered phosphors. Light-emitting diode_sentence_152

PFS assists in red light generation, and is used in conjunction with conventional Ce:YAG phosphor. Light-emitting diode_sentence_153

In LEDs with PFS phosphor, some blue light passes through the phosphors, the Ce:YAG phosphor converts blue light to green and red light, and the PFS phosphor converts blue light to red light. Light-emitting diode_sentence_154

The color temperature of the LED can be controlled by changing the concentration of the phosphors. Light-emitting diode_sentence_155

The 'whiteness' of the light produced is engineered to suit the human eye. Light-emitting diode_sentence_156

Because of metamerism, it is possible to have quite different spectra that appear white. Light-emitting diode_sentence_157

The appearance of objects illuminated by that light may vary as the spectrum varies. Light-emitting diode_sentence_158

This is the issue of color rendition, quite separate from color temperature. Light-emitting diode_sentence_159

An orange or cyan object could appear with the wrong color and much darker as the LED or phosphor does not emit the wavelength it reflects. Light-emitting diode_sentence_160

The best color rendition LEDs use a mix of phosphors, resulting in less efficiency and better color rendering. Light-emitting diode_sentence_161

RGB systems Light-emitting diode_section_9

Mixing red, green, and blue sources to produce white light needs electronic circuits to control the blending of the colors. Light-emitting diode_sentence_162

Since LEDs have slightly different emission patterns, the color balance may change depending on the angle of view, even if the RGB sources are in a single package, so RGB diodes are seldom used to produce white lighting. Light-emitting diode_sentence_163

Nonetheless, this method has many applications because of the flexibility of mixing different colors, and in principle, this mechanism also has higher quantum efficiency in producing white light. Light-emitting diode_sentence_164

There are several types of multicolor white LEDs: , tri-, and tetrachromatic white LEDs. Light-emitting diode_sentence_165

Several key factors that play among these different methods include color stability, color rendering capability, and luminous efficacy. Light-emitting diode_sentence_166

Often, higher efficiency means lower color rendering, presenting a trade-off between the luminous efficacy and color rendering. Light-emitting diode_sentence_167

For example, the dichromatic white LEDs have the best luminous efficacy (120 lm/W), but the lowest color rendering capability. Light-emitting diode_sentence_168

Although tetrachromatic white LEDs have excellent color rendering capability, they often have poor luminous efficacy. Light-emitting diode_sentence_169

Trichromatic white LEDs are in between, having both good luminous efficacy (>70 lm/W) and fair color rendering capability. Light-emitting diode_sentence_170

One of the challenges is the development of more efficient green LEDs. Light-emitting diode_sentence_171

The theoretical maximum for green LEDs is 683 lumens per watt but as of 2010 few green LEDs exceed even 100 lumens per watt. Light-emitting diode_sentence_172

The blue and red LEDs approach their theoretical limits. Light-emitting diode_sentence_173

Multicolor LEDs also offer a new means to form light of different colors. Light-emitting diode_sentence_174

Most perceivable colors can be formed by mixing different amounts of three primary colors. Light-emitting diode_sentence_175

This allows precise dynamic color control. Light-emitting diode_sentence_176

However, this type of LED's emission power decays exponentially with rising temperature, resulting in a substantial change in color stability. Light-emitting diode_sentence_177

Such problems inhibit industrial use. Light-emitting diode_sentence_178

Multicolor LEDs without phosphors cannot provide good color rendering because each LED is a narrowband source. Light-emitting diode_sentence_179

LEDs without phosphor, while a poorer solution for general lighting, are the best solution for displays, either backlight of LCD, or direct LED based pixels. Light-emitting diode_sentence_180

Dimming a multicolor LED source to match the characteristics of incandescent lamps is difficult because manufacturing variations, age, and temperature change the actual color value output. Light-emitting diode_sentence_181

To emulate the appearance of dimming incandescent lamps may require a feedback system with color sensor to actively monitor and control the color. Light-emitting diode_sentence_182

Phosphor-based LEDs Light-emitting diode_section_10

This method involves coating LEDs of one color (mostly blue LEDs made of InGaN) with phosphors of different colors to form white light; the resultant LEDs are called phosphor-based or phosphor-converted white LEDs (pcLEDs). Light-emitting diode_sentence_183

A fraction of the blue light undergoes the Stokes shift, which transforms it from shorter wavelengths to longer. Light-emitting diode_sentence_184

Depending on the original LED's color, various color phosphors are used. Light-emitting diode_sentence_185

Using several phosphor layers of distinct colors broadens the emitted spectrum, effectively raising the color rendering index (CRI). Light-emitting diode_sentence_186

Phosphor-based LEDs have efficiency losses due to heat loss from the Stokes shift and also other phosphor-related issues. Light-emitting diode_sentence_187

Their luminous efficacies compared to normal LEDs depend on the spectral distribution of the resultant light output and the original wavelength of the LED itself. Light-emitting diode_sentence_188

For example, the luminous efficacy of a typical YAG yellow phosphor based white LED ranges from 3 to 5 times the luminous efficacy of the original blue LED because of the human eye's greater sensitivity to yellow than to blue (as modeled in the luminosity function). Light-emitting diode_sentence_189

Due to the simplicity of manufacturing, the phosphor method is still the most popular method for making high-intensity white LEDs. Light-emitting diode_sentence_190

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 presently on the market are manufactured using phosphor light conversion. Light-emitting diode_sentence_191

Among the challenges being faced to improve the efficiency of LED-based white light sources is the development of more efficient phosphors. Light-emitting diode_sentence_192

As of 2010, the most efficient yellow phosphor is still the YAG phosphor, with less than 10% Stokes shift loss. Light-emitting diode_sentence_193

Losses attributable to internal optical losses due to re-absorption in the LED chip and in the LED packaging itself account typically for another 10% to 30% of efficiency loss. Light-emitting diode_sentence_194

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. Light-emitting diode_sentence_195

For instance, the efficiency can be raised by adapting better package design or by using a more suitable type of phosphor. Light-emitting diode_sentence_196

Conformal coating process is frequently used to address the issue of varying phosphor thickness. Light-emitting diode_sentence_197

Some phosphor-based white LEDs encapsulate InGaN blue LEDs inside phosphor-coated epoxy. Light-emitting diode_sentence_198

Alternatively, the LED might be paired with a remote phosphor, a preformed polycarbonate piece coated with the phosphor material. Light-emitting diode_sentence_199

Remote phosphors provide more diffuse light, which is desirable for many applications. Light-emitting diode_sentence_200

Remote phosphor designs are also more tolerant of variations in the LED emissions spectrum. Light-emitting diode_sentence_201

A common yellow phosphor material is cerium-doped yttrium aluminium garnet (Ce:YAG). Light-emitting diode_sentence_202

White LEDs can also be made by coating near-ultraviolet (NUV) LEDs with a mixture of high-efficiency europium-based phosphors that emit red and blue, plus copper and aluminium-doped zinc sulfide (ZnS:Cu, Al) that emits green. Light-emitting diode_sentence_203

This is a method analogous to the way fluorescent lamps work. Light-emitting diode_sentence_204

This method is less efficient than blue LEDs with YAG:Ce phosphor, as the Stokes shift is larger, so more energy is converted to heat, but yields light with better spectral characteristics, which render color better. Light-emitting diode_sentence_205

Due to the higher radiative output of the ultraviolet LEDs than of the blue ones, both methods offer comparable brightness. Light-emitting diode_sentence_206

A concern is that UV light may leak from a malfunctioning light source and cause harm to human eyes or skin. Light-emitting diode_sentence_207

Other white LEDs Light-emitting diode_section_11

Another method used to produce experimental white light 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. Light-emitting diode_sentence_208

A new style of wafers composed of gallium-nitride-on-silicon (GaN-on-Si) is being used to produce white LEDs using 200-mm silicon wafers. Light-emitting diode_sentence_209

This avoids the typical costly sapphire substrate in relatively small 100- or 150-mm wafer sizes. Light-emitting diode_sentence_210

The sapphire apparatus must be coupled with a mirror-like collector to reflect light that would otherwise be wasted. Light-emitting diode_sentence_211

It was predicted that since 2020, 40% of all GaN LEDs are made with GaN-on-Si. Light-emitting diode_sentence_212

Manufacturing large sapphire material is difficult, while large silicon material is cheaper and more abundant. Light-emitting diode_sentence_213

LED companies shifting from using sapphire to silicon should be a minimal investment. Light-emitting diode_sentence_214

Organic light-emitting diodes (OLEDs) Light-emitting diode_section_12

Main article: Organic light-emitting diode Light-emitting diode_sentence_215

In an organic light-emitting diode (OLED), the electroluminescent material composing the emissive layer of the diode is an organic compound. Light-emitting diode_sentence_216

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. Light-emitting diode_sentence_217

The organic materials can be small organic molecules in a crystalline phase, or polymers. Light-emitting diode_sentence_218

The potential advantages of OLEDs include thin, low-cost displays with a low driving voltage, wide viewing angle, and high contrast and color gamut. Light-emitting diode_sentence_219

Polymer LEDs have the added benefit of printable and flexible displays. Light-emitting diode_sentence_220

OLEDs have been used to make visual displays for portable electronic devices such as cellphones, digital cameras, lighting and televisions. Light-emitting diode_sentence_221

Types Light-emitting diode_section_13

LEDs are made in different packages for different applications. Light-emitting diode_sentence_222

A single or a few LED junctions may be packed in one miniature device for use as an indicator or pilot lamp. Light-emitting diode_sentence_223

An LED array may include controlling circuits within the same package, which may range from a simple resistor, blinking or color changing control, or an addressable controller for RGB devices. Light-emitting diode_sentence_224

Higher-powered white-emitting devices will be mounted on heat sinks and will be used for illumination. Light-emitting diode_sentence_225

Alphanumeric displays in dot matrix or bar formats are widely available. Light-emitting diode_sentence_226

Special packages permit connection of LEDs to optical fibers for high-speed data communication links. Light-emitting diode_sentence_227

Miniature Light-emitting diode_section_14

These are mostly single-die LEDs used as indicators, and they come in various sizes from 2 mm to 8 mm, through-hole and surface mount packages. Light-emitting diode_sentence_228

Typical current ratings range from around 1 mA to above 20 mA. Light-emitting diode_sentence_229

Multiple LED dies attached to a flexible backing tape form an LED strip light. Light-emitting diode_sentence_230

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. Light-emitting diode_sentence_231

The encapsulation may also be clear or tinted to improve contrast and viewing angle. Light-emitting diode_sentence_232

Infrared devices may have a black tint to block visible light while passing infrared radiation. Light-emitting diode_sentence_233

Ultra-high-output LEDs are designed for viewing in direct sunlight Light-emitting diode_sentence_234

5 V and 12 V LEDs are ordinary miniature LEDs that have a series resistor for direct connection to a 5 V or 12 V supply. Light-emitting diode_sentence_235

High-power Light-emitting diode_section_15

See also: Solid-state lighting, LED lamp, and Thermal management of high-power LEDs Light-emitting diode_sentence_236

High-power LEDs (HP-LEDs) or high-output 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. Light-emitting diode_sentence_237

Some can emit over a thousand lumens. Light-emitting diode_sentence_238

LED power densities up to 300 W/cm have been achieved. Light-emitting diode_sentence_239

Since overheating is destructive, the HP-LEDs must be mounted on a heat sink to allow for heat dissipation. Light-emitting diode_sentence_240

If the heat from an HP-LED is not removed, the device fails in seconds. Light-emitting diode_sentence_241

One HP-LED can often replace an incandescent bulb in a flashlight, or be set in an array to form a powerful LED lamp. Light-emitting diode_sentence_242

Some well-known HP-LEDs in this category are the Nichia 19 series, Lumileds Rebel Led, Osram Opto Semiconductors Golden Dragon, and Cree X-lamp. Light-emitting diode_sentence_243

As of September 2009, some HP-LEDs manufactured by Cree now exceed 105 lm/W. Light-emitting diode_sentence_244

Examples for Haitz's law—which predicts an exponential rise in light output and efficacy of LEDs over time—are the CREE XP-G series LED, which achieved 105 lm/W in 2009 and the Nichia 19 series with a typical efficacy of 140 lm/W, released in 2010. Light-emitting diode_sentence_245

AC-driven Light-emitting diode_section_16

LEDs developed by Seoul Semiconductor can operate on AC power without a DC converter. Light-emitting diode_sentence_246

For each half-cycle, part of the LED emits light and part is dark, and this is reversed during the next half-cycle. Light-emitting diode_sentence_247

The efficacy of this type of HP-LED is typically 40 lm/W. Light-emitting diode_sentence_248

A large number of LED elements in series may be able to operate directly from line voltage. Light-emitting diode_sentence_249

In 2009, Seoul Semiconductor released a high DC voltage LED, named as 'Acrich MJT', capable of being driven from AC power with a simple controlling circuit. Light-emitting diode_sentence_250

The low-power dissipation of these LEDs affords them more flexibility than the original AC LED design. Light-emitting diode_sentence_251

Application-specific variations Light-emitting diode_section_17

Power sources Light-emitting diode_section_18

Main article: LED power sources Light-emitting diode_sentence_252

The current in an LED or other diodes rises exponentially with the applied voltage (see Shockley diode equation), so a small change in voltage can cause a large change in current. Light-emitting diode_sentence_253

Current through the LED must be regulated by an external circuit such as a constant current source to prevent damage. Light-emitting diode_sentence_254

Since most common power supplies are (nearly) constant-voltage sources, LED fixtures must include a power converter, or at least a current-limiting resistor. Light-emitting diode_sentence_255

In some applications, the internal resistance of small batteries is sufficient to keep current within the LED rating. Light-emitting diode_sentence_256

Electrical polarity Light-emitting diode_section_19

Main article: Electrical polarity of LEDs Light-emitting diode_sentence_257

Unlike a traditional incandescent lamp, an LED will light only when voltage is applied in the forward direction of the diode. Light-emitting diode_sentence_258

No current flows and no light is emitted if voltage is applied in the reverse direction. Light-emitting diode_sentence_259

If the reverse voltage exceeds the breakdown voltage, a large current flows and the LED will be damaged. Light-emitting diode_sentence_260

If the reverse current is sufficiently limited to avoid damage, the reverse-conducting LED is a useful noise diode. Light-emitting diode_sentence_261

Safety and health Light-emitting diode_section_20

Certain blue LEDs and cool-white LEDs can exceed 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". Light-emitting diode_sentence_262

One study showed no evidence of a risk in normal use at domestic illuminance, and that caution is only needed for particular occupational situations or for specific populations. Light-emitting diode_sentence_263

In 2006, the International Electrotechnical Commission published IEC 62471 Photobiological safety of lamps and lamp systems, replacing the application of early laser-oriented standards for classification of LED sources. Light-emitting diode_sentence_264

While LEDs have the advantage over fluorescent lamps, in that they do not contain mercury, they may contain other hazardous metals such as lead and arsenic. Light-emitting diode_sentence_265

In 2016 the American Medical Association (AMA) issued a statement concerning the possible adverse influence of blueish street lighting on the sleep-wake cycle of city-dwellers. Light-emitting diode_sentence_266

Industry critics claim exposure levels are not high enough to have a noticeable effect. Light-emitting diode_sentence_267

Advantages Light-emitting diode_section_21

Light-emitting diode_unordered_list_0

  • Efficiency: LEDs emit more lumens per watt than incandescent light bulbs. The efficiency of LED lighting fixtures is not affected by shape and size, unlike fluorescent light bulbs or tubes.Light-emitting diode_item_0_0
  • Color: LEDs can emit light of an intended color without using any color filters as traditional lighting methods need. This is more efficient and can lower initial costs.Light-emitting diode_item_0_1
  • Size: LEDs can be very small (smaller than 2 mm) and are easily attached to printed circuit boards.Light-emitting diode_item_0_2
  • Warmup time: LEDs light up very quickly. A typical red indicator LED achieves full brightness in under a microsecond. LEDs used in communications devices can have even faster response times.Light-emitting diode_item_0_3
  • Cycling: 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.Light-emitting diode_item_0_4
  • Dimming: LEDs can very easily be dimmed either by pulse-width modulation or lowering the forward current. This pulse-width modulation is why 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.Light-emitting diode_item_0_5
  • Cool light: In contrast to most light sources, 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.Light-emitting diode_item_0_6
  • Slow failure: LEDs mainly fail by dimming over time, rather than the abrupt failure of incandescent bulbs.Light-emitting diode_item_0_7
  • Lifetime: LEDs can have a relatively long useful life. One report estimates 35,000 to 50,000 hours of useful life, though time to complete failure may be shorter or longer. Fluorescent tubes typically are rated at about 10,000 to 25,000 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.Light-emitting diode_item_0_8
  • Shock resistance: LEDs, being solid-state components, are difficult to damage with external shock, unlike fluorescent and incandescent bulbs, which are fragile.Light-emitting diode_item_0_9
  • Focus: The solid package of the LED can be designed to focus its light. Incandescent and fluorescent sources often require an external reflector to collect light and direct it in a usable manner. For larger LED packages total internal reflection (TIR) lenses are often used to the same effect. However, when large quantities of light are needed many light sources are usually deployed, which are difficult to focus or collimate towards the same target.Light-emitting diode_item_0_10

Disadvantages Light-emitting diode_section_22

Light-emitting diode_unordered_list_1

  • Temperature dependence: LED performance largely depends on the ambient temperature of the operating environment – or thermal management properties. Overdriving an LED in high ambient temperatures may result in overheating the LED package, eventually leading to device failure. An adequate heat sink is needed to maintain long life. This is especially important in automotive, medical, and military uses where devices must operate over a wide range of temperatures, and require low failure rates.Light-emitting diode_item_1_11
  • Voltage sensitivity: 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).Light-emitting diode_item_1_12
  • Color rendition: Most cool-white 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, red surfaces being rendered particularly poorly by typical phosphor-based cool-white LEDs. The same is true with green surfaces. The quality of color rendition of an LED is measured by the Color Rendering Index (CRI).Light-emitting diode_item_1_13
  • Area light source: Single LEDs do not approximate a point source of light giving a spherical light distribution, but rather a lambertian distribution. So, LEDs are difficult to apply to uses needing a spherical light field; however, different fields of light can be manipulated by the application of different optics or "lenses". LEDs cannot provide divergence below a few degrees.Light-emitting diode_item_1_14
  • Light pollution: Because white LEDs emit more short wavelength light than sources such as high-pressure sodium vapor lamps, the increased blue and green sensitivity of scotopic vision means that white LEDs used in outdoor lighting cause substantially more sky glow.Light-emitting diode_item_1_15
  • Efficiency droop: The efficiency of LEDs decreases as the electric current increases. Heating also increases with higher currents, which compromises LED lifetime. These effects put practical limits on the current through an LED in high power applications.Light-emitting diode_item_1_16
  • Impact on wildlife: 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. LED lighting near beaches, particularly intense blue and white colors, can disorient turtle hatchlings and make them wander inland instead. The use of "Turtle-safe lighting" LEDs that emit only at narrow portions of the visible spectrum is encouraged by conservancy groups in order to reduce harm.Light-emitting diode_item_1_17
  • Use in winter conditions: Since they do not give off much heat in comparison to incandescent lights, LED lights used for traffic control can have snow obscuring them, leading to accidents.Light-emitting diode_item_1_18
  • Thermal runaway: Parallel strings of LEDs will not share current evenly due to the manufacturing tolerances in their forward voltage. Running two or more strings from a single current source may result in LED failure as the devices warm up. If forward voltage binning is not possible, a circuit is required to ensure even distribution of current between parallel strands.Light-emitting diode_item_1_19

Applications Light-emitting diode_section_23

LED uses fall into four major categories: Light-emitting diode_sentence_268

Light-emitting diode_unordered_list_2

  • Visual signals where light goes more or less directly from the source to the human eye, to convey a message or meaningLight-emitting diode_item_2_20
  • Illumination where light is reflected from objects to give visual response of these objectsLight-emitting diode_item_2_21
  • Measuring and interacting with processes involving no human visionLight-emitting diode_item_2_22
  • Narrow band light sensors where LEDs operate in a reverse-bias mode and respond to incident light, instead of emitting lightLight-emitting diode_item_2_23

Indicators and signs Light-emitting diode_section_24

Lighting Light-emitting diode_section_25

Main article: LED lamp Light-emitting diode_sentence_269

With the development of high-efficiency and high-power LEDs, it has become possible to use LEDs in lighting and illumination. Light-emitting diode_sentence_270

To encourage the shift to LED lamps and other high-efficiency lighting, in 2008 the US Department of Energy created the L Prize competition. Light-emitting diode_sentence_271

The Philips Lighting North America LED bulb won the first competition on August 3, 2011, after successfully completing 18 months of intensive field, lab, and product testing. Light-emitting diode_sentence_272

Efficient lighting is needed for sustainable architecture. Light-emitting diode_sentence_273

As of 2011, some LED bulbs provide up to 150 lm/W and even inexpensive low-end models typically exceed 50 lm/W, so that a 6-watt LED could achieve the same results as a standard 40-watt incandescent bulb. Light-emitting diode_sentence_274

The lower heat output of LEDs also reduces demand on air conditioning systems. Light-emitting diode_sentence_275

Worldwide, LEDs are rapidly adopted to displace less effective sources such as incandescent lamps and CFLs and reduce electrical energy consumption and its associated emissions. Light-emitting diode_sentence_276

Solar powered LEDs are used as street lights and in architectural lighting. Light-emitting diode_sentence_277

The mechanical robustness and long lifetime are used in automotive lighting on cars, motorcycles, and bicycle lights. Light-emitting diode_sentence_278

LED street lights are employed on poles and in parking garages. Light-emitting diode_sentence_279

In 2007, the Italian village of Torraca was the first place to convert its street lighting to LEDs. Light-emitting diode_sentence_280

Cabin lighting on recent Airbus and Boeing jetliners uses LED lighting. Light-emitting diode_sentence_281

LEDs are also being used in airport and heliport lighting. Light-emitting diode_sentence_282

LED airport fixtures currently include medium-intensity runway lights, runway centerline lights, taxiway centerline and edge lights, guidance signs, and obstruction lighting. Light-emitting diode_sentence_283

LEDs are also used as a light source for DLP projectors, and to backlight LCD televisions (referred to as LED TVs) and laptop displays. Light-emitting diode_sentence_284

RGB LEDs raise the color gamut by as much as 45%. Light-emitting diode_sentence_285

Screens for TV and computer displays can be made thinner using LEDs for backlighting. Light-emitting diode_sentence_286

LEDs are small, durable and need little power, so they are used in handheld devices such as flashlights. Light-emitting diode_sentence_287

LED strobe lights or camera flashes operate at a safe, low voltage, instead of the 250+ volts commonly found in xenon flashlamp-based lighting. Light-emitting diode_sentence_288

This is especially useful in cameras on mobile phones, where space is at a premium and bulky voltage-raising circuitry is undesirable. Light-emitting diode_sentence_289

LEDs are used for infrared illumination in night vision uses including security cameras. Light-emitting diode_sentence_290

A ring of LEDs around a video camera, aimed forward into a retroreflective background, allows chroma keying in video productions. Light-emitting diode_sentence_291

LEDs are used in mining operations, as cap lamps to provide light for miners. Light-emitting diode_sentence_292

Research has been done to improve LEDs for mining, to reduce glare and to increase illumination, reducing risk of injury to the miners. Light-emitting diode_sentence_293

LEDs are increasingly finding uses in medical and educational applications, for example as mood enhancement. Light-emitting diode_sentence_294

NASA has even sponsored research for the use of LEDs to promote health for astronauts. Light-emitting diode_sentence_295

Data communication and other signalling Light-emitting diode_section_26

See also: Li-Fi, fibre optics, Visible light communication, and Optical disc Light-emitting diode_sentence_296

Light can be used to transmit data and analog signals. Light-emitting diode_sentence_297

For example, lighting white LEDs can be used in systems assisting people to navigate in closed spaces while searching necessary rooms or objects. Light-emitting diode_sentence_298

Assistive listening devices in many theaters and similar spaces use arrays of infrared LEDs to send sound to listeners' receivers. Light-emitting diode_sentence_299

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 cables to the very high bandwidth fiber links that form the Internet backbone. Light-emitting diode_sentence_300

For some time, computers were commonly equipped with IrDA interfaces, which allowed them to send and receive data to nearby machines via infrared. Light-emitting diode_sentence_301

Because LEDs can cycle on and off millions of times per second, very high data bandwidth can be achieved. Light-emitting diode_sentence_302

For that reason, Visible Light Communication (VLC) has been proposed as an alternative to the increasingly competitive radio bandwidth. Light-emitting diode_sentence_303

By operating in the visible part of the electromagnectic spectrum, data can be transmitted without occupying the frequencies of radio communications. Light-emitting diode_sentence_304

The main characteristic of VLC, lies on the incapacity of light to surpass physical opaque barriers. Light-emitting diode_sentence_305

This characteristic can be considered a weak point of VLC, due to the susceptibility of interference from physical objects, but is also one of its many strengths: unlike radio waves, light waves are confined in the encolsed spaces they are transmitted, which enforces a physical safety barrier that requires a receptor of that signal to have physical access to the place where the transmission is occurring. Light-emitting diode_sentence_306

A promising application of VLC lies on the Indoor Positioning System (IPS), an analogous to the GPS built to operate in enclosed spaces where the satellite transmissions that allow the GPS operation are hard to reach. Light-emitting diode_sentence_307

For instance, commercial buildings, shopping malls, parking garages, as well as subways and tunnel systems are all possible applications for VLC-based indoor positioning systems. Light-emitting diode_sentence_308

Additionally, once the VLC lamps are able to perform lighting at the same time as data transmission, it can simply occupy the installation of traditional single-function lamps. Light-emitting diode_sentence_309

Other applications for VLC involve communication between appliances of a smart home or office. Light-emitting diode_sentence_310

With increasing IoT-capable devices, connectivity through traditional radio waves might be subjected to interference. Light-emitting diode_sentence_311

However, light bulbs with VLC capabilities would be able to transmit data and commands for such devices. Light-emitting diode_sentence_312

Machine vision systems Light-emitting diode_section_27

Main article: Machine vision Light-emitting diode_sentence_313

Machine vision systems often require bright and homogeneous illumination, so features of interest are easier to process. Light-emitting diode_sentence_314

LEDs are often used. Light-emitting diode_sentence_315

Barcode scanners are the most common example of machine vision applications, and many of those scanners use red LEDs instead of lasers. Light-emitting diode_sentence_316

Optical computer mice use LEDs as a light source for the miniature camera within the mouse. Light-emitting diode_sentence_317

LEDs are useful for machine vision because they provide a compact, reliable source of light. Light-emitting diode_sentence_318

LED lamps can be turned on and off to suit the needs of the vision system, and the shape of the beam produced can be tailored to match the system's requirements. Light-emitting diode_sentence_319

Biological detection Light-emitting diode_section_28

The discovery of radiative recombination in Aluminum Gallium Nitride (AlGaN) alloys by U.S. Light-emitting diode_sentence_320 Army Research Laboratory (ARL) led to the conceptualization of UV light emitting diodes (LEDs) to be incorporated in light induced fluorescence sensors used for biological agent detection. Light-emitting diode_sentence_321

In 2004, the Edgewood Chemical Biological Center (ECBC) initiated the effort to create a biological detector named TAC-BIO. Light-emitting diode_sentence_322

The program capitalized on Semiconductor UV Optical Sources (SUVOS) developed by the Defense Advanced Research Projects Agency (DARPA). Light-emitting diode_sentence_323

UV induced fluorescence is one of the most robust techniques used for rapid real time detection of biological aerosols. Light-emitting diode_sentence_324

The first UV sensors were lasers lacking in-field-use practicality. Light-emitting diode_sentence_325

In order to address this, DARPA incorporated SUVOS technology to create a low cost, small, lightweight, low power device. Light-emitting diode_sentence_326

The TAC-BIO detector's response time was one minute from when it sensed a biological agent. Light-emitting diode_sentence_327

It was also demonstrated that the detector could be operated unattended indoors and outdoors for weeks at a time. Light-emitting diode_sentence_328

Aerosolized biological particles will fluoresce and scatter light under a UV light beam. Light-emitting diode_sentence_329

Observed fluorescence is dependent on the applied wavelength and the biochemical fluorophores within the biological agent. Light-emitting diode_sentence_330

UV induced fluorescence offers a rapid, accurate, efficient and logistically practical way for biological agent detection. Light-emitting diode_sentence_331

This is because the use of UV fluorescence is reagent less, or a process that does not require an added chemical to produce a reaction, with no consumables, or produces no chemical byproducts. Light-emitting diode_sentence_332

Additionally, TAC-BIO can reliably discriminate between threat and non-threat aerosols. Light-emitting diode_sentence_333

It was claimed to be sensitive enough to detect low concentrations, but not so sensitive that it would cause false positives. Light-emitting diode_sentence_334

The particle counting algorithm used in the device converted raw data into information by counting the photon pulses per unit of time from the fluorescence and scattering detectors, and comparing the value to a set threshold. Light-emitting diode_sentence_335

The original TAC-BIO was introduced in 2010, while the second generation TAC-BIO GEN II, was designed in 2015 to be more cost efficient as plastic parts were used. Light-emitting diode_sentence_336

Its small, light-weight design allows it to be mounted to vehicles, robots, and unmanned aerial vehicles. Light-emitting diode_sentence_337

The second generation device could also be utilized as an environmental detector to monitor air quality in hospitals, airplanes, or even in households to detect fungus and mold. Light-emitting diode_sentence_338

Other applications Light-emitting diode_section_29

The light from LEDs can be modulated very quickly so they are used extensively in optical fiber and free space optics communications. Light-emitting diode_sentence_339

This includes remote controls, such as for television sets, where infrared LEDs are often used. Light-emitting diode_sentence_340

Opto-isolators use an LED combined with a photodiode or phototransistor to provide a signal path with electrical isolation between two circuits. Light-emitting diode_sentence_341

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. Light-emitting diode_sentence_342

An optoisolator also lets information be transferred between circuits that do not share a common ground potential. Light-emitting diode_sentence_343

Many sensor systems rely on light as the signal source. Light-emitting diode_sentence_344

LEDs are often ideal as a light source due to the requirements of the sensors. Light-emitting diode_sentence_345

The Nintendo Wii's sensor bar uses infrared LEDs. Light-emitting diode_sentence_346

Pulse oximeters use them for measuring oxygen saturation. Light-emitting diode_sentence_347

Some flatbed scanners use arrays of RGB LEDs rather than the typical cold-cathode fluorescent lamp as the light source. Light-emitting diode_sentence_348

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. Light-emitting diode_sentence_349

Further, its sensors only need be monochromatic, since at any one time the page being scanned is only lit by one color of light. Light-emitting diode_sentence_350

Since LEDs can also be used as photodiodes, they can be used for both photo emission and detection. Light-emitting diode_sentence_351

This could be used, for example, in a touchscreen that registers reflected light from a finger or stylus. Light-emitting diode_sentence_352

Many materials and biological systems are sensitive to, or dependent on, light. Light-emitting diode_sentence_353

Grow lights use LEDs to increase photosynthesis in plants, and bacteria and viruses can be removed from water and other substances using UV LEDs for sterilization. Light-emitting diode_sentence_354

Deep UV LEDs, with a spectra range 247 nm to 386 nm, have other applications, such as water/air purification, surface disinfection, epoxy curing, free-space nonline-of-sight communication, high performance liquid chromatography, UV curing and printing, phototherapy, medical/ analytical instrumentation, and DNA absorption. Light-emitting diode_sentence_355

LEDs have also been used as a medium-quality voltage reference in electronic circuits. Light-emitting diode_sentence_356

The forward voltage drop (about 1.7 V for a red LED or 1.2V for an infrared) can be used instead of a Zener diode in low-voltage regulators. Light-emitting diode_sentence_357

Red LEDs have the flattest I/V curve above the knee. Light-emitting diode_sentence_358

Nitride-based LEDs have a fairly steep I/V curve and are useless for this purpose. Light-emitting diode_sentence_359

Although LED forward voltage is far more current-dependent than a Zener diode, Zener diodes with breakdown voltages below 3 V are not widely available. Light-emitting diode_sentence_360

The progressive miniaturization of low-voltage lighting technology, such as LEDs and OLEDs, suitable to incorporate into low-thickness materials has fostered experimentation in combining light sources and wall covering surfaces for interior walls in the form of LED wallpaper. Light-emitting diode_sentence_361

Light-emitting diode_unordered_list_3

  • Light-emitting diode_item_3_24
  • Light-emitting diode_item_3_25
  • Light-emitting diode_item_3_26

Research and development Light-emitting diode_section_30

Key challenges Light-emitting diode_section_31

LEDs require optimized efficiency to hinge on ongoing improvements such as phosphor materials and quantum dots. Light-emitting diode_sentence_362

The process of down-conversion (the method by which materials convert more-energetic photons to different, less energetic colors) also needs improvement. Light-emitting diode_sentence_363

For example, the red phosphors that are used today are thermally sensitive and need to be improved in that aspect so that they do not color shift and experience efficiency drop-off with temperature. Light-emitting diode_sentence_364

Red phosphors could also benefit from a narrower spectral width to emit more lumens and becoming more efficient at converting photons. Light-emitting diode_sentence_365

In addition, work remains to be done in the realms of current efficiency droop, color shift, system reliability, light distribution, dimming, thermal management, and power supply performance. Light-emitting diode_sentence_366

Potential technology Light-emitting diode_section_32

Perovskite LEDs (PLEDs) Light-emitting diode_section_33

A new family of LEDs are based on the semiconductors called perovskites. Light-emitting diode_sentence_367

In 2018, less than four years after their discovery, the ability of perovskite LEDs (PLEDs) to produce light from electrons already rivaled those of the best performing OLEDs. Light-emitting diode_sentence_368

They have a potential for cost-effectiveness as they can be processed from solution, a low-cost and low-tech method, which might allow perovskite-based devices that have large areas to be made with extremely low cost. Light-emitting diode_sentence_369

Their efficiency is superior by eliminating non-radiative losses, in other words, elimination of recombination pathways that do not produce photons; or by solving outcoupling problem (prevalent for thin-film LEDs) or balancing charge carrier injection to increase the EQE (external quantum efficiency). Light-emitting diode_sentence_370

The most up-to-date PLED devices have broken the performance barrier by shooting the EQE above 20%. Light-emitting diode_sentence_371

In 2018, Cao et al. Light-emitting diode_sentence_372

and Lin et al. Light-emitting diode_sentence_373

independently published two papers on developing perovskite LEDs with EQE greater than 20%, which made these two papers a mile-stone in PLED development. Light-emitting diode_sentence_374

Their device have similar planar structure, i.e. the active layer (perovskite) is sandwiched between two electrodes. Light-emitting diode_sentence_375

To achieve a high EQE, they not only reduced non-radiative recombination, but also utilized their own, subtly different methods to improve the EQE. Light-emitting diode_sentence_376

In the work of Cao et al. Light-emitting diode_sentence_377

, researchers targeted the outcoupling problem, which is that the optical physics of thin-film LEDs causes the majority of light generated by the semiconductor to be trapped in the device. Light-emitting diode_sentence_378

To achieve this goal, they demonstrated that solution-processed perovskites can spontaneously form submicrometre-scale crystal platelets, which can efficiently extract light from the device. Light-emitting diode_sentence_379

These perovskites are formed via the introduction of amino acid additives into the perovskite precursor solutions. Light-emitting diode_sentence_380

In addition, their method is able to passivate perovskite surface defects and reduce nonradiative recombination. Light-emitting diode_sentence_381

Therefore, by improving the light outcoupling and reducing nonradiative losses, Cao and his colleagues successfully achieved PLED with EQE up to 20.7%. Light-emitting diode_sentence_382

In Lin and his colleague's work, however, they used a different approach to generate high EQE. Light-emitting diode_sentence_383

Instead of modifying the microstructure of perovskite layer, they chose to adopt a new strategy for managing the compositional distribution in the device——an approach that simultaneously provides high luminescence and balanced charge injection. Light-emitting diode_sentence_384

In other words, they still used flat emissive layer, but tried to optimize the balance of electrons and holes injected into the perovskite, so as to make the most efficient use of the charge carriers. Light-emitting diode_sentence_385

Moreover, in the perovskite layer, the crystals are perfectly enclosed by MABr additive (where MA is CH3NH3). Light-emitting diode_sentence_386

The MABr shell passivates the nonradiative defects that would otherwise be present perovskite crystals, resulting in reduction of the nonradiative recombination. Light-emitting diode_sentence_387

Therefore, by balancing charge injection and decreasing nonradiative losses, Lin and his colleagues developed PLED with EQE up to 20.3%. Light-emitting diode_sentence_388

Two-way LEDs Light-emitting diode_section_34

Devices called "nanorods" are a form of LEDs that can also detect and absorb light. Light-emitting diode_sentence_389

They consist of a quantum dot directly contacting two semiconductor materials (instead of just one as in a traditional LED). Light-emitting diode_sentence_390

One semiconductor allows movement of positive charge and one allows movement of negative charge. Light-emitting diode_sentence_391

They can emit light, sense light, and collect energy. Light-emitting diode_sentence_392

The nanorod gathers electrons while the quantum dot shell gathers positive charges so the dot emits light. Light-emitting diode_sentence_393

When the voltage is switched the opposite process occurs and the dot absorbs light. Light-emitting diode_sentence_394

By 2017 the only color developed was red. Light-emitting diode_sentence_395

See also Light-emitting diode_section_35

Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Light-emitting diode.