Light-emitting diode

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Light-emitting diode (LED)
RBG-LED.jpg
Blue, green, and red LEDs in 5 mm diffused case
Working principle Electroluminescence
Invented H. J. Round (1907) [1]
Oleg Losev (1927) [2]
James R. Biard (1961) [3]
Nick Holonyak (1962) [4]
First productionOctober 1962
Pin configuration Anode and cathode
Electronic symbol
LED symbol.svg
Parts of a conventional LED. The flat bottom surfaces of the anvil and post embedded inside the epoxy act as anchors, to prevent the conductors from being forcefully pulled out via mechanical strain or vibration. LED, 5mm, green (en).svg
Parts of a conventional LED. The flat bottom surfaces of the anvil and post embedded inside the epoxy act as anchors, to prevent the conductors from being forcefully pulled out via mechanical strain or vibration.
Close up image of a surface mount LED Surface mount LED close up image.png
Close up image of a surface mount LED
A bulb-shaped modern retrofit LED lamp with aluminum heat sink, a light diffusing dome and E27 screw base, using a built-in power supply working on mains voltage Br20 1.jpg
A bulb-shaped modern retrofit LED lamp with aluminum heat sink, a light diffusing dome and E27 screw base, using a built-in power supply working on mains voltage

A light-emitting diode (LED) is a semiconductor light source that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. 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. [5] White light is obtained by using multiple semiconductors or a layer of light-emitting phosphor on the semiconductor device. [6]

Contents

Appearing as practical electronic components in 1962, the earliest LEDs emitted low-intensity infrared (IR) light. [7] Infrared LEDs are used in remote-control circuits, such as those used with a wide variety of consumer electronics. The first visible-light LEDs were of low intensity and limited to red. Modern LEDs are available across the visible, ultraviolet (UV), and infrared wavelengths, with high light output.

Early LEDs were often used as indicator lamps, replacing small incandescent bulbs, and in seven-segment displays. Recent developments have produced high-output white light LEDs suitable for room and outdoor area lighting. LEDs have led to new displays and sensors, while their high switching rates are useful in advanced communications technology.

LEDs have many advantages over incandescent light sources, including lower energy consumption, longer lifetime, improved physical robustness, smaller size, and faster switching. 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. [8]

Unlike a laser, the light emitted from an LED is neither spectrally coherent nor even highly monochromatic. However, its spectrum is sufficiently narrow that it appears to the human eye as a pure (saturated) color. [9] [10] Also unlike most lasers, its radiation is not spatially coherent, so it cannot approach the very high brightnesses characteristic of lasers.

History

Discoveries and early devices

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

Electroluminescence as a phenomenon was discovered in 1907 by the British experimenter H. J. Round of Marconi Labs, using a crystal of silicon carbide and a cat's-whisker detector. [11] [12] Russian inventor Oleg Losev reported creation of the first LED in 1927. [13] His research was distributed in Soviet, German and British scientific journals, but no practical use was made of the discovery for several decades. [14] [15]

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. In his publications, Destriau often referred to luminescence as Losev-Light. Destriau worked in the laboratories of Madame Marie Curie, also an early pioneer in the field of luminescence with research on radium. [16] [17]

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. [18]

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. [19] [20]

Rubin Braunstein [21] of the 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  kelvins.

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 for optical communication applications.

A 1962 Texas Instruments SNX-100 GaAs LED contained in a TO-18 transistor metal case SNX100.jpg
A 1962 Texas Instruments SNX-100 GaAs LED contained in a TO-18 transistor metal case

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. [7] 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 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. After establishing the priority of their work based on engineering notebooks predating submissions from G.E. Labs, RCA Research Labs, IBM Research Labs, Bell Labs, and Lincoln Lab at MIT, the U.S. patent office issued the two inventors the patent for the GaAs infrared light-emitting diode (U.S. Patent US3293513), the first practical LED. [7] Immediately after filing the patent, Texas Instruments (TI) began a project to manufacture infrared diodes. 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. [7] In October 1963, TI announced the first commercial hemispherical LED, the SNX-110. [25]

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. [26] Holonyak and Bevacqua reported this LED in the journal Applied Physics Letters on December 1, 1962. [27] [28] M. George Craford, [29] 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. [30] 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. [31]

Initial commercial development

The first commercial visible-wavelength LEDs were commonly used as replacements for incandescent and neon indicator lamps, and in seven-segment displays, [32] 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). Until 1968, visible and infrared LEDs were extremely costly, in the order of US$200 per unit, and so had little practical use. [33]

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. [34] 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, [35] and they collaborated with Monsanto Company on developing the first usable LED products. [36] The first usable LED products were HP's LED display and Monsanto's LED indicator lamp, both launched in 1968. [36] Monsanto was the first organization to mass-produce visible LEDs, using GaAsP in 1968 to produce red LEDs suitable for indicators. [33] Monsanto had previously offered to supply HP with GaAsP, but HP decided to grow its own GaAsP. [33] 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. [34] 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. [37]

Atalla left HP and joined Fairchild Semiconductor in 1969. [38] He was the vice president and general manager of the Microwave & Optoelectronics division, [39] from its inception in May 1969 up until November 1971. [40] He continued his work on LEDs, proposing they could be used for indicator lights and optical readers in 1971. [41] In the 1970s, commercially successful LED devices at less than five cents each were produced by Fairchild Optoelectronics. These devices employed compound semiconductor chips fabricated with the planar process (developed by Jean Hoerni, [42] [43] based on Atalla's surface passivation method [44] [45] ). 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. [46] LED producers continue to use these methods. [47]

LED display of a TI-30 scientific calculator (ca. 1978), which uses plastic lenses to increase the visible digit size TI-30-LED-Display-3682e1.jpg
LED display of a TI-30 scientific calculator (ca. 1978), which uses plastic lenses to increase the visible digit size

The early red LEDs were bright enough only for use as indicators, as the light output was not enough to illuminate an area. Readouts in calculators were so small that plastic lenses were built over each digit to make them legible. Later, other colors became widely available and appeared in appliances and equipment.

Early LEDs were packaged in metal cases similar to those of transistors, with a glass window or lens to let the light out. Modern indicator LEDs are packed in transparent molded plastic cases, tubular or rectangular in shape, and often tinted to match the device color. Infrared devices may be dyed, to block visible light. More complex packages have been adapted for efficient heat dissipation in high-power LEDs. Surface-mounted LEDs further reduce the package size. LEDs intended for use with fiber optics cables may be provided with an optical connector.

Blue LED

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. [48] [49] At the time Maruska was on leave from RCA Laboratories, where he collaborated with Jacques Pankove on related work. 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. [50] [51] In 1974 the U.S. Patent Office awarded Maruska, Rhines and Stanford professor David Stevenson a patent for their work in 1972 (U.S. Patent US3819974 A). Today, magnesium-doping of gallium nitride remains the basis for all commercial blue 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). [52] SiC LEDs had very low efficiency, no more than about 0.03%, but did emit in the blue portion of the visible light spectrum. [53] [54]

In the late 1980s, key breakthroughs in GaN epitaxial growth and p-type doping [55] 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 using a new two-step process in 1991. [56]

Two years later, in 1993, high-brightness blue LEDs were demonstrated by Shuji Nakamura of Nichia Corporation using a gallium nitride growth process. [57] [58] [59] 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. This new development revolutionized LED lighting, making high-power blue light sources practical, leading to the development of technologies like Blu-ray [ citation needed ].

Nakamura was awarded the 2006 Millennium Technology Prize for his invention. [60] Nakamura, Hiroshi Amano and Isamu Akasaki were awarded the Nobel Prize in Physics in 2014 for the invention of the blue LED. [61] 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. [62]

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).

In 2001 [63] and 2002, [64] processes for growing gallium nitride (GaN) LEDs on silicon were successfully demonstrated. In January 2012, Osram demonstrated high-power InGaN LEDs grown on silicon substrates commercially, [65] and GaN-on-silicon LEDs are in production at Plessey Semiconductors. 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). Samsung, the University of Cambridge, and Toshiba are performing research into GaN on Si LEDs. Toshiba has stopped research, possibly due to low yields. [66] [67] [68] [69] [70] [71] [72] 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. Epitaxy (or patterned sapphire) can be carried out with nanoimprint lithography. [73] [74] [75] [76] [77] [78] [79] GaN is often deposited using Metalorganic vapour-phase epitaxy (MOCVD), and it also utilizes Lift-off.

White LEDs and the illumination breakthrough

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. The attainment of high efficiency blue LEDs was quickly followed by the development of the first white LED. In this device a Y
3
Al
5
O
12
:Ce (known as "YAG" or Ce:YAG phosphor) cerium doped phosphor coating produces yellow light through fluorescence. The combination of that yellow with remaining blue light appears white to the eye. Using different phosphors produces green and red light through fluorescence. 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.[ citation needed ]

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

The first white LEDs were expensive and inefficient. However, the light output of LEDs has increased exponentially. 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. This trend in increased output has been called Haitz's law after Dr. Roland Haitz. [80]

Light output and efficiency of blue and near-ultraviolet LEDs rose and the cost of reliable devices fell. This led to relatively high-power white-light LEDs for illumination, which are replacing incandescent and fluorescent lighting. [81] [82]

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. [83] [84] However, commercially available LEDs have an efficiency of up to 223 lm/w as of 2018. [85] [86] [87] A previous record of 135lm/w was achieved by Nichia in 2010. [88] 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. [89]

The LED chip is encapsulated inside a small, plastic, white mold. It can be encapsulated using resin (polyurethane-based), silicone, or epoxy containing (powdered) Cerium doped YAG phosphor. 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. 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. In this case, the bond wire(s) are attached to the ITO film that has been deposited in the LEDs. 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.[ citation needed ]

Physics of light production and emission

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". The wavelength of the light depends on the energy band gap of the semiconductors used. 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.[ citation needed ]

Colors

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

Blue and ultraviolet

Blue LEDs Blue light emitting diodes over a proto-board.jpg
Blue LEDs
External video
Herb Maruska original blue LED College of New Jersey Sarnoff Collection.png
Nuvola apps kaboodle.svg “The Original Blue LED”, Science History Institute

Blue LEDs have an active region consisting of one or more InGaN quantum wells sandwiched between thicker layers of GaN, called cladding layers. By varying the relative In/Ga fraction in the 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 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.[ citation needed ]

With AlGaN and AlGaInN, even shorter wavelengths are achievable. 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. While substantially more expensive, shorter-wavelength diodes are commercially available for wavelengths down to 240 nm. [90] 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. Recent research has shown that commercially available UVA LEDs (365 nm) are already effective disinfection and sterilization devices. [91] UV-C wavelengths were obtained in laboratories using aluminium nitride (210 nm), [92] boron nitride (215 nm) [93] [94] and diamond (235 nm). [95]

White

There are two primary ways of producing white light-emitting diodes. 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. 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. The yellow phosphor is cerium-doped YAG crystals suspended in the package or coated on the LED. This YAG phosphor causes white LEDs to appear yellow when off, and the space between the crystals allow some blue light to pass through. Alternatively, white LEDs may use other phosphors like manganese(IV)-doped potassium fluorosilicate (PFS) or other engineered phosphors. PFS assists in red light generation, and is used in conjunction with conventional Ce:YAG phosphor. 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. The color temperature of the LED can be controlled by changing the concentration of the phosphors. [96] [97] [98]

The 'whiteness' of the light produced is engineered to suit the human eye. Because of metamerism, it is possible to have quite different spectra that appear white. 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. 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. The best color rendition LEDs use a mix of phosphors, resulting in less efficiency and better color rendering.[ citation needed ]

RGB systems

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 Spectrum.svg
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 RGB LED.jpg
RGB LED

Mixing red, green, and blue sources to produce white light needs electronic circuits to control the blending of the colors. 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. Nonetheless, this method has many applications because of the flexibility of mixing different colors, [99] and in principle, this mechanism also has higher quantum efficiency in producing white light. [100]

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 have the best luminous efficacy (120 lm/W), but the lowest color rendering capability. Although tetrachromatic white LEDs have excellent color rendering capability, they often have poor luminous efficacy. Trichromatic white LEDs are in between, having both good luminous efficacy (>70 lm/W) and fair color rendering capability.[ citation needed ]

One of the challenges is the development of more efficient green LEDs. The theoretical maximum for green LEDs is 683 lumens per watt but as of 2010 few green LEDs exceed even 100 lumens per watt. The blue and red LEDs approach their theoretical limits.[ citation needed ]

Multicolor LEDs also offer 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. However, this type of LED's emission power decays exponentially with rising temperature, [101] resulting in a substantial change in color stability. Such problems inhibit industrial use. Multicolor LEDs without phosphors cannot provide good color rendering because each LED is a narrowband source. 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.

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. To emulate the appearance of dimming incandescent lamps may require a feedback system with color sensor to actively monitor and control the color. [102]

Phosphor-based LEDs

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

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). [103] A fraction of the blue light undergoes the Stokes shift, which transforms it from shorter wavelengths to longer. Depending on the original LED's color, various color phosphors are used. Using several phosphor layers of distinct colors broadens the emitted spectrum, effectively raising the color rendering index (CRI). [104]

Phosphor-based LEDs have efficiency losses due to heat loss from the Stokes shift and also other phosphor-related issues. 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. 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). 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 presently on the market are manufactured using phosphor light conversion.[ citation needed ]

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 phosphor, with less than 10% Stokes shift loss. Losses attributable to internal optical losses due to re-absorption in the LED chip and in the LED packaging itself account typically for another 10% to 30% of efficiency loss. Currently, in the area of phosphor LED development, much effort is being spent on optimizing these devices to higher light output and higher operation temperatures. For instance, the efficiency can be raised by adapting better package design or by using a more suitable type of phosphor. Conformal coating process is frequently used to address the issue of varying phosphor thickness.[ citation needed ]

Some phosphor-based white LEDs encapsulate InGaN blue LEDs inside phosphor-coated epoxy. Alternatively, the 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 emissions spectrum. A common yellow phosphor material is cerium-doped yttrium aluminium garnet (Ce3+:YAG).[ citation needed ]

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

Other white LEDs

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. [105]

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. This avoids the typical costly sapphire substrate in relatively small 100- or 150-mm wafer sizes. [106] The sapphire apparatus must be coupled with a mirror-like collector to reflect light that would otherwise be wasted. It was predicted that since 2020, 40% of all GaN LEDs are made with GaN-on-Si. Manufacturing large sapphire material is difficult, while large silicon material is cheaper and more abundant. LED companies shifting from using sapphire to silicon should be a minimal investment. [107]

Organic light-emitting diodes (OLEDs)

In an organic light-emitting diode (OLED), the electroluminescent material composing 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. [108] The organic materials can be small organic molecules in a crystalline phase, or polymers. [109]

The potential advantages of OLEDs include thin, low-cost displays with a low driving voltage, wide viewing angle, and high contrast and color gamut. [110] Polymer LEDs have the added benefit of printable and flexible displays. [111] [112] [113] OLEDs have been used to make visual displays for portable electronic devices such as cellphones, digital cameras, lighting and televisions. [109] [110]

Types

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 such as those used for lighting and backlighting are generally found in surface-mount technology (SMT) packages (not shown). Verschiedene LEDs.jpg
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 such as those used for lighting and backlighting are generally found in surface-mount technology (SMT) packages (not shown).

LEDs are made in different packages for different applications. A single or a few LED junctions may be packed in one miniature device for use as an indicator or pilot lamp. 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. Higher-powered white-emitting devices will be mounted on heat sinks and will be used for illumination. Alphanumeric displays in dot matrix or bar formats are widely available. Special packages permit connection of LEDs to optical fibers for high-speed data communication links.

Miniature

Photo of miniature surface mount LEDs in most common sizes. They can be much smaller than a traditional 5 mm lamp type LED, shown on the upper left corner. Single and multicolor surface mount miniature LEDs in most common sizes.jpg
Photo of miniature surface mount 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 package with gold wire bonding details. Very small 1.6x1.6x0.35 mm RGB Surface Mount LED EAST1616RGBA2.jpg
Very small (1.6×1.6×0.35 mm) red, green, and blue surface mount miniature LED package with gold wire bonding details.

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. [114] Typical current ratings range from around 1 mA to above 20 mA. Multiple LED dies attached to a flexible backing tape form an LED strip light.[ citation needed ]

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. Infrared devices may have a black tint to block visible light while passing infrared radiation.[ citation needed ]

Ultra-high-output LEDs are designed for viewing in direct sunlight[ citation needed ]

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.[ citation needed ]

High-power

High-power light-emitting diodes attached to an LED star base (Luxeon, Lumileds) 2007-07-24 High-power light emitting diodes (Luxeon, Lumiled).jpg
High-power light-emitting diodes attached to an LED star base (Luxeon, Lumileds)

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. Some can emit over a thousand lumens. [115] [116] LED power densities up to 300 W/cm2 have been achieved. Since overheating is destructive, the HP-LEDs must be mounted on a heat sink to allow for heat dissipation. If the heat from an HP-LED is not removed, the device fails in seconds. One HP-LED can often replace an incandescent bulb in a flashlight, or be set in an array to form a powerful LED lamp.

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. As of September 2009, some HP-LEDs manufactured by Cree now exceed 105 lm/W. [117]

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 [117] and the Nichia 19 series with a typical efficacy of 140 lm/W, released in 2010. [118]

AC-driven

LEDs developed by Seoul Semiconductor can operate on AC power without a DC converter. For each half-cycle, part of the LED emits light and part is dark, and this is reversed during the next half-cycle. The efficacy of this type of HP-LED is typically 40 lm/W. [119] A large number of LED elements in series may be able to operate directly from line voltage. In 2009, Seoul Semiconductor released a high DC voltage LED, 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 design. [120]

Application-specific variations

Flashing

Flashing LEDs are used as attention seeking indicators without requiring external electronics. Flashing LEDs resemble standard LEDs but they contain an integrated voltage regulator and a multivibrator circuit that causes the 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

Bi-color LEDs contain two different LED emitters in one case. There are two types of these. One type consists of two dies connected to the same two leads antiparallel 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.

RGB Tri-color

Tri-color LEDs contain three different LED emitters in one case. Each emitter is connected to a separate lead so they can be controlled independently. A four-lead arrangement is typical with one common lead (anode or cathode) and an additional lead for each color. Others, however, have only two leads (positive and negative) and have a built-in electronic controller.

RGB-SMD-LED RGB-SMD-LED.jpg
RGB-SMD-LED

RGB LEDs consist of one red, one green, and one blue LED. [121] By independently adjusting each of the three, RGB LEDs are capable of producing a wide color gamut. Unlike dedicated-color LEDs, however, these do not produce pure wavelengths. Modules may not be optimized for smooth color mixing.

Decorative-multicolor

Decorative-multicolor LEDs incorporate several emitters of different colors supplied by only two lead-out wires. Colors are switched internally by varying the supply voltage.

Alphanumeric

Composite image of an 11 x 44 LED matrix lapel name tag display using 1608/0603-type SMD LEDs. Top: A little over half of the 21 x 86 mm display. Center: Close-up of LEDs in ambient light. Bottom: LEDs in their own red light. Macro photo of LED matrix.jpg
Composite image of an 11 × 44 LED matrix lapel name tag display using 1608/0603-type SMD LEDs. Top: A little over half of the 21 × 86 mm display. Center: Close-up of LEDs in ambient light. Bottom: LEDs in their own red light.

Alphanumeric 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 5×7 pixels per character. Seven-segment 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 displays.

Digital RGB

Digital RGB addressable LEDs contain their own "smart" control electronics. In addition to power and ground, these provide connections for data-in, data-out, clock and sometimes a strobe signal. These are connected in a daisy chain. Data sent to the first LED of the chain can control the brightness and color of each 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 matrices. Some even have refresh rates in the kHz range, allowing for basic video applications. These devices are known by their part number (WS2812 being common) or a brand name such as NeoPixel.

Filament

An LED filament consists of multiple LED chips connected in series on a common longitudinal substrate that forms a thin rod reminiscent of a traditional incandescent filament. [122] These are being used as a low-cost decorative alternative for traditional light bulbs that are being phased out in many countries. The filaments use a rather high voltage, allowing them to work efficiently 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 need. [123] Usually, they are packaged in bulb similar to the lamps they were designed to replace, and filled with inert gas to remove heat efficiently.

Chip-on-board arrays

Surface-mounted LEDs are frequently produced in chip on board (COB) arrays, allowing better heat dissipation than with a single LED of comparable luminous output. [124] The LEDs can be arranged around a cylinder, and are called "corn cob lights" because of the rows of yellow LEDs. [125]

Considerations for use

Power sources

Simple LED circuit with resistor for current limiting LED circuit.svg
Simple LED circuit with resistor for current limiting

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. Current through the LED must be regulated by an external circuit such as a constant current source to prevent damage. Since most common power supplies are (nearly) constant-voltage sources, LED fixtures must include a power converter, or at least a current-limiting resistor. In some applications, the internal resistance of small batteries is sufficient to keep current within the LED rating.[ citation needed ]

Electrical polarity

Unlike a traditional incandescent lamp, an LED will light only when voltage is applied in the forward direction of the diode. No current flows and no light is emitted if voltage is applied in the reverse direction. If the reverse voltage exceeds the breakdown voltage, a large current flows and the LED will be damaged. If the reverse current is sufficiently limited to avoid damage, the reverse-conducting LED is a useful noise diode.[ citation needed ]

Safety and health

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". [126] One study showed no evidence of a risk in normal use at domestic illuminance, [127] and that caution is only needed for particular occupational situations or for specific populations. [128] 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. [129]

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. [130]

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. Industry critics claim exposure levels are not high enough to have a noticeable effect. [131]

Advantages

Disadvantages

Applications

Daytime running light LEDs of an automobile LED DaytimeRunningLights.jpg
Daytime running light LEDs of an automobile

LED uses fall into four major categories:

Indicators and signs

The low energy consumption, low maintenance and small size of LEDs has led to uses as status indicators and displays on a variety of equipment and installations. Large-area 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 and green LED traffic signals Red and green traffic signals, Stamford Road, Singapore - 20111210.jpg
Red and green LED traffic signals

One-color light is well suited for traffic lights and signals, exit signs, emergency vehicle lighting, ships' navigation lights, and LED-based Christmas lights

Because of their long life, fast switching times, and visibility in broad daylight due to their high output and focus, LEDs have been used in automotive brake lights and turn signals. The use in brakes improves safety, due to a great reduction in the time needed to light fully, or faster rise time, about 0.1 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 are not pulsed at a fast enough frequency, they can create a phantom array, where ghost images of the LED appear if the eyes quickly scan across the array. White LED headlamps are beginning to appear. Using LEDs has styling advantages because 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 for LED art.

Lighting

With the development of high-efficiency and high-power LEDs, it has become possible to use LEDs in lighting and illumination. To encourage the shift to LED lamps and other high-efficiency lighting, in 2008 the US Department of Energy created the L Prize competition. The Philips Lighting North America LED bulb won the first competition on August 3, 2011, after successfully completing 18 months of intensive field, lab, and product testing. [156]

Efficient lighting is needed for sustainable architecture. 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. The lower heat output of LEDs also reduces demand on air conditioning systems. 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. Solar powered LEDs are used as street lights and in architectural lighting.

The mechanical robustness and long lifetime are used in automotive lighting on cars, motorcycles, and bicycle lights. LED street lights are employed on poles and in parking garages. In 2007, the Italian village of Torraca was the first place to convert its street lighting to LEDs. [157]

Cabin lighting on recent Airbus and Boeing jetliners uses LED lighting. LEDs are also being used in airport and heliport lighting. LED airport fixtures currently include medium-intensity runway lights, runway centerline lights, taxiway centerline and edge lights, guidance signs, and obstruction lighting.

LEDs are also used as a light source for DLP projectors, and to backlight LCD televisions (referred to as LED TVs) and laptop displays. RGB LEDs raise the color gamut by as much as 45%. Screens for TV and computer displays can be made thinner using LEDs for backlighting. [158]

LEDs are small, durable and need little power, so they are used in handheld devices such as flashlights. 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 are used for infrared illumination in night vision uses including security cameras. A ring of LEDs around a video camera, aimed forward into a retroreflective background, allows chroma keying in video productions.

LED for miners, to increase visibility inside mines LED for mines.jpg
LED for miners, to increase visibility inside mines
Los Angeles Vincent Thomas Bridge illuminated with blue LEDs Los Angeles Bridge.jpg
Los Angeles Vincent Thomas Bridge illuminated with blue LEDs

LEDs are used in mining operations, as cap lamps to provide light for miners. Research has been done to improve LEDs for mining, to reduce glare and to increase illumination, reducing risk of injury to the miners. [159]

LEDs are increasingly finding uses in medical and educational applications, for example as mood enhancement. [160] NASA has even sponsored research for the use of LEDs to promote health for astronauts. [161]

Data communication and other signalling

Light can be used to transmit data and analog signals. For example, lighting white LEDs can be used in systems assisting people to navigate in closed spaces while searching necessary rooms or objects. [162]

Assistive listening devices in many theaters and similar spaces use arrays of infrared 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 cables to the very high bandwidth fiber links that form the Internet backbone. For some time, computers were commonly equipped with IrDA interfaces, which allowed them to send and receive data to nearby machines via infrared.

Because LEDs can cycle on and off millions of times per second, very high data bandwidth can be achieved. [163] For that reason, Visible Light Communication (VLC) has been proposed as an alternative to the increasingly competitive radio bandwidth [164] . By operating in the visible part of the electromagnectic spectrum, data can be transmitted without occupying the frequencies of radio communications.

The main characteristic of VLC, lies on the incapacity of light to surpass physical opaque barriers. 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 strenghts: 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. [164]

A promising applications of VLC lies on the Indoor Positioning System (IPS), an analogous to the GPS built to operate in enclosed spaces where the sattelite transmissions that allow the GPS operation are hard to reach. For instance, commercial buildings, shoppings malls, parking garages, as well as subways and tunnel systems are all possible applications for VLC-based indoor positioning systems. 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.

Other applications for VLC involve communication between appliances of a smart home or office. With increasing IoT-capable devices, connectivity through traditional radio waves might be subjected to interference [165] . However, light bulbs with VLC capabilities would be able to transmit data and commands for such devices.

Machine vision systems

Machine vision systems often require bright and homogeneous illumination, so features of interest are easier to process. LEDs are often used.

Barcode scanners are the most common example of machine vision applications, and many of those scanners use red LEDs instead of lasers. Optical computer mice use LEDs as a light source for the miniature camera within the mouse.

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

Biological detection

The discovery of radiative recombination in Aluminum Gallium Nitride (AlGaN) alloys by U.S. 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. [166] [167] [168] In 2004, the Edgewood Chemical Biological Center (ECBC) initiated the effort to create a biological detector named TAC-BIO. The program capitalized on Semiconductor UV Optical Sources (SUVOS) developed by the Defense Advanced Research Projects Agency (DARPA). [168]

UV induced fluorescence is one of the most robust techniques used for rapid real time detection of biological aerosols. [168] The first UV sensors were lasers lacking in-field-use practicality. In order to address this, DARPA incorporated SUVOS technology to create a low cost, small, lightweight, low power device. The TAC-BIO detector's response time was one minute from when it sensed a biological agent. It was also demonstrated that the detector could be operated unattended indoors and outdoors for weeks at a time. [168]

Aerosolized biological particles will fluoresce and scatter light under a UV light beam. Observed fluorescence is dependent on the applied wavelength and the biochemical fluorophores within the biological agent. UV induced fluorescence offers a rapid, accurate, efficient and logistically practical way for biological agent detection. 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. [168]

Additionally, TAC-BIO can reliably discriminate between threat and non-threat aerosols. It was claimed to be sensitive enough to detect low concentrations, but not so sensitive that it would cause false positives. 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. [169]

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. Its small, light-weight design allows it to be mounted to vehicles, robots, and unmanned aerial vehicles. 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. [170] [171]

Other applications

LED costume for stage performers LED Costume by Beo Beyond.jpg
LED costume for stage performers
LED wallpaper by Meystyle Digitally printed LED wallpaper Dolomites.jpg
LED wallpaper by Meystyle

The light from 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 television sets, where infrared LEDs are often used. Opto-isolators use an LED combined with a photodiode or phototransistor to provide a signal path with electrical isolation between two circuits. This is especially useful in medical equipment where the signals from a low-voltage sensor circuit (usually battery-powered) in contact with a living organism must be electrically isolated from any possible electrical failure in a recording or monitoring device operating at potentially dangerous voltages. An optoisolator also 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 are often ideal as a light source due to the requirements of the sensors. 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 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 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. [172] Many materials and biological systems are sensitive to, or dependent on, light. Grow lights use LEDs to increase photosynthesis in plants, [173] and bacteria and viruses can be removed from water and other substances using UV LEDs for sterilization. [91]

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. [167] [174]

LEDs have also been used as a medium-quality voltage reference in electronic circuits. 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. Red LEDs have the flattest I/V curve above the knee. Nitride-based LEDs have a fairly steep I/V curve and are useless for this purpose. Although LED forward voltage is far more current-dependent than a 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 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.

Research and development

Key challenges

LEDs require optimized efficiency to hinge on ongoing improvements such as phosphor materials and quantum dots. [175]

The process of down-conversion (the method by which materials convert more-energetic photons to different, less energetic colors) also needs improvement. 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. Red phosphors could also benefit from a narrower spectral width to emit more lumens and becoming more efficient at converting photons. [176]

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. [175]

Potential technology

Perovskite LEDs (PLEDs)

A new family of LEDs are based on the semiconductors called perovskites. 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. [177] 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. 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). The most up-to-date PLED devices have broken the performance barrier by shooting the EQE above 20%. [178]

In 2018, Cao et al. and Lin et al. independently published two papers on developing perovskite LEDs with EQE greater than 20%, which made these two papers a mile-stone in PLED development. Their device have similar planar structure, i.e. the active layer (perovskite) is sandwiched between two electrodes. To achieve a high EQE, they not only reduced non-radiative recombination, but also utilized their own, subtly different methods to improve the EQE. [178]

In Cao and his colleagues' work, they targeted to solve 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. [179] 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. These perovskites are formed via the introduction of amino acid additives into the perovskite precursor solutions. In addition, their method is able to passivate perovskite surface defects and reduce nonradiative recombination. Therefore, by improving the light outcoupling and reducing nonradiative losses, Cao and his colleagues successfully achieved PLED with EQE up to 20.7%. [180]

In Lin and his colleague's work, however, they used a different approach to generate high EQE. 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. 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. Moreover, in the perovskite layer, the crystals are perfectly enclosed by MABr additive (where MA is CH3NH3). The MABr shell passivates the nonradiative defects that would otherwise be present perovskite crystals, resulting in reduction of the nonradiative recombination. Therefore, by balancing charge injection and decreasing nonradiative losses, Lin and his colleagues developed PLED with EQE up to 20.3%. [181]

Two-way LEDs

Devices called "nanorods" are a form of LEDs that can also detect and absorb light. They consist of a quantum dot directly contacting two semiconductor materials (instead of just one as in a traditional LED). One semiconductor allows movement of positive charge and one allows movement of negative charge. They can emit light, sense light, and collect energy. The nanorod gathers electrons while the quantum dot shell gathers positive charges so the dot emits light. When the voltage is switched the opposite process occurs and the dot absorbs light. By 2017 the only color developed was red. [182]

See also

Related Research Articles

Electroluminescence

Electroluminescence (EL) is an optical phenomenon and electrical phenomenon in which a material emits light in response to the passage of an electric current or to a strong electric field. This is distinct from black body light emission resulting from heat (incandescence), a chemical reaction (chemiluminescence), sound (sonoluminescence), or other mechanical action (mechanoluminescence).

Phosphor

A phosphor, most generally, is a substance that exhibits the phenomenon of luminescence; it emits light when exposed to some type of radiant energy. The term is used both for fluorescent or phosphorescent substances which glow on exposure to ultraviolet or visible light, and cathodoluminescent substances which glow when struck by an electron beam in a cathode ray tube.

Fluorescent lamp Light source

A fluorescent lamp, or fluorescent tube, is a low-pressure mercury-vapor gas-discharge lamp that uses fluorescence to produce visible light. An electric current in the gas excites mercury vapor, which produces short-wave ultraviolet light that then causes a phosphor coating on the inside of the lamp to glow. A fluorescent lamp converts electrical energy into useful light much more efficiently than incandescent lamps. The typical luminous efficacy of fluorescent lighting systems is 50–100 lumens per watt, several times the efficacy of incandescent bulbs with comparable light output.

Neon lamp Light source based on gas discharge

A neon lamp is a miniature gas discharge lamp. The lamp typically consists of a small glass capsule that contains a mixture of neon and other gases at a low pressure and two electrodes. When sufficient voltage is applied and sufficient current is supplied between the electrodes, the lamp produces an orange glow discharge. The glowing portion in the lamp is a thin region near the cathode; the larger and much longer neon signs are also glow discharges, but they use the positive column which is not present in the ordinary neon lamp. Neon glow lamps were widely used as indicator lamps in the displays of electronic instruments and appliances.

Wide-bandgap semiconductors are semiconductor materials which have a relatively large band gap compared to conventional semiconductors. Conventional semiconductors like silicon have a bandgap in the range of 1 - 1.5 electronvolt (eV), whereas wide-bandgap materials have bandgaps in the range of 2 - 4 eV. Generally, wide-bandgap semiconductors have electronic properties which fall in between those of conventional semiconductors and insulators.

Gallium nitride

Gallium nitride (GaN) is a binary III/V direct bandgap semiconductor commonly used in light-emitting diodes since the 1990s. The compound is a very hard material that has a Wurtzite crystal structure. Its wide band gap of 3.4 eV affords it special properties for applications in optoelectronic, high-power and high-frequency devices. For example, GaN is the substrate which makes violet (405 nm) laser diodes possible, without use of nonlinear optical frequency-doubling.

Mercury-vapor lamp Electric lighting source

A mercury-vapor lamp is a gas-discharge lamp that uses an electric arc through vaporized mercury to produce light. The arc discharge is generally confined to a small fused quartz arc tube mounted within a larger borosilicate glass bulb. The outer bulb may be clear or coated with a phosphor; in either case, the outer bulb provides thermal insulation, protection from the ultraviolet radiation the light produces, and a convenient mounting for the fused quartz arc tube.

Germicidal lamp Ultraviolet C light-emitting device

A germicidal lamp is an electric light that produces ultraviolet C (UVC) light. This short-wave ultraviolet light disrupts DNA base pairing, causing formation of pyrimidine dimers, and leads to the inactivation of bacteria, viruses, and protozoa. It can also be used to produce ozone for water disinfection.

Electrodeless lamp

The internal electrodeless lamp or induction lamp is a gas discharge lamp in which an electric or magnetic field transfers the power required to generate light from outside the lamp envelope to the gas inside. This is in contrast to a typical gas discharge lamp that uses internal electrodes connected to the power supply by conductors that pass through the lamp envelope. Eliminating the internal electrodes provides two advantages:

Architectural lighting design Field within architecture, interior design and electrical engineering

Architectural lighting design is a field within architecture, interior design and electrical engineering that is concerned with the design of lighting systems, including natural light, electric light, or both, to serve human needs.

Blue laser laser that emits electromagnetic radiation

A blue laser is a laser that emits electromagnetic radiation with a wavelength between 360 and 480 nanometers, which the human eye sees as blue or violet.

Indium gallium nitride

Indium gallium nitride is a semiconductor material made of a mix of gallium nitride (GaN) and indium nitride (InN). It is a ternary group III/group V direct bandgap semiconductor. Its bandgap can be tuned by varying the amount of indium in the alloy. InxGa1−xN has a direct bandgap span from the infrared for InN to the ultraviolet of GaN. The ratio of In/Ga is usually between 0.02/0.98 and 0.3/0.7.

Aluminium gallium indium phosphide is a semiconductor material that provides a platform for the development of novel multi-junction photovoltaics and optoelectronic devices, as it spans a direct bandgap from deep ultraviolet to infrared.

Aluminium gallium nitride (AlGaN) is a semiconductor material. It is any alloy of aluminium nitride and gallium nitride.

Nichia Corporation is a Japanese chemical engineering and manufacturing company headquartered in Anan, Japan with global subsidiaries. It specializes in the manufacturing and distribution of phosphors, including light-emitting diodes (LEDs), laser diodes, battery materials, and calcium chloride.

LED lamp light source

An LED lamp or LED light bulb is an electric light for use in light fixtures that produces light using one or more light-emitting diodes (LEDs). LED lamps have a lifespan many times longer than equivalent incandescent lamps, and are significantly more efficient than most fluorescent lamps, with some manufacturers claiming LED chips with a luminous efficacy of up to 303 lumens per watt. However, LED lamps require an electronic LED driver circuit when operated from mains power lines, and losses from this circuit means that the efficiency of the lamp is lower than the efficiency of the LED chips it uses. The most efficient commercially available LED lamps have efficiencies of 200 lumens per watt (Lm/W). The LED lamp market is projected to grow by more than twelve-fold over the next decade, from $2 billion in the beginning of 2014 to $25 billion in 2023, a compound annual growth rate (CAGR) of 25%. As of 2016, many LEDs use only about 10% of the energy an incandescent lamp requires.

LED filament

An LED filament light bulb is an LED lamp which is designed to resemble a traditional incandescent light bulb with visible filaments for aesthetic and light distribution purposes, but with the high efficiency of light emitting diodes (LEDs). It produces its light using LED filaments, which are series-connected strings of diodes that resemble, in appearance, the filaments of incandescent light bulbs. They are direct replacements for conventional clear incandescent bulbs, as they are made with the same envelope shapes, the same bases that fit the same sockets, and work at the same supply voltage. They may be used for their appearance, similar when lit to a clear incandescent bulb, or for their wide angle of light distribution, typically 300°. They are also more efficient than many other LED lamps.

microLED, also known as micro-LED, mLED or µLED, is an emerging flat-panel display technology. microLED displays consist of arrays of microscopic LEDs forming the individual pixel elements. When compared with widespread LCD technology, microLED displays offer better contrast, response times, and energy efficiency.

In light-emitting diode physics, the recombination of electrons and electron holes in a semiconductor produce light, a process called "electroluminescence". The wavelength of the light produced depends on the energy band gap of the semiconductors used. 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. An LED is a long-lived light source, but certain mechanisms can cause slow loss of efficiency of the device or sudden failure. The wavelength of the light emitted is a function of the band gap of the semiconductor material used; materials such as gallium arsenide, and others, with various trace doping elements, are used to produce different colors of light. Another type of LED uses a quantum dot which can have its properties and wavelength adjusted by its size. Light-emitting diodes are widely used in indicator and display functions, and white LEDs are displacing other technologies for general illumination purposes.

References

  1. "HJ Round was a pioneer in the development of the LED". www.myledpassion.com.
  2. "The life and times of the LED — a 100-year history" (PDF). The Optoelectronics Research Centre, University of Southampton. April 2007. Archived from the original (PDF) on September 15, 2012. Retrieved September 4, 2012.
  3. US Patent 3293513, "Semiconductor Radiant Diode", James R. Biard and Gary Pittman, Filed on Aug. 8th, 1962, Issued on Dec. 20th, 1966.
  4. "Inventor of Long-Lasting, Low-Heat Light Source Awarded $500,000 Lemelson-MIT Prize for Invention". Washington, D.C. Massachusetts Institute of Technology. April 21, 2004. Archived from the original on October 9, 2011. Retrieved December 21, 2011.
  5. Edwards, Kimberly D. "Light Emitting Diodes" (PDF). University of California at Irvine . p. 2. Retrieved January 12, 2019.
  6. Lighting Research Center. "How is white light made with LEDs?". Rensselaer Polytechnic Institute . Retrieved January 12, 2019.
  7. 1 2 3 4 Okon, Thomas M.; Biard, James R. (2015). "The First Practical LED" (PDF). EdisonTechCenter.org. Edison Tech Center . Retrieved February 2, 2016.
  8. Peláez, E. A; Villegas, E. R (2007). LED power reduction trade-offs for ambulatory pulse oximetry. 2007 29th Annual International Conference of the IEEE Engineering in Medicine and Biology Society. 2007. pp. 2296–9. doi:10.1109/IEMBS.2007.4352784. ISBN   978-1-4244-0787-3. PMID   18002450. S2CID   34626885.
  9. "LED Basics | Department of Energy". www.energy.gov. Retrieved October 22, 2018.
  10. "LED Spectral Distribution". optiwave.com. July 25, 2013. Retrieved June 20, 2017.
  11. Round, H. J. (1907). "A note on carborundum". Electrical World. 19: 309.
  12. Margolin J. "The Road to the Transistor". jmargolin.com.
  13. Losev, O. V. (1927). "Светящийся карборундовый детектор и детектирование с кристаллами" [Luminous carborundum detector and detection with crystals]. Телеграфия и Телефония без Проводов [Wireless Telegraphy and Telephony] (in Russian). 5 (44): 485–494. English translation: Losev, O. V. (November 1928). "Luminous carborundum detector and detection effect and oscillations with crystals". Philosophical Magazine. 7th series. 5 (39): 1024–1044. doi:10.1080/14786441108564683.
  14. Zheludev, N. (2007). "The life and times of the LED: a 100-year history" (PDF). Nature Photonics. 1 (4): 189–192. Bibcode:2007NaPho...1..189Z. doi:10.1038/nphoton.2007.34. Archived from the original (PDF) on May 11, 2011. Retrieved April 11, 2007.
  15. Lee, Thomas H. (2004). The design of CMOS radio-frequency integrated circuits . Cambridge University Press. p.  20. ISBN   978-0-521-83539-8.
  16. Destriau, G. (1936). "Recherches sur les scintillations des sulfures de zinc aux rayons". Journal de Chimie Physique. 33: 587–625. doi:10.1051/jcp/1936330587.
  17. McGraw-Hill Concise Encyclopedia of Physics: electroluminescence. (n.d.) McGraw-Hill Concise Encyclopedia of Physics. (2002).
  18. "Brief history of LEDs" (PDF).
  19. Lehovec, K; Accardo, C. A; Jamgochian, E (1951). "Injected Light Emission of Silicon Carbide Crystals". Physical Review. 83 (3): 603–607. Bibcode:1951PhRv...83..603L. doi:10.1103/PhysRev.83.603. Archived from the original on December 11, 2014.
  20. Lehovec, K; Accardo, C. A; Jamgochian, E (1953). "Injected Light Emission of Silicon Carbide Crystals". Physical Review. 89 (1): 20–25. Bibcode:1953PhRv...89...20L. doi:10.1103/PhysRev.89.20.
  21. "Rubin Braunstein". UCLA. Archived from the original on March 11, 2011. Retrieved January 24, 2012.
  22. Braunstein, Rubin (1955). "Radiative Transitions in Semiconductors". Physical Review. 99 (6): 1892–1893. Bibcode:1955PhRv...99.1892B. doi:10.1103/PhysRev.99.1892.
  23. Kroemer, Herbert (September 16, 2013). "The Double-Heterostructure Concept: How It Got Started". Proceedings of the IEEE. 101 (10): 2183–2187. doi:10.1109/JPROC.2013.2274914. S2CID   2554978.
  24. Matzen, W. T. ed. (March 1963) "Semiconductor Single-Crystal Circuit Development," Texas Instruments Inc., Contract No. AF33(616)-6600, Rept. No ASD-TDR-63-281.
  25. Carr, W. N.; G. E. Pittman (November 1963). "One-watt GaAs p-n junction infrared source". Applied Physics Letters. 3 (10): 173–175. Bibcode:1963ApPhL...3..173C. doi:10.1063/1.1753837.
  26. Kubetz, Rick (May 4, 2012). "Nick Holonyak, Jr., six decades in pursuit of light". University of Illinois. Retrieved July 7, 2020.
  27. Holonyak Nick; Bevacqua, S. F. (December 1962). "Coherent (Visible) Light Emission from Ga(As1−x Px) Junctions". Applied Physics Letters. 1 (4): 82. Bibcode:1962ApPhL...1...82H. doi:10.1063/1.1753706. Archived from the original on October 14, 2012.
  28. Wolinsky, Howard (February 5, 2005). "U. of I.'s Holonyak out to take some of Edison's luster". Chicago Sun-Times. Archived from the original on March 28, 2006. Retrieved July 29, 2007.
  29. Perry, T. S. (1995). "M. George Craford [biography]". IEEE Spectrum. 32 (2): 52–55. doi:10.1109/6.343989.
  30. "Brief Biography — Holonyak, Craford, Dupuis" (PDF). Technology Administration. Archived from the original (PDF) on August 9, 2007. Retrieved May 30, 2007.
  31. Pearsall, T. P.; Miller, B. I.; Capik, R. J.; Bachmann, K. J. (1976). "Efficient, Lattice-matched, Double Heterostructure LEDs at 1.1 mm from GaxIn1−xAsyP1−y by Liquid-phase Epitaxy". Appl. Phys. Lett. 28 (9): 499. Bibcode:1976ApPhL..28..499P. doi:10.1063/1.88831.
  32. Rostky, George (March 1997). "LEDs cast Monsanto in Unfamiliar Role". Electronic Engineering Times (EETimes) (944).
  33. 1 2 3 Schubert, E. Fred (2003). "1". Light-Emitting Diodes. Cambridge University Press. ISBN   978-0-8194-3956-7.
  34. 1 2 Borden, Howard C.; Pighini, Gerald P. (February 1969). "Solid-State Displays" (PDF). Hewlett-Packard Journal : 2–12.
  35. House, Charles H.; Price, Raymond L. (2009). The HP Phenomenon: Innovation and Business Transformation. Stanford University Press. pp. 110–1. ISBN   9780804772617.
  36. 1 2 Kramer, Bernhard (2003). Advances in Solid State Physics. Springer Science & Business Media. p. 40. ISBN   9783540401506.
  37. "Hewlett-Packard 5082-7000". The Vintage Technology Association. Retrieved August 15, 2019.
  38. Bassett, Ross Knox (2007). To the Digital Age: Research Labs, Start-up Companies, and the Rise of MOS Technology. Johns Hopkins University Press. p. 328. ISBN   9780801886393.
  39. Annual Report (PDF). Fairchild Camera and Instrument Corporation. 1969. p. 6.
  40. "Solid State Technology". Solid State Technology. Cowan Publishing Corporation. 15: 79. 1972. Dr. Atalla was general manager of the Microwave & Optoelectronics division from its inception in May 1969 until November 1971 when it was incorporated into the Semiconductor Components Group.
  41. "Laser Focus with Fiberoptic Communications". Laser Focus with Fiberoptic Communications. Advanced Technology Publication. 7: 28. 1971. Its chief, John Atalla — Greene's predecessor at Hewlett-Packard — sees early applications for LEDs in small displays, principally for indicator lights. Because of their compatibility with integrated circuits, these light emitters can be valuable in fault detection. “Reliability has already been demonstrated beyond any doubt,” Atalla continues. “No special power supplies are required. Design takes no time, you just put the diode in. So introduction becomes strictly an economic question." Bright Outlook for Optical Readers Atalla is particularly sanguine about applications of diodes in high-volume optical readers.
  42. US 3025589,"Method of Manufacturing Semiconductor Devices",issued Mar 20, 1962
  43. Patent number: 3025589 Retrieved May 17, 2013
  44. Lojek, Bo (2007). History of Semiconductor Engineering . Springer Science & Business Media. pp.  120& 321–323. ISBN   9783540342588.
  45. Bassett, Ross Knox (2007). To the Digital Age: Research Labs, Start-up Companies, and the Rise of MOS Technology. Johns Hopkins University Press. p. 46. ISBN   9780801886393.
  46. Bausch, Jeffrey (December 2011). "The Long History of Light Emitting Diodes". Hearst Business Communications.
  47. Park, S. -I.; Xiong, Y.; Kim, R. -H.; Elvikis, P.; Meitl, M.; Kim, D. -H.; Wu, J.; Yoon, J.; Yu, C. -J.; Liu, Z.; Huang, Y.; Hwang, K. -C.; Ferreira, P.; Li, X.; Choquette, K.; Rogers, J. A. (2009). "Printed Assemblies of Inorganic Light-Emitting Diodes for Deformable and Semitransparent Displays" (PDF). Science. 325 (5943): 977–981. Bibcode:2009Sci...325..977P. CiteSeerX   10.1.1.660.3338 . doi:10.1126/science.1175690. PMID   19696346. S2CID   8062948. Archived from the original (PDF) on October 24, 2015.
  48. "Nobel Shocker: RCA Had the First Blue LED in 1972". IEEE Spectrum. October 9, 2014
  49. "Oregon tech CEO says Nobel Prize in Physics overlooks the actual inventors". The Oregonian. October 16, 2014
  50. Schubert, E. Fred (2006) Light-emitting diodes 2nd ed., Cambridge University Press. ISBN   0-521-86538-7 pp. 16–17
  51. Maruska, H. (2005). "A Brief History of GaN Blue Light-Emitting Diodes". LIGHTimes Online – LED Industry News. Archived June 11, 2012, at the Wayback Machine
  52. Major Business and Product Milestones. Cree.com. Retrieved on March 16, 2012. Archived April 13, 2011, at the Wayback Machine
  53. Edmond, John A.; Kong, Hua-Shuang; Carter, Calvin H. (April 1, 1993). "Blue LEDs, UV photodiodes and high-temperature rectifiers in 6H-SiC". Physica B: Condensed Matter. 185 (1): 453–460. Bibcode:1993PhyB..185..453E. doi:10.1016/0921-4526(93)90277-D. ISSN   0921-4526.
  54. "History & Milestones". Cree.com. Cree . Retrieved September 14, 2015.
  55. "GaN-based blue light emitting device development by Akasaki and Amano" (PDF). Takeda Award 2002 Achievement Facts Sheet. The Takeda Foundation. April 5, 2002. Retrieved November 28, 2007.
  56. Moustakas, Theodore D. U.S. Patent 5686738A "Highly insulating monocrystalline gallium nitride thin films " Issue date: Mar 18, 1991
  57. Nakamura, S.; Mukai, T.; Senoh, M. (1994). "Candela-Class High-Brightness InGaN/AlGaN Double-Heterostructure Blue-Light-Emitting-Diodes". Appl. Phys. Lett. 64 (13): 1687. Bibcode:1994ApPhL..64.1687N. doi:10.1063/1.111832.
  58. Nakamura, Shuji. "Development of the Blue Light-Emitting Diode". SPIE Newsroom. Retrieved September 28, 2015.
  59. Iwasa, Naruhito; Mukai, Takashi and Nakamura, Shuji U.S. Patent 5,578,839 "Light-emitting gallium nitride-based compound semiconductor device" Issue date: November 26, 1996
  60. 2006 Millennium technology prize awarded to UCSB's Shuji Nakamura. Ia.ucsb.edu (June 15, 2006). Retrieved on August 3, 2019.
  61. Overbye, Dennis (October 7, 2014). "Nobel Prize in Physics". The New York Times .
  62. Brown, Joel (December 7, 2015). "BU Wins $13 Million in Patent Infringement Suit". BU Today. Retrieved December 7, 2015.
  63. Dadgar, A.; Alam, A.; Riemann, T.; Bläsing, J.; Diez, A.; Poschenrieder, M.; Strassburg, M.; Heuken, M.; Christen, J.; Krost, A. (2001). "Crack-Free InGaN/GaN Light Emitters on Si(111)". Physica Status Solidi A. 188: 155–158. doi:10.1002/1521-396X(200111)188:1<155::AID-PSSA155>3.0.CO;2-P.
  64. Dadgar, A.; Poschenrieder, M.; BläSing, J.; Fehse, K.; Diez, A.; Krost, A. (2002). "Thick, crack-free blue light-emitting diodes on Si(111) using low-temperature AlN interlayers and in situ Si\sub x]N\sub y] masking". Applied Physics Letters. 80 (20): 3670. Bibcode:2002ApPhL..80.3670D. doi:10.1063/1.1479455.
  65. "Success in research: First gallium-nitride LED chips on silicon in pilot stage" (PDF). Archived from the original (PDF) on September 15, 2012. Retrieved 2012-09-15.. www.osram.de, January 12, 2012.
  66. Lester, Steve (2014) Role of Substrate Choice on LED Packaging. Toshiba America Electronic Components.
  67. GaN on Silicon — Cambridge Centre for Gallium Nitride. Gan.msm.cam.ac.uk. Retrieved on 2018-07-31.
  68. Bush, Steve. (2016-06-30) Toshiba gets out of GaN-on-Si leds. Electronicsweekly.com. Retrieved on 2018-07-31.
  69. Nunoue, Shin-ya; Hikosaka, Toshiki; Yoshida, Hisashi; Tajima, Jumpei; Kimura, Shigeya; Sugiyama, Naoharu; Tachibana, Koichi; Shioda, Tomonari; Sato, Taisuke; Muramoto, Eiji; Onomura, Masaaki (2013). "LED manufacturing issues concerning gallium nitride-on-silicon (GaN-on-Si) technology and wafer scale up challenges". 2013 IEEE International Electron Devices Meeting. pp. 13.2.1–13.2.4. doi:10.1109/IEDM.2013.6724622. ISBN   978-1-4799-2306-9. S2CID   23448056.
  70. Wright, Maury (2 May 2016) Samsung's Tarn reports progress in CSP and GaN-on-Si LEDs. LEDs Magazine.
  71. Increasing The Competitiveness Of The GaN-on-silicon LED. compoundsemiconductor.net (30 March 2016).
  72. Samsung To Focus on Silicon-based LED Chip Technology in 2015. LED Inside (17 March 2015).
  73. Keeping, Steven. (2013-01-15) Material and Manufacturing Improvements. DigiKey. Retrieved on 2018-07-31.
  74. Keeping, Steven. (2014-12-09) Manufacturers Shift Attention to Light Quality to Further LED Market Share Gains. DigiKey. Retrieved on 2018-07-31.
  75. Keeping, Steven. (2013-09-24) Will Silicon Substrates Push LED Lighting. DigiKey. Retrieved on 2018-07-31.
  76. Keeping, Steven. (2015-03-24) Improved Silicon-Substrate LEDs Address High Solid-State Lighting Costs. DigiKey. Retrieved on 2018-07-31.
  77. Development of the Nano-Imprint Equipment ST50S-LED for High-Brightness LED. Toshiba Machine (2011-05-18). Retrieved on 2018-07-31.
  78. The use of sapphire in mobile device and LED industries: Part 2 | Solid State Technology. Electroiq.com (2017-09-26). Retrieved on 2018-07-31.
  79. Epitaxy. Applied Materials. Retrieved on 2018-07-31.
  80. "Haitz's law". Nature Photonics . 1 (1): 23. 2007. Bibcode:2007NaPho...1...23.. doi: 10.1038/nphoton.2006.78 .
  81. Morris, Nick (June 1, 2006). "LED there be light, Nick Morris predicts a bright future for LEDs". Electrooptics.com.
  82. "The LED Illumination Revolution". Forbes . February 27, 2008.
  83. Press Release, Official Nobel Prize website, 7 October 2014
  84. Cree First to Break 300 Lumens-Per-Watt Barrier. Cree.com (2014-03-26). Retrieved on 2018-07-31.
  85. LM301B | SAMSUNG LED | Samsung LED Global Website. Samsung.com. Retrieved on 2018-07-31.
  86. Samsung Achieves 220 Lumens per Watt with New Mid-Power LED Package. Samsung.com (2017-06-16). Retrieved on 2018-07-31.
  87. LED breakthrough promises ultra efficient luminaires | Lux Magazine. Luxreview.com (2018-01-19). Retrieved on 2018-07-31.
  88. "White LEDs with super-high luminous efficacy could satisfy all general lighting needs". phys.org.
  89. LED bulb efficiency expected to continue improving as cost declines. U.S. Energy Information Administration (March 19, 2014)
  90. Cooke, Mike (April–May 2010). "Going Deep for UV Sterilization LEDs" (PDF). Semiconductor Today. 5 (3): 82. Archived from the original (PDF) on May 15, 2013.
  91. 1 2 Mori, M.; Hamamoto, A.; Takahashi, A.; Nakano, M.; Wakikawa, N.; Tachibana, S.; Ikehara, T.; Nakaya, Y.; Akutagawa, M.; Kinouchi, Y. (2007). "Development of a new water sterilization device with a 365 nm UV-LED". Medical & Biological Engineering & Computing. 45 (12): 1237–1241. doi:10.1007/s11517-007-0263-1. PMID   17978842. S2CID   2821545.
  92. Taniyasu, Y.; Kasu, M.; Makimoto, T. (2006). "An aluminium nitride light-emitting diode with a wavelength of 210 nanometres". Nature. 441 (7091): 325–328. Bibcode:2006Natur.441..325T. doi:10.1038/nature04760. PMID   16710416. S2CID   4373542.
  93. Kubota, Y.; Watanabe, K.; Tsuda, O.; Taniguchi, T. (2007). "Deep Ultraviolet Light-Emitting Hexagonal Boron Nitride Synthesized at Atmospheric Pressure". Science. 317 (5840): 932–934. Bibcode:2007Sci...317..932K. doi: 10.1126/science.1144216 . PMID   17702939.
  94. Watanabe, K.; Taniguchi, T.; Kanda, H. (2004). "Direct-bandgap properties and evidence for ultraviolet lasing of hexagonal boron nitride single crystal". Nature Materials. 3 (6): 404–409. Bibcode:2004NatMa...3..404W. doi:10.1038/nmat1134. PMID   15156198. S2CID   23563849.
  95. Koizumi, S.; Watanabe, K.; Hasegawa, M.; Kanda, H. (2001). "Ultraviolet Emission from a Diamond pn Junction". Science. 292 (5523): 1899–1901. Bibcode:2001Sci...292.1899K. doi:10.1126/science.1060258. PMID   11397942. S2CID   10675358.
  96. "Seeing Red with PFS Phosphor".
  97. "GE Lighting manufactures PFS red phosphor for LED display backlight applications". March 31, 2015.
  98. "GE TriGain® Phosphor Technology | GE".
  99. Moreno, I.; Contreras, U. (2007). "Color distribution from multicolor LED arrays". Optics Express. 15 (6): 3607–3618. Bibcode:2007OExpr..15.3607M. doi:10.1364/OE.15.003607. PMID   19532605. S2CID   35468615.
  100. Yeh, Dong-Ming; Huang, Chi-Feng; Lu, Chih-Feng; Yang, Chih-Chung. "Making white-light-emitting diodes without phosphors | SPIE Homepage: SPIE". spie.org. Retrieved April 7, 2019.
  101. Schubert, E. Fred; Kim, Jong Kyu (2005). "Solid-State Light Sources Getting Smart" (PDF). Science. 308 (5726): 1274–1278. Bibcode:2005Sci...308.1274S. doi:10.1126/science.1108712. PMID   15919985. S2CID   6354382. Archived from the original (PDF) on February 5, 2016.
  102. Nimz, Thomas; Hailer, Fredrik; Jensen, Kevin (November 2012). "Sensors and Feedback Control of Multicolor LED Systems". Led Professional Review : Trends & Technologie for Future Lighting Solutions. LED Professional (34): 2–5. ISSN   1993-890X. Archived from the original (PDF) on April 29, 2014.
  103. Tanabe, S.; Fujita, S.; Yoshihara, S.; Sakamoto, A.; Yamamoto, S. (2005). Ferguson, Ian T; Carrano, John C; Taguchi, Tsunemasa; Ashdown, Ian E (eds.). "YAG glass-ceramic phosphor for white LED (II): luminescence characteristics" (PDF). Proceedings of SPIE. Fifth International Conference on Solid State Lighting. 5941: 594112. Bibcode:2005SPIE.5941..193T. doi:10.1117/12.614681. S2CID   38290951. Archived from the original (PDF) on May 11, 2011.
  104. Ohno, Y. (2004). Ferguson, Ian T; Narendran, Nadarajah; Denbaars, Steven P; Carrano, John C (eds.). "Color rendering and luminous efficacy of white LED spectra" (PDF). Proc. SPIE. Fourth International Conference on Solid State Lighting. 5530: 89. Bibcode:2004SPIE.5530...88O. doi:10.1117/12.565757. S2CID   122777225. Archived from the original (PDF) on May 11, 2011.
  105. Whitaker, Tim (December 6, 2002). "Joint venture to make ZnSe white LEDs" . Retrieved January 3, 2009.
  106. Next-Generation GaN-on-Si White LEDs Suppress Costs, Electronic Design, 19 November 2013
  107. GaN-on-Silicon LEDs Forecast to Increase Market Share to 40 Percent by 2020, iSuppli, 4 December 2013
  108. Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; MacKay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. (1990). "Light-emitting diodes based on conjugated polymers". Nature. 347 (6293): 539–541. Bibcode:1990Natur.347..539B. doi:10.1038/347539a0. S2CID   43158308.
  109. 1 2 Kho, Mu-Jeong; Javed, T.; Mark, R.; Maier, E.; David, C (March 4, 2008). Final Report: OLED Solid State Lighting. Kodak European Research. Cambridge Science Park, Cambridge, UK.
  110. 1 2 Bardsley, J. N. (2004). "International OLED Technology Roadmap". IEEE Journal of Selected Topics in Quantum Electronics. 10 (1): 3–4. Bibcode:2004IJSTQ..10....3B. doi:10.1109/JSTQE.2004.824077. S2CID   30084021.
  111. Hebner, T. R.; Wu, C. C.; Marcy, D.; Lu, M. H.; Sturm, J. C. (1998). "Ink-jet printing of doped polymers for organic light emitting devices". Applied Physics Letters. 72 (5): 519. Bibcode:1998ApPhL..72..519H. doi:10.1063/1.120807. S2CID   119648364.
  112. Bharathan, J.; Yang, Y. (1998). "Polymer electroluminescent devices processed by inkjet printing: I. Polymer light-emitting logo". Applied Physics Letters. 72 (21): 2660. Bibcode:1998ApPhL..72.2660B. doi:10.1063/1.121090. S2CID   44128025.
  113. Gustafsson, G.; Cao, Y.; Treacy, G. M.; Klavetter, F.; Colaneri, N.; Heeger, A. J. (1992). "Flexible light-emitting diodes made from soluble conducting polymers". Nature. 357 (6378): 477–479. Bibcode:1992Natur.357..477G. doi:10.1038/357477a0. S2CID   4366944.
  114. LED-design. Elektor.com. Retrieved on March 16, 2012. Archived August 31, 2012, at the Wayback Machine
  115. "Luminus Products". Luminus Devices. Archived from the original on July 25, 2008. Retrieved October 21, 2009.
  116. "Luminus Products CST-90 Series Datasheet" (PDF). Luminus Devices. Archived from the original (PDF) on March 31, 2010. Retrieved October 25, 2009.
  117. 1 2 "Xlamp Xp-G Led". Cree.com. Cree, Inc. Archived from the original on March 13, 2012. Retrieved March 16, 2012.
  118. High Power Point Source White Led NVSx219A. Nichia.co.jp, November 2, 2010.
  119. "Seoul Semiconductor launches AC LED lighting source Acrich". LEDS Magazine. November 17, 2006. Retrieved February 17, 2008.
  120. 1 2 Visibility, Environmental, and Astronomical Issues Associated with Blue-Rich White Outdoor Lighting (PDF). International Dark-Sky Association. May 4, 2010. Archived from the original (PDF) on January 16, 2013.
  121. Ting, Hua-Nong (June 17, 2011). 5th Kuala Lumpur International Conference on Biomedical Engineering 2011: BIOMED 2011, 20–23 June 2011, Kuala Lumpur, Malaysia. Springer Science & Business Media. ISBN   9783642217296.
  122. "The Next Generation of LED Filament Bulbs". LEDInside.com. Trendforce. Retrieved October 26, 2015.
  123. "LED Filaments" . Retrieved October 26, 2015.
  124. Handbook on the Physics and Chemistry of Rare Earths: Including Actinides. Elsevier Science. August 1, 2016. p. 89. ISBN   978-0-444-63705-5.
  125. "Corn Lamps: What Are They & Where Can I Use Them?". Shine Retrofits. September 1, 2016. Retrieved December 30, 2018.
  126. "Blue LEDs: A health hazard?". texyt.com. January 15, 2007. Retrieved September 3, 2007.
  127. Some evidences that white LEDs are toxic for human at domestic radiance?. Radioprotection (2017-09-12). Retrieved on 2018-07-31.
  128. Point, S. and Barlier-Salsi, A. (2018) LEDs lighting and retinal damage, technical information sheets, SFRP
  129. https://www.ledsmagazine.com/articles/2012/11/led-based-products-must-meet-photobiological-safety-standards-part-2-magazine.html, LED Based Products Must Meet Photobilogical Safety Standards, retrieved 2019 Jan 17
  130. Lim, S. R.; Kang, D.; Ogunseitan, O. A.; Schoenung, J. M. (2011). "Potential Environmental Impacts of Light-Emitting Diodes (LEDs): Metallic Resources, Toxicity, and Hazardous Waste Classification". Environmental Science & Technology. 45 (1): 320–327. Bibcode:2011EnST...45..320L. doi:10.1021/es101052q. PMID   21138290.
  131. https://www.ledroadwaylighting.com/fr/nouvelles/612-response-to-the-american-medical-association-statement-on-high-intensity-street-lighting.html Respnse to the AMA Statement on High Intensity Street Lighting, retrieved January 17, 2019
  132. "Solid-State Lighting: Comparing LEDs to Traditional Light Sources". eere.energy.gov. Archived from the original on May 5, 2009.
  133. "Dialight Micro LED SMD LED "598 SERIES" Datasheet" (PDF). Dialight.com. Archived from the original (PDF) on February 5, 2009.
  134. "Data Sheet — HLMP-1301, T-1 (3 mm) Diffused LED Lamps". Avago Technologies. Retrieved May 30, 2010.
  135. Narra, Prathyusha; Zinger, D.S. (2004). An effective LED dimming approach. Industry Applications Conference, 2004. 39th IAS Annual Meeting. Conference Record of the 2004 IEEE. 3. pp. 1671–1676. doi:10.1109/IAS.2004.1348695. ISBN   978-0-7803-8486-6. S2CID   16372401.
  136. "Lifetime of White LEDs". Archived from the original on April 10, 2009. Retrieved 2009-04-10., US Department of Energy
  137. Lifetime of White LEDs. US Department of Energy. (PDF) . Retrieved on March 16, 2012.
  138. "In depth: Advantages of LED Lighting". energy.ltgovernors.com.
  139. "LED Light Bars For Off Road Illumination". Larson Electronics.
  140. The Led Museum. The Led Museum. Retrieved on March 16, 2012.
  141. Worthey, James A. "How White Light Works". LRO Lighting Research Symposium, Light and Color. Retrieved October 6, 2007.
  142. Hecht, E. (2002). Optics (4 ed.). Addison Wesley. p.  591. ISBN   978-0-19-510818-7.
  143. Stevenson, Richard (August 2009) The LED’s Dark Secret: Solid-state lighting won't supplant the lightbulb until it can overcome the mysterious malady known as droop. IEEE Spectrum
  144. "LEDs: Good for prizes, bad for insects". news.sciencemag.org. October 7, 2014. Retrieved October 7, 2014.
  145. Pawson, S. M.; Bader, M. K.-F. (2014). "LED Lighting Increases the Ecological Impact of Light Pollution Irrespective of Color Temperature". Ecological Applications. 24 (7): 1561–1568. doi:10.1890/14-0468.1. PMID   29210222.
  146. "Scientist's new database can help protect wildlife from harmful hues of LED lights". USC News. June 12, 2018. Retrieved December 16, 2019.
  147. "Information About Sea Turtles: Threats from Artificial Lighting – Sea Turtle Conservancy" . Retrieved December 16, 2019.
  148. "Stoplights' Potentially Deadly Winter Problem". ABC News. January 8, 2010.
  149. "LED Traffic Lights Can't Melt Snow, Ice".
  150. https://www.ledsmagazine.com/articles/print/volume-6/issue-2/features/led-design-forum-avoiding-thermal-runaway-when-driving-multiple-led-strings-magazine.html Avoiding therma runaway when driving multiple LED strings, retrieved January 17, 2019
  151. European Photonics Industry Consortium (EPIC). This includes use in data communications over fiber optics as well as "broadcast" data or signaling.
  152. Mims, Forrest M. III. "An Inexpensive and Accurate Student Sun Photometer with Light-Emitting Diodes as Spectrally Selective Detectors".
  153. "Water Vapor Measurements with LED Detectors". cs.drexel.edu (2002).
  154. Dziekan, Mike (February 6, 2009) "Using Light-Emitting Diodes as Sensors". soamsci.or. Archived May 31, 2013, at the Wayback Machine
  155. Ben-Ezra, Moshe; Wang, Jiaping; Wilburn, Bennett; Xiaoyang Li; Le Ma (2008). "An LED-only BRDF measurement device". 2008 IEEE Conference on Computer Vision and Pattern Recognition. pp. 1–8. CiteSeerX   10.1.1.165.484 . doi:10.1109/CVPR.2008.4587766. ISBN   978-1-4244-2242-5. S2CID   206591080.
  156. "L-Prize U.S. Department of Energy", L-Prize Website, August 3, 2011
  157. LED There Be Light, Scientific American, March 18, 2009
  158. Eisenberg, Anne (June 24, 2007). "In Pursuit of Perfect TV Color, With L.E.D.'s and Lasers". New York Times. Retrieved April 4, 2010.
  159. "CDC – NIOSH Publications and Products – Impact: NIOSH Light-Emitting Diode (LED) Cap Lamp Improves Illumination and Decreases Injury Risk for Underground Miners". cdc.gov. 2011. doi: 10.26616/NIOSHPUB2011192 . Retrieved May 3, 2013.Cite journal requires |journal= (help)
  160. Janeway, Kimberly (December 12, 2014). "LED lightbulbs that promise to help you sleep". Consumer Reports. Retrieved May 10, 2018.
  161. "LED Device Illuminates New Path to Healing" (Press release). nasa.gov. Retrieved January 30, 2012.
  162. Fudin, M. S.; Mynbaev, K. D.; Aifantis, K. E.; Lipsanen H.; Bougrov, V. E.; Romanov, A. E. (2014). "Frequency characteristics of modern LED phosphor materials". Scientific and Technical Journal of Information Technologies, Mechanics and Optics. 14 (6).
  163. Green, Hank (October 9, 2008). "Transmitting Data Through LED Light Bulbs". EcoGeek. Archived from the original on December 12, 2008. Retrieved February 15, 2009.
  164. 1 2 Dimitrov, Svilen; Haas, Harald (2015). Principles of LED Light Communications: Towards Networked Li-Fi. Cambridge: Cambridge University Press. doi:10.1017/cbo9781107278929. ISBN   978-1-107-04942-0.
  165. "Cisco Annual Internet Report - Cisco Annual Internet Report (2018–2023) White Paper". Cisco. Retrieved October 21, 2020.
  166. Sampath, A. V.; Reed, M. L; Moe, C.; Garrett, G. A.; Readinger, E. D.; Sarney, W. L; Shen, H.; Wraback, M.; Chua, C. (December 1, 2009), "THE EFFECTS OF INCREASING AlN MOLE FRACTION ON THE PERFORMANCE OF AlGaN ACTIVE REGIONS CONTAINING NANOMETER SCALE COMPOSITIONALLY INHOMOGENEITIES", Advanced High Speed Devices, Selected Topics in Electronics and Systems, WORLD SCIENTIFIC, Volume 51, pp. 69–76, doi:10.1142/9789814287876_0007, ISBN   9789814287869
  167. 1 2 Liao, Yitao; Thomidis, Christos; Kao, Chen-kai; Moustakas, Theodore D. (February 21, 2011). "AlGaN based deep ultraviolet light emitting diodes with high internal quantum efficiency grown by molecular beam epitaxy". Applied Physics Letters. 98 (8): 081110. Bibcode:2011ApPhL..98h1110L. doi: 10.1063/1.3559842 . ISSN   0003-6951.
  168. 1 2 3 4 5 Cabalo, Jerry; DeLucia, Marla; Goad, Aime; Lacis, John; Narayanan, Fiona; Sickenberger, David (October 2, 2008). Carrano, John C; Zukauskas, Arturas (eds.). "Overview of the TAC-BIO detector". Optically Based Biological and Chemical Detection for Defence IV. International Society for Optics and Photonics. 7116: 71160D. Bibcode:2008SPIE.7116E..0DC. doi:10.1117/12.799843. S2CID   108562187.
  169. Poldmae, Aime; Cabalo, Jerry; De Lucia, Marla; Narayanan, Fiona; Strauch III, Lester; Sickenberger, David (September 28, 2006). Carrano, John C; Zukauskas, Arturas (eds.). "Biological aerosol detection with the tactical biological (TAC-BIO) detector". Optically Based Biological and Chemical Detection for Defence III. SPIE. 6398: 63980E. doi:10.1117/12.687944. S2CID   136864366.
  170. "Army advances bio-threat detector". www.army.mil. Retrieved October 10, 2019.
  171. Kesavan, Jana; Kilper, Gary; Williamson, Mike; Alstadt, Valerie; Dimmock, Anne; Bascom, Rebecca (February 1, 2019). "Laboratory validation and initial field testing of an unobtrusive bioaerosol detector for health care settings". Aerosol and Air Quality Research. 19 (2): 331–344. doi: 10.4209/aaqr.2017.10.0371 . ISSN   1680-8584.
  172. Dietz, P. H.; Yerazunis, W. S.; Leigh, D. L. (2004). "Very Low-Cost Sensing and Communication Using Bidirectional LEDs".Cite journal requires |journal= (help)
  173. Goins, G. D.; Yorio, N. C.; Sanwo, M. M.; Brown, C. S. (1997). "Photomorphogenesis, photosynthesis, and seed yield of wheat plants grown under red light-emitting diodes (LEDs) with and without supplemental blue lighting". Journal of Experimental Botany. 48 (7): 1407–1413. doi: 10.1093/jxb/48.7.1407 . PMID   11541074.
  174. Gaska, R.; Shur, M. S.; Zhang, J. (October 2006). "Physics and Applications of Deep UV LEDs". 2006 8th International Conference on Solid-State and Integrated Circuit Technology Proceedings: 842–844. doi:10.1109/ICSICT.2006.306525. ISBN   1-4244-0160-7. S2CID   17258357.
  175. 1 2 "LED R&D Challenges". Energy.gov. Retrieved March 13, 2019.
  176. "JULY 2015 POSTINGS". Energy.gov. Retrieved March 13, 2019.
  177. Di, Dawei; Romanov, Alexander S.; Yang, Le; Richter, Johannes M.; Rivett, Jasmine P. H.; Jones, Saul; Thomas, Tudor H.; Abdi Jalebi, Mojtaba; Friend, Richard H.; Linnolahti, Mikko; Bochmann, Manfred (April 14, 2017). "High-performance light-emitting diodes based on carbene-metal-amides" (PDF). Science. 356 (6334): 159–163. arXiv: 1606.08868 . Bibcode:2017Sci...356..159D. doi:10.1126/science.aah4345. ISSN   0036-8075. PMID   28360136. S2CID   206651900.
  178. 1 2 Armin, Ardalan; Meredith, Paul (October 2018). "LED technology breaks performance barrier". Nature. 562 (7726): 197–198. Bibcode:2018Natur.562..197M. doi: 10.1038/d41586-018-06923-y . PMID   30305755.
  179. Cho, Sang-Hwan; Song, Young-Woo; Lee, Joon-gu; Kim, Yoon-Chang; Lee, Jong Hyuk; Ha, Jaeheung; Oh, Jong-Suk; Lee, So Young; Lee, Sun Young; Hwang, Kyu Hwan; Zang, Dong-Sik (August 18, 2008). "Weak-microcavity organic light-emitting diodes with improved light out-coupling". Optics Express. 16 (17): 12632–12639. Bibcode:2008OExpr..1612632C. doi:10.1364/OE.16.012632. ISSN   1094-4087. PMID   18711500.
  180. Cao, Yu; Wang, Nana; Tian, He; Guo, Jingshu; Wei, Yingqiang; Chen, Hong; Miao, Yanfeng; Zou, Wei; Pan, Kang; He, Yarong; Cao, Hui (October 2018). "Perovskite light-emitting diodes based on spontaneously formed submicrometre-scale structures". Nature. 562 (7726): 249–253. Bibcode:2018Natur.562..249C. doi: 10.1038/s41586-018-0576-2 . ISSN   1476-4687. PMID   30305742.
  181. Lin, Kebin; Xing, Jun; Quan, Li Na; de Arquer, F. Pelayo García; Gong, Xiwen; Lu, Jianxun; Xie, Liqiang; Zhao, Weijie; Zhang, Di; Yan, Chuanzhong; Li, Wenqiang (October 2018). "Perovskite light-emitting diodes with external quantum efficiency exceeding 20 per cent". Nature. 562 (7726): 245–248. Bibcode:2018Natur.562..245L. doi:10.1038/s41586-018-0575-3. ISSN   1476-4687. PMID   30305741. S2CID   52958604.
  182. Hiolski, Emma (February 9, 2017). "Watch: Phones of the future could detect your gestures without touch, thanks to two-way LEDs". Science | AAAS. Retrieved March 13, 2019.

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