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`Post-Grant Review of U.S. Patent No. D799,100
`
`
`Exhibit 1015
`
`
`Section 1
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`Exhibit 1015
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`Light-emitting diode - Wikipedia, the free encyclopedia
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`https://web.archive.org/web/20120406160107/https://en.wikipedia.org/wi...
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`Light-emitting diode
`
`From Wikipedia, the free encyclopedia
`
`A light-emitting diode (LED) is a semiconductor light source.[3] LEDs are used as
`indicator lamps in many devices and are increasingly used for other lighting. Introduced
`as a practical electronic component in 1962,[4] early LEDs emitted low-intensity red
`light, but modern versions are available across the visible, ultraviolet, and infrared
`wavelengths, with very high brightness.
`
`When a light-emitting diode is forward-biased (switched on), electrons are able to
`recombine with electron holes within the device, releasing energy in the form of photons.
`This effect is called electroluminescence and the color of the light (corresponding to the
`energy of the photon) is determined by the energy gap of the semiconductor. LEDs are
`often small in area (less than 1 mm2), and integrated optical components may be used to
`shape its radiation pattern.[5] LEDs present many advantages over incandescent light
`sources including lower energy consumption, longer lifetime, improved robustness,
`smaller size, and faster switching. LEDs powerful enough for room lighting are relatively
`expensive and require more precise current and heat management than compact
`fluorescent lamp sources of comparable output.
`
`Light-emitting diodes are used in applications as diverse as aviation lighting, automotive
`lighting, advertising, general lighting, and traffic signals. LEDs have allowed new text,
`video displays, and sensors to be developed, while their high switching rates are also
`useful in advanced communications technology. Infrared LEDs are also used in the
`remote control units of many commercial products including televisions, DVD players,
`and other domestic appliances.
`
`Contents
`
`1 History
`1.1 Discoveries and early devices
`1.2 Practical use
`1.3 Continuing development
`2 Technology
`2.1 Physics
`2.2 Refractive index
`2.2.1 Transition coatings
`2.3 Efficiency and operational parameters
`2.4 Lifetime and failure
`3 Colors and materials
`3.1 Ultraviolet and blue LEDs
`3.2 White light
`3.2.1 RGB systems
`3.2.2 Phosphor-based LEDs
`3.2.3 Other white LEDs
`3.3 Organic light-emitting diodes (OLEDs)
`3.4 Quantum dot LEDs (experimental)
`4 Types
`4.1 Miniature
`4.2 Mid-range
`4.3 High-power
`4.4 Application-specific variations
`5 Considerations for use
`5.1 Power sources
`5.2 Electrical polarity
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`Light-emitting diode
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`Red, pure green and blue LEDs of the 5mm
`diffused type
`
`Type
`
`Passive, optoelectronic
`
`Invented
`
`Working principle Electroluminescence
`Nick Holonyak Jr. (1962)[1]
`1968[2]
`Electronic symbol
`
`First production
`
`Pin configuration
`
`anode and cathode
`
`Parts of an LED. Although not directly
`labeled, the flat bottom surfaces of the
`anvil and post embedded inside the epoxy
`act as anchors, to prevent the conductors
`from being forcefully pulled out from
`mechanical strain or vibration.
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`Exhibit 1015
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`Light-emitting diode - Wikipedia, the free encyclopedia
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`https://web.archive.org/web/20120406160107/https://en.wikipedia.org/wi...
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`5.3 Safety and health
`5.4 Advantages
`5.5 Disadvantages
`6 Applications
`6.1 Indicators and signs
`6.2 Lighting
`6.3 Smart lighting
`6.4 Sustainable lighting
`6.4.1 Energy consumption
`6.4.2 Economically sustainable
`6.5 Other applications
`6.6 Light sources for machine vision systems
`7 See also
`8 References
`9 Further reading
`10 External links
`
`LED retrofit "bulb" with aluminium
`heatsink, a diffusing dome and E27 base,
`using a built-in power supply working on
`mains voltage
`
`History
`
`Discoveries and early devices
`
`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.[6][7] Russian
`Oleg Vladimirovich Losev reported creation of the first LED in 1927.[8][9] His research was
`distributed in Russian, German and British scientific journals, but no practical use was made of the
`discovery for several decades.[10][11] Rubin Braunstein[12] of the Radio Corporation of America
`reported on infrared emission from gallium arsenide (GaAs) and other semiconductor alloys in
`1955.[13] Braunstein observed infrared emission generated by simple diode structures using gallium
`antimonide (GaSb), GaAs, indium phosphide (InP), and silicon-germanium (SiGe) alloys at room
`temperature and at 77 kelvin.
`
`In 1961 American experimenters Robert Biard and Gary Pittman, working at Texas Instruments,[14]
`found that GaAs emitted infrared radiation when electric current was applied and received the patent
`for the infrared LED.
`
`Green electroluminescence from a
`point contact on a crystal of SiC
`recreates H. J. Round's original
`experiment from 1907.
`
`The first practical visible-spectrum (red) LED was developed in 1962 by Nick Holonyak Jr., while working at General Electric Company.[4]
`Holonyak is seen as the "father of the light-emitting diode".[15] M. George Craford,[16] 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.[17] In 1976, T. P. Pearsall created the
`first high-brightness, high-efficiency LEDs for optical fiber telecommunications by inventing new semiconductor materials specifically
`adapted to optical fiber transmission wavelengths.[18]
`
`Until 1968, visible and infrared LEDs were extremely costly, on the order of US$200 per unit, and so had little practical use.[2] The
`Monsanto Company was the first organization to mass-produce visible LEDs, using gallium arsenide phosphide (GaAsP) in 1968 to
`produce red LEDs suitable for indicators.[2] Hewlett Packard (HP) introduced LEDs in 1968, initially using GaAsP supplied by Monsanto.
`The technology proved to have major uses for alphanumeric displays and was integrated into HP's early handheld calculators. In the 1970s
`commercially successful LED devices at less than five cents each were produced by Fairchild Optoelectronics. These devices employed
`compound semiconductor chips fabricated with the planar process invented by Dr. Jean Hoerni at Fairchild Semiconductor.[19] 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. These methods continue to be used by LED producers.[20]
`
`Practical use
`
`The first commercial LEDs were commonly used as replacements for incandescent and neon indicator lamps, and in seven-segment
`displays,[21] first in expensive equipment such as laboratory and electronics test equipment, then later in such appliances as TVs, radios,
`telephones, calculators, and even watches (see list of signal uses). These red LEDs were bright enough only for use as indicators, as the light
`output was not enough to illuminate an area. Readouts in calculators were so small that plastic lenses were built over each digit to make
`them legible. Later, other colors grew widely available and also appeared in appliances and equipment. As LED materials technology grew
`more advanced, light output rose, while maintaining efficiency and reliability at acceptable levels. The invention and development of the
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`LED display of a TI-30 scientific
`calculator (ca. 1978), which uses plastic
`
`lenses to increase the visible digit size
`
`high-power white-light LED led to use for illumination, which is fast replacing incandescent
`and fluorescent lighting[22][23] (see list of illumination applications). Most LEDs were made in
`the very common 5 mm T1¾ and 3 mm T1 packages, but with rising power output, it has grown
`increasingly necessary to shed excess heat to maintain reliability,[24] so more complex packages
`have been adapted for efficient heat dissipation. Packages for state-of-the-art high-power LEDs
`bear little resemblance to early LEDs.
`
`Continuing development
`
`The first high-brightness blue LED was demonstrated by Shuji Nakamura of Nichia
`Corporation and was based on InGaN,[25] borrowing on critical developments in GaN
`nucleation on sapphire substrates and the demonstration of p-type doping of GaN, which
`were developed by Isamu Akasaki and H. Amano in Nagoya.[citation needed] In 1995,
`Alberto Barbieri at the Cardiff University Laboratory (GB) investigated the efficiency
`and reliability of high-brightness LEDs and demonstrated a very impressive result by
`using a transparent contact made of indium tin oxide (ITO) on (AlGaInP/GaAs) LED.
`The existence of blue LEDs and high-efficiency LEDs quickly led to the development of
`the first white LED, which employed a Y3Al5O12:Ce, or "YAG", phosphor coating to
`mix yellow (down-converted) light with blue to produce light that appears white.
`Nakamura was awarded the 2006 Millennium Technology Prize for his invention.[26]
`
`The development of LED technology has caused their efficiency and light output to rise
`exponentially, with a doubling occurring about every 36 months since the 1960s, in a
`way similar to Moore's law. The advances are in general attributed to the parallel
`development of other semiconductor technologies and advances in optics and material
`science. This trend is called Haitz's law after Dr. Roland Haitz.[27]
`
`Illustration of Haitz's law. Light output per LED as
`a function of production year; note the logarithmic
`scale on the vertical axis
`
`In February 2008, a luminous efficacy of 300 lumens of visible light per watt of radiation (not per electrical watt) and warm-light emission
`was achieved by using nanocrystals.[28]
`
`In 2001[29] and 2002,[30] processes for growing gallium nitride (GaN) LEDs on silicon were successfully demonstrated, yielding high
`power LEDs reported in January 2012.[31] Epitaxy costs could be reduced by up to 90% using six-inch silicon wafers instead of two-inch
`sapphire wafers.[32]
`
`In 2011, Zhong Li Wang from the Georgia Institute of Technology discovered that the energy efficiency of Piezoelectric UV LED's can be
`increased by 400% (from 2% to 8%) by using zinc oxide nanowires.[33]
`
`Technology
`
`Physics
`
`The LED consists of a chip of semiconducting material doped with impurities to create a
`p-n junction. As in other diodes, current flows easily from the p-side, or anode, to the
`n-side, or cathode, but not in the reverse direction. Charge-carriers—electrons and
`holes—flow into the junction from electrodes with different voltages. When an electron
`meets a hole, it falls into a lower energy level, and releases energy in the form of a
`photon.
`
`The wavelength of the light emitted, and thus its color depends on the band gap energy
`of the materials forming the p-n junction. In silicon or germanium diodes, the electrons
`and holes recombine by a non-radiative transition, which produces no optical emission,
`because these are indirect band gap materials. The materials used for the LED have a
`direct band gap with energies corresponding to near-infrared, visible, or near-ultraviolet
`light.
`
`The inner workings of an LED
`
`LED development began with infrared and red devices made with gallium arsenide. Advances in materials science have enabled making
`devices with ever-shorter wavelengths, emitting light in a variety of colors.
`
`LEDs are usually built on an n-type substrate, with an electrode attached to the p-type layer deposited on its surface. P-type substrates,
`while less common, occur as well. Many commercial LEDs, especially GaN/InGaN, also use sapphire substrate.
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`Most materials used for LED production have very high refractive indices. This means
`that much light will be reflected back into the material at the material/air surface
`interface. Thus, light extraction in LEDs is an important aspect of LED production,
`subject to much research and development.
`
`Refractive index
`
`Bare uncoated semiconductors such as silicon exhibit a very high refractive index
`relative to open air, which prevents passage of photons at sharp angles relative to the air-
`contacting surface of the semiconductor. This property affects both the light-emission
`efficiency of LEDs as well as the light-absorption efficiency of photovoltaic cells. The
`refractive index of silicon is 4.24, while air is 1.0002926.[35]
`
`In general, a flat-surface uncoated LED semiconductor chip will emit light only
`perpendicular to the semiconductor's surface, and a few degrees to the side, in a cone
`shape referred to as the light cone, cone of light,[36] or the escape cone.[37] The
`maximum angle of incidence is referred to as the critical angle. When this angle is
`exceeded, photons no longer penetrate the semiconductor but are instead reflected both
`internally inside the semiconductor crystal and externally off the surface of the crystal as
`if it were a mirror.[37]
`
`Internal reflections can escape through other crystalline faces, if the incidence angle is
`low enough and the crystal is sufficiently transparent to not re-absorb the photon
`emission. But for a simple square LED with 90-degree angled surfaces on all sides, the
`faces all act as equal angle mirrors. In this case the light can not escape and is lost as
`waste heat in the crystal.[37]
`
`A convoluted chip surface with angled facets similar to a jewel or fresnel lens can
`increase light output by allowing light to be emitted perpendicular to the chip surface
`while far to the sides of the photon emission point.[38]
`
`The ideal shape of a semiconductor with maximum light output would be a microsphere
`with the photon emission occurring at the exact center, with electrodes penetrating to the
`center to contact at the emission point. All light rays emanating from the center would
`be perpendicular to the entire surface of the sphere, resulting in no internal reflections.
`A hemispherical semiconductor would also work, with the flat back-surface serving as a
`mirror to back-scattered photons.[39]
`
`Transition coatings
`
`Many LED semiconductor chips are potted in clear or colored molded plastic shells. The
`plastic shell has three purposes:
`
`1.
`2.
`
`3.
`
`Mounting the semiconductor chip in devices is easier to accomplish.
`The tiny fragile electrical wiring is physically supported and protected from
`damage.
`The plastic acts as a refractive intermediary between the relatively high-index
`semiconductor and low-index open air.[40]
`
`The third feature helps to boost the light emission from the semiconductor by acting as a
`diffusing lens, allowing light to be emitted at a much higher angle of incidence from the
`light cone than the bare chip is able to emit alone.
`
`Efficiency and operational parameters
`
`Typical indicator LEDs are designed to operate with no more than 30–60 milliwatts
`(mW) of electrical power. Around 1999, Philips Lumileds introduced power LEDs
`capable of continuous use at one watt. These LEDs used much larger semiconductor die
`sizes to handle the large power inputs. Also, the semiconductor dies were mounted onto
`metal slugs to allow for heat removal from the LED die.
`
`One of the key advantages of LED-based lighting sources is high luminous efficacy.
`
`I-V diagram for a diode. An LED will begin to emit
`light when the on-voltage is exceeded. Typical on
`voltages are 2–3 volts.
`
`Idealized example of light emission cones in a
`semiconductor, for a single point-source emission
`zone. The left illustration is for a fully translucent
`wafer, while the right illustration shows the half-
`cones formed when the bottom layer is fully
`opaque. The light is actually emitted equally in all
`directions from the point-source, so the areas
`between the cones shows the large amount of
`trapped light energy that is wasted as heat.[34]
`
`The light emission cones of a real LED wafer are
`far more complex than a single point-source light
`emission. The light emission zone is typically a
`two-dimensional plane between the wafers. Every
`atom across this plane has an individual set of
`emission cones.
`
`Drawing the billions of overlapping cones is
`impossible, so this is a simplified diagram showing
`the extents of all the emission cones combined. The
`larger side cones are clipped to show the interior
`features and reduce image complexity; they would
`extend to the opposite edges of the two-dimensional
`emission plane.
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`Light-emitting diode - Wikipedia, the free encyclopedia
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`White LEDs quickly matched and overtook the efficacy of standard incandescent lighting systems. In 2002, Lumileds made five-watt LEDs
`available with a luminous efficacy of 18–22 lumens per watt (lm/W). For comparison, a conventional incandescent light bulb of 60–100
`watts emits around 15 lm/W, and standard fluorescent lights emit up to 100 lm/W. A recurring problem is that efficacy falls sharply with
`rising current. This effect is known as droop and effectively limits the light output of a given LED, raising heating more than light output for
`higher current.[41][42][43]
`
`In September 2003, a new type of blue LED was demonstrated by the company Cree Inc. to provide 24 mW at 20 milliamperes (mA). This
`produced a commercially packaged white light giving 65 lm/W at 20 mA, becoming the brightest white LED commercially available at the
`time, and more than four times as efficient as standard incandescents. In 2006, they demonstrated a prototype with a record white LED
`luminous efficacy of 131 lm/W at 20 mA. Nichia Corporation has developed a white LED with luminous efficacy of 150 lm/W at a forward
`current of 20 mA.[44] Cree's XLamp XM-L LEDs, commercially available in 2011, produce 100 lumens per watt at their full power of 10
`watts, and up to 160 lumens/watt at around 2 watts input power.
`
`Practical general lighting needs high-power LEDs, of one watt or more. Typical operating currents for such devices begin at 350 mA.
`
`Note that these efficiencies are for the LED chip only, held at low temperature in a lab. Lighting works at higher temperature and with drive
`circuit losses, so efficiencies are much lower. United States Department of Energy (DOE) testing of commercial LED lamps designed to
`replace incandescent lamps or CFLs showed that average efficacy was still about 46 lm/W in 2009 (tested performance ranged from
`17 lm/W to 79 lm/W).[45]
`
`Cree issued a press release on February 3, 2010 about a laboratory prototype LED achieving 208 lumens per watt at room temperature. The
`correlated color temperature was reported to be 4579 K.[46]
`
`Lifetime and failure
`
`Main article: List of LED failure modes
`
`Solid state devices such as LEDs are subject to very limited wear and tear if operated at low currents and at low temperatures. Many of the
`LEDs made in the 1970s and 1980s are still in service today. Typical lifetimes quoted are 25,000 to 100,000 hours, but heat and current
`settings can extend or shorten this time significantly. [47]
`
`The most common symptom of LED (and diode laser) failure is the gradual lowering of light output and loss of efficiency. Sudden failures,
`although rare, can occur as well. Early red LEDs were notable for their short lifetime. With the development of high-power LEDs the
`devices are subjected to higher junction temperatures and higher current densities than traditional devices. This causes stress on the material
`and may cause early light-output degradation. To quantitatively classify lifetime in a standardized manner it has been suggested to use the
`terms L75 and L50, which is the time it will take a given LED to reach 75% and 50% light output respectively.[48]
`
`Like other lighting devices, LED performance is temperature dependent. Most manufacturers’ published ratings of LEDs are for an
`operating temperature of 25 °C. LEDs used outdoors, such as traffic signals or in-pavement signal lights, and that are utilized in climates
`where the temperature within the luminaire gets very hot, could result in low signal intensities or even failure.[49]
`
`LED light output rises at lower temperatures, leveling off depending on type at around −30C.[citation needed] Thus, LED technology may be a
`good replacement in uses such as supermarket freezer lighting[50][51][52] and will last longer than other technologies. Because LEDs emit
`less heat than incandescent bulbs, they are an energy-efficient technology for uses such as freezers. However, because they emit little heat,
`ice and snow may build up on the LED luminaire in colder climates.[49] This lack of waste heat generation has been observed to cause
`sometimes significant problems with street traffic signals and airport runway lighting in snow-prone areas, although some research has been
`done to try to develop heat sink technologies to transfer heat to other areas of the luminaire.[53]
`
`Colors and materials
`
`Conventional LEDs are made from a variety of inorganic semiconductor materials, the following table shows the available colors with
`wavelength range, voltage drop and material:
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`Color Wavelength [nm] Voltage drop [ΔV]
`
`Infrared
`
`λ > 760
`
`ΔV < 1.9
`
`Red
`
`610 < λ < 760
`
`1.63 < ΔV < 2.03
`
`Orange
`
`590 < λ < 610
`
`2.03 < ΔV < 2.10
`
`Yellow
`
`570 < λ < 590
`
`2.10 < ΔV < 2.18
`
`Green
`
`500 < λ < 570
`
`1.9[54] < ΔV < 4.0
`
`Blue
`
`450 < λ < 500
`
`2.48 < ΔV < 3.7
`
`Violet
`
`400 < λ < 450
`
`2.76 < ΔV < 4.0
`
`Purple
`
`multiple types
`
`2.48 < ΔV < 3.7
`
`Ultraviolet λ < 400
`
`3.1 < ΔV < 4.4
`
`Pink
`
`multiple types
`
`ΔV ~ 3.3[60]
`
`White
`
`Broad spectrum ΔV = 3.5
`
`Ultraviolet and blue LEDs
`
`Semiconductor material
`Gallium arsenide (GaAs)
`Aluminium gallium arsenide (AlGaAs)
`Aluminium gallium arsenide (AlGaAs)
`Gallium arsenide phosphide (GaAsP)
`Aluminium gallium indium phosphide (AlGaInP)
`Gallium(III) phosphide (GaP)
`Gallium arsenide phosphide (GaAsP)
`Aluminium gallium indium phosphide (AlGaInP)
`Gallium(III) phosphide (GaP)
`Gallium arsenide phosphide (GaAsP)
`Aluminium gallium indium phosphide (AlGaInP)
`Gallium(III) phosphide (GaP)
`Indium gallium nitride (InGaN) / Gallium(III) nitride (GaN)
`Gallium(III) phosphide (GaP)
`Aluminium gallium indium phosphide (AlGaInP)
`Aluminium gallium phosphide (AlGaP)
`Zinc selenide (ZnSe)
`Indium gallium nitride (InGaN)
`Silicon carbide (SiC) as substrate
`Silicon (Si) as substrate – (under development)
`Indium gallium nitride (InGaN)
`Dual blue/red LEDs,
`blue with red phosphor,
`or white with purple plastic
`Diamond (235 nm)[55]
`Boron nitride (215 nm)[56][57]
`Aluminium nitride (AlN) (210 nm)[58]
`Aluminium gallium nitride (AlGaN)
`Aluminium gallium indium nitride (AlGaInN) – (down to 210 nm)[59]
`Blue with one or two phosphor layers:
`yellow with red, orange or pink phosphor added afterwards,
`or white with pink pigment or dye. [61]
`Blue/UV diode with yellow phosphor
`
`Current bright blue LEDs are based on the wide band gap semiconductors GaN (gallium nitride) and
`InGaN (indium gallium nitride). They can be added to existing red and green LEDs to produce the
`impression of white light, though white LEDs today rarely use this principle.
`
`The first blue LEDs using gallium nitride were made in 1971 by Jacques Pankove at RCA
`Laboratories.[62] These devices had too little light output to be of practical use and research into
`gallium nitride devices slowed. In August 1989, Cree Inc. introduced the first commercially
`available blue LED based on the indirect bandgap semiconductor, silicon carbide.[63] SiC LEDs had
`very low efficiency, no more than about 0.03%, but did emit in the blue portion of the visible light
`spectrum.
`
`In the late 1980s, key breakthroughs in GaN epitaxial growth and p-type doping[64] ushered in the
`modern era of GaN-based optoelectronic devices. Building upon this foundation, in 1993 high-
`brightness blue LEDs were demonstrated. Efficiency (light energy produced vs. electrical energy
`used) reached 10%.[65] High-brightness blue LEDs invented by Shuji Nakamura of Nichia
`Corporation using gallium nitride revolutionized LED lighting, making high-power light sources
`practical.
`
`By the late 1990s, blue LEDs had become widely available. They have an active region consisting of
`
`Blue LEDs
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`one or more InGaN quantum wells sandwiched between thicker layers of GaN, called cladding layers. By varying the relative InN-GaN
`fraction in the InGaN quantum wells, the light emission can be varied from violet to amber. AlGaN aluminium gallium nitride of varying
`AlN fraction can be used to manufacture the cladding and quantum well layers for ultraviolet LEDs, but these devices have not yet reached
`the level of efficiency and technological maturity of the InGaN-GaN blue/green devices. If the active quantum well layers are GaN, instead
`of alloyed InGaN or AlGaN, the device will emit near-ultraviolet light with wavelengths around 350–370 nm. Green LEDs manufactured
`from the InGaN-GaN system are far more efficient and brighter than green LEDs produced with non-nitride material systems.
`
`With nitrides containing aluminium, most often AlGaN and AlGaInN, even shorter wavelengths are achievable. Ultraviolet LEDs in a range
`of wavelengths are becoming available on the market. Near-UV emitters at wavelengths around 375–395 nm are already cheap and often
`encountered, for example, as black light lamp replacements for inspection of anti-counterfeiting UV watermarks in some documents and
`paper currencies. Shorter-wavelength diodes, while substantially more expensive, are commercially available for wavelengths down to
`247 nm.[66] As the photosensitivity of microorganisms approximately matches the absorption spectrum of DNA, with a peak at about
`254 nm, UV LED emitting at 250–270 nm are to be 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.[67]
`
`Deep-UV wavelengths were obtained in laboratories using aluminium nitride (210 nm),[58] boron nitride (215 nm)[56][57] and diamond
`(235 nm).[55]
`
`White light
`
`There are two primary ways of producing high-intensity white-light using LEDs. One is to use individual LEDs that emit three primary
`colors[68]—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, much in the same way a fluorescent light bulb works.
`
`Due to metamerism, it is possible to have quite different spectra that appear white.
`
`RGB systems
`
`White light can be formed by mixing differently colored lights; the most
`common method is to use red, green, and blue (RGB). Hence the method is
`called multi-color white LEDs (sometimes referred to as RGB LEDs). Because
`these need electronic circuits to control the blending and diffusion of different
`colors, and because the individual color LEDs typically have slightly different
`emission patterns (leading to variation of the color depending on direction) even
`if they are made as a single unit, these are seldom used to produce white
`lighting. Nevertheless, this method is particularly interesting in many uses
`because of the flexibility of mixing different colors,[69] and, in principle, this
`mechanism also has higher quantum efficiency in producing white light.
`
`There are several types of multi-color 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 will mean lower color rendering, presenting a trade-off
`between the luminous efficiency and color rendering. For example, the
`dichromatic white LEDs have the best luminous efficacy (120 lm/W), but the
`lowest color rendering capability. However, although tetrachromatic white LEDs
`have excellent color rendering capability, they often have poor luminous
`efficiency. Trichromatic white LEDs are in between, having both good luminous efficacy (>70 lm/W) and fair color rendering capability.
`
`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.
`
`Multi-color LEDs offer not merely another means to form white light but a new means to form light of different colors. Most perceivable
`colors can be formed by mixing different amounts of three primary colors. This allows precise dynamic color control. As more effort is
`devoted to investigating this method, multi-color LEDs should have profound influence on the fundamental method that we use to produce
`and control light color. However, before this type of LED can play a role on the market, several technical problems need solving. These
`include that this type of LED's emission power decays exponentially with rising temperature,[70] resulting in a substantial change in color
`stability. Such problems inhibit and may preclude industrial use. Thus, many new package designs aimed at solving this problem have been
`proposed and their results are now being reproduced by researchers and scientists.
`
`Phosphor-based LEDs
`
`This method involves coating LEDs of one color (mostly blue LEDs made of InGaN) with phosphor of different colors to form white light;
`the resultant LEDs are called phosphor-based white LEDs.[71] A fraction of the blue light undergoes the Stokes shift being transformed
`from shorter wavelengths to longer. Depending on the color of the original LED, phosphors of different colors can be employed. If several
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`Exhibit 1015
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`Light-emitting diode - Wikipedia, the free encyclopedia
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`https://web.archive.org/web/20120406160107/https://en.wikipedia.org/wi...
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`phosphor layers of distinct colors are applied, the emitted spectrum is
`broadened, effectively raising the color rendering index (CRI) value of a given
`LED.[72]
`
`Phosphor-based LEDs efficiency losses are due to the heat loss from the Stokes
`shift and also other phosphor-related degradation issues. Its efficiencies
`compared to normal LEDs are dependent on the spectral distribution of the
`resultant light output and the original wavelength of the LED itself. The
`efficiency of a typical YAG-based yellow phosphor converted white LED ranges
`from 3 to 5 times the efficiency of the original blue LED. 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.
`
`Among the challenges being faced to improve the efficiency of LED-based
`white light sources are the development of more efficient phosphors as well as
`the development of more efficient green LEDs. The theoretical maximum for
`green LEDs is at 683 lumens per watt but today few Green LEDs exceed even
`100 lumens per watt. Today the most efficient yellow phosphor is still the YAG phosphor, with less than 10% Stoke shift loss. Losses
`attributable to internal optical losses due to re-absorption in the LED chip and in the LED packaging itself account typically for another
`10% to 30% of efficiency loss. Currently, in the area of phosphor LED development, much effort is being spent on optimizing these devices
`to higher light output and higher operation temperatures. For instance, the efficiency can be raised by adapting better package design or by
`using a more suitable type of phosphor. Conformal coating process is frequently used to address the issue of varying phosphor thickness.
`
`Spectrum of a “white” LED clearly showing blue light
`directly emitted by the GaN-based LED (peak at about 465
`nm) and the more broadband Stokes-shifted light emitted by
`the Ce3+:YAG phosphor, which emits at roughly 500–700
`nm.
`
`The phosphor-based white LEDs encapsulate InGaN blue LEDs inside phosphor coated epoxy. A common yellow phosphor material is
`cerium-doped yttrium aluminium garnet (Ce3+:YAG).
`
`White LEDs can also be made by coating near-ultraviolet (NUV) LEDs with a mixture of high-efficiency europium-based phosphors that
`emit red and blue, plus copper and aluminium-