Light Emitting Diode | Dissertation

tripping Emitting rectifying tube DissertationIntroductionA absolved-emitting rectifying valve( lead) is a semiconductor machination unit leisurely source. leads ar apply as indicator lamps in many devices and ar progressively utilize for proterozoic(a) excitation. Introduced as a hard-nosed negatronic component part in 1962, early guides emitted commencement-intensity loss fainthearted, plainly modern versions atomic number 18 visible(prenominal) across thevisible, ultra chromatic and infra carmine leisurely sort out wavelengths, with truly eminent luminance.When a well-defined-emitting diodeis preliminary aslant (switched on), electrons argon able to recombine with electron holes within the device, releasing energy in the form of photons. This effect is c everyedelectroluminescenceand the blazonof the unhorse ( interchangeable to the energy of the photon) is indomitable by the energy gap of the semiconductor. An guide is often underage in b ea (less(prenominal) than 1mm2), and integpaced optical components may be employ to flesh its radiation pattern. leads present manyadvantagesover candent light sources includinglower energy consumption, longerlifetime, ameliorate robustness, littler size, faster switching, and greater durability and dependableness. takes powerful enough for room lighting are comparatively expensive and require more(prenominal) precise true andheat managementthan compactfluorescent lampsources of comparable takings.Light-emitting diodes are go ford in applications as diverse as replacements foraviation lighting,automotive lighting(particularly brake lamps, turn signals and indicators) as surface as intraffic signals. The compact size, the disaster of narrow bandwidth, switching speed, and extreme reliability of leads has in allowed refreshing text and television set displays and sensors to be developed, while their extravagantly switching rates are alike commitful in advanced co mmunications applied science.Infra fierceLEDs are also used in theremote controlunits of many commercial products including televisions, DVD p levels, and other(a) domestic appliances. tarradiddleDiscoveries and early devicesGreen electroluminescence from a point contact on a crystal ofSiCrecreatesH. J. Rounds original experiment from 1907.Electroluminescenceas a phenomenon was discove passing in 1907 by the British experimenterH. J. RoundofMarconi Labs, utilise a crystal ofsilicon carbideand acats-whisker detector.RussianOleg Vladimirovich Losevreported on the creation of a starting signal base LED in 1927.His research was distributed in Russian, German and British scientific journals, but no practical use was do of the stripping for several decades. Rubin Braunstein of theRadio Corporation of Americareported on infrared sack fromgallium arsenide(GaAs) and other semiconductor alloys in 1955.Braunstein observed infrared electric arc generated by simple diode structures usin ggallium antimonide(GaSb), GaAs,indium phosphide(InP), andsilicon-germanium(SiGe) alloys at room temperature and at 77kelvin.In 1961, American experimenters Robert Biard and Gary Pittman working(a) atTexas Instruments, ready that GaAs emitted infrared radiation when electric period was applied and received the patent for the infrared LED.The premier practical visible-spectrum (red) LED was developed in 1962 byNick Holonyak Jr., while working atGeneral Electric Company.Holonyak is seen as the father of the light-emitting diode.M. George Craford,a former graduate student of Holonyak, invented the first yellow LED and improved the igniter of red and red-orange LEDs by a factor of ten in 1972. In 1976, T.P. Pearsall created the first proud-brightness, high faculty LEDs for optical fiber telecommunications by inventing new semiconductor solids specifically equal to optical fiber transmission wavelengths.Until 1968, visible and infrared LEDs were extremely costly, on the order of US $200 per unit, and so had little practical use.TheMonsanto Companywas the first organization to mass- rear visible LEDs, using gallium arsenide phosphide in 1968 to produce red LEDs suitable for indicators. Hewlett Packard(HP) introduced LEDs in 1968, initially using GaAsP supplied by Monsanto. The engineering proved to have major uses for alphanumeric displays and was integrated into HPs early hand-held calculators. In the 1970s commercially successful LED devices at few than five cents each were produced by F channelisechild Optoelectronics. These devices employed compound semiconductor cut outs put on with theplanar processinvented by Dr. Jean Hoerni atFairchild Semiconductor.The combination of planar processing for chip guile and innovative packaging methods enabled the team at Fairchild led by optoelectronics lead up Thomas Brandt to grasp the needed cost reductions. These methods continue to be used by LED producers.History Of LEDs and LED TechnologyLight Emitting D iode (LED)Light Emitting Diode (LED) is essentially a PN junction semiconductor diode that emits a monochromatic ( wizard color) light when operated in a forward biased direction. The basic structure of an LED consists of the weaken or light emitting semiconductor stuff, a lead frame where the soften is actually placed, and the encapsulation epoxy which surrounds and protects the die (Figure 1).The first commercially usable LEDs were developed in the 1960s by combining three primary elements gallium, arsenic and phosphorus (GaAsP) to obtain a 655nm red light source. Although the luminous intensity was very low with brightness levels of some 1-10mcd 20mA, they subdued found use in a grade of applications, primarily as indicators. Following GaAsP, hurly burly, or gallium phosphide, red LEDs were developed. These devices were found to exhibit very high quantum efficiencies, however, they played only a forgivable role in the product of new applications for LEDs. This was rec eivable to devil reasons First, the 700nm wavelength firing is in a spectral region where the sensitivity level of the gentle pump is very low (Figure 2) and therefore, it does non appear to be very bright scour though the might is high (the human eye is some responsive to yellow- unripened light). Second, this high susceptibility is only achieved at low currents. As the current increases, the aptitude decreases. This proves to be a trauma to users more(prenominal)(prenominal) as outdoor means sign reconstructrs who typically multiplex their LEDs at high currents to achieve brightness levels similar to that of DC incessant operation. As a event, GaP red LEDs are soon used in only a limited number of applications.As LED engine room progressed through the 1970s, additional colors and wavelengths became available. The most coarse squares were GaP green and red, GaAsP orange or high efficiency red and GaAsP yellow, all of which are still used directly (Table3). The trend towards more practical applications was also beginning to develop. LEDs were found in such products as calculators, digital watches and test equipment. Although the reliability of LEDs has always been superior to that of candent, neon etcetera, the failure rate of early devices was some(prenominal) higher than current technology now achieves. This was due in part to the actual component assembly that was primarily manual(a) in nature. Individual operators per make such tasks as dispensing epoxy, placing the die into position, and mixing epoxy all by hand. This resulted in defects such as epoxy slops which caused VF (forward voltage) and VR (reverse voltage) leakage or even shorting of the PN junction. In addition, the branch methods and poppycocks used were not as refined as they are today. High numbers of defects in the crystal, substrate and epitaxial layers resulted in reduced efficiency and shorter device lifetimes.Gallium aluminum ArsenideIt wasnt until the 19 80s when a new material, GaAlAs (gallium aluminum arsenide) was developed, that a rapid growth in the use ofLEDsbegan to occur. GaAlAs technology provided superior performance over antecedently availableLEDs. The brightness was over 10 times greater than standardLEDsdue to increased efficiency and multi-layer, heterojunction grapheme structures. The voltage required for operation was lower resulting in a total power savings. TheLEDscould also be easily pulsed or multiplexed. This allowed their use in variable message and outdoor signs.LEDswere also designed into such applications as bar code scanners, fiber optic data transmission systems, and medical equipment. Although this was a major breakthrough inLEDtechnology, there were still meaningful drawbacks to GaAlAs material. First, it was only available in a red 660nm wavelength. Second, the light rig degradation of GaAlAs is greater than that of standard technology. It has long been a misconception withLEDsthat light fruit allo w decrease by 50% after 100,000 hours of operation. In fact, well-nigh GaAlAsLEDsmay decrease by 50% after only 50,000 -70,000 hours of operation. This is especially accepted in high temperature and/or high humidity environments. Also during this time, yellow, green and orange saw only a minor overture in brightness and efficiency which was primarily due to improvements in crystal growth and optics design. The basic structure of the material remained relatively unchanged.To overcome these unvoiced issues new technology was needed.LEDdesigners turned to laser diode technology for solutions. In replicate of latitude with the rapid victimisations inLEDtechnology, laser diode technology had also been making progress. In the late 1980s laser diodes with fruit in the visible spectrum began to be commercially produced for applications such as bar code readers, measurement and alignment systems and next generation storage systems.LEDdesigners looked to using similar techniques to p roduce high brightness and high reliabilityLEDs. This led to the development of InGaAlP (Indium Gallium Aluminum Phosphide) visibleLEDs. The use of InGaAlP as the luminescent material allowed flexibility in the design ofLEDoutput color simply by adjusting the size of the energy band gap. Thus, green, yellow, orange and redLEDsall could be produced using the same basic technology. Additionally, light output degradation of InGaAlP material is remarkablely improved even at elevated temperature and humidity.Current Developments of LED TechnologyInGaAlPLEDstook a further leap in brightness with a new development by Toshiba, a leading manufacturer ofLEDs. Toshiba, using the MOCVD (Metal Oxide Chemical Vapor Deposition) growth process, was able to produce a device structure that reflected 90% or more of the generated light traveling from the agile layer to the substrate back as useful light output (Figure 4). This allowed for an virtually two-fold increase in theLEDluminance over conven tional devices.LEDperformance was further improved by introducing a current blocking layer into theLEDstructure (Figure 5). This blocking layer essentially channels the current through the device to achieve intermit device efficiency.As a result of these developments, much of the growth forLEDsin the mid-nineties allow for be concentrated in three main areas The first is in traffic control devices such as stop lights, prosy signals, barricade lights and road hazard signs. The second is in variable message signs such as the one located in Times square toes red-hot York which displays commodities, news and other information. The third concentration would be in automotive applications.The visibleLEDhas come a long way since its introduction almost 30 years ago and has tho to show any signs of subnormality down. A BlueLED, which has only recently become available in outturn quantities, leave result in an entire generation of new applications. BlueLEDsbecause of their high photo n energies (2.5eV) and relatively low eye sensitivity have always been difficult to manufacture. In addition the technology necessary to fabricate theseLEDsis very diametric and far less advanced than standardLEDmaterials. The meritlessLEDsavailable today consist of GaN (gallium nitride) and SiC (silicon carbide) spin with brightness levels in excess of 1000mcd 20mA for GaN devices. Since lamentable is one of the primary colors, (the other two being red and green), full color solid stateLEDsigns, TVs etc. will soon become commercially available. Full colorLEDsigns have already been manufactured on a small prototype basis, however, due to the high price of inexorableLEDs, it is still not practical on a extensive(p) scale. Other applications for no-goodLEDsinclude medical diagnostic equipment and photolithography.LED ColorsIt is also achievable to produce other colors using the same basic GaN technology and growth processes. For example, a high brightness green (approximatel y 500nm)LEDhas been developed that is currently being evaluated for use as a replacement to the green lightbulb in traffic lights. Other colors including purple and atomic number 6-clad are also possible. With the recent introduction of blueLEDs, it is now possible to produce discolor by selectively combining the proper combination of red, green and blue light. This process however, requires sophisticated software and hardware design to implement. In addition, the brightness level is low and the overall light output of each RGB die being used degrades at a different rate resulting in an eventual color unbalance. Another approach being taken to achieve white light output, is to use a phosphor layer (Yttrium Aluminum Garnet) on the surface of a blueLED.In summary,LEDshave gone from infancy to adolescence and are experiencing slightly of the most rapid market growth of their lifetime. By using InGaAlP material with MOCVD as the growth process, combined with economic delivery of g enerated light and efficient use of injected current, around of the brightest, most efficient and most reliableLEDsare now available. This technology together with other novelLEDstructures will ensure wide application ofLEDs. New developments in the blue spectrum and on white light output will also guarantee the continued increase in applications of these economical light sources.Practical useThe first commercial LEDs were harshly used as replacements forincandescentandneonindicator lamps, and inseven-segment displays,first in expensive equipment such as laboratory and electronics test equipment, wherefore later in such appliances as TVs, radios, telephones, calculators, and even watches (see list ofsignal 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 pliant lenses were built over each digit to make them legible. Later, other colors grew widely available a nd 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 high power white light LED led to use for illumination, which is fast replacing incandescent and fluorescent lighting. (see list ofillumination applications). close LEDs were made in the very common 5mm T1 and 3mm T1 packages, but with rising power output, it has grown increasingly necessary to plainten excess heat to maintain reliability,so more complex packages have been equal for efficient heat dissipation. Packages for state-of-the-arthigh power LEDsbear little resemblance to early LEDs.chronic developmentThe first high-brightness blue LED was demonstrated byShuji NakamuraofNichia Corporationand was establish onInGaNborrowing on critical developments inGaNnucleation on sapphire substrates and the demonstration of p-type doping of GaN which were developed byIsamu Akasakiand H. Amano inNagoya. In 1995,Alberto Barbieriat theCardiff UniversityLaboratory (GB) investigated the efficiency and reliability of high-brightness LEDs and demonstrated a very impressive result by using a transparent contact made ofindium tin oxide(ITO) on (AlGaInP/GaAs) LED. The existence of blue LEDs and high efficiency LEDs quickly led to the development of the firstwhite LED, which employed aY3Al5O12Ce, or YAG, phosphor coating to mix yellow (down-converted) light with blue to produce light that appears white. Nakamura was awarded the 2006Millennium Technology Prizefor his invention.The development of LED technology has caused their efficiency and light output torise exponentially, with a doubling occurring about every 36 months since the 1960s, in a way similar toMoores law. The advances are planetaryly attributed to the parallel development of other semiconductor technologies and advances in optics and material science. This trend is normally calledHaitzs Lawafter Dr. Roland Haitz.In February 2008, 300lumensof visible light per wattluminous force(not per electrical watt) and warm-light emission was achieved by usingnanocrystals.In 2009, a process for growing gallium nitride (GaN) LEDs on silicon has been reported.Epitaxycosts could be reduced by up to 90% using six-inch silicon wafers sooner of two-inch sapphire wafers.Illustration of Haitzs Law. Light output per LED as a function of production year, note the logarithmic scale on the straight axisTechnologyPhysicsThe LED consists of a chip of conductive materialdopedwith impurities to create ap-n junction. As in other diodes, current flows easily from the p-side, oranode, to the n-side, orcathode, but not in the reverse direction. Charge-carrierselectronsandholesflow into the junction fromelectrodeswith different voltages. When an electron meets a hole, it falls into a lowerenergy level, and releasesenergyin the form of a photon.Thewavelengthof the light emitted, and therefrom its color depends on theband gapenergy of the materials forming thep-n junction. Insiliconor germaniumdiodes, the electrons and holes recombine by anon-radiative transitionwhich produces no optical emission, because these are indirect band gapmaterials. The materials used for the LED have adirect band gapwith energies corresponding to near-infrared, visible or near-ultraviolet light.LED development began with infrared and red devices made withgallium arsenide. Advances inmaterials sciencehave enabled making devices with ever-shorter wavelengths, emitting light in a variety of colors.LEDs are ordinarily 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 usesapphiresubstrate.Most materials used for LED production have very highrefr agile indices. This means that much light will be reflected back into the material at the material/air surface i nterface. Thus,light extraction in LEDsis an important aspect of LED production, subject to much research and development.The inner workings of an LED I-V diagram for adiode. An LED will begin to emit light when the on-voltageis exceeded. veritable(prenominal) on voltages are 2-3volts.Refractive IndexIdealized example of light emission cones in a semiconductor, for a single point-source emission zone. The left metaphor 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.The light emission cones of a rattling LED wafer are far more complex than a single point-source light emission. Typically the light emission zone is a 2D plane between the wafers. Across this 2D plane, there is potently a separate set of emission cones for e very atom. Drawing the billions of overlapping cones is impossible, so this is a simplified diagram showing the extents of all the emission cones combined. The bigger side cones are clipped to show the interior features and reduce enter complexity they would extend to the opposite edges of the 2D emission plane.Bare uncoated semiconductors such assiliconexhibit a very highrefractive indexrelative to open air, which prevents passage of photons at crispy 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 ofphotovoltaic cells. The refractive index of silicon is 4.24, while air is 1.00002926.Generally a flat-surfaced uncoated LED semiconductor chip will only emit light perpendicular to the semiconductors surface, and a few degrees to the side, in a cone shape referred to as thelight cone,cone of light,or theescape cone.The maximumangle of relative incidenceis referred to as thecritical 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 amirror.Internal reflectionscan 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. tho for a simple square LED with 90-degree angled surfaces on all sides, the faces all act as equal angle mirrors. In this model the light cannot escape and is lost as waste heat in the crystal.A convoluted chip surface with angledfacetssimilar to a jewel orfresnel lenscan increase light output by allowing light to be emitted perpendicular to the chip surface while far to the sides of the photon emission point.The ideal shape of a semiconductor with maximum light output would be amicrospherewith the photon emission occurring at the exact center, with el ectrodes penetrating to the center to contact at the emission point. each(prenominal) 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.Transition coatingsMany LED semiconductor chips arepottedin clear or colored molded p weatheric shells. The plastic shell has three purposes1. Mounting the semiconductor chip in devices is easier to accomplish.2. The flyspeck fragile electrical wiring is physically supported and protected from damage3. The plastic acts as a refractive intermediary between the relatively high-index semiconductor and low-index open air.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 usable parametersTypical indicator LEDs are designed to operate with no more than 30-60mWof electrical power. Around 1999,Philips Lumiledsintroduced power LEDs capable of continuous use at oneW. 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 is its high force,dubious-discussas measured by its light output per unit power input. pureness LEDs quickly matched and overtook the efficacy of standard incandescent lighting systems. In 2002, Lumileds made five-watt LEDs available with aluminous efficacyof 18-22 lumens per watt (lm/W). For comparison, a conventional 60-100 Wincandescent light bulbemits around 15 lm/W, and standardfluorescent lightsemit up to 100 lm/W. A recurring problem is that efficacy falls sharply with rising current. This effect is known asdroopand effectively limits the light output of a given(p) LED, raising heating more than light output for higher current.In September 2003, a new type of blue LED was demonstrated by the companyCree Inc.to provide 24mW at 20milliamperes(mA). This produced a commercially packaged white light giving 65 lm/W at 20 mA, become 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. Also,Seoul Semiconductorplans for 135 lm/W by 2007 and cxlv lm/W by 2008,which would be nearing an order of magnitude improvement over standard incandescents and better than even standard fluorescents.Nichia Corporationhas developed a white LED with luminous efficacy of 150 lm/W at a forward current of 20 mA.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 efficiencie s are for the LED chip only, held at low temperature in a lab. Lighting whole shebang 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 orCFLsshowed that average efficacy was still about 46 lm/W in 2009 (tested performance ranged from 17lm/W to 79lm/W).Cree issued a press release on February 3, 2010 about a laboratory prototype LED achieving 208 lumens per watt at room temperature. The correlatedcolor temperaturewas reported to be 4579K.Lifetime and failureMain phraseList of LED failure modesSolid state devices such as LEDs are subject to very limitedwear and tearif 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.The most common symptom o f LED (anddiode 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 higherjunction temperaturesand 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 appearance 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. deal other lighting devices, LED performance is temperature dependent. Most manufacturers published ratings of LEDs are for an operating temperature of 25C. 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 intensi ties or even failure.LED light output actually rises at colder temperatures (leveling off depending on type at around 30C). Consequently, LED technology may be a untroubled replacement in uses such as supermarket freezer lightingand 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.This escape 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 conveyancing heat to other areas of the luminaire.Ultraviolet and blue LEDsBlueLEDs.Blue LEDs are based on the wideband gapsemiconductors GaN (gallium nitride) andInGaN(indium gallium nitride). They can be added to existing red and green LE Ds to produce the impression of white light, though white LEDs today rarely use this principle.The first blue LEDs were made in 1971 by Jacques Pankove (inventor of the gallium nitride LED) atRCA Laboratories.These devices had too little light output to be of much practical use. In August of 1989, Cree Inc. introduced the first commercially available blue LED.In the late 1980s, key breakthroughs in GaNepitaxialgrowth andp-typedoping ushered in the modern era of GaN-based optoelectronic devices. construction upon this foundation, in 1993 high brightness blue LEDs were demonstrated.By the late 1990s, blue LEDs had become widely available. They have an active region consisting of one or more InGaNquantum wellssandwiched between thicker layers of GaN, called cladding layers. By varying the relative InN-GaN split in the InGaN quantum wells, the light emission can be varied from violet to amber. AlGaNaluminium gallium nitrideof 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 350-370nm. 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 oftenAlGaNandAlGaInN, even shorter wavelengths are achievable. Ultraviolet LEDs in a range of wavelengths are becoming available on the market. Near-UV emitters at wavelengths around 375-395nm are already punk and often encountered, for example, asblack lightlamp replacements for inspection of anti-counterfeitingUV watermarks in some documents and paper currencies. Shorter wavelength diodes, while advantageously more expensive, are commercially available for wavelengths dow n to 247nm.As the photosensitivity of microorganisms approximately matches the absorption spectrum ofDNA, with a peak at about 260nm, UV LED emitting at 250-270nm are to be expected in prospective disinfection and sterilization devices. Recent research has shown that commercially available UVA LEDs (365nm) are already effective disinfection and sterilization devices.Deep-UV wavelengths were obtained in laboratories usingaluminium nitride(210nm),boron nitride(215nm)anddiamond(235nm).White lightThere are two primary ways of producing high intensity white-light using LEDs. One is to use individual LEDs that emit threeprimary colorsred, green, and blueand 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 tometamerism, it is possible to have instead different spectra that appear white.RGB systemsCombined spectral curves for blue, yellow-green, and high brightness red solid-state semiconductor LEDs.FWHMspectral bandwidth is approximately 24-27 nm for all three colors.White lightcan be formed by mixing differently colored lights, the most common method is to usered, green and blue(RGB). Hence the