Light Emitting Diode - By Ravinder Puranam

8/9/2019 Light Emitting Diode - By Ravinder Puranam

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Light-emitting diode

Blue, green and red LEDs.A light-emitting diode (LED) is a semiconductor diode thatemits incoherent narrow-spectrum light when electrically biased in the forward directionof the p-n junction. This effect is a form of electroluminescence. An LED is usually a

small area source, often with extra optics added to the chip that shapes its radiationpattern.[1] The color of the emitted light depends on the composition and condition of thesemiconducting material used, and can be infrared, visible, or near-ultraviolet. An LEDcan be used as a regular household light source.

1 History

In the early 20th century, Henry Round of Marconi Labs first noted that a semiconductorjunction could produce light. Russian Oleg Vladimirovich Losev independently createdthe first LED in the mid 1920s; his research, though distributed in Russian, German andBritish scientific journals, was ignored. Rubin Braunstein of the Radio Corporation of

America reported on infrared emission from gallium arsenide (GaAs) and othersemiconductor alloys in 1955. Experimenters at Texas Instruments, Bob Biard and GaryPittman, found in 1961 that gallium arsenide gave off infrared (invisible) light whenelectric current was applied. Biard and Pittman were able to establish the priority of theirwork and received the patent for the infrared light-emitting diode. Nick Holonyak Jr.,then of the General Electric Company and later with the University of Illinois at Urbana-Champaign, developed the first practical visible-spectrum LED in 1962 and is seen as the"father of the light-emitting diode". Holonyak's former graduate student, M. GeorgeCraford, invented in 1972 the first yellow LED and 10x brighter red and red-orangeLEDs. Shuji Nakamura of Nichia of Japan demonstrated the first high-brightness blueLED based on InGaN, borrowing on critical developments in GaN nucleation on sapphire

substrates and the demonstration of p-type doping of GaN which were developed by I.Akasaki and H. Amano in Nagoya. The existence of the blue LED led quickly to the firstwhite LED, which employed a Y3Al5O12:Ce, or "YAG", phosphor coating to mixyellow (down-converted) light with blue to produce light that appears white. Nakamurawas awarded the 2006 Millennium Technology Prize for his invention.

2 LED technology

2.1 Physical function

Like a normal diode, an LED consists of a chip of semiconducting material impregnated,or 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-carrierselectrons and holesflow into the junction from electrodes withdifferent voltages. When an electron meets a hole, it falls into a lower energy level, andreleases energy in the form of a photon.

The wavelength of the light emitted, and therefore its color, depends on the band gapenergy of the materials forming the p-n junction. In silicon or germanium diodes, theelectrons and holes recombine by a non-radiative transition which produces no optical

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emission, because these are indirect band gap materials. The materials used for an LEDhave a direct band gap with energies corresponding to near-infrared, visible or near-ultraviolet light.

LED development began with infrared and red devices made with gallium arsenide.

Advances in materials science have made possible the production of devices with ever-shorter wavelengths, producing light in a variety of colors.

LEDs are usually built on an n-type substrate, with an electrode attached to the p-typelayer deposited on its surface. P-type substrates, while less common, occur as well. Manycommercial LEDs, especially GaN/InGaN, also use sapphire substrate. Substrates that aretransparent to the emitted wavelength, and backed by a reflective layer, increase the LEDefficiency. The refractive index of the package material should match the index of thesemiconductor, otherwise the produced light gets partially reflected back into thesemiconductor, where it may be absorbed and turned into additional heat, thus loweringthe efficiency. This type of reflection also occurs at the surface of the package if the LED

is coupled to a medium with a different refractive index such as a glass fiber or air. Therefractive index of most LED semiconductors is quite high, so in almost all cases theLED is coupled into a much lower-index medium. The large index difference makes thereflection quite substantial (per the Fresnel coefficients), and this is usually one of thedominant causes of LED inefficiency. Often more than half of the emitted light iseflected back at the LED-package and package-air interfaces. The reflection is mostcommonly reduced by using a dome-shaped (half-sphere) package with the diode in thecenter so that the outgoing light rays strike the surface perpendicularly, at which anglethe reflection is minimized. An anti-reflection coating may be added as well. Thepackage may be cheap plastic, which may be colored, but this is only for cosmeticreasons or to improve the contrast ratio; the color of the packaging does not substantiallyaffect the color of the light emitted. Other strategies for reducing the impact of theinterface reflections include designing the LED to reabsorb and reemit the reflected light(called photon recycling) and manipulating the microscopic structure of the surface toreduce the reflectance, either by introducing random roughness or by creatingprogrammed moth eye surface patterns.

Conventional LEDs are made from a variety of inorganic semiconductor materials,producing the following colors:

Aluminium gallium arsenide (AlGaAs) red and infraredAluminium gallium phosphide (AlGaP) greenAluminium gallium indium phosphide (AlGaInP) high-brightness orange-red,orange, yellow, and greenGallium arsenide phosphide (GaAsP) red, orange-red, orange, and yellowGallium phosphide (GaP) red, yellow and greenGallium nitride (GaN) green, pure green (or emerald green), and blue alsowhite (if it has an AlGaN Quantum Barrier)Indium gallium nitride (InGaN) near ultraviolet, bluish-green and blueSilicon carbide (SiC) as substrate blueSilicon (Si) as substrate blue (under development)

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2.3 White LEDs

Blue LEDs can be added to existing red and green LEDs to produce the impression ofwhite light, though white LEDs today rarely use this principle. Most "white" LEDs in production today are based on an InGaN-GaN structure, and emit blue light of

wavelengths between 450 nm and 470 nm blue GaN. These GaN-based, InGaN-active-layer LEDs are covered by a yellowish phosphor coating usually made of cerium-dopedyttrium aluminum garnet (Ce3+:YAG) crystals which have been powdered and bound ina type of viscous adhesive. The LED chip emits blue light, part of which is efficientlyconverted to a broad spectrum centered at about 580 nm (yellow) by the Ce3+:YAG. Thesingle crystal form of Ce3+:YAG is actually considered a scintillator rather than aphosphor. Since yellow light stimulates the red and green receptors of the eye, theresulting mix of blue and yellow light gives the appearance of white, the resulting shadeoften called "lunar white". This approach was developed by Nichia and was used by themfrom 1996 for manufacturing of white LEDs. The pale yellow emission of theCe3+:YAG can be tuned by substituting the cerium with other rare earth elements such as

terbium and gadolinium and can even be further adjusted by substituting some or all ofthe aluminum in the YAG with gallium. Due to the spectral characteristics of the diode,the red and green colors of objects in its blue yellow light are not as vivid as in broad-spectrum light. Manufacturing variations and varying thicknesses in the phosphor makethe LEDs produce light with different color temperatures, from warm yellowish to coldbluish; the LEDs have to be sorted during manufacture by their actual characteristics.Philips Lumileds patented conformal coating process addresses the issue of varyingphosphor thickness, giving the white LEDs a more consistent spectrum of white light.Spectrum of a "white" LED clearly showing blue light which is directly emitted by theGaN-based LED (peak at about 465 nanometers) and the more broadband stokes shifted

light emitted by the Ce3+:YAG phosphor which extends from around 500 to 700nanometers.White LEDs can also be made by coating near ultraviolet (NUV) emittingLEDs with a mixture of high efficiency europium-based red and blue emitting phosphorsplus green emitting copper and aluminum doped zinc sulfide (ZnS:Cu, Al). This is amethod analogous to the way fluorescent lamps work. However the ultraviolet lightcauses photodegradation to the epoxy resin and many other materials used in LEDpackaging, causing manufacturing challenges and shorter lifetimes. This method is lessefficient than the blue LED with YAG:Ce phosphor, as the Stokes shift is larger and moreenergy is therefore converted to heat, but yields light with better spectral characteristics,which render color better. Due to the higher radiative output of the ultraviolet LEDs thanof the blue ones, both approaches offer comparable brightness. The newest method used

to produce white light LEDs uses no phosphors at all and is based on homoepitaxiallygrown zinc selenide (ZnSe) on a ZnSe substrate which simultaneously emits blue lightfrom its active region and yellow light from the substrate. A new technique developed byMichael Bowers, a graduate student at Vanderbilt University in Nashville, involvescoating a blue LED with quantum dots that glow white in response to the blue light fromthe LED. This technique produces a warm, yellowish-white light similar to that producedby incandescent bulbs.

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2.4 Organic light-emitting diodes (OLEDs)

Combined spectral curves for blue, yellow-green, and high brightness red solid-statesemiconductor LEDs. FWHM spectral bandwidth is approximately 2427 nanometres for

all three colors.If the emitting layer material of an LED is an organic compound, it isknown as an Organic Light Emitting Diode (OLED). To function as a semiconductor, theorganic emitting material must have conjugated pi bonds. The emitting material can be asmall organic molecule in a crystalline phase, or a polymer. Polymer materials can beflexible; such LEDs are known as PLEDs or FLEDs. Compared with regular LEDs,OLEDs are lighter, and polymer LEDs can have the added benefit of being flexible.Some possible future applications of OLEDs could be:

Inexpensive, flexible displaysLight sourcesWall decorationsLuminous cloth

OLEDs have been used to produce visual displays for portable electronic devices such ascellphones, digital cameras, and MP3 players. Larger displays have been demonstrated,but their life expectancy is still far too short (


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It should be noted that high-power ( 1 Watt) LEDs are necessary for practical generallighting applications. Typical operating currents for these devices begin at 350 mA. Thehighest efficiency high-power white LED is claimed by Philips Lumileds Lighting Co.with a luminous efficacy of 115 lm/W (350 mA).Today, OLEDs operate at substantially

lower efficiency than inorganic (crystalline) LEDs. The best luminous efficacy of anOLED so far is about 10% of the theoretical maximum of 683, or about 68 lm/W. Theseclaim to be much cheaper to fabricate than inorganic LEDs, and large arrays of them canbe deposited on a screen using simple printing methods to create a color graphicaldisplay.

2.6 Failure modes

The most common way for LEDs (and diode lasers) to fail is the gradual lowering of lightoutput and loss of efficiency. However, sudden failures can occur as well. Themechanism of degradation of the active region, where the radiative recombination occurs,

involves nucleation and growth of dislocations; this requires a presence of an existingdefect in the crystal and is accelerated by heat, high current density, and emitted light.Gallium arsenide and aluminum gallium arsenide are more susceptible to this mechanismthan gallium arsenide phosphide and indium phosphide. Due to different properties of theactive regions, gallium nitride and indium gallium nitride are virtually insensitive tothis kind of defect; however, high current density can cause electromigration of atoms outof the active regions, leading to emergence of dislocations and point defects, acting asnonradiative recombination centers and producing heat instead of light. Ionizing radiationcan lead to the creation of such defects as well, which leads to issues with radiationhardening of circuits containing LEDs (e.g., in optoisolators). Early red LEDs werenotable for their short lifetime.White LEDs often use one or more phosphors. The

phosphors tend to degrade with heat and age, losing efficiency and causing changes in theproduced light color.

Pink LEDs often use an organic phosphor formulation which may degrade after just a fewhours of operation causing a major shift in output color.High electrical currents atelevated temperatures can cause diffusion of metal atoms from the electrodes into theactive region. Some materials, notably indium tin oxide and silver, are subject toelectromigration. In some cases, especially with GaN/InGaN diodes, a barrier metal layeris used to hinder the electromigration effects. Mechanical stresses, high currents, andcorrosive environment can lead to formation of whiskers, causing short circuits. High-power LEDs are susceptible to current crowding, nonhomogenous distribution of the

current density over the junction. This may lead to creation of localized hot spots, whichposes risk of thermal runaway. Nonhomogenities in the substrate, causing localized lossof thermal conductivity, aggravate the situation; most common ones are voids caused byincomplete soldering, or by electromigration effects and Kirkendall voiding. Thermalrunaway is a common cause of LED failures.

Laser diodes may be subject to catastrophic optical damage, when the light outputexceeds a critical level and causes melting of the facet. Some materials of the plastic

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package tend to yellow when subjected to heat, causing partial absorption (and thereforeloss of efficiency) of the affected wavelengths. Sudden failures are most often caused bythermal stresses. When the epoxy resin used in packaging reaches its glass transitiontemperature, it starts rapidly expanding, causing mechanical stresses on thesemiconductor and the bonded contact, weakening it or even tearing it off. Conversely,

very low temperatures can cause cracking of the packaging.

Electrostatic discharge (ESD) may cause immediate failure of the semiconductorjunction, a permanent shift of its parameters, or latent damage causing increased rate ofdegradation. LEDs and lasers grown on sapphire substrate are more susceptible to ESDdamage.

3 Considerations in use

Close-up of a typical LED in its case, showing the internal structure. Unlike incandescentlight bulbs, which light up regardless of the electrical polarity, LEDs will only light with

positive electrical polarity. When the voltage across the p-n junction is in the correctdirection, a significant current flows and the device is said to be forward-biased. If thevoltage is of the wrong polarity, the device is said to be reverse biased, very little currentflows, and no light is emitted. Some LEDs can be operated on an alternating currentvoltage, but they will only light with positive voltage, causing the LED to turn on and offat the frequency of the AC supply.While the only 100% accurate way to determine thepolarity of an LED is to examine its datasheet, these methods are usually reliable:

sign:+polarity:positivenegativeterminal:anode (A)cathode (K)leads:longshort

exterior:roundflatinterior:smalllargewiring:redblack

Less reliable methods of determining polarity are:sign:+marking:nonestripepin:12PCB:roundsquare

Because the voltage versus current characteristics of an LED are much like any diode

(that is, current approximately an exponential function of voltage), a small voltagechange results in a huge change in current. Added to deviations in the process this meansthat a voltage source may barely make one LED light while taking another of the sametype beyond its maximum ratings and potentially destroying it. Since the voltage islogarithmically related to the current it can be considered to remain largely constant overthe LEDs operating range. Thus the power can be considered to be almost proportional tothe current. In order to keep power nearly constant with variations in supply and LEDcharacteristics, the power supply should be a "current source", that is, it should supply an

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almost constant current. If high efficiency is not required (e.g., in most indicatorapplications), an approximation to a current source made by connecting the LED in serieswith a current limiting resistor to a constant voltage source is generally used. Most LEDshave low reverse breakdown voltage ratings, so they will also be damaged by an appliedreverse voltage of more than a few volts. Since some manufacturers don't follow the

indicator standards above, if possible the data sheet should be consulted before hookingup an LED, or the LED may be tested in series with a resistor on a sufficiently lowvoltage supply to avoid the reverse breakdown. If it is desired to drive an LED directlyfrom an AC supply of more than the reverse breakdown voltage then it may be protectedby placing a diode (or another LED) in inverse parallel.

LEDs can be purchased with built in series resistors. These can save PCB space and areespecially useful when building prototypes or populating a PCB in a way other than itsdesigners intended. However the resistor value is set at the time of manufacture,removing one of the key methods of setting the LED's intensity. To increase efficiency(or to allow intensity control without the complexity of a DAC), the power may be

applied periodically or intermittently; so long as the flicker rate is greater than the humanflicker fusion threshold, the LED will appear to be continuously lit. Provided there issufficient voltage available, multiple LEDs can be connected in series with a singlecurrent limiting resistor. Parallel operation is generally problematic. The LEDs have to beof the same type in order to have a similar forward voltage. Even then, variations in themanufacturing process can make the odds of satisfactory operation low.

Bicolor LED units contain two diodes, one in each direction (that is, two diodes ininverse parallel) and each a different color (typically red and green), allowing two-coloroperation or a range of apparent colors to be created by altering the percentage of timethe voltage is in each polarity. Other LED units contain two or more diodes (of differentcolors) arranged in either a common anode or common cathode configuration. These canbe driven to different colors without reversing the polarity. LEDs are usually constantlyilluminated when a current passes through them, but flashing LEDs are also available.Flashing LEDs resemble standard LEDs but they contain an integrated multivibratorcircuit inside which causes the LED to flash with a typical period of one second. Thistype of LED comes most commonly as red, yellow, or green. Most flashing LEDs emitlight of a single wavelength, but multicolored flashing LEDs are available too. Generally,for newer common standard LEDs in 3 mm or 5 mm packages, the following forward DCpotential differences are typically measured. The forward potential difference dependingon the LED's chemistry, temperature, and on the current (values here are for approx. 20milliamperes, a commonly found maximum value).

ColorPotential DifferenceInfrared1.6 VRed1.8 V to 2.1 VOrange2.2 VYellow2.4 VGreen2.6 VBlue3.0 V to 3.5 VWhite3.0 V to 3.5 V

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Ultraviolet3.5 V

Many LEDs are rated at 5 V maximum reverse voltage.

3.1 Advantages of using LEDs

This section does not cite any references or sources.Please improve this section by adding citations to reliable sources.Unverifiable material may be challenged and removed. (tagged sinceNovember 2006)

LED schematic symbol

LEDs produce more light per watt than do incandescent bulbs; this is useful in batterypowered or energy-saving devices.

LEDs can emit light of an intended color without the use of color filters that traditional

lighting methods require. This is more efficient and can lower initial costs. The solid package of an LED can be designed to focus its light. Incandescent and fluorescentsources often require an external reflector to collect light and direct it in a usablemanner. When used in applications where dimming is required,

LEDs do not change their color tint as the current passing through them is lowered,unlike incandescent lamps, which turn yellow.

LEDs are ideal for use in applications that are subject to frequent on-off cycling, unlikefluorescent lamps that burn out more quickly when cycled frequently, or HID lamps thatrequire a long time before restarting.

LEDs, being solid state components, are difficult to damage with external shock.Fluorescent and incandescent bulbs are easily broken if dropped on the ground.

LEDs have an extremely long life span. One manufacturer has calculated the ETTF(Estimated Time To Failure) for their LEDs to be between 100,000 and 1,000,000 hours.[17] Fluorescent tubes typically are rated at about 30,000 hours, and incandescent lightbulbs at 1,000-2,000 hours.

LEDs mostly fail by dimming over time, rather than the abrupt burn-out of incandescentbulbs.

LEDs light up very quickly. A typical red indicator LED will achieve full brightness inmicroseconds; LEDs used in communications devices can have even faster responsetimes.

LEDs can be very small and are easily populated onto printed circuit boards.

LEDs do not contain mercury, while compact fluorescent lamps do.

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LEDs are produced in an array of shapes and sizes. The 5 mm cylindrical package (red,fifth from the left) is the most common, estimated at 80% of world production. The colorof 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 haveclear housings. There are also LEDs in extremely tiny packages, such as those found on

blinkies

3.2 Disadvantages of using LEDs

LEDs are currently more expensive, price per lumen, on an initial capital cost basis, thanmore conventional lighting technologies. The additional expense partially stems from therelatively low lumen output and the drive circuitry and power supplies needed. However,when considering the total cost of ownership (including energy and maintenance costs),LEDs far surpass incandescent or halogen sources and begin to threaten compactfluorescent lamps.

LED performance largely depends on the ambient temperature of the operatingenvironment. Driving an LED hard in high ambient temperatures may result inoverheating of the LED package, eventually leading to device failure. Adequate heat-sinking is required to maintain long life. This is especially important when consideringautomotive, medical, and military applications where the device must operate over a largerange of temperatures, and are required to have a low failure rate.

LEDs must be supplied with the correct current. This can involve shunt resistors orregulated power supplies.

LEDs typically cast light in one direction at a narrow angle compared to an incandescentor fluorescent lamp of the same lumen level. The spectrum of some white LEDs differssignificantly from a black body radiator, such as the sun or an incandescent light. Thespike at 460 nm and dip at 500 nm can cause the color of objects to be perceiveddifferently under LED illumination than other light sources.

LEDs cannot be used in applications that need a sharply directive and collimated beam oflight. LEDs are not capable of providing directivity below a few degrees. In such casesLASERs (or LED lasers) may be a better option. There is increasing concern that blueLEDs and white LEDs are now capable of exceeding safe limits of the so-called blue-light hazard as defined in eye safety specifications such as ANSI/IESNA RP-27.1-05:

Recommended Practice for Photobiological Safety for Lamp and Lamp Systems.[19]

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4 LED applications

LED panel light source used in an experiment on plant growth. The findings of such

experiments may be used to grow food in space on long duration missions. Light sourcesfor machine vision systems.

Old calculator LED display.

Flashlights and lanterns that utilise white LEDs are becoming increasingly popular due totheir durability and longer battery life. Single high-brightness LED with a glass lenscreates a bright carrier beam that can stream DVD-quality video over considerabledistances. The device, RONJA, can be built very simply by enthusiasts.

4.1 List of LED applications

LED lights on an Audit List of LED applications

Some of these applications are further elaborated upon in the following text.Architectural lightingStatus indicators on all sorts of equipmentTraffic lights and signalsLight source for machine vision systems, requiring bright, focused,homogeneous and possibly strobed illumination.Exit signsMotorcycle and Bicycle lights

Toys and recreational sporting goods, such as the FlashflightRailroad crossing signalsContinuity indicatorsFlashlights, including some mechanically powered models.Emergency vehicle lightingElevator Push Button LightingThin, lightweight message displays at airports and railway stations and asdestination displays for trains, buses, trams and ferries.Red or yellow LEDs are used in indicator and alphanumeric displays inenvironments where night vision must be retained: aircraft cockpits, submarineand ship bridges, astronomy observatories, and in the field, e.g. night time

animal watching and military field use.Red, yellow, green, and blue LEDs can be used for model railroadingapplicationsRemote controls, such as for TVs and VCRs, often use infrared LEDs.In optical fiber and Free Space Optics communications.In dot matrix arrangements for displaying messages.Glowlights, as a more expensive but longer lasting and reusable alternative toGlowsticks.



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Grow lights composed of LEDs are more efficient, both because LEDs producemore lumens per watt than other alternatives, and also because they can betuned to the specific wavelengths plants can make the most use of.Movement sensors, for example in optical computer miceBecause of their long life and fast switching times, LEDs have been used for

automotive high-mounted brake lights and truck and bus brake lights and turnsignals for some time, but many high-end vehicles are now starting to use LEDsfor their entire rear light clusters. Besides the gain in reliability, thishas styling advantages because LEDs are capable of forming much thinner lightsthan incandescent lamps with parabolic reflectors. The significant improvementin the time taken to light up (perhaps 0.5s faster than an incandescent bulb)improves safety by giving drivers more time to react. It has been reportedthat at normal highway speeds this equals one car length increased reactiontime for the car behind.Backlighting for LCD televisions and displays. The availability of LEDs inspecific colors (RGB) enables a full-spectrum light source which expands the

color gamut by as much as 45%.New stage lighting equipment is being developed with LED sources in primaryred-green-blue arrangements.Lumalive, a photonic textileLED-based Christmas lights have been available since 2002, but are only nowbeginning to gain in popularity and acceptance due to their higher initialpurchase cost when compared to similar incandescent-based Christmas lights.For example, as of 2006, a set of 50 incandescent lights might cost US$2,while a similar set of 50 LED lights might cost US$10. The purchase cost canbe even higher for single-color sets of LED lights with rare orrecently-introduced colors, such as purple, pink or white. Regardless of thehigher initial purchase price, the total cost of ownership for LED Christmaslights would eventually be lower than the TCO for similar incandescentChristmas lights [citation needed] since an LED requires much less power tooutput the same amount of light as a similar incandescent bulb.LED phototherapy for acne using blue or red LEDs has been proven tosignificantly reduce acne over a 3 month period.[citation needed]As a medium quality voltage reference in electronic circuits. The forwardvoltage drop (e.g., about 1.7 V for a normal red LED) can be used instead of aZener diode in low-voltage regulators. Although LED forward voltage is muchmore current-dependent than a good Zener, Zener diodes are not available belowvoltages of about 3 V.Computers, for hard drive activity and power on. Some custom computers featureLED accent lighting to draw attention to a given component. Many computermanufactuers use LEDs to tell the user its current state. One example would bethe Mac, which tells its user when it is asleep by fading the LED activitylights in and out, in and out.Light bulbsLanterns



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4.2 Optoisolators and optocouplers

Optocoupler schematic showing LED and phototransistorAn LED may be combined witha photodiode or phototransistor in a single electronic device to provide a signal path with

electrical isolation between two circuits. An optoisolator will have typical breakdownvoltages between the input and output circuits of typically 500 to 3000 volts. This isespecially useful in medical equipment where the signals from a low voltage sensorcircuit (usually battery powered) in contact with a living organism must be electricalyisolated from any possible electrical failure in a recording or montoring device operatingat potentially dangerous voltages. An optoisolator also allows information to betransferred between circuits not sharing a common ground potential. An optocoupler maynot have such high breakdown voltages and may even share a ground between input andoutput, but both types are useful in preventing electrical noise, particularly commonmode electrical noise, on a sensor circuit from being transferred to the receiving circuit(where it may adversely affect the operation or durability of various components) and/or

transferring a noisy signal. Optoisolators are also used in the feedback circuit of a DC toDC converter, allowing power to be transferred while retaining electrical isolationbetween the input and output.

4.3 Light sources for machine vision systems

Machine vision systems often require bright and homogeneous illumination, so featuresof interest are easier to process. LEDs are often used to this purpose, and this field ofapplication is likely to remain one of the major application areas until price drops lowenough to make signalling and illumination applications more widespread. LEDsconstitute a nearly ideal light source for machine vision systems for several main reasons:Size of illuminated field is usually comparatively small and Vision systems or smart

camera are quite expensive, so cost of LEDs is usually a minor concern, compared tosignaling applications.

LED elements tend to be small and can be placed with high density over flat or evenshaped substrates (PCBs etc) so that bright and homogeneous sources can be designedwhich direct light from tightly controlled directions on inspected parts.

LEDs often have or can be used with small, inexpensive lenses and diffusers, helping toachieve high light densities and very good lighting control and homogeneity.

LEDs can be easily strobed (in the microsecond range and below) and synchronized;

their power also has reached high enough levels that sufficiently high intensity can beobtained, allowing well lit images even with very short light pulses: this is often used inorder to obtain crisp and sharp "still" images of fast moving parts.

LEDs come in several different colors and wavelengths, easily allowing to use the bestcolor for each application, where different color may provide better visibility of featuresof interest. Having a precisely known spectrum allows tightly matched filters to be usedto separate informative bandwidth or to reduce disturbing effect of ambient light.



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LEDs usually operate at comparatively low working temperatures, simplifying heatmanagement and dissipation, therefore allowing plastic lenses, filters and diffusers to beused. Waterproof units can also easily be designed, allowing for use in harsh or wetenvironments (food, beverage, oil industries).

LED sources can be shaped in several main configurations (spot lights for reflectiveillumination; ring lights for coaxial illumination; backlights for contour illumination;linear assemblies; flat, large format panels; dome sources for diffused, omnidirectionalillumination).Very compact designs are possible, allowing for small LED illuminators to be integratedwithin smart cameras and vision sensors.

5 History

5.1 Discovery

The first known report of a light-emitting solid-state diode was made in 1907 by theBritish experimenter H. J. Round. However, no practical use was made of the discoveryfor several decades.[20] Independently, Oleg Vladimirovich Losev published "Luminouscarborundum [silicon carbide] detector and detection with crystals" in the Russian journalTelegrafiya i Telefoniya bez Provodov (Wireless Telegraphy and Telephony).[2] Losev'swork languished for decades.

The first practical LED was invented by Nick Holonyak, Jr., in 1962 while he was atGeneral Electric Company. The first LEDs became commercially available in late 1960s,and were red. They were commonly used as replacements for incandescent indicators,and in seven-segment displays, first in expensive equipment such as laboratory and

electronics test equipment, then later in such appliances as TVs, radios, telephones,calculators, and even watches. These red LEDs were bright enough only for use asindicators, as the light output was not enough to illuminate an area. Later, other colorsbecame widely available and also appeared in appliances and equipment. As the LEDmaterials technology became more advanced, the light output was increased, and LEDsbecame bright enough to be used for illumination. Most LEDs were made in the verycommon 5 mm T1-3/4 and 3 mm T1 packages, but with higher power, it has becomeincreasingly necessary to get rid of the heat, so the packages have become more complexand adapted for heat dissipation.

Packages for state-of-the-art high power LEDs bear little resemblance to early

LEDs (see, for example, Philips Lumileds).



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5.2 LED panels

The 1,500 foot long LED display on the Fremont Street Experience is currently thelargest in the world.There are two types of LED panels: conventional, using discrete

LEDs, and surface mounted device (SMD) panels. Most outdoor screens and some indoorscreens are built around discrete LEDs, also known as individually mounted LEDs. Acluster of red, green, and blue diodes is driven together to form a full-color pixel, usuallysquare in shape. These pixels are spaced evenly apart and are measured from center tocenter for absolute pixel resolution. The largest LED display in the world is over 1,500foot (457.2 m) long and is located in Las Vegas, Nevada covering the Fremont StreetExperience. Most indoor screens on the market are built using SMD technologya trendthat is now extending to the outdoor market. An SMD pixel consists of red, green, andblue diodes mounted on a chipset, which is then mounted on the driver PC board. Theindividual diodes are smaller than a pinhead and are set very close together. Thedifference is that the maximum viewing distance is reduced by 25% from the discrete

diode screen with the same resolution.

LED panels allow for smaller sets of interchangeable LEDs to be one largedisplay.Indoor use generally requires a screen that is based on SMD technology and has aminimum brightness of 600 candelas per square meter (unofficially called nits). This willusually be more than sufficient for corporate and retail applications, but under highambient-brightness conditions, higher brightness may be required for visibility. Fashionand auto shows are two examples of high-brightness stage lighting that may requirehigher LED brightness. Conversely, when a screen may appear in a shot on a televisionshow, the requirement will often be for lower brightness levels with lower colortemperatures (common displays have a white point of 6500 to 9000 K, which is much

bluer than the common lighting on a television production set).A large LED screen in Razorback StadiumFor outdoor use, at least 2,000 nits are requiredfor most situations, whereas higher brightness types of up to 5,000 nits cope even betterwith direct sunlight on the screen. (The brightness of LED panels can be reduced fromthe designed maximum, if required.) Suitable locations for large display panels areidentified by factors such as line of sight, local authority planning requirements (if theinstallation is to become semi-permanent), vehicular access (trucks carrying the screen,truck-mounted screens, or cranes), cable runs for power and video (accounting for bothdistance and health and safety requirements), power, suitability of the ground for thelocation of the screen (if there are no pipes, shallow drains, caves, or tunnels that may not

be able to support heavy loads), and overhead obstructions.

5.2.1 Early LED flat panel TV history

Perhaps the first recorded flat LED television screen prototype to be developed was byJames P. Mitchell in 1977. The modular, scalable display was enabled by MV50 LEDsand newly available TTL (transistor transistor logic) memory addressing circuittechnology. The prototype and paper was displayed at an Engineering Exposition in



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Anaheim May 1978, and organized by the Science Service in Washington D.C. The LEDflat panel TV display received special recognition by NASA, General MotorsCorporation, and area universities including The University of California Irvine, RobertM. Saunders Prof. of Engineering and IEEE President 1977. Additionally, technology business representatives from the U.S. and overseas witnessed operation of the

monochromatic LED flat panel television display. The prototype remains operational. AnLCD (liquid crystal display) matrix design was also presented in the accompanyingscientific paper, as a future television display method using a similar scanning designmethod.The early display prototype was red monochromatic. Low-cost efficient blueLEDs did not emerge until the early 1990s, completing the RGB color triad. High-brightness colors gradually emerged in the 1990s enabling new designs for outdoorsignage and huge video displays for billboards and stadiums.

5.3 Multi-touch sensing

Since LEDs share some basic physical properties with photodiodes, which also use p-n

junctions with band gap energies in the visible light wavelengths, they can also be usedfor photo detection. These properties have been known for some time, but more recentlyso-called bidirectional LED matrices have been proposed as a method of touch-sensing.In 2003, Dietz, Yerazunis, and Leigh published a paper describing the use of LEDs ascheap sensor devices. In this usage, various LEDs in the matrix are quickly switched onand off. LEDs that are on shine light onto a user's fingers or a stylus. LEDs that are offfunction as photodiodes to detect reflected light from the fingers or stylus. The voltagethus induced in the reverse-biased LEDs can then be read by a microprocessor, whichinterprets the voltage peaks and then also uses them elsewhere.

5.4 Electronics Portal

Photometry (optics) Main Photometry/Radiometry article explains technical termsLED lamp solid state lighting (SSL)BlinkiesThrowiesLED circuitNixie tubeLight Up the World FoundationLumalive, a photonic textile


Light Emitting Diode - By Ravinder Puranam

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