History, Development and Uses for Light Emitting Diodes

Introduction

A light-emitting diode (LED) is a semiconductor light source. LEDs are used as indicator lamps in many devices and are increasingly used for other lighting. Introduced as a practical electronic component in 1962, early LEDs emitted low-intensity red light, but modern versions are available across thevisible, 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 colorof the light (corresponding to the energy of the photon) is determined by the energy gap of the semiconductor. An LED is often small in area (less than 1mm2), and integrated optical components may be used to shape its radiation pattern.LEDs present many advantages over incandescent light sources including lower energy consumption, longer lifetime, improved robustness, smaller size, faster switching, and greater durability and reliability. 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 replacements for aviation lighting, automotive lighting (particularly brake lamps, turn signals and indicators) as well as intraffic signals. The compact size, the possibility of narrow bandwidth, switching speed, and extreme reliability of LEDs has allowed new text and 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.

History

Discoveries and early devices

Green electroluminescence from a point contact on a crystal of SiCre creates H. J. Round’s original experiment from 1907.

Electroluminescence as a phenomenon was discovered in 1907 by the British experimenter H. J. Round of Marconi Labs, using a crystal of silicon carbide and a cat’s-whisker detector. Russian Oleg Vladimirovich Losev reported on the creation of a first LED in 1927. His research was distributed in Russian, German and British scientific journals, but no practical use was made of the discovery for several decades. Rubin Braunstein of the Radio Corporation of America reported on infrared emission from gallium arsenide (GaAs) and other semiconductor alloys in 1955. 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, found that GaAs emitted infrared radiation when electric current was applied and received the patent for the infrared LED.

The first practical visible-spectrum (red) LED was developed in 1962 by Nick Holonyak Jr., while working at General 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 brightness of red and red-orange LEDs by a factor of ten in 1972. 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.

Until 1968, visible and infrared LEDs were extremely costly, on the order of US $200 per unit, and so had little practical use.The Monsanto Company was the first organization to mass-produce 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 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 fewer 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. 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.

History Of LEDs and LED Technology

Light Emitting Diode (LED)

Light Emitting Diode (LED) is essentially a PN junction semiconductor diode that emits a monochromatic (single color) light when operated in a forward biased direction. The basic structure of an LED consists of the die or light emitting semiconductor material, a lead frame where the die is actually placed, and the encapsulation epoxy which surrounds and protects the die (Figure 1).

The first commercially usable LEDs were developed in the 1960’s 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 approximately 1-10mcd @ 20mA, they still found use in a variety of applications, primarily as indicators. Following GaAsP, GaP, or gallium phosphide, red LEDs were developed. These devices were found to exhibit very high quantum efficiencies, however, they played only a minor role in the growth of new applications for LEDs. This was due to two reasons: First, the 700nm wavelength emission is in a spectral region where the sensitivity level of the human eye is very low (Figure 2) and therefore, it does not “appear” to be very bright even though the efficiency is high (the human eye is most responsive to yellow-green light). Second, this high efficiency is only achieved at low currents. As the current increases, the efficiency decreases. This proves to be a disadvantage to users such as outdoor message sign manufacturers who typically multiplex their LEDs at high currents to achieve brightness levels similar to that of DC continuous operation. As a result, GaP red LEDs are currently used in only a limited number of applications.

As LED technology progressed through the 1970’s, additional colors and wavelengths became available. The most common materials were GaP green and red, GaAsP orange or high efficiency red and GaAsP yellow, all of which are still used today (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 incandescent, neon etc., the failure rate of early devices was much higher than current technology now achieves. This was due in part to the actual component assembly that was primarily manual in nature. Individual operators performed such tasks as dispensing epoxy, placing the die into position, and mixing epoxy all by hand. This resulted in defects such as “epoxy slop” which caused VF (forward voltage) and VR (reverse voltage) leakage or even shorting of the PN junction. In addition, the growth methods and materials 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 Arsenide

It wasn’t until the 1980’s 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 previously availableLEDs. The brightness was over 10 times greater than standardLEDsdue to increased efficiency and multi-layer, heterojunction type 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 significant drawbacks to GaAlAs material. First, it was only available in a red 660nm wavelength. Second, the light output degradation of GaAlAs is greater than that of standard technology. It has long been a misconception withLEDsthat light output will decrease by 50% after 100,000 hours of operation. In fact, some GaAlAsLEDsmay decrease by 50% after only 50,000 -70,000 hours of operation. This is especially true in high temperature and/or high humidity environments. Also during this time, yellow, green and orange saw only a minor improvement 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 difficult issues new technology was needed.LEDdesigners turned to laser diode technology for solutions. In parallel with the rapid developments inLEDtechnology, laser diode technology had also been making progress. In the late 1980’s laser diodes with output 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 produce 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 significantly improved even at elevated temperature and humidity.

Current Developments of LED Technology

In GaAlP LEDs took a further leap in brightness with a new development by Toshiba, a leading manufacturer of LEDs. 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 active layer to the substrate back as useful light output (Figure 4). This allowed for an almost two-fold increase in the LED luminance over conventional devices. LED performance was further improved by introducing a current blocking layer into the LED structure (Figure 5). This blocking layer essentially channels the current through the device to achieve better device efficiency.

As a result of these developments, much of the growth for LEDs in the 1990’s will be concentrated in three main areas: The first is in traffic control devices such as stop lights, pedestrian signals, barricade lights and road hazard signs. The second is in variable message signs such as the one located in Times Square New York which displays commodities, news and other information. The third concentration would be in automotive applications.

The visible LED has come a long way since its introduction almost 30 years ago and has yet to show any signs of slowing down. A Blue LED, which has only recently become available in production quantities, will result in an entire generation of new applications. Blue LEDs because of their high photon energies (>2.5eV) and relatively low eye sensitivity have always been difficult to manufacture. In addition the technology necessary to fabricate theseLEDsis very different and far less advanced than standard LED materials. The blueLEDsavailable today consist of GaN (gallium nitride) and SiC (silicon carbide) construction with brightness levels in excess of 1000mcd @ 20mA for GaN devices. Since blue is one of the primary colors, (the other two being red and green), full color solid stateLEDsigns, TV’s etc. will soon become commercially available. Full color LED signs have already been manufactured on a small prototype basis, however, due to the high price of blueLEDs, it is still not practical on a large scale. Other applications for blue LEDs include medical diagnostic equipment and photolithography.

LED Colors

It is also possible to produce other colors using the same basic GaN technology and growth processes. For example, a high brightness green (approximately 500nm)LEDhas been developed that is currently being evaluated for use as a replacement to the green bulb in traffic lights. Other colors including purple and white are also possible. With the recent introduction of blue LEDs, it is now possible to produce white 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 blue LED.

In summary, LEDs have gone from infancy to adolescence and are experiencing some of the most rapid market growth of their lifetime. By using InGaAlP material with MOCVD as the growth process, combined with efficient delivery of generated light and efficient use of injected current, some of the brightest, most efficient and most reliable LEDs are now available. This technology together with other novel LED structures will ensure wide application of LEDs. 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 use

The first commercial LEDs were commonly used as replacements forincandescentandneonindicator lamps, and inseven-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 (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 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 high power white light LED led to use for illumination, which is fast replacing incandescent and fluorescent lighting. (see list ofillumination applications). Most LEDs were made in the very common 5mm T1¾ and 3mm T1 packages, but with rising power output, it has grown increasingly necessary to shed excess heat to maintain reliability,so more complex packages have been adapted for efficient heat dissipation. Packages for state-of-the-arthigh power LEDsbear little resemblance to early LEDs.

Continuing development

The first high-brightness blue LED was demonstrated byShuji NakamuraofNichia Corporationand was based 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 aY3Al5O12:Ce, 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 toMoore’s law. The advances are generally attributed to the parallel development of other semiconductor technologies and advances in optics and material science. This trend is normally calledHaitz’s Lawafter Dr. Roland Haitz.

In February 2008, 300lumensof visible light per wattluminous efficacy(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 instead of two-inch sapphire wafers.

Illustration of Haitz’s Law. Light output per LED as a function of production year, note the logarithmic scale on the vertical axis

Technology

Physics

The LED consists of a chip of semiconducting 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-carriers—electronsandholes—flow 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 thus 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 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 usesapphiresubstrate.

Most materials used for LED production have very highrefractive 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.

The inner workings of an LED I-V diagram for adiode. An LED will begin to emit light when the on-voltageis exceeded. Typical on voltages are 2-3volts.

Refractive Index

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.

The light emission cones of a real 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 effectively a separate set of emission cones for every 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 larger side cones are clipped to show the interior features and reduce image 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 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 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 semiconductor’s 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 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. 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 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 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.

Transition coatings

Many LED semiconductor chips arepottedin clear or colored molded plastic shells. The plastic shell has three purposes:

1. Mounting the semiconductor chip in devices is easier to accomplish.

2. The tiny fragile electrical wiring is physically supported and protected from damage

3. 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 operational parameters

Typical 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 efficacy,[dubious-discuss]as measured by its light output per unit power input. White 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 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, 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. Also,Seoul Semiconductorplans for 135 lm/W by 2007 and 145 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 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 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 failure

Main article: List of LED failure modes

Solid 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 of 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 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.

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.

LED light output actually rises at colder temperatures (leveling off depending on type at around −30C). Consequently, LED technology may be a good 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 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.

Ultraviolet and blue LEDs

BlueLEDs

Blue LEDs are based on the wideband gapsemiconductors GaN (gallium nitride) andInGaN(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 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. Building 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 fraction 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 cheap and often encountered, for example, asblack lightlamp replacements for inspection of anti-counterfeitingUV watermarks in some documents and paper currencies. Shorter wavelength diodes, while substantially more expensive, are commercially available for wavelengths down 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 light

There are two primary ways of producing high intensity white-light using LEDs. One is to use individual LEDs that emit threeprimary colors—red, green, and blue—and then mix all the colors to form white light. The other is to use a phosphor material to convert monochromatic light from a blue or UV LED to broad-spectrum white light, much in the same way a fluorescent light bulb works.

Due tometamerism, it is possible to have quite different spectra that appear white.

RGB systems

Combined 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 method is called multi-colored white LED's (sometimes referred to as Red Green Blue LED's). Because these need electronic circuits to control the blending anddiffusionof different colors, 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,[60]and, in principle, this mechanism also has higher quantum efficiency in producing white light.

There are several types of multi-colored white LEDs:di-,tri-, andtetrachromaticwhite LEDs. Several key factors that play among these different methods, include color stability,color renderingcapability, andluminous efficacy. Often higher efficiency will mean lower color rendering, presenting a tradeoff 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. Conversely, althoughtetrachromaticwhite 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.

Multi-color LEDs offer not merely another means to form white light, but a new means to form light of different colors. Mostperceivable colorscan 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 which 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 powerdecays exponentiallywith rising temperature,[61]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

Spectrum of a “white” LED clearly showing blue light which is directly emitted by the GaN-based LED (peak at about 465 nm) and the more broadbandStokes-shiftedlight emitted by the Ce3+:YAG phosphor which emits at roughly 500-700 nm.

This method involvescoatingan LED of one color (mostly blue LED made of InGaN) withphosphorof different colors to form white light; the resultant LEDs are calledphosphor-based white LEDs.A fraction of the blue light undergoes theStokes shiftbeing transformed from shorter wavelengths to longer. Depending on the color of the original LED, phosphors of different colors can be employed. If several phosphor layers of distinct colors are applied, the emitted spectrum is broadened, effectively raising thecolor rendering index(CRI) value of a given LED.

Phosphor based LEDs have a lower efficiency than normal LEDs due to the heat loss from the Stokes shift and also other phosphor-related degradation issues. However, the phosphor method is still the most popular method for makinghigh intensitywhite 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 complexRGBsystem, and the majority of high intensity white LEDs presently on the market are manufactured using phosphor light conversion.

The greatest barrier to high efficiency is the seemingly unavoidable Stokes energy loss. However, 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. Philips Lumileds' patented conformal coating process addresses the issue of varying phosphor thickness, giving the white LEDs a more homogeneous white light.[64]With development ongoing, the efficiency of phosphor based LEDs generally rises with each new product announcement.

The phosphor based white LEDs encapsulate InGaN blue LEDs inside phosphor coated epoxy. A common yellow phosphor material is cerium-dopedyttrium aluminium garnet(Ce3+ YAG).

White LEDs can also be made bycoatingnearultraviolet(NUV) emitting LEDs with a mixture of high efficiencyeuropium-based red and blue emitting phosphors plus green emitting copper and aluminium doped zinc sulfide (ZnS:Cu, Al). This is a method analogous to the wayfluorescent lampswork. This method is less efficient than the blue LED with YAG:Ce phosphor, as theStokes shiftis larger, so more energy is converted to heat, but yields light with better spectral characteristics, which render color better. Due to the higher radiative output of the ultraviolet LEDs than of the blue ones, both methods offer comparable brightness. A concern is that UV light may leak from a malfunctioning light source and cause harm to human eyes or skin.

Other white LEDs

Another method used to produce experimental white light LEDs used no phosphors at all and was based onhomoepitaxiallygrownzinc selenide(ZnSe) on a ZnSe substrate which simultaneously emitted blue light from its active region and yellow light from the substrate.

Organic light-emitting diodes (OLEDs)

Organic light-emitting diode

Demonstration of a flexible OLED device

In an organic light emitting diode (OLED), theelectroluminescentmaterial comprising the emissive layer of the diode is anorganic compound. The organic material is electrically conductive due to thedelocalizationof pi electrons caused byconjugationover all or part of the molecule, and the material therefore functions as anorganic semiconductor. The organic materials can be small organicmoleculesin acrystallinephase, orpolymers.

The potential advantages of OLEDs include thin, low cost displays with a low driving voltage, wide viewing angle and high contrast and colour gamut. Polymer LEDs have the added benefit of printableandflexibledisplays. OLEDs have been used to make visual displays for portable electronic devices such as cellphones, digital cameras, and MP3 players while possible future uses include lighting and televisions.

Quantum dot LEDs (experimental)

A new method developed by Michael Bowers, a graduate student atVanderbilt Universityin Nashville, involves coating a blue LED withquantum dotsthat glow white in response to the blue light from the LED. This method emits a warm, yellowish-white light similar to that made byincandescent bulbs.

Quantum dots (QD) aresemiconductornanocrystals that possess unique optical properties. Their emission color can be tuned from the visible throughout the infrared spectrum. This allows quantum dot LEDs to create almost any color on theCIEdiagram. This provides more color options and better color rendering than white LEDs. Quantum dot LEDs are available in the same package types as traditionalphosphorbased LEDs.

In September 2009NanocoGroup announced that it has signed a joint development agreement with a major Japanese electronics company under which it will design and develop quantum dots for use in light emitting diodes (LEDs) in liquid crystal display (LCD) televisions.

The major difficulty in using quantum dots based LEDs is the insufficient stability of QDs under prolonged irradiation. In February 2011 scientists at PlasmaChem GmbH could synthesize quantum dots for LED applications and build a light converter on their basis, which could efficiently convert light from blue to any other color for many hundred hours. Such QDs can be used to emit visible or near infrared light of any wavelength being excited by light with a shorter wavelength.

LEDs are produced in a variety of shapes and sizes. The 5mm cylindrical package (red, fifth from the left) is the most common, estimated at 80% of world production. The color of the plastic lens is often the same as the actual color of light emitted, but not always. For instance, purple plastic is often used forinfraredLEDs, and most blue devices have clear housings. There are also LEDs inSMT packages, such as those found onblinkiesand on cell phone keypads (not shown).

The main types of LEDs are miniature, high power devices and custom designs such as alphanumeric or multi-color.

Miniature

Different sized LEDs. 8 mm, 5 mm and 3 mm, with a wooden match-stick for scale.

Main article:Miniature light-emitting diode

These are mostly single-die LEDs used as indicators, and they come in various-sizes from 2mm to 8mm,through-holeandsurface mountpackages. They are usually simple in design, not requiring any separate cooling body.Typical current ratings ranges from around 1 mA to above 20 mA. The small scale sets a natural upper boundary on power consumption due to heat caused by the high current density and need forheat sinking.

A greensurface-mountLED mounted on a circuit board.

Mid-range

Medium power LEDs are often through-hole mounted and used when an output of a few lumen is needed. They sometimes have the diode mounted to four leads (two cathode leads, two anode leads) for better heat conduction and carry an integrated lens. An example of this is the Superflux package, from Philips Lumileds. These LEDs are most commonly used in light panels, emergency lighting and automotive tail-lights. Due to the larger amount of metal in the LED, they are able to handle higher currents (around 100 mA). The higher current allows for the higher light output required for tail-lights and emergency lighting.

High power

See also:Solid-state lightingandLED lamp

High-power light emiting diodes (Luxeon,Lumileds)

High power LEDs (HPLED) can be driven at currents from hundreds of mA to more than an ampere, compared with the tens of mA for other LEDs. Some can emit over a thousand lumens.Since overheating is destructive, the HPLEDs must be mounted on a heat sink to allow for heat dissipation. If the heat from a HPLED is not removed, the device will fail in seconds. One HPLED can often replace an incandescent bulb in atorch, or be set in an array to form a powerfulLED lamp.

Some well-known HPLEDs in this category are the Lumileds Rebel Led, Osram Opto Semiconductors Golden Dragon and Cree X-lamp. As of September 2009 some HPLEDs manufactured byCree Inc.now exceed 105 lm/W(e.g. the XLamp XP-G LED chip emitting Cool White light) and are being sold in lamps intended to replace incandescent, halogen, and even fluorescent lights, as LEDs grow more cost competitive.

LEDs have been developed by Seoul Semiconductor that can operate on AC power without the need for a DC converter. For each half cycle, part of the LED emits light and part is dark, and this is reversed during the next half cycle. The efficacy of this type of HPLED is typically 40 lm/W.A large number of LED elements in series may be able to operate directly from line voltage. In 2009 Seoul Semiconductor released a high DC voltage capable of being driven from AC power with a simple controlling circuit. The low power dissipation of these LEDs affords them more flexibility than the original AC LED design.

Application-specific variations

§ Flashing LEDsare used as attention seeking indicators without requiring external electronics. Flashing LEDs resemble standard LEDs but they contain an integratedmultivibratorcircuit which causes the LED to flash with a typical period of one second. In diffused lens LEDs this is visible as a small black dot. Most flashing LEDs emit light of one color, but more sophisticated devices can flash between multiple colors and even fade through a color sequence using RGB color mixing.

Calculator LED display, 1970s

§ Bi-color LEDsare actually two different LEDs in one case. They consist of two dies connected to the same two leadsantiparallelto each other. Current flow in one direction emits one color, and current in the opposite direction emits the other color. Alternating the two colors with sufficient frequency causes the appearance of a blended third color. For example, a red/green LED operated in this fashion will color blend to emit a yellow appearance.

§ Tri-color LEDsare two LEDs in one case, but the two LEDs are connected to separate leads so that the two LEDs can be controlled independently and lit simultaneously. A three-lead arrangement is typical with one common lead (anode or cathode).

§ RGB LEDscontain red, green and blue emitters, generally using a four-wire connection with one common lead (anode or cathode). These LEDs can have either common positive or common negative leads. Others however, have only two leads (positive and negative) and have a built in tinyelectronic control unit.

§ Alphanumeric LED displaysare available inseven-segmentandstarburstformat. Seven-segment displays handle all numbers and a limited set of letters. Starburst displays can display all letters. Seven-segment LED displays were in widespread use in the 1970s and 1980s, but rising use ofliquid crystal displays, with their lower power needs and greater display flexibility, has reduced the popularity of numeric and alphanumeric LED displays.

Considerations for use

Power sources

Main article: LED power sources

The current/voltage characteristic of an LED is similar to other diodes, in that the current is dependent exponentially on the voltage (seeShockley diode equation). This means that a small change in voltage can cause a large change in current. If the maximum voltage rating is exceeded by a small amount, the current rating may be exceeded by a large amount, potentially damaging or destroying the LED. The typical solution is to useconstant currentpower supplies, or driving the LED at a voltage much below the maximum rating. Since most common power sources (batteries, mains) are not constant current sources, most LED fixtures must include a power converter. However, theI/Vcurve of nitride-based LEDs is quite steep above the knee and gives anIfof a few milliamperes at aVfof 3 V, making it possible to power a nitride-based LED from a 3 V battery such as acoin cellwithout the need for a current limiting resistor.

Electrical polarity

Main article: Electrical polarity of LEDs

As with all diodes, current flows easily from p-type to n-type material.However, no current flows and no light is emitted if a small voltage is applied in the reverse direction. If the reverse voltage grows large enough to exceed thebreakdown voltage, a large current flows and the LED may be damaged. If the reverse current is sufficiently limited to avoid damage, the reverse-conducting LED is a usefulnoise diode.

Safety and health

The vast majority of devices containing LEDs are "safe under all conditions of normal use", and so are classified as "Class 1 LED product"/"LED Klasse 1". At present, only a few LEDs—extremely bright LEDs that also have a tightly focused viewing angle of 8° or less—could, in theory, cause temporary blindness, and so are classified as "Class 2".In general,laser safetyregulations—and the "Class 1", "Class 2", etc. system—also apply to LEDs.

While LEDs have the advantage overfluorescent lampsthat they do not containmercury, they may contain other hazardous metals such asleadandarsenic. A study published in 2011 states: "According to federal standards, LEDs are not hazardous except for low-intensity red LEDs, which leached Pb [lead] at levels exceeding regulatory limits (186 mg/L; regulatory limit: 5). However, according to California regulations, excessive levels of copper (up to 3892 mg/kg; limit: 2500), Pb (up to 8103 mg/kg; limit: 1000),nickel(up to 4797 mg/kg; limit: 2000), orsilver(up to 721 mg/kg; limit: 500) render all except low-intensity yellow LEDs hazardous.".

Advantages

§ Efficiency:LEDs emit more light per watt thanincandescent light bulbs.Their efficiency is not affected by shape and size, unlike fluorescent light bulbs or tubes.

§ Color:LEDs can emit light of an intended color without using any color filters as traditional lighting methods need. This is more efficient and can lower initial costs.

§ Size:LEDs can be very small (smaller than 2mm2) and are easily populated onto printed circuit boards.

§ On/Off time:LEDs light up very quickly. A typical red indicator LED will achieve full brightness in under amicrosecond.LEDs used in communications devices can have even faster response times.

§ Cycling:LEDs are ideal for uses subject to frequent on-off cycling, unlike fluorescent lamps that fail faster when cycled often, orHID lampsthat require a long time before restarting.

§ Dimming:LEDs can very easily bedimmedeither bypulse-width modulationor lowering the forward current.

§ Cool light:In contrast to most light sources, LEDs radiate very little heat in the form ofIRthat can cause damage to sensitive objects or fabrics. Wasted energy is dispersed as heat through the base of the LED.

§ Slow failure:LEDs mostly fail by dimming over time, rather than the abrupt failure of incandescent bulbs.[88]

§ Lifetime:LEDs can have a relatively long useful life. One report estimates 35,000 to 50,000 hours of useful life, though time to complete failure may be longer.[89]Fluorescent tubes typically are rated at about 10,000 to 15,000 hours, depending partly on the conditions of use, and incandescent light bulbs at 1,000-2,000 hours.

§ Shock resistance:LEDs, being solid state components, are difficult to damage with external shock, unlike fluorescent and incandescent bulbs which are fragile.

§ Focus:The solid package of the LED can be designed to focus its light. Incandescent and fluorescent sources often require an external reflector to collect light and direct it in a usable manner.

Disadvantages

§ High initial price:LEDs are currently more expensive, price per lumen, on an initial capital cost basis, than most conventional lighting technologies. The additional expense partially stems from the relatively low lumen output and the drive circuitry and power supplies needed.

§ Temperature dependence:LED performance largely depends on the ambient temperature of the operating environment. Over-driving an LED in high ambient temperatures may result in overheating the LED package, eventually leading to device failure. Adequateheat sinkingis needed to maintain long life. This is especially important in automotive, medical, and military uses where devices must operate over a wide range of temperatures, and need low failure rates.

§ Voltage sensitivity:LEDs must be supplied with the voltage above the threshold and a current below the rating. This can involve series resistors or current-regulated power supplies.[90]

§ Light quality:Most cool-white LEDshave spectra that differ significantly from ablack bodyradiator like the sun or an incandescent light. The spike at 460nm and dip at 500nm can cause the color of objects to beperceived differentlyunder cool-white LED illumination than sunlight or incandescent sources, due tometamerism,red surfaces being rendered particularly badly by typical phosphor based cool-white LEDs. However, the color rendering properties of common fluorescent lamps are often inferior to what is now available in state-of-art white LEDs.

§ Area light source:LEDs do not approximate a “point source” of light, but rather alambertiandistribution. So LEDs are difficult to apply to uses needing a spherical light field. LEDs cannot provide divergence below a few degrees. In contrast, lasers can emit beams with divergences of 0.2 degrees or less.

§ Blue hazard:There is a concern thatblue LEDsand cool-white LEDsare now capable of exceeding safe limits of the so-calledblue-light hazardas defined in eye safety specifications such as ANSI/IESNA RP-27.1-05: Recommended Practice for Photobiological Safety for Lamp and Lamp Systems.

§ Electrical Polarity:Unlikeincandescentlight bulbs, which illuminate regardless of the electricalpolarity, LEDs will only light with correct electrical polarity.

§ Blue pollution:Because cool-white LEDs(i.e., LEDs with highcolor temperature) emit proportionally more blue light than conventional outdoor light sources such as high-pressuresodium vapor lamps, the strong wavelength dependence ofRayleigh scatteringmeans that cool-white LEDs can cause morelight pollutionthan other light sources. TheInternational Dark-Sky Association discourages using white light sources with correlated color temperature above 3,000 K.

§ Droop: Theefficiencyof LEDs tends to decrease as one increasescurrent.

Applications

LED lighting in the aircraft cabin of anAirbus A320 Enhanced.

A large LED display behind adisc jockey.

LEDdestination signson buses, one with a colored route number.

LED digital display that can display 4 digits along with points.

Traffic lightusing LED

Western Australia Policecar using LED

Printhead of an OkiLED printer

LEDdaytime running lightsof Audi A4

LED panel light source used in an experiment onplantgrowth. The findings of such experiments may be used to grow food in space on long duration missions.

LED Illumination.

LED lights reacting dynamically to video feed viaAmBX.

LED uses fall into four major categories:

§ Visual signals where light goes more or less directly from the source to the human eye, to convey a message or meaning.

§ Illuminationwhere light is reflected from objects to give visual response of these objects.

§ Measuring and interacting with processes involving no human vision.[100]

§ Narrow band light sensors where LEDs operate in a reverse-bias mode and respond to incident light, instead of emitting light.

For more than 70 years, until the LED, practically all lighting was incandescent and fluorescent with the first fluorescent light only being commercially available after the1939 World's Fair.

Indicators and signs

Thelow energy consumption, low maintenance and small size of modern LEDs has led to uses as status indicators and displays on a variety of equipment and installations. Large-areaLED displaysare used as stadium displays and as dynamic decorative displays. Thin, lightweight message displays are used at airports and railway stations, and asdestination displaysfor trains, buses, trams, and ferries.

One-color light is well suited fortraffic lightsand signals,exit signs,emergency vehicle lighting, ships' navigation lights orlanterns(chromacity and luminance standards being set under the Convention on the International Regulations for Preventing Collisions at Sea 1972, Annex I and the CIE) andLED-based Christmas lights. In cold climates, LED traffic lights may remain snow covered.[101]Red or yellow LEDs are used in indicator and alphanumeric displays in environments where night vision must be retained: aircraft cockpits, submarine and ship bridges, astronomy observatories, and in the field, e.g. night time animal watching and military field use.

Because of their long life and fast switching times, LEDs have been used in brake lights for carshigh-mounted brake lights, trucks, and buses, and in turn signals for some time, but many vehicles now use LEDs for their rear light clusters. The use in brakes improves safety, due to a great reduction in the time needed to light fully, or faster rise time, up to 0.5 second faster than an incandescent bulb. This gives drivers behind more time to react. It is reported that at normal highway speeds, this equals one car length equivalent in increased time to react. In a dual intensity circuit (i.e., rear markers and brakes) if the LEDs are not pulsed at a fast enough frequency, they can create aphantom array, where ghost images of the LED will appear if the eyes quickly scan across the array. White LED headlamps are starting to be used. Using LEDs has styling advantages because LEDs can form much thinner lights than incandescent lamps withparabolic reflectors.

Due to the relative cheapness of low output LEDs, they are also used in many temporary uses such asglowsticks,throwies, and the photonictextileLumalive. Artists have also used LEDs forLED art.

Weather/all-hazards radioreceivers withSpecific Area Message Encoding(SAME) have three LEDs: red for warnings, orange for watches, and yellow for advisories & statements whenever issued.

Lighting

Main article: LED lamp

With the development of high efficiency and high power LEDs it has become possible to use LEDs in lighting and illumination. Replacementlight bulbshave been made, as well as dedicated fixtures andLED lamps. LEDs are used asstreet lightsand in otherarchitectural lightingwhere color changing is used. The mechanical robustness and long lifetime is used inautomotive lightingon cars, motorcycles and onbicycle lights.

LED street lightsare employed on poles and in parking garages. In 2007, the Italian villageTorracawas the first place to convert its entire illumination system to LEDs.[102]

LEDs are used inaviation lighting.Airbushas used LED lighting in theirAirbus A320 Enhancedsince 2007, and Boeing plans its use in the787. LEDs are also being used now in airport and heliport lighting. LED airport fixtures currently include medium-intensity runway lights, runway centerline lights, taxiway centerline & edge lights, guidance signs and obstruction lighting.

LEDs are also suitable forbacklightingforLCDtelevisions and lightweightlaptopdisplays and light source forDLPprojectors (SeeLED TV). RGB LEDs raise the colorgamutby as much as 45%. Screens for TV and computer displays can be made thinner using LEDs for backlighting.[103]

LEDs are used increasingly in aquarium lights. Particularly for reef aquariums, LED lights provide an efficient light source with less heat output to help maintain optimal aquarium temperatures. LED-based aquarium fixtures also have the advantage of being manually adjustable to emit a specific color-spectrum for ideal coloration of corals, fish, and invertebrates while optimizing photosynthetically active radiation (PAR) which raises growth and sustainability of photosynthetic life such as corals, anemones, clams, and macroalgae. These fixtures can be electronically programmed to simulate various lighting conditions throughout the day, reflecting phases of the sun and moon for a dynamic reef experience. LED fixtures typically cost up to five times as much as similarly rated fluorescent or high-intensity discharge lighting designed for reef aquariums and are not as high output to date.

The lack of IR/heat radiation makes LEDs ideal forstage lightsusing banks of RGB LEDs that can easily change color and decrease heating from traditional stage lighting, as well as medical lighting where IR-radiation can be harmful. In energy conservation, LED's lower heat output also means air conditioning(cooling) systems have less heat to dispose of, reducing carbon emmissions.

LEDs are small, durable and need little power, so they are used in hand held devices such asflashlights. LEDstrobe lightsorcamera flashesoperate at a safe, low voltage, instead of the 250+ volts commonly found inxenonflashlamp-based lighting. This is especially useful in cameras onmobile phones, where space is at a premium and bulky voltage-raising circuitry is undesirable.

LEDs are used for infrared illumination innight visionuses includingsecurity cameras. A ring of LEDs around avideo camera, aimed forward into aretroreflectivebackground, allowschroma keyinginvideo productions.

LED's are now used commonly in all market areas from commercial to home use (standard lighting and AV installations, stage and theatrical, architectural and public spaces, in fact anywhere and everywhere that artificial light is used.

In many countries incandescent lighting for homes and offices is no longer available and building regulations insist on new premises being fitted out at day one with LED fixtures and fittings.

Increasingly the adaptability of colour LED's are finding uses in medical and educational applications such as mood enhancement and new technologies, such asAmBX, for the control of colour LED's have been developed to exploit LED versatility.Nasahas even sponsored research for the use of LED's to promote health for astronauts.[104]

Smart lighting

Light can be used to transmitbroadbanddata, which is already implemented inIrDAstandards using infrared LEDs. Because LEDs cancycle on and offmillions of times per second, they can bewirelesstransmitters andaccess pointsfordatatransport.[105]Laserscan also bemodulatedin this manner.

Sustainable lighting

Efficient lighting is needed forsustainable architecture. A 13 watt LED lamp emits 450 to 650 lumens.[106]which is equivalent to a standard 40 watt incandescent bulb.[107]A standard 40 W incandescent bulb has an expected lifespan of 1,000 hours while an LED can continue to operate with reduced efficiency for more than 50,000 hours, 50 times longer than the incandescent bulb.

Energy consumption

One kilowatt-hour of electricity will cause 1.34pounds (610g) ofCO2emission.[108]Assuming the average light bulb is on for 10 hours a day, one 40-watt incandescent bulb will cause 196pounds (89kg) ofCO2emission per year. The 13-watt LED equivalent will only cause 63pounds (29kg) ofCO2over the same time span. A building's carbon footprint from lighting can be reduced by 68% by exchanging all incandescent bulbs for new LEDs in warm climates. In cold climates, the energy saving may be lower, since more heating is needed to compensate for the lower temperature.

Economically sustainable

LED light bulbs could be a cost-effective option for lighting a home or office space because of their very long lifetimes. Consumer use of LEDs as a replacement for conventional lighting system is currently hampered by the high cost and low efficiency of available products. 2009 DOE testing results showed an average efficacy of 35 lm/W, below that of typicalCFLs, and as low as 9 lm/W, worse than standard incandescents.[106]The high initial cost of the commercial LED bulb is due to the expensivesapphiresubstratewhich is key to the production process. The sapphire apparatus must be coupled with a mirror-like collector to reflect light that would otherwise be wasted.

Non-visual applications

The light from LEDs can be modulated very quickly so they are used extensively inoptical fiberandFree Space Opticscommunications. This includeremote controls, such as for TVs and VCRs, where infrared LEDs are often used.Opto-isolatorsuse an LED combined with aphotodiodeorphototransistorto provide a signal path with electrical isolation between two circuits. This is especially useful in medical equipment where the signals from a low-voltagesensorcircuit (usually battery powered) in contact with a living organism must be electrically isolated from any possible electrical failure in a recording or monitoring device operating at potentially dangerous voltages. An optoisolator also allows information to be transferred between circuits not sharing a common ground potential.

Many sensor systems rely on light as the signal source. LEDs are often ideal as a light source due to the requirements of the sensors. LEDs are used asmovement sensors, for example inoptical computer mice. The NintendoWii's sensor bar uses infrared LEDs. Inpulse oximetersfor measuringoxygen saturation. Some flatbed scanners use arrays of RGB LEDs rather than the typicalcold-cathode fluorescent lampas the light source. Having independent control of three illuminated colors allows the scanner to calibrate itself for more accurate color balance, and there is no need for warm-up. Further, its sensors only need be monochromatic, since at any one time the page being scanned is only lit by one color of light.Touch sensing: Since LEDs can also be used asphotodiodes, they can be used for both photo emission and detection. This could be used in for example a touch-sensing screen that register reflected light from a finger orstylus.[109]

Many materials and biological systems are sensitive to, or dependent on light.Grow lightsuse LEDs to increasephotosynthesisinplants[110]and bacteria and viruses can be removed from water and other substances usingUVLEDs forsterilization.[58]Other uses are asUV curingdevices for some ink and coating methods, and inLED printers.

Plant growers are interested in LEDs because they are more energy efficient, emit less heat (can damage plants close to hot lamps), and can provide the optimum light frequency for plant growth and bloom periods compared to currently used grow lights:HPS(high pressure sodium),MH(metal halide) orCFL/low-energy. However, LEDs have not replaced these grow lights due to higher price. As mass production and LED kits develop, the LED products will become cheaper.

LEDs have also been used as a medium qualityvoltage referencein electronic circuits. The forward voltage drop (e.g., about 1.7 V for a normal red LED) can be used instead of aZener diodein low-voltage regulators. Red LEDs have the flattestI/Vcurve above the knee. Nitride-based LEDs have a fairly steepI/Vcurve and are useless for this purpose. Although LED forward voltage is far more current-dependent than a good Zener, Zener diodes are not widely available below voltages of about 3 V.

Light sources for machine vision systems

Machine visionsystems often require bright and homogeneous illumination, so features of interest are easier to process. LEDs are often used for this purpose, and this is likely to remain one of their major uses until price drops low enough to make signaling and illumination uses more widespread.Barcode scannersare the most common example of machine vision, and many low cost ones use red LEDs instead of lasers. Optical computer mice are also another example of LEDs in machine vision, as it is used to provide an even light source on the surface for the miniature camera within the mouse. LEDs constitute a nearly ideal light source formachine visionsystems for several reasons:

The size of the illuminated field is usually comparatively small and machine vision systems are often quite expensive, so the cost of the light source is usually a minor concern. However, it might not be easy to replace a broken light source placed within complex machinery, and here the long service life of LEDs is a benefit.

LED elements tend to be small and can be placed with high density over flat or even-shaped substrates (PCBs etc.) so that bright and homogeneous sources can be designed which direct light from tightly controlled directions on inspected parts. This can often be obtained with small, low-cost lenses and diffusers, helping to achieve high light densities with control over lighting levels and homogeneity. LED sources can be shaped in several configurations (spot lights for reflective illumination; ring lights for coaxial illumination; back lights for contour illumination; linear assemblies; flat, large format panels; dome sources for diffused, omnidirectional illumination).

LEDs can be easily strobed (in the microsecond range and below) and synchronized with imaging. High-power LEDs are available allowing well lit images even with very short light pulses. This is often used to obtain crisp and sharp “still” images of quickly moving parts.

LEDs come in several different colors and wavelengths, allowing easy use of the best color for each need, where different color may provide better visibility of features of interest. Having a precisely known spectrum allows tightly matched filters to be used to separate informative bandwidth or to reduce disturbing effects of ambient light. LEDs usually operate at comparatively low working temperatures, simplifying heat management and dissipation. This allows using plastic lenses, filters, and diffusers. Waterproof units can also easily be designed, allowing use in harsh or wet environments (food, beverage, oil industries).