LED - Light Emitting Diode

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 applications.

Introduced as a practical electronic component in 1962 ( Nick Holonyak Jr. invented the first practically useful visible in 1962 while working as a consulting scientist at a General Electric Company laboratory and has been called "the father of the light-emitting diode ), early LEDs emitted low-intensity red light, but modern versions are available across the visible spectrum, ultraviolet and infrared wavelengths, with very high brightness.

When a LED is forward biased (switched on), electrons are able to recombine with electron holes within the device, releasing energy in the form of photons. This effect is called electroluminescence and the color of the light (corresponding to the energy of the photon) is determined by the energy gap of the semiconductor. LEDs are often small in area (less than 1 mm²), and integrated optical components may be used to shape its radiation pattern.

LEDs present many advantages over incandescent and fluorescent light sources including lower energy consumption, longer lifetime, improved robustness, smaller size, faster switching, ease of regulate the brightness and good color stability.

LED available

Conventional LEDs are made from a variety of inorganic semiconductor materials, and are available in a lot of wavwlenght and colors. The following list shows the available colors with wavelength range, voltage drop and material used for construction

Infrared LED

Wavelength : 1000-760 nm

Forward Voltage: 1.5-1.9 V

Construction Material : Gallium arsenide (GaAs) / Aluminium gallium arsenide (AlGaAs)

Red LED

Wavelength : 610-760 nm

Forward Voltage: 1.5-2.0 V

Construction Material : Aluminium gallium arsenide (AlGaAs) / Gallium arsenide phosphide (GaAsP) / Aluminium gallium indium phosphide (AlGaInP) /Gallium(III) phosphide (GaP)

Orange LED

Wavelength : 590-610 nm

Forward Voltage : 2.0-2.1 V

Construction Material : Gallium arsenide phosphide (GaAsP) / Aluminium gallium indium phosphide (AlGaInP) / Gallium(III) phosphide (GaP)

Yellow LED

Wavelength : 570-590 nm

Forward Voltage : 2.10-2.20 V

Construction Material : Gallium arsenide phosphide (GaAsP) / Aluminium gallium indium phosphide (AlGaInP) /Gallium(III) phosphide (GaP)

Green LED

Wavelength : 500-570 nm

Forward Voltage : 2.00-4.00 V

Construction Material : Indium gallium nitride (InGaN) / Gallium(III) nitride (GaN) / Gallium(III) phosphide (GaP) / Aluminium gallium indium phosphide (AlGaInP) / Aluminium gallium phosphide (AlGaP)

Blue LED

Wavelength : 450-500 nm

Forward Voltage : 2.50-3.70 V

Construction Material : Zinc selenide (ZnSe) / Indium gallium nitride (InGaN)

Violet LED

Wavelength : 400-450 nm

Forward Voltage : 2.80-4.0 V

Construction Material : Indium gallium nitride (InGaN)

Ultraviolet LED

Wavelength : 350-400 nm

Forvard Voltage : 3.2-4.4 V

Construction Material : --

White LED

Wavelength : not defined

Forward Voltage : 3.2-3.7 V

Construction Material : Blue LED with yellow phosphor

Ultraviolet and blue LEDs

File:Blue light emitting diodes over a proto-board.jpg

Blue LEDs.

Current bright blue LEDs are based on the wide band gap semiconductors GaN (gallium nitride) and InGaN (indium gallium nitride). They can be added to existing red and green LEDs to produce the impression of white light, though white LEDs today rarely use this principle.

The first blue LEDs using gallium nitride were made in 1971 by Jacques Pankove at RCA Laboratories.<ref>E. Fred Schubert Light-emitting diodes 2nd ed., Cambridge University Press, 2006 ISBN 0521865387 page 16-17</ref> These devices had too little light output to be of practical use and research into gallium nitride devices slowed. In August 1989, Cree Inc. introduced the first commercially available blue LED based on the indirect bandgap semiconductor, silicon carbide.<ref>Template:Cite web</ref> SiC LEDs had very low effiency, no more than about 0.03%, but did emit in the blue portion of the visible light spectrum.

In the late 1980s, key breakthroughs in GaN epitaxial growth and p-type doping<ref>Template:Cite web</ref> ushered in the modern era of GaN-based optoelectronic devices. Building upon this foundation, in 1993 high brightness blue LEDs were demonstrated. Efficiency (light energy produced vs. electrical energy used) reached 10%.<ref>Template:US patent "Light-emitting gallium nitride-based compound semiconductor device" Nakamura et al., Issue date: November 26, 1996</ref> High-brightness blue LEDs invented by Shuji Nakamura of Nichia Corporation using gallium nitride revolutionized LED lighting, making high-power light sources practical.

By the late 1990s, blue LEDs had become widely available. They have an active region consisting of one or more InGaN quantum wells sandwiched between thicker layers of GaN, called cladding layers. By varying the relative InN-GaN fraction in the InGaN quantum wells, the light emission can be varied from violet to amber. AlGaN aluminium gallium nitride of varying AlN fraction can be used to manufacture the cladding and quantum well layers for ultraviolet LEDs, but these devices have not yet reached the level of efficiency and technological maturity of the InGaN-GaN blue/green devices. If the active quantum well layers are GaN, instead of alloyed InGaN or AlGaN, the device will emit near-ultraviolet light with wavelengths around 350–370 nm. Green LEDs manufactured from the InGaN-GaN system are far more efficient and brighter than green LEDs produced with non-nitride material systems.

With nitrides containing aluminium, most often AlGaN and AlGaInN, even shorter wavelengths are achievable. Ultraviolet LEDs in a range of wavelengths are becoming available on the market. Near-UV emitters at wavelengths around 375–395 nm are already cheap and often encountered, for example, as black light lamp replacements for inspection of anti-counterfeiting UV watermarks in some documents and paper currencies. Shorter wavelength diodes, while substantially more expensive, are commercially available for wavelengths down to 247 nm.<ref>Sensor Electronic Technology, Inc.: Nitride Products Manufacturer</ref> As the photosensitivity of microorganisms approximately matches the absorption spectrum of DNA, with a peak at about 260 nm, UV LED emitting at 250–270 nm are to be expected in prospective disinfection and sterilization devices. Recent research has shown that commercially available UVA LEDs (365 nm) are already effective disinfection and sterilization devices.<ref name="water sterilization">Template:Cite journal</ref>

Deep-UV wavelengths were obtained in laboratories using aluminium nitride (210 nm),<ref name=aln/> boron nitride (215 nm)<ref name=BN/><ref name=bn2/> and diamond (235 nm).<ref name=dia/>

White light

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

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

RGB systems

File:Red-YellowGreen-Blue LED spectra.png

Combined spectral curves for blue, yellow-green, and high brightness red solid-state semiconductor LEDs. FWHM spectral bandwidth is approximately 24–27 nm for all three colors.

White light can be formed by mixing differently colored lights; the most common method is to use red, green and blue (RGB). Hence the method is called multi-colored white LEDs (sometimes referred to as Red Green Blue LEDs). Because these need electronic circuits to control the blending and diffusion of 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,<ref>Template:Cite journal</ref> 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-, and tetrachromatic white LEDs. Several key factors that play among these different methods, include color stability, color rendering capability, and luminous efficacy. Often higher efficiency will mean lower color rendering, presenting a trade off between the luminous efficiency and color rendering. For example, the dichromatic white LEDs have the best luminous efficacy (120 lm/W), but the lowest color rendering capability. Conversely, although tetrachromatic white LEDs have excellent color rendering capability, they often have poor luminous efficiency. Trichromatic white LEDs are in between, having both good luminous efficacy (>70 lm/W) and fair color rendering capability.

Multi-color LEDs offer not merely another means to form white light, but a new means to form light of different colors. Most perceivable colors can be formed by mixing different amounts of three primary colors. This allows precise dynamic color control. As more effort is devoted to investigating this method, multi-color LEDs should have profound influence on the fundamental method 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 power decays exponentially with rising temperature,<ref>Template:Cite journal</ref> 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

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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 broadband Stokes-shifted light emitted by the Ce³⁺:YAG phosphor which emits at roughly 500–700 nm.

This method involves coating an LED of one color (mostly blue LED made of InGaN) with phosphor of different colors to form white light; the resultant LEDs are called phosphor-based white LEDs.<ref>Template:Cite journal</ref> A fraction of the blue light undergoes the Stokes shift being transformed from shorter wavelengths to longer. Depending on the color of the original LED, phosphors of different colors can be employed. If several phosphor layers of distinct colors are applied, the emitted spectrum is broadened, effectively raising the color rendering index (CRI) value of a given LED.<ref>Template:Cite journal</ref>

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 making high intensity white LEDs. The design and production of a light source or light fixture using a monochrome emitter with phosphor conversion is simpler and cheaper than a complex RGB system, and the majority of high intensity white LEDs presently on the market are manufactured using phosphor light conversion.

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.<ref>Template:Cite patent</ref> 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-doped yttrium aluminium garnet (Ce³⁺:YAG).

White LEDs can also be made by coating near ultraviolet (NUV) emitting LEDs with a mixture of high efficiency europium-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 way fluorescent lamps work. This method is less efficient than the blue LED with YAG:Ce phosphor, as the Stokes shift is larger, so more energy is converted to heat, but yields light with better spectral characteristics, which render color better. Due to the higher radiative output of the ultraviolet LEDs than of the blue ones, both methods offer comparable brightness. A concern is that UV light may leak from a malfunctioning light source and cause harm to human eyes or skin.

Other white LEDs

Another method used to produce experimental white light LEDs used no phosphors at all and was based on homoepitaxially grown zinc selenide (ZnSe) on a ZnSe substrate which simultaneously emitted blue light from its active region and yellow light from the substrate.<ref>Template:Cite web</ref>

Organic light-emitting diodes (OLEDs)

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Demonstration of a flexible OLED device

In an organic light emitting diode (OLED), the electroluminescent material comprising the emissive layer of the diode is an organic compound. The organic material is electrically conductive due to the delocalization of pi electrons caused by conjugation over all or part of the molecule, and the material therefore functions as an organic semiconductor.<ref>Template:Cite journal</ref> The organic materials can be small organic molecules in a crystalline phase, or polymers.

The potential advantages of OLEDs include thin, low cost displays with a low driving voltage, wide viewing angle and high contrast and color gamut.<ref name=bardsley>Template:Cite journal</ref> Polymer LEDs have the added benefit of printable<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> and flexible<ref>Template:Cite journal</ref> displays. 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.<ref name=bardsley />

Quantum dot LEDs (experimental)

Quantum dots (QD) are semiconductor nanocrystals that possess unique optical properties.<ref name=MITqdot2002>Quantum-dot LED may be screen of choice for future electronics Massachusetts Institute of Technology News Office, December 18, 2002</ref> Their emission color can be tuned from the visible throughout the infrared spectrum. This allows quantum dot LEDs to create almost any color on the CIE diagram. This provides more color options and better color rendering than white LEDs.Template:Citation needed Quantum dot LEDs are available in the same package types as traditional phosphor based LEDs.Template:Citation needed One example of this is a method developed by Michael Bowers, at Vanderbilt University in Nashville, involving coating a blue LED with quantum dots that glow white in response to the blue light from the LED. This method emits a warm, yellowish-white light similar to that made by incandescent bulbs.<ref>Template:Cite news</ref> Quantum dots are also being considered for use in white light emitting diodes in liquid crystal display (LCD) televisions.<ref>Nanoco Signs Agreement with Major Japanese Electronics Company, 23 September 2009</ref>

The major difficulty in using quantum dots based LEDs is the insufficient stability of QDs under prolonged irradiation.Template:Citation needed 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.Template:Citation needed Such QDs can be used to emit visible or near infrared light of any wavelength being excited by light with a shorter wavelength.

Types

File:Verschiedene LEDs.jpg

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

File:LED 3528 vs 5050 SMD Chips.png

Difference between 3528 and 5050 LED SMD Chips

<ref>http://www.flexfireleds.com/pages/Comparison-between-3528-LEDs-and-5050-LEDs.html</ref>

Miniature

File:LEDs 8 5 3mm.JPG

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

These are mostly single-die LEDs used as indicators, and they come in various-sizes from 2 mm to 8 mm, through-hole and surface mount packages. They usually don't use a separate heat sink.<ref>LED-design</ref> Typical current ratings ranges from around 1 mA to above 20 mA. The small size sets a natural upper boundary on power consumption due to heat caused by the high current density and need for heat sinking.

File:Arduino led-4.jpg

A green surface-mount LED mounted on an Arduino circuit board.

Common package shapes include round, with a domed or flat top, rectangular wtih a flat top (as used in bar-graph displays), and triangular or square with a flat top The encapsulation may also be clear or tinted to improve contrast and viewing angle.

There are three main categories of miniature single die LEDs:

• Low current — typically rated for 2 mA at around 2 V (approximately 4 mW consumption).
• Standard — 20 mA LEDs at around 2 V (approximately 40 mW) for red, orange, yellow & green, and 20 mA at 4–5 V (approximately 100 mW) for blue, violet and white.
• Ultra-high output — 20 mA at approximately 2 V or 4–5 V, designed for viewing in direct sunlight.

Five- and twelve-volt LEDs are ordinary miniature LEDs that incorporate a suitable series resistor for direct connection to a 5 V or 12 V supply.

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

File:2007-07-24 High-power light emiting diodes (Luxeon, Lumiled).jpg

High-power light emitting 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.<ref>Template:Cite web</ref><ref>Template:Cite web</ref> 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 a torch, or be set in an array to form a powerful LED 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 by Cree Inc. now exceed 105 lm/W <ref>Template:Cite web</ref> (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.<ref>Template:Cite web</ref> 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.<ref>Template:Cite book</ref>

Application-specific variations

• Flashing LEDs are used as attention seeking indicators without requiring external electronics. Flashing LEDs resemble standard LEDs but they contain an integrated multivibrator circuit 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.

File:LED DISP.JPG

Calculator LED display, 1970s.

• Bi-color LEDs are actually two different LEDs in one case. They consist of two dies connected to the same two leads antiparallel to each other. Current flow in one direction emits one color, and current in the opposite direction emits the other color. 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 LEDs are 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).Template:Citation needed

• RGB LEDs contain 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 tiny electronic control unit.

• Alphanumeric LED displays are available in seven-segment and starburst format. 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 of liquid 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

Template:Main The current/voltage characteristic of an LED is similar to other diodes, in that the current is dependent exponentially on the voltage (see Shockley 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 use constant current power 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, the I/V curve of nitride-based LEDs is quite steep above the knee and gives an I_f of a few milliamperes at a V_f of 3 V, making it possible to power a nitride-based LED from a 3 V battery such as a coin cell without the need for a current limiting resistor.

Electrical polarity

Template:Main As with all diodes, current flows easily from p-type to n-type material.<ref>Template:Cite book</ref> 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 the breakdown 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 useful noise 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".<ref>"Visible LED Device Classifications"</ref> In general, laser safety regulations—and the "Class 1", "Class 2", etc. system—also apply to LEDs.<ref> "Eye Safety and LED (Light Emitting Diode) diffusion": "The relevant standard for LED lighting is EN 60825-1:2001 (Safety of laser products) ... The standard states that throughout the standard ”light emitting diodes (LED) are included whenever the word “laser” is used.”" </ref>

While LEDs have the advantage over fluorescent lamps that they do not contain mercury, they may contain other hazardous metals such as lead and arsenic. 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), or silver (up to 721 mg/kg; limit: 500) render all except low-intensity yellow LEDs hazardous.".<ref name=Limetal2011>Template:Cite journal Free full-text.</ref>

Advantages

• Efficiency: LEDs emit more light per watt than incandescent light bulbs.<ref>Template:Cite web</ref> 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 2 mm²<ref>Template:Cite web

</ref>) 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 a microsecond.<ref>Template:Cite web</ref> 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, or HID lamps that require a long time before restarting.
• Dimming: LEDs can very easily be dimmed either by pulse-width modulation or lowering the forward current.<ref>Template:Cite journal</ref>
• Cool light: In contrast to most light sources, LEDs radiate very little heat in the form of IR that 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.<ref>Template:Cite web</ref>
• 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.<ref>Department of Energy</ref> 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. Adequate heat sinking is 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.<ref>The Led Museum</ref>
• Light quality: Most cool-white LEDs have spectra that differ significantly from a black body radiator like the sun or an incandescent light. The spike at 460 nm and dip at 500 nm can cause the color of objects to be perceived differently under cool-white LED illumination than sunlight or incandescent sources, due to metamerism,<ref>Template:Cite web</ref> 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 a lambertian distribution. 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.<ref>Template:Cite book</ref>

• Electrical Polarity: Unlike incandescent light bulbs, which illuminate regardless of the electrical polarity, LEDs will only light with correct electrical polarity.

• Blue hazard: There is a concern that blue LEDs and cool-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.<ref>Template:Cite news</ref><ref>Template:Cite web</ref>

• Blue pollution: Because cool-white LEDs (i.e., LEDs with high color temperature) emit proportionally more blue light than conventional outdoor light sources such as high-pressure sodium vapor lamps, the strong wavelength dependence of Rayleigh scattering means that cool-white LEDs can cause more light pollution than other light sources. The International Dark-Sky Association discourages using white light sources with correlated color temperature above 3,000 K.<ref name="IDA">

Template:Cite book</ref>Template:Failed verification

• Droop: The efficiency of LEDs tends to decrease as one increases current.<ref>Template:Cite journal</ref><ref>Template:Cite web</ref><ref>Template:Cite web</ref><ref>EnergyDaily: The LED's dark secret</ref>

Applications

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LED lighting in the aircraft cabin of an Airbus A320 Enhanced.

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A large LED display behind a disc jockey.

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LED destination signs on buses, one with a colored route number.

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

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Traffic light using LED

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Western Australia Police car with LEDs used in its high-mounted brake light, its rear window and roof-mounted flashing Police vehicle lights and roof-mounted road user information display.

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LED daytime running lights of Audi A4

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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.

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LED Illumination.

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LED lights reacting dynamically to video feed via AmBX.

In general, all the LED products can be divided into two major parts, the public lighting and indoor lighting.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.
• Illumination where light is reflected from objects to give visual response of these objects.
• Measuring and interacting with processes involving no human vision.<ref>EPIC European Photonics Industry Consortium.</ref>
• 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 the 1939 World's Fair.

Lighting

Template:Main With the development of high efficiency and high power LEDs it has become possible to use LEDs in lighting and illumination. Replacement light bulbs have been made, as well as dedicated fixtures and LED lamps. LEDs are used as street lights and in other architectural lighting where color changing is used. The mechanical robustness and long lifetime is used in automotive lighting on cars, motorcycles and on bicycle lights.

LED street lights are employed on poles and in parking garages. In 2007, the Italian village Torraca was the first place to convert its entire illumination system to LEDs.<ref>LED There Be Light, Scientific American, March 18, 2009</ref>

LEDs are used in aviation lighting. Airbus has used LED lighting in their Airbus A320 Enhanced since 2007, and Boeing plans its use in the 787. 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 for backlighting for LCD televisions and lightweight laptop displays and light source for DLP projectors (See LED TV). RGB LEDs raise the color gamut by as much as 45%. Screens for TV and computer displays can be made thinner using LEDs for backlighting.<ref>Template:Cite news</ref>

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 for stage lights using 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, LEDs lower heat output also means air conditioning(cooling) systems have less heat to dispose of, reducing carbon dioxide emissions.

LEDs are small, durable and need little power, so they are used in hand held devices such as flashlights. LED strobe lights or camera flashes operate at a safe, low voltage, instead of the 250+ volts commonly found in xenon flashlamp-based lighting. This is especially useful in cameras on mobile phones, where space is at a premium and bulky voltage-raising circuitry is undesirable.

LEDs are used for infrared illumination in night vision uses including security cameras. A ring of LEDs around a video camera, aimed forward into a retroreflective background, allows chroma keying in video productions.

LED’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, wherever 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 fittingsTemplate:Fact.

Increasingly the adaptability of color LED’s are finding uses in medical and educational applications such as mood enhancement and new technologies, such as AmBX, for the control of color LED’s have been developed to exploit LED versatility. NASA has even sponsored research for the use of LEDs to promote health for astronauts. <ref>http://www.sti.nasa.gov/tto/Spinoff2008/hm_3.html</ref>

Smart lighting

Light can be used to transmit broadband data, which is already implemented in IrDA standards using infrared LEDs. Because LEDs can cycle on and off millions of times per second, they can be wireless transmitters and access points for data transport.<ref>Template:Cite news</ref> Lasers can also be modulated in this manner.

Sustainable lighting

Efficient lighting is needed for sustainable architecture. In 2009, a typical 13 watt LED lamp emitted 450 to 650 lumens.<ref name="doe2009"> Template:Cite book</ref> which is equivalent to a standard 40 watt incandescent bulb. In 2011, LEDs have become more efficient, so that a 6 Watt LED can easily achieve the same results. <ref>Template:Cite web</ref> 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 Template:Convert of Template:Chem emission.<ref>Template:Cite web</ref> Assuming the average light bulb is on for 10 hours a day, one 40-watt incandescent bulb will cause Template:Convert of Template:Chem emission per year. The 6-watt LED equivalent will only cause Template:Convert of Template:Chem over the same time span. A building’s carbon footprint from lighting can be reduced by 85% by exchanging all incandescent bulbs for new LEDs.

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 typical CFLs, and as low as 9 lm/W, worse than standard incandescents.<ref name="doe2009"/> However, as of 2011 there are LED bulbs available as efficient as 150 lm/W and even inexpensive low-end models typically exceed 50 lm/W. The high initial cost of the commercial LED bulb is due to the expensive sapphire substrate which 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.

Other applications

The light from LEDs can be modulated very quickly so they are used extensively in optical fiber and Free Space Optics communications. This include remote controls, such as for TVs and VCRs, where infrared LEDs are often used. Opto-isolators use an LED combined with a photodiode or phototransistor to provide a signal path with electrical isolation between two circuits. This is especially useful in medical equipment where the signals from a low-voltage sensor circuit (usually battery powered) in contact with a living organism must be electrically isolated from any possible electrical failure in a recording or monitoring device operating at potentially dangerous voltages. An optoisolator also 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 as movement sensors, for example in optical computer mice. The Nintendo Wii's sensor bar uses infrared LEDs. Pulse oximeters use them for measuring oxygen saturation. Some flatbed scanners use arrays of RGB LEDs rather than the typical cold-cathode fluorescent lamp as the light source. Having independent control of three illuminated colors allows the scanner to calibrate itself for more accurate color balance, and there is no need for warm-up. Further, its sensors only need be monochromatic, since at any one time the page being scanned is only lit by one color of light. Touch sensing: Since LEDs can also be used as photodiodes, 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 or stylus.<ref>Template:Cite journal </ref>

Many materials and biological systems are sensitive to, or dependent on light. Grow lights use LEDs to increase photosynthesis in plants<ref>Template:Cite journal</ref> and bacteria and viruses can be removed from water and other substances using UV LEDs for sterilization.<ref name="water sterilization"/> Other uses are as UV curing devices for some ink and coating methods, and in LED 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) or CFL/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 quality voltage reference in electronic circuits. The forward voltage drop (e.g., about 1.7 V for a normal red LED) can be used instead of a Zener diode in low-voltage regulators. Red LEDs have the flattest I/V curve above the knee. Nitride-based LEDs have a fairly steep I/V curve and are useless for this purpose. Although LED forward voltage is far more current-dependent than a good Zener, Zener diodes are not widely available below voltages of about 3 V.

Light sources for machine vision systems

Machine vision systems 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 scanners are 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 for machine vision systems 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).

See also

• Display examples
• Laser diode
• LED circuit
• LED lamp
• LED as light sensor
• Luminous efficacy
• Nixie tube
• Photovoltaics
• Seven-segment display
• Solar lamp
• Solid-state lighting (SSL)

References

Further reading

• Template:Cite book

External links

• Template:Dmoz
• Promises and Limitations of Light-Emitting Diodes A concise University of California, Berkeley summary of the history, operation, benefits and limitations of LEDs
• Rensselaer Electrical Engineering Department LED information arranged in textbook form, aimed at introductory to advanced audience
• Interfacing LED to Micro controller