A light-emitting diode, usually called an LED (pronounced /ˌɛliːˈdiː/),[1] is a semiconductor diode that emits incoherent narrow-spectrum light when electrically biased in the forward direction of the p-n junction, as in the common LED circuit. A semiconductor' is a Solid material that has Electrical conductivity in between a conductor and an insulator; it can vary over that Dioden2jpg|thumb|right|150px|Figure 2 Various semiconductor diodes In Physics, coherence is a property of waves that enables stationary (i Light, or visible light, is Electromagnetic radiation of a Wavelength that is visible to the Human eye (about 400–700 Biasing in Electronics is the method of establishing predetermined Voltages and/or Currents at various points of a circuit to set an appropriate A p-n junction is a junction formed by combining P-type and N-type Semiconductors together in very close contact In Electronics, the basic LED circuit is an electrical circuit used to power a Light-emitting diode (LED This effect is a form of electroluminescence. Electroluminescence (EL is an Optical phenomenon and Electrical phenomenon in which a material emits light in response to an Electric current passed through
An LED is usually a small area light source, often with optics added to the chip to shape its radiation pattern and assist in reflection. [2][3] LEDs are often used as small indicator lights on electronic devices and increasingly in higher power applications such as flashlights and area lighting. The color of the emitted light depends on the composition and condition of the semiconducting material used, and can be infrared, visible, or ultraviolet. Infrared ( IR) radiation is Electromagnetic radiation whose Wavelength is longer than that of Visible light, but shorter than that of Ultraviolet ( UV) light is Electromagnetic radiation with a Wavelength shorter than that of Visible light, but longer than X-rays LEDs can also be used as a regular household light source. Besides lighting, interesting applications include sterilization of water and disinfection of devices. [4]
Contents |
History
In the early 20th century, Henry Round of Marconi Labs first noted that a semiconductor junction would produce light. Captain Henry Joseph Round ( 2 June 1881, Kingswinford, Staffordshire, England &ndash 17 August 1966, Bognor The Marconi Company Ltd was founded by Guglielmo Marconi in 1897 as The Wireless Telegraph & Signal Company (sometimes presented as Wireless Russian Oleg Vladimirovich Losev independently created the first LED in the mid 1920s; his research, though distributed in Russian, German and British scientific journals, was ignored. Oleg Vladimirovich Losev (Олег Владимирович Лосев ( 10 May 1903 - 22 January 1942) was a Scientist and [5] Rubin Braunstein of the Radio Corporation of America reported on infrared emission from gallium arsenide (GaAs) and other semiconductor alloys in 1955. RCA Corporation, founded as Radio Corporation of America, was an electronics company in existence from 1919 to 1986 Gallium arsenide ( GaAs) is a compound of two elements Gallium and Arsenic. [6] Experimenters at Texas Instruments, Bob Biard[7] and Gary Pittman, found in 1961 that gallium arsenide gave off infrared radiation when electric current was applied. Texas Instruments ( better known in the electronics industry (and popularly as TI, is an American company based in Dallas, Texas, USA Biard and Pittman were able to establish the priority of their work and received the patent for the infrared light-emitting diode. Dioden2jpg|thumb|right|150px|Figure 2 Various semiconductor diodes Nick Holonyak Jr., then of the General Electric Company and later with the University of Illinois at Urbana-Champaign, developed the first practical visible-spectrum LED in 1962[8] and is seen as the "father of the light-emitting diode". Nick Holonyak Jr (born in Zeigler Illinois on November 3, 1928) invented the first visible LED in 1962 while working as a consulting This article is about the flagship campus For other uses and locations of University of Illinois, see University of Illinois (disambiguation The University of [9] Holonyak's former graduate student, M. George Craford, invented in 1972 the first yellow LED and 10x brighter red and red-orange LEDs. [10]
Shuji Nakamura of Nichia Corporation of Japan demonstrated the first high-brightness blue LED based on InGaN, borrowing on critical developments in GaN nucleation on sapphire substrates and the demonstration of p-type doping of GaN which were developed by I. The is a Japanese chemical manufacturing company that is most widely known for producing Phosphors, including Light-emitting diodes (LEDs Indium gallium nitride ( InGaN, x1-x is a Semiconductor material made of a mix of Gallium nitride (GaN and Indium nitride Gallium nitride ( is a very hard material commonly used in bright LEDs since the 1990s Akasaki and H. Amano in Nagoya. In the 1995 Alberto Barbieri at the Cardiff University Laboratory (GB) investigated the Efficiency and Reliability of high-brightness LED demonstrating very high result by using a transparent contact made by indium tin oxide (ITO) on (AlGaInP/GaAs) LED. Uses ITO is mainly used to make transparent conductive coatings for Liquid crystal displays Flat panel displays Plasma displays touch panels The existence of the blue LED and high efficiency quickly carried to the first white LED, which employed a Y3Al5O12:Ce, or "YAG", phosphor coating to mix yellow (down-converted) light with blue to produce light that appears white. Nakamura was awarded the 2006 Millennium Technology Prize for his invention. The Millennium Technology Prize (Millennium-teknologiapalkinto is Finland's recognition for innovators that aim to improve quality of life and raise its profile as a high-tech [11]
Discovery
The first known report of a light-emitting solid-state diode was made in 1907 by the British experimenter H. J. Round. Year 1907 ( MCMVII) was a Common year starting on Tuesday (link will display the full calendar of the Gregorian calendar (or a Common year Captain Henry Joseph Round ( 2 June 1881, Kingswinford, Staffordshire, England &ndash 17 August 1966, Bognor However, no practical use was made of the discovery for several decades. [12] Independently, Oleg Vladimirovich Losev published "Luminous carborundum [silicon carbide] detector and detection with crystals" in the Russian journal Telegrafiya i Telefoniya bez Provodov (Wireless Telegraphy and Telephony). Oleg Vladimirovich Losev (Олег Владимирович Лосев ( 10 May 1903 - 22 January 1942) was a Scientist and [5] Losev's work languished for decades.
The first practical LED was invented by Nick Holonyak, Jr. , in 1962 while he was at General Electric Company. The first LEDs became commercially available in late 1960s, and were red. They were commonly used as replacements for incandescent indicators, and in seven-segment displays, first in expensive equipment such as laboratory and electronics test equipment, then later in such appliances as TVs, radios, telephones, calculators, and even watches. A seven-segment display (abbreviation " 7-seg (ment display" less commonly known as a seven-segment indicator, is a form of electronic Display device These red LEDs were bright enough only for use as indicators, as the light output was not enough to illuminate an area. Later, other colors became widely available and also appeared in appliances and equipment. As the LED materials technology became more advanced, the light output was increased, maintaining the efficiency and the reliability to an acceptable level, therefore LEDs became bright enough to be used for illumination.
Most LEDs were made in the very common 5 mm T1³⁄₄ and 3 mm T1 packages, but with higher power, it has become increasingly necessary to get rid of the heat in order to keep good reliability, so the packages have become more complex and adapted for heat dissipation. Packages for state-of-the-art high power LEDs bear little resemblance to early LEDs (see, for example, Philips Lumileds). Philips Lumileds Lighting Company is the manufacturer of Luxeon a high-power light-emitting diode ( LED)
LED panels
There are two types of LED panels: conventional, using discrete LEDs, and surface mounted device (SMD) panels. Surface-mount technology ( SMT) is a method for constructing electronic circuits in which the components (SMC or Surface Mounted Components are mounted directly onto Most outdoor screens and some indoor screens are built around discrete LEDs, also known as individually mounted LEDs. A cluster of red, green, and blue diodes is driven together to form a full-color pixel, usually square in shape. These pixels are spaced evenly apart and are measured from center to center for absolute pixel resolution. The largest LED display in the world is over 1,500 foot (457. 2 m) long and is located in Las Vegas, Nevada covering the Fremont Street Experience. Las Vegas ( Spanish: "The Meadows" is the most populous City in the state of Nevada, the seat of Clark County, and an internationally Nevada ( is a state located in the western region of the United States of America. The Fremont Street Experience (FSE is a Pedestrian mall and attraction in downtown Las Vegas, Nevada.
Most indoor screens on the market are built using SMD technology—a trend that is now extending to the outdoor market. An SMD pixel consists of red, green, and blue diodes mounted on a chipset, which is then mounted on the driver PC board. The individual diodes are smaller than a pinhead and are set very close together. The difference is that the maximum viewing distance is reduced by 25% from the discrete diode screen with the same resolution.
Indoor use generally requires a screen that is based on SMD technology and has a minimum brightness of 600 candelas per square meter (cd/m², sometimes informally called nits). The candela (kanˈdɛlə /-ˈdiːlə/ symbol cd) is the SI base unit of Luminous intensity; that is power emitted by a light source in a particular M^2 redirects here For other uses see M². CM2 redirects here "Nit (unit" redirects here For the unit of information entropy see Nat (information. This will usually be more than sufficient for corporate and retail applications, but under high ambient-brightness conditions, higher brightness may be required for visibility. Fashion and auto shows are two examples of high-brightness stage lighting that may require higher LED brightness. Conversely, when a screen may appear in a shot on a television show, the requirement will often be for lower brightness levels with lower color temperatures (common displays have a white point of 6500 to 9000 K, which is much bluer than the common lighting on a television production set).
For outdoor use, at least 2,000 cd/m² is required for most situations, whereas higher-brightness types of up to 5,000 cd/m² cope even better with direct sunlight on the screen. (The brightness of LED panels can be reduced from the designed maximum, if required. )
Suitable locations for large display panels are identified by factors such as line of sight, local authority planning requirements (if the installation is to become semi-permanent), vehicular access (trucks carrying the screen, truck-mounted screens, or cranes), cable runs for power and video (accounting for both distance and health and safety requirements), power, suitability of the ground for the location of the screen (if there are no pipes, shallow drains, caves, or tunnels that may not be able to support heavy loads), and overhead obstructions.
Flat Panel LED TV History
The first known recorded flat panel LED television screen prototype was developed by James P. Mitchell in 1977. The modular, scalable display was enabled by MV50 LEDs and newly available TTL (transistor transistor logic) memory addressing circuit technology. The prototype and paper were displayed at an Engineering Exposition in Anaheim May 1978, organized by the Science Service in Washington D. C. The LED TV display received special awards and recognition from NASA, General Motors Corporation and area universities including Robert M. The National Aeronautics and Space Administration ( NASA, ˈnæsə is an agency of the United States government, responsible for the nation's public space program General Motors Corporation ( GM) ( is a multinational automobile manufacturer founded in 1908 and headquartered in the United States. Saunders of The University of California Irvine, Professor of Engineering and National IEEE President 1977. Additionally, technology and business representatives from the U. S. and overseas witnessed operation of the monochromatic LED television display. The prototype remains operational. An LCD (liquid crystal display) matrix design was also presented in the accompanying scientific paper as a future alternate television display method using a similar array scanning design.
The early display prototype was red monochromatic. Low-cost efficient blue LEDs did not emerge until the early 1990s, completing the desired RGB color triad. High-brightness colors gradually emerged in the 1990s enabling new designs for outdoor signage and huge video displays for billboards and stadiums.
Touch sensing
Since LEDs share some basic physical properties with photodiodes, which also use p-n junctions with band gap energies in the visible light wavelengths, they can also be used for photo detection. A photodiode is a type of Photodetector capable of converting Light into either current or Voltage, depending upon the mode of operation These properties have been known for some time, but more recently so-called bidirectional LED matrices have been proposed as a method of touch-sensing. A Touch Screen is a display which can detect the presence and location of a touch within the display area In 2003, Dietz, Yerazunis, and Leigh published a paper describing the use of LEDs as cheap sensor devices.
In this usage, various LEDs in the matrix are quickly switched on and off. LEDs that are on shine light onto a user's fingers or a stylus. LEDs that are off function as photodiodes to detect reflected light from the fingers or stylus. The voltage thus induced in the reverse-biased LEDs can then be read by a microprocessor, which interprets the voltage peaks and then also uses them elsewhere.
LED technology
Physical function
Like a normal diode, the LED consists of a chip of semiconducting material impregnated, or doped, with impurities to create a p-n junction. Dioden2jpg|thumb|right|150px|Figure 2 Various semiconductor diodes In Semiconductor production doping is the process of intentionally introducing impurities into an extremely pure (also referred to as intrinsic) semiconductor to A p-n junction is a junction formed by combining P-type and N-type Semiconductors together in very close contact As in other diodes, current flows easily from the p-side, or anode, to the n-side, or cathode, but not in the reverse direction. An anode is an Electrode through which Electric current flows into a polarized electrical device A cathode is an Electrode through which (positive Electric current flows out of a polarized electrical device Charge-carriers—electrons and holes—flow into the junction from electrodes with different voltages. The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J An electron hole is the conceptual and mathematical Opposite of an Electron, useful in the study of Physics and Chemistry. An electrode is an Electrical conductor used to make contact with a nonmetallic part of a circuit (e Electrical tension (or voltage after its SI unit, the Volt) is the difference of electrical potential between two points of an electrical When an electron meets a hole, it falls into a lower energy level, and releases energy in the form of a photon. A quantum mechanical system or particle that is bound, confined spacially can only take on certain discrete values of energy as opposed to classical particles which In Physics and other Sciences energy (from the Greek grc ἐνέργεια - Energeia, "activity operation" from grc ἐνεργός In Physics, the photon is the Elementary particle responsible for electromagnetic phenomena
The wavelength of the light emitted, and therefore its color, depends on the band gap energy of the materials forming the p-n junction. In Physics wavelength is the distance between repeating units of a propagating Wave of a given Frequency. In Solid state physics and related applied fields a band gap, also called an energy gap or bandgap, is an energy range in a solid where no electron states In silicon or germanium diodes, the electrons and holes recombine by a non-radiative transition which produces no optical emission, because these are indirect band gap materials. Silicon (ˈsɪlɪkən or /ˈsɪlɪkɒn/ silicium is the Chemical element that has the symbol Si and Atomic number 14 Germanium (dʒɚˈmeɪniəm is a Chemical element with the symbol Ge and Atomic number 32 In Semiconductor Physics, an indirect bandgap is a Bandgap in which the minimum energy in the Conduction band is shifted by a The materials used for the LED have a direct band gap with energies corresponding to near-infrared, visible or near-ultraviolet light. In Semiconductor Physics, a direct Bandgap means that the minimum energy of the Conduction band lies directly above the maximum energy of the
LED development began with infrared and red devices made with gallium arsenide. Gallium arsenide ( GaAs) is a compound of two elements Gallium and Arsenic. Advances in materials science have made possible the production of devices with ever-shorter wavelengths, producing light in a variety of colors. Materials Science or Materials Engineering is an interdisciplinary field involving the properties of matter and its applications to various areas of Science and In Physics wavelength is the distance between repeating units of a propagating Wave of a given Frequency.
LEDs are usually built on an n-type substrate, with an electrode attached to the p-type layer deposited on its surface. P-type substrates, while less common, occur as well. Many commercial LEDs, especially GaN/InGaN, also use sapphire substrate. Sapphire (antique greek hyacinthos refers to gem varieties of the mineral Corundum, an Aluminium oxide (Al2O3 when it is a color other than Substrates that are transparent to the emitted wavelength, and backed by a reflective layer, and light spreading layer, increase the LED efficiency. The refractive index of the package material should match the index of the semiconductor, otherwise the produced light gets partially reflected back into the semiconductor, where it may be absorbed and turned into additional heat, thus lowering the efficiency. The refractive index (or index of Refraction) of a medium is a measure for how much the speed of light (or other waves such as sound waves is reduced inside the medium This type of reflection also occurs at the surface of the package if the LED is coupled to a medium with a different refractive index such as a glass fiber or air. The refractive index of most LED semiconductors is quite high, so in almost all cases the LED is coupled into a much lower-index medium. The large index difference makes the reflection quite substantial (per the Fresnel coefficients), and this is usually one of the dominant causes of LED inefficiency. Often more than half of the emitted light is reflected back at the LED-package and package-air interfaces. The reflection is most commonly reduced by using a dome-shaped (half-sphere) package with the diode in the center so that the outgoing light rays strike the surface perpendicularly, at which angle the reflection is minimized. An anti-reflection coating may be added as well. Anti-reflective or antireflection (AR coatings are a type of Optical coating applied to the surface of lenses and other optical devices to reduce The package may be cheap plastic, which may be colored, but this is only for cosmetic reasons or to improve the contrast ratio; the color of the packaging does not substantially affect the color of the light emitted. Other strategies for reducing the impact of the interface reflections include designing the LED to reabsorb and reemit the reflected light (called photon recycling) and manipulating the microscopic structure of the surface to reduce the reflectance, either by introducing random roughness or by creating programmed moth eye surface patterns.
Conventional LEDs are made from a variety of inorganic semiconductor materials, producing the following colors:
- Aluminium gallium arsenide (AlGaAs) — red and infrared
- Aluminium gallium phosphide (AlGaP) — green
- Aluminium gallium indium phosphide (AlGaInP) — high-brightness orange-red, orange, yellow, and green
- Gallium arsenide phosphide (GaAsP) — red, orange-red, orange, and yellow
- Gallium phosphide (GaP) — red, yellow and green
- Gallium nitride (GaN) — green, pure green (or emerald green), and blue also white (if it has an AlGaN Quantum Barrier)
- Indium gallium nitride (InGaN) — 450–470 nm — near ultraviolet, bluish-green and blue
- Silicon carbide (SiC) as substrate — blue
- Silicon (Si) as substrate — blue (under development)
- Sapphire (Al2O3) as substrate — blue
- Zinc selenide (ZnSe) — blue
- Diamond (C) — ultraviolet
- Aluminium nitride (AlN), aluminium gallium nitride (AlGaN), aluminium gallium indium nitride (AlGaInN) — near to far ultraviolet (down to 210 nm[13])
With this wide variety of colors, arrays of multicolor LEDs can be designed to produce unconventional color patterns. A semiconductor' is a Solid material that has Electrical conductivity in between a conductor and an insulator; it can vary over that Aluminium gallium arsenide (also aluminum gallium arsenide) ( Al x Ga 1-x As) is a Semiconductor material with very Infrared ( IR) radiation is Electromagnetic radiation whose Wavelength is longer than that of Visible light, but shorter than that of Aluminium gallium phosphide (, also GaAlP) a Phosphide of Aluminium and Gallium, is a Semiconductor material. Aluminium gallium indium phosphide (, also AlInGaP, InGaAlP, etc Gallium arsenide phosphide () is a Semiconductor material, an alloy of Gallium arsenide and Gallium phosphide. The colour orange occurs Yellow is the Color evoked by light that stimulates both the L and M (long and medium wavelength Cone cells of the Retina about equally Gallium phosphide () a Phosphide of Gallium, is a compound Semiconductor material with an indirect Band gap Gallium nitride ( is a very hard material commonly used in bright LEDs since the 1990s Blue is a Colour, the Perception of which is evoked by Indium gallium nitride ( InGaN, x1-x is a Semiconductor material made of a mix of Gallium nitride (GaN and Indium nitride Silicon carbide ( is a compound of Silicon and Carbon bonded together to form Ceramics but it also occurs in nature as the extremely rare mineral Silicon (ˈsɪlɪkən or /ˈsɪlɪkɒn/ silicium is the Chemical element that has the symbol Si and Atomic number 14 Sapphire (antique greek hyacinthos refers to gem varieties of the mineral Corundum, an Aluminium oxide (Al2O3 when it is a color other than Zinc selenide ( Zn[[Selenium Se]] is a light yellow binary solid compound In Mineralogy, diamond is the allotrope of carbon where the carbon atoms are arranged in Aluminium nitride ( Al[[Nitrogen N]] is a Nitride of Aluminium. Aluminium gallium nitride ( AlGaN) is a Semiconductor material. Ultraviolet ( UV) light is Electromagnetic radiation with a Wavelength shorter than that of Visible light, but longer than X-rays [14]
Ultraviolet and blue LEDs
Blue LEDs are based on the wide band gap semiconductors GaN (gallium nitride) and InGaN (indium gallium nitride). In Solid state physics and related applied fields a band gap, also called an energy gap or bandgap, is an energy range in a solid where no electron states Gallium nitride ( is a very hard material commonly used in bright LEDs since the 1990s 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. White is a Color, the perception which is evoked by Light that stimulates all three types of color sensitive Cone cells in the Human eye
The first blue LEDs were made in 1971 by Jacques Pankove (inventor of the gallium nitride LED) at RCA Laboratories. Year 1971 ( MCMLXXI) was a Common year starting on Friday (link will display full calendar of the 1971 Gregorian calendar. RCA Corporation, founded as Radio Corporation of America, was an electronics company in existence from 1919 to 1986 [15] However, these devices were too feeble to be of much practical use. In the late 1980s, key breakthroughs in GaN epitaxial growth and p-type doping by Isamu Akasaki and Hiroshi Amano (Nagoya, Japan)[16] ushered in the modern era of GaN-based optoelectronic devices. is a Japanese Scientist, best known for inventing p-n junction blue LEDs using Gallium nitride ( GaN) in as early as 1989 first in the world Building upon this foundation, in 1993 high brightness blue LEDs were demonstrated through the work of Shuji Nakamura at Nichia Corporation. Year 1993 ( MCMXCIII) was a Common year starting on Friday (link will display full 1993 Gregorian calendar) The is a Japanese chemical manufacturing company that is most widely known for producing Phosphors, including Light-emitting diodes (LEDs [17]
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. A quantum well is a Potential well that confines particles which were originally free to move in three dimensions to two dimensions forcing them to occupy a planar region 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. Aluminium gallium nitride ( AlGaN) is a Semiconductor material. If the active quantum well layers are GaN, as opposed to 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. Aluminium gallium nitride ( AlGaN) is a Semiconductor material. 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. A Black light or UV Light is a lamp emitting Electromagnetic radiation that is almost exclusively in the soft near ultraviolet range and emits A counterfeit is an imitation that is made usually with the intent to deceptively represent its content or origins Shorter wavelength diodes, while substantially more expensive, are commercially available for wavelengths down to 247 nm. [18] As the photosensitivity of microorganisms approximately matches the absorption spectrum of DNA, with a peak at about 260 nm, UV LEDs emitting at 250–270 nm are to be expected in prospective disinfection and sterilisation devices. Deoxyribonucleic acid ( DNA) is a Nucleic acid that contains the genetic instructions used in the development and functioning of all known Recent research has shown that commercially available UVA LEDs (365 nm) are already effective disinfection and sterilisation devices. [4]
Wavelengths down to 210 nm were obtained in laboratories using aluminium nitride. Aluminium nitride ( Al[[Nitrogen N]] is a Nitride of Aluminium.
While not actually LEDs as such, ordinary NPN bipolar transistor will emit violet light if its emitter-base junction is subjected to non-destructive reverse breakdown. This is easy to demonstrate by filing the top off a metal-can transistor (BC107, 2N2222 or similar) and biasing it well above emitter-base breakdown (≥ 20 V) via a current-limiting resistor.
White LEDs
There are two ways of producing high intensity white-light LED. White is a Color, the perception which is evoked by Light that stimulates all three types of color sensitive Cone cells in the Human eye One is to first produce individual LEDs that emit three primary colors[19] – red, green, and blue, and then mix all the colors to produce white light. White is a Color, the perception which is evoked by Light that stimulates all three types of color sensitive Cone cells in the Human eye Hence the product is called multi-colored white LEDs (sometimes referred to as RGB LEDs). Because its mechanism is involved with sophisticated electro-optical design to control the blend and diffusion of different colors, this approach has rarely been used to mass produce white LEDs in the industry. Nevertheless this method is particularly interesting to many researchers and scientists because of the flexibility of mixing different colors. In principle, this mechanism also has higher quantum efficiency in producing white light. On the other hand, the second method of producing white LED is involved with coating a LED of one color (mostly blue LED made of InGaN) with phosphor coating of a different color to produce white light. Coating is a Covering that is applied to an object The aim of applying coatings is to improve surface properties of a bulk material usually referred to as a substrate A phosphor is a substance that exhibits the phenomenon of Phosphorescence (sustained glowing after exposure to energized particles such as Electrons Depending on the color of the original LED, phosphors of different colors can also be employed. A phosphor is a substance that exhibits the phenomenon of Phosphorescence (sustained glowing after exposure to energized particles such as Electrons By applying several phosphor layers of distinct colors, we can effectively increase the color rendering index (CRI) value of a given LED. The term CRI will be defined more elegantly in the following section. Because this method of producing white LEDs heavily employs the usage of phosphor, the resultant LEDs are called phosphor based white LEDs. A phosphor is a substance that exhibits the phenomenon of Phosphorescence (sustained glowing after exposure to energized particles such as Electrons Although easier to be manufactured than multi-colored LEDs, phosphor based LEDs have a lower quantum efficiency and other phosphor-related degradation issues. A phosphor is a substance that exhibits the phenomenon of Phosphorescence (sustained glowing after exposure to energized particles such as Electrons However, it is still the most popular technique of manufacturing high intensity white LEDs as well as high intensity LEDs of other colors because it requires much easier material processing and therefore suits today’s applications. In Physics, intensity is a measure of the time-averaged Energy Flux. Much effort has been spent on optimizing the operating environment, namely temperature and current, for this type of LED.
There are several types of multi-colored white LEDs: di-, tri-, and tetrachromatic white LEDs. Numerical prefixes are usually derived from the words for numbers in various languages most commonly Greek and Latin, although this is not always the case Numerical prefixes are usually derived from the words for numbers in various languages most commonly Greek and Latin, although this is not always the case Tetrachromacy is the condition of possessing four independent channels for conveying Color information or possessing four different cones. Several key factors that play among these different approaches include color stability, color rendering capability, and luminous efficiency. Stability can refer to Aircraft flight Stability (aircraft In atmospheric fluid dynamics atmospheric stability, a measure of the turbulence Luminous efficiency is a term expressing the luminous flux per unit electrical input power. It is a key factor in discussing energy efficiency. In principle, if perfect solid-state lighting devices can be fabricated, the same level of luminance can be achieved by using merely 1/20 of the energy that incandescent lighting source requires. Incandescence is the emission of Light (visible Electromagnetic radiation) from a hot body due to its temperature Color stability is a self-explanatory term which means the stability of color. Color rendering capability is hard to grasp without being traced back to its origin. In 1777, George Palmer first found that an object’s perceived color strongly depends on the illumination source. George Palmer may refer to George Palmer (businessman (1818&ndash1897 Quaker entrepreneur and biscuit manufacturer of Reading England father [20] He discovered that by varying the illumination sources, an object’s color appeared differently. Because of their conflicting nature, there is always a trade off between the luminous efficiency and color rendering. For example, the dichromatic white LEDs have the best luminous efficiency (120 lm/W), but the lowest color rendering capability. Oppositely although tetrachromatic white LEDs have excellent color rendering capability, they often have poor luminous efficiency. Tetrachromacy is the condition of possessing four independent channels for conveying Color information or possessing four different cones. Trichromatic white LEDs are in between, having both good luminous efficiency (>70 lm/W) and fair color rendering capability.
Phosphor based white LEDs encapsulate InGaN blue LEDs inside of a phosphor coated epoxy. A common yellow phosphor material is cerium-doped yttrium aluminum garnet (Ce3+:YAG). Cerium (ˈsɪəriəm is a Chemical element with the symbol Ce and Atomic number 58 In Semiconductor production doping is the process of intentionally introducing impurities into an extremely pure (also referred to as intrinsic) semiconductor to YAG redirects here For the IATA airport code see Fort Frances Municipal Airport. Although the phosphor based white LEDs have a relatively easier mechanism, they reach the fundamental limitation due to the unavoidable Stokes energy loss6, a loss that occurs when short wavelength photons are converted to long wavelength photons. In Physics wavelength is the distance between repeating units of a propagating Wave of a given Frequency. In Physics, the photon is the Elementary particle responsible for electromagnetic phenomena Regardless this technique of manufacturing is adopted by most of the LED industry because of its low cost and high output. All the high intensity white LEDs now on the market are manufactured by this method.
What multi-color LEDs offer is not merely another solution of producing white light, but is a whole new technique of producing light of different colors. In principle, all colors in the visible spectrum can be produced by mixing different amount of three primary colors, and this makes it possible to produce precise dynamic color control as well. As more effort is devoted to investigating this technique, 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 LEDs can truly play a role on the market, several technical problems need to be solved. These certainly include that this type of LEDs’ emission power decays exponentially] with increasing temperature,[21] resulting in a substantial change in color stability. Such problem is not acceptable for industrial usage. Therefore, many new package designs aiming to solve this problem have been proposed, and their results are being reproduced by researchers and scientists.
On the other hand, phosphor based white LEDs are the optimal solution to produce high intensity white light. Since its simplified mechanism, this type of LEDs has attracted much interest from the lighting industry. Because of their more stable performance over a range of temperatures, prototypes as well as products based on this phosphor based mechanism have already appeared on the market. And more high intensity white LEDs are expected to be produced in the near future. In Physics, intensity is a measure of the time-averaged Energy Flux. However, the biggest challenge these phosphor based white LEDs face is solving the seemingly unavoidable Stokes energy loss. A phosphor is a substance that exhibits the phenomenon of Phosphorescence (sustained glowing after exposure to energized particles such as Electrons Again this can be done by adapting a better package design or by replacing 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 consistent spectrum of white light. Philips Lumileds Lighting Company is the manufacturer of Luxeon a high-power light-emitting diode ( LED)
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 aluminum doped zinc sulfide (ZnS:Cu, Al). Coating is a Covering that is applied to an object The aim of applying coatings is to improve surface properties of a bulk material usually referred to as a substrate Ultraviolet ( UV) light is Electromagnetic radiation with a Wavelength shorter than that of Visible light, but longer than X-rays Europium (jʊˈroʊpiəm is a Chemical element with the symbol Eu and Atomic number 63 This is a method analogous to the way fluorescent lamps work. A fluorescent lamp or fluorescent tube is a Gas-discharge lamp that uses Electricity to excite mercury Vapor. However, the ultraviolet light causes photodegradation to the epoxy resin and many other materials used in LED packaging, causing manufacturing challenges and shorter lifetimes. Ultraviolet ( UV) light is Electromagnetic radiation with a Wavelength shorter than that of Visible light, but longer than X-rays Photodegradation is degradation of a photodegradable Molecule caused by the absorption of Photons, particularly those Wavelengths found In Chemistry, epoxy or polyepoxide is a Thermosetting Epoxide Polymer that cures (polymerizes and crosslinks when mixed with a This method is less efficient than the blue LED with YAG:Ce phosphor, as the Stokes shift is larger and more energy is therefore converted to heat, but yields light with better spectral characteristics, which render color better. Stokes shift is the difference (in Wavelength or Frequency units between positions of the band maxima of the absorption and emission Due to the higher radiative output of the ultraviolet LEDs than of the blue ones, both approaches offer comparable brightness.
The newest method used to produce white light LEDs uses no phosphors at all and is based on homoepitaxially grown zinc selenide (ZnSe) on a ZnSe substrate which simultaneously emits blue light from its active region and yellow light from the substrate
A new technique developed by Michael Bowers, a graduate student at Vanderbilt University in Nashville, involves coating a blue LED with quantum dots that glow white in response to the blue light from the LED. Zinc selenide ( Zn[[Selenium Se]] is a light yellow binary solid compound Vanderbilt University is a private, Nonsectarian, Coeducational Research University in Nashville, Tennessee, A quantum dot is a Semiconductor whose Excitons are confined in all three Spatial dimensions. This technique produces a warm, yellowish-white light similar to that produced by incandescent bulbs. The incandescent light bulb, incandescent lamp or incandescent light globe is a source of electric Light that works by Incandescence, (a general [22]
Quantum Dot LEDs
Quantum Dots are semiconductor nanocrystals that possess unique optical properties. A quantum dot is a Semiconductor whose Excitons are confined in all three Spatial dimensions. A semiconductor' is a Solid material that has Electrical conductivity in between a conductor and an insulator; it can vary over that [23] 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. Established in 1913 and based in Vienna, Austria, the International Commission on Illumination (usually known as the CIE for its French name Commission This provides more color options and better color rendering white LEDs. Quantum dot LEDs are available in the same package types as traditional phosphor based LEDs. A phosphor is a substance that exhibits the phenomenon of Phosphorescence (sustained glowing after exposure to energized particles such as Electrons
Organic light-emitting diodes (OLEDs)
If the emitting layer material of the LED is an organic compound, it is known as an Organic Light Emitting Diode (OLED). An organic compound is any member of a large class of Chemical compounds whose Molecules contain Carbon. An Organic Light Emitting Diode ( OLED) also Light Emitting Polymer ( LEP) and Organic Electro Luminescence ( OEL) is any To function as a semiconductor, the organic emitting material must have conjugated pi bonds. A chemically conjugated system is a system of atoms Covalently bonded with alternating single and multiple (e The emitting material can be a small organic molecule in a crystalline phase, or a polymer. In Chemistry, a molecule is defined as a sufficiently stable electrically neutral group of at least two Atoms in a definite arrangement held together by In Materials science, a crystal is a Solid in which the constituent Atoms Molecules or Ions are packed in a regularly ordered repeating In the Physical sciences a phase is a Set of states of a macroscopic physical system that have relatively uniform chemical composition and physical properties A polymer is a large Molecule ( Macromolecule) composed of repeating Structural units typically connected by Covalent Chemical bonds Polymer materials can be flexible; such LEDs are known as PLEDs or FLEDs.
Compared with regular LEDs, OLEDs are lighter, and polymer LEDs can have the added benefit of being flexible. Some possible future applications of OLEDs could be:
- Inexpensive, flexible displays
- Light sources
- Wall decorations
- Luminous cloth
OLEDs have been used to produce visual displays for portable electronic devices such as cellphones, digital cameras, and MP3 players. A textile is a flexible material comprised of a network of natural or artificial Fibres often referred to as thread or Yarn. Larger displays have been demonstrated, but their life expectancy is still far too short (<1,000 hours) to be practical.
Efficiency and operational parameters
Most typical LEDs are designed to operate with no more than 30–60 milliwatts (mW) of electrical power. The watt (symbol W) is the SI derived unit of power, equal to one Joule of energy per Second. Around 1999, Philips Lumileds introduced power LEDs capable of continuous use at one watt (W). Philips Lumileds Lighting Company is the manufacturer of Luxeon a high-power light-emitting diode ( LED) The watt (symbol W) is the SI derived unit of power, equal to one Joule of energy per Second. 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 efficiency, as measured by its light output per unit power input. White LEDs quickly matched and overtook the efficiency of standard incandescent lighting systems. In 2002, Lumileds made five-watt LEDs available with a luminous efficacy of 18–22 lumens per watt (lm/W). Philips Lumileds Lighting Company is the manufacturer of Luxeon a high-power light-emitting diode ( LED) Luminous efficacy is a property of Light sources which indicates what portion of the emitted Electromagnetic radiation is usable for human vision. The lumen (symbol lm) is the SI unit of Luminous flux, a measure of the perceived power of Light. For comparison, a conventional 60–100 W incandescent lightbulb produces around 15 lm/W, and standard fluorescent lights produce up to 100 lm/W. (The luminous efficacy article discusses these comparisons in more detail. Luminous efficacy is a property of Light sources which indicates what portion of the emitted Electromagnetic radiation is usable for human vision. )
In September 2003, a new type of blue LED was demonstrated by the company Cree, Inc. to provide 24 mW at 20 milliamperes (mA). Cree Inc is a Durham North Carolina based American Corporation which manufactures Semiconductor materials and devices The ampere, in practice often shortened to amp, (symbol A is a unit of Electric current, or amount of Electric charge per second 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 Semiconductor has plans for 135 lm/W by 2007 and 145 lm/W by 2008, which would be approaching an order of magnitude improvement over standard incandescents and better even than standard fluorescents. Seoul Semiconductor Co Ltd is a Korean company manufacturing LED devices [24] Nichia Corporation has developed a white light LED with luminous efficacy of 150 lm/W at a forward current of 20 mA. The is a Japanese chemical manufacturing company that is most widely known for producing Phosphors, including Light-emitting diodes (LEDs [25] In May 2008, 130lu/W is available from Chinese LED manufacturers.
It should be noted that high-power (≥ 1 W) LEDs are necessary for practical general lighting applications. Typical operating currents for these devices begin at 350 mA. The highest efficiency high-power white LED is claimed by Philips Lumileds Lighting Co. with a luminous efficacy of 115 lm/W (350 mA).
Potential of OLEDs
Today, OLEDs operate at substantially lower efficiency than inorganic (crystalline) LEDs. The best luminous efficacy of an OLED so far is about 10% of the theoretical maximum of 683 for “white” light, or about 68 lm/W. These claim to be much cheaper to fabricate than inorganic LEDs, and large arrays of them can be deposited on a screen using simple printing methods to create a color graphical display.
Failure modes
The most common way for LEDs (and diode lasers) to fail is the gradual lowering of light output and loss of efficiency. A laser diode is a Laser where the active medium is a Semiconductor similar to that found in a Light-emitting diode. However, sudden failures can occur as well.
The mechanism of degradation of the active region, where the radiative recombination occurs, involves nucleation and growth of dislocations; this requires a presence of an existing defect in the crystal and is accelerated by heat, high current density, and emitted light. Nucleation is the onset of a Phase transition in a small region In Materials science, a dislocation is a Crystallographic defect, or irregularity within a Crystal structure. Gallium arsenide and aluminium gallium arsenide are more susceptible to this mechanism than gallium arsenide phosphide and indium phosphide. Gallium arsenide ( GaAs) is a compound of two elements Gallium and Arsenic. Aluminium gallium arsenide (also aluminum gallium arsenide) ( Al x Ga 1-x As) is a Semiconductor material with very Gallium arsenide phosphide () is a Semiconductor material, an alloy of Gallium arsenide and Gallium phosphide. Indium phosphide ( is a binary Semiconductor composed of Indium and Phosphorus. Due to different properties of the active regions, gallium nitride and indium gallium nitride are virtually insensitive to this kind of defect; however, high current density can cause electromigration of atoms out of the active regions, leading to emergence of dislocations and point defects, acting as nonradiative recombination centers and producing heat instead of light. Gallium nitride ( is a very hard material commonly used in bright LEDs since the 1990s Indium gallium nitride ( InGaN, x1-x is a Semiconductor material made of a mix of Gallium nitride (GaN and Indium nitride Electromigration is the transport of material caused by the gradual movement of the Ions in a conductor due to the momentum transfer between conducting electrons and Ionizing radiation can lead to the creation of such defects as well, which leads to issues with radiation hardening of circuits containing LEDs (e. Image talkNew_radiation_symbol_ISO_21482svg for details --> Ionizing radiation Radiation hardening is a method of designing and testing electronic components and systems to make them resistant to damage or malfunctions caused by high-energy Subatomic particles g. , in optoisolators). In Electronics, an opto-isolator (or optical isolator, optocoupler, photocoupler, or photoMOS) Early red LEDs were notable for their short lifetime.
White LEDs often use one or more phosphors. A phosphor is a substance that exhibits the phenomenon of Phosphorescence (sustained glowing after exposure to energized particles such as Electrons The phosphors tend to degrade with heat and age, losing efficiency and causing changes in the produced light color. Pink LEDs often use an organic phosphor formulation which may degrade after just a few hours of operation causing a major shift in output color.
High electrical currents at elevated temperatures can cause diffusion of metal atoms from the electrodes into the active region. Some materials, notably indium tin oxide and silver, are subject to electromigration with the conseguence of leakage current and non radiative recombination along the chip edges. Uses ITO is mainly used to make transparent conductive coatings for Liquid crystal displays Flat panel displays Plasma displays touch panels Silver (ˈsɪlvɚ is a Chemical element with the symbol " Ag " (argentum from the Ancient Greek: ἀργήντος - argēntos gen In some cases, especially with GaN/InGaN diodes, a barrier metal layer is used to hinder the electromigration effects. A barrier metal is a material used in Integrated circuits to chemically isolate Semiconductors from soft metal interconnects while maintaining an electrical connection Mechanical stresses, high currents, and corrosive environment can lead to formation of whiskers, causing short circuits. Metal whiskering is a Crystalline metallurgical phenomenon involving the spontaneous growth of tiny filiform hairs from a Metallic surface
High-power LEDs are susceptible to current crowding, nonhomogenous distribution of the current density over the junction. Current crowding (also current crowding effect, or CCE) is a nonhomogenous distribution of Current density through a conductor or semiconductor especially This may lead to creation of localized hot spots, which poses risk of thermal runaway. HotSpot is the primary Java Virtual Machine for desktops and servers produced by Sun Microsystems. Thermal runaway refers to a situation where an increase in temperature changes the conditions in a way that causes a further increase in temperature leading to a destructive result Nonhomogenities in the substrate, causing localized loss of thermal conductivity, aggravate the situation; most common ones are voids caused by incomplete soldering, or by electromigration effects and Kirkendall voiding. In Physics, thermal conductivity, k is the property of a material that indicates its ability to conduct Heat. Soldering is a process in which two or more Metal items are joined together by melting and flowing a filler metal into the joint the filler metal having a relatively low The Kirkendall effect is the migration of markers that occurs when markers are placed at the interface between an Alloy and a Metal, and the whole is heated to a temperature Thermal runaway is a common cause of LED failures.
Laser diodes may be subject to catastrophic optical damage, when the light output exceeds a critical level and causes melting of the facet. A laser diode is a Laser where the active medium is a Semiconductor similar to that found in a Light-emitting diode. Catastrophic optical damage ( COD) is a failure mode of high-power Semiconductor lasers It occurs when the Semiconductor junction is overloaded by exceeding
Some materials of the plastic package tend to yellow when subjected to heat, causing partial absorption (and therefore loss of efficiency) of the affected wavelengths.
Sudden failures are most often caused by thermal stresses. When the epoxy resin used in packaging reaches its glass transition temperature, it starts rapidly expanding, causing mechanical stresses on the semiconductor and the bonded contact, weakening it or even tearing it off. In Chemistry, epoxy or polyepoxide is a Thermosetting Epoxide Polymer that cures (polymerizes and crosslinks when mixed with a The glass transition temperature, T g is the temperature at which an Amorphous solid, such as Glass or a Polymer, becomes brittle Wire bonding is a method of making interconnections between a microchip and other electronics as part of semiconductor device fabrication. Conversely, very low temperatures can cause cracking of the packaging.
Electrostatic discharge (ESD) may cause immediate failure of the semiconductor junction, a permanent shift of its parameters, or latent damage causing increased rate of degradation. Electrostatic discharge ( ESD) is the sudden and momentary Electric current that flows between two objects at different Electrical potentials The term is LEDs and lasers grown on sapphire substrate are more susceptible to ESD damage. Sapphire (antique greek hyacinthos refers to gem varieties of the mineral Corundum, an Aluminium oxide (Al2O3 when it is a color other than
Research on DNA
The DNA in salmon sperm has recently been discovered to amplify the effects and quality of an LED light. [1]
See research at University of Cincinnati: http://www.uc.edu/news/NR.asp?id=7089
Articles from the UK: http://www.theregister.co.uk/2007/09/18/led_salmon_dna_product_enhancement/
Considerations in use
Unlike incandescent light bulbs, which light up regardless of the electrical polarity, LEDs will only light with correct electrical polarity. The incandescent light bulb, incandescent lamp or incandescent light globe is a source of electric Light that works by Incandescence, (a general In Physics, polarity is a description of an attribute typically a binary attribute (one with two values or a Vector (a direction When the voltage across the p-n junction is in the correct direction, a significant current flows and the device is said to be forward-biased. If the voltage is of the wrong polarity, the device is said to be reverse biased, very little current flows, and no light is emitted. Some LEDs can be operated on an alternating current voltage, but they will only light with positive voltage, causing the LED to turn on and off at the frequency of the AC supply. An alternating current ( AC) is an Electric current whose direction reverses cyclically as opposed to Direct current, whose direction remains constant
While the only definitive way to determine the polarity of the LED is to examine its datasheet, these methods are usually reliable:
| sign: | + | - |
| terminal: | anode (A) | cathode (K) |
| leads: | long | short |
| exterior: | round | flat |
| interior: | small | large |
| wiring: | red | black |
Less reliable methods of determining polarity are:
| sign: | + | - |
| marking: | none | stripe |
| pin: | 1 | 2 |
| PCB: | round | square |
While it is not an officially reliable method, it is almost universally true that the cup that holds the LED die corresponds to the cathode. A printed circuit board, or PCB, is used to mechanically support and electrically connect Electronic components using conductive pathways or traces It is strongly recommended to apply a safe voltage and observe the illumination as a test regardless of what method is used to determine the polarity.
Because the voltage versus current characteristics of the LED are much like any diode (that is, current approximately an exponential function of voltage), a small voltage change results in a huge change in current. Dioden2jpg|thumb|right|150px|Figure 2 Various semiconductor diodes Added to deviations in the process this means that a voltage source may barely make one LED light while taking another of the same type beyond its maximum ratings and potentially destroying it. A voltage source is any device or system that produces an Electromotive force between its terminals OR derives a secondary voltage from a primary
Since the voltage is logarithmically related to the current it can be considered to remain largely constant over the LED's operating range. Thus the power can be considered to be essentially proportional to the current. In order to keep power nearly constant with variations in supply and LED characteristics, the power supply should be a “current source”, that is, it should supply an almost constant current. A current source is an electrical or electronic device that delivers or absorbs electric current If high efficiency is not required (e. g. , in most indicator applications), an approximation to a current source is made by connecting the LED in series with a current limiting resistor to a regulated voltage source.
Most LEDs have low reverse breakdown voltage ratings, so they will also be damaged by an applied reverse voltage of more than a few volts. Breakdown Voltage of an Insulator is the minimum voltage that causes a portion of an insulator to become electrically conductive. Since some manufacturers don't follow the indicator standards above, if possible the data sheet should be consulted before hooking up the LED, or the LED may be tested in series with a resistor on a sufficiently low voltage supply to avoid the reverse breakdown. A datasheet is a Document summarizing the performance and other characteristics of a component (e |- align = "center"| |width = "25"| | |- align = "center"| || Potentiometer |- align = "center"| | | |- align = "center"| Resistor| | If it is desired to drive the LED directly from an AC supply of more than the reverse breakdown voltage then it may be protected by placing a diode (or another LED) in inverse parallel. In electronics two antiparallel or inverse-parallel devices are connected in parallel but with their polarities reversed
LEDs can be purchased with built in series resistors. These can save PCB space and are especially useful when building prototypes or populating a PCB in a way other than its designers intended. A printed circuit board, or PCB, is used to mechanically support and electrically connect Electronic components using conductive pathways or traces A prototype is an original type form or instance of something serving as a typical example basis or standard for other things of the same category However, the resistor value is set at the time of manufacture, removing one of the key methods of setting the LED's intensity. To increase efficiency (or to allow intensity control without the complexity of a DAC), the power may be applied periodically or intermittently; so long as the flicker rate is greater than the human flicker fusion threshold, the LED will appear to be continuously lit. In Electronics, a digital-to-analog converter ( DAC or D-to-A) is a device for converting a digital (usually binary code to an Analog signal The flicker fusion threshold (or flicker fusion rate) is a concept in the Psychophysics of vision.
Multiple LEDs can be connected in series with a single current limiting resistor provided the source voltage is greater than the sum of the individual LED threshold voltages. If two or more circuit components are connected end to end like a daisy chain it is said they are connected in series. Parallel operation is also possible but can be more problematic. If two or more circuit components are connected end to end like a daisy chain it is said they are connected in series. Parallel LEDs must have closely matched forward voltages (Vf) in order to have equal branch currents and, therefore, equal light output. Variations in the manufacturing process can make it difficult to obtain satisfactory operation when connecting some types of LEDs in parallel. [26]
Bicolor LED units contain two diodes, one in each direction (that is, two diodes in inverse parallel) and each a different color (typically red and green), allowing two-color operation or a range of apparent colors to be created by altering the percentage of time the voltage is in each polarity. Red is any of a number of similar Colors evoked by light consisting predominantly of the longest wavelengths of Light discernible by the human eye in the wavelength Green is a Color, the perception of which is evoked by light having a spectrum dominated by energy with a Wavelength of roughly 520–570- nm. Other LED units contain two or more diodes (of different colors) arranged in either a common anode or common cathode configuration. These can be driven to different colors without reversing the polarity, however, more than two electrodes (leads) are required.
LEDs are usually constantly illuminated when a current passes through them, but flashing LEDs are also available. Flashing LEDs resemble standard LEDs but they contain an integrated multivibrator circuit inside which causes the LED to flash with a typical period of one second. A multivibrator is an Electronic circuit used to implement a variety of simple two-state systems such as Oscillators Timers and flip-flops It This type of LED comes most commonly as red, yellow, or green. Most flashing LEDs emit light of a single wavelength, but multicolored flashing LEDs are available too.
Generally, for newer common standard LEDs in 3 mm or 5 mm packages, the following forward DC potential differences are typically measured. The forward potential difference depending on the LED's chemistry, temperature, and on the current (values here are for approx. 20 mA, a commonly-found maximum value).
| Color | Potential Difference (Vf) |
| Infrared | 1. 6 V |
| Red | 1. 8–2. 1 V |
| Orange | 2. 2 V |
| Yellow | 2. 4 V |
| Green | 2. 6 V |
| Blue | 3. 0–3. 5 V |
| White | 3. 0–3. 5 V |
| Ultraviolet | 3. 5 V |
Many LEDs are rated at 3 V maximum reverse potential.
LEDs also behave as photocells, and will generate a current depending on the ambient light. Photosensors or photodetectors are Sensors of Light or other Electromagnetic energy They are not efficient as photocells, and will only produce a few microamperes (µA), but will produce a surprising electrical potential—as much as 2 or 3 V. This is enough to operate an amplifier or a CMOS logic gate. This effect can be used to make an inexpensive light sensor, for example to decide when to turn on the LED illuminator.
Advantages of using LEDs
- LEDs produce more light per watt than incandescent bulbs; this is useful in battery powered or energy-saving devices. [27]
- LEDs can emit light of an intended color without the use of color filters that traditional lighting methods require. This is more efficient and can lower initial costs.
- The solid package of 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.
- When used in applications where dimming is required, LEDs do not change their color tint as the current passing through them is lowered, unlike incandescent lamps, which turn yellow.
- LEDs are ideal for use in applications that are subject to frequent on-off cycling, unlike fluorescent lamps that burn out more quickly when cycled frequently, or HID lamps that require a long time before restarting.
- LEDs, being solid state components, are difficult to damage with external shock. Fluorescent and incandescent bulbs are easily broken if dropped on the ground.
- LEDs 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. [28] Fluorescent tubes typically are rated at about 30,000 hours, and incandescent light bulbs at 1,000–2,000 hours.
- LEDs mostly fail by dimming over time, rather than the abrupt burn-out of incandescent bulbs. [29]
- LEDs light up very quickly. A typical red indicator LED will achieve full brightness in microseconds; Philips Lumileds technical datasheet DS23 for the Luxeon Star states “less than 100ns. ” LEDs used in communications devices can have even faster response times.
- LEDs can be very small and are easily populated onto printed circuit boards.
- LEDs do not contain mercury, unlike compact fluorescent lamps. Mercury (ˈmɜrkjʊri also called quicksilver or hydrargyrum, is a Chemical element with the symbol Hg ( Latinized hydrargyrum
- Due to the human eye's visual persistence LED's can be pulse width or duty cycle modulated in order to save power or achieve an apparent higher brightness for a given power input. The eye will tend to perceive the peak current light level rather than the average current light level when the modulation rate is higher than approximately 1000 hertz and the duty cycle is greater than 15 to 20%. This is also useful when applied to the multiplexing used in 7-segment displays.
Disadvantages of using LEDs
- LEDs are currently more expensive, price per lumen, on an initial capital cost basis, than more conventional lighting technologies. The additional expense partially stems from the relatively low lumen output and the drive circuitry and power supplies needed. However, when considering the total cost of ownership (including energy and maintenance costs), LEDs far surpass incandescent or halogen sources and begin to threaten compact fluorescent lamps. In December 2007, scientists at Glasgow University claimed to have found a way to make Light Emitting Diodes brighter and use less power than energy efficient light bulbs currently on the market by imprinting holes into billions of LEDs in a new and cost effective method using a process known as nanoimprint lithography. The University of Glasgow (Oilthigh Ghlaschu was founded in 1451 in Glasgow, Scotland and along with its contemporary institutions the University of St Andrews Nanoimprint lithography is a novel method of fabricating nanometer scale patterns [30]
- LED performance largely depends on the ambient temperature of the operating environment. Over-driving the LED in high ambient temperatures may result in overheating of the LED package, eventually leading to device failure. Adequate heat-sinking is required to maintain long life. A heat sink (or heatsink) is an environment or object that absorbs and dissipates heat from another object using Thermal contact (either direct or radiant This is especially important when considering automotive, medical, and military applications where the device must operate over a large range of temperatures, and is required to have a low failure rate.
- LEDs must be supplied with the correct current. This can involve series resistors or current-regulated power supplies. [31]
- The spectrum of some white LEDs differs significantly from a black body radiator, such as the sun or an incandescent light. In Physics, a black body is an object that absorbs all light that falls on it The spike at 460 nm and dip at 500 nm can cause the color of objects to be perceived differently under LED illumination than sunlight or incandescent sources, due to metamerism. Color vision is the capacity of an organism or machine to distinguish objects based on the Wavelengths (or frequencies) of the Light they reflect or emit This article is about metamerism as it relates to Colorimetry. [32] Color rendering properties of common fluorescent lamps are often inferior to what is now available in state-of-art white LEDs.
- LEDs do not approximate a “point source” of light, so cannot be used in applications needing a highly collimated beam. LEDs are not capable of providing divergence below a few degrees. This is contrasted with commercial ruby lasers with divergences of 0. 2 degrees or less. [33] This can be corrected by using lenses and other optical devices.
- There is increasing concern that blue LEDs and white LEDs are now capable of exceeding safe limits of the so-called blue-light hazard as defined in eye safety specifications such as ANSI/IESNA RP-27. 1-05: Recommended Practice for Photobiological Safety for Lamp and Lamp Systems. [34][35]
Types
There are three main types of LEDs: miniature, alphanumeric, and illumination.
Miniature LEDs
These are mostly single-die LEDs used as indicators, and they come in various-size packages:
- surface mount
- 2 mm
- 3 mm (T1)
- 5 mm (T1³⁄₄)
- Other sizes are also available, but less common.
Common package shapes:
- Round, dome top
- Round, flat top
- Rectangular, flat top (often seen in LED bar-graph displays)
- Triangular or square, flat top
The encapsulation may also be clear or semi opaque 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.
Multi-color LEDs
A “bi-color LED” is actually two different LEDs in one case. It consists of two dies connected to the same two leads but in opposite directions. Current flow in one direction produces one color, and current in the other direction produces the another color. Alternating the two colors with sufficient frequency causes the appearance of a third color.
A “tri-color LED” is also 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.
RGB LEDs contain red, green and blue emitters, generally using a four-wire connection with one common (anode or cathode).
Five- and twelve-volt LEDs
These are miniature LEDs incorporating a series resistor, and may be connected directly to a 5 V or 12 V supply. |- align = "center"| |width = "25"| | |- align = "center"| || Potentiometer |- align = "center"| | | |- align = "center"| Resistor| |
Flashing LEDs
These miniature LEDs flash when connected to 5 V or 12 V. Used as attention seeking indicators where it is desired to avoid the complexity of external electronics.
Alphanumeric LEDs
LED displays are available in seven-segment and starburst format. A seven-segment display (abbreviation " 7-seg (ment display" less commonly known as a seven-segment indicator, is a form of electronic Display device A fourteen-segment display (sometimes referred to as a starburst display or a " Union Jack " display is a type of display based on 14 segments that can 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 increasing use of liquid crystal displays, with their lower power consumption and greater display flexibility, has reduced the popularity of numeric and alphanumeric LED displays.
Lighting LEDs
LED lamps (also called LED bars or Illuminators) are usually clusters of LEDs in a suitable housing. A LED lamp is a type of solid state lighting (SSL that utilizes Light-emitting diodes (LEDs as a source of illumination rather than Electrical filaments They come in different shapes, among them the light bulb shape with a large E27 Edison screw and MR16 shape with a bi-pin base. The incandescent light bulb, incandescent lamp or incandescent light globe is a source of electric Light that works by Incandescence, (a general The Edison screw fitting is a system of Light bulb connectors developed by Thomas Edison in 1909 under the Mazda trademark MR16 (sometimes referred to as MR-16) is a standard format for halogen Reflector lamps made by a variety of manufacturers Other models might have a small Edison E14 fitting, GU5.3 (Bipin cap) or GU10 (bayonet socket). The Edison screw fitting is a system of Light bulb connectors developed by Thomas Edison in 1909 under the Mazda trademark A bipin or bi-pin, (sometimes referred to as 2-pin bipin cap or bipin socket is a standard from the IEC for lamp fittings A bayonet mount or bayonet connector is a fastening mechanism that relies on mated surfaces a male side with one or more pins or slots and a female receptor with This includes low-voltage (typically 12 V halogen-like) varieties and replacements for regular AC mains (120-240 V AC) lighting. Currently the latter are less widely available but this is changing rapidly.
Seoul Semiconductor Co., Ltd produces LEDs that can run directly from mains power without the need for a DC converter. Seoul Semiconductor Co Ltd is a Korean company manufacturing LED devices For each half cycle part of the LED diode emits light and part is dark, and this is reversed during the next half cycle. Current efficiency is 80 lm/W. [36]
LED applications
List of LED applications
Some of these applications are further elaborated upon in the following text.
Devices, medical applications, clothing, toys
- Remote controls, such as for TVs and VCRs, often use infrared LEDs. A remote control is an electronic device used for the remote operation of a Machine. Infrared ( IR) radiation is Electromagnetic radiation whose Wavelength is longer than that of Visible light, but shorter than that of
- Glowlights, as a more expensive but longer lasting and reusable alternative to Glowsticks. A glow stick is a single-use translucent Plastic tube containing isolated substances which when combined are capable of producing Light through a Chemical reaction
- Movement sensors, for example in optical computer mice
- The Nintendo Wii's sensor bar uses infrared LEDs. In Computing, a mouse (plural mice, mouse devices, or mouses) Infrared ( IR) radiation is Electromagnetic radiation whose Wavelength is longer than that of Visible light, but shorter than that of
- In optical fiber and Free Space Optics communications. An optical fiber (or fibre) is a Glass or Plastic fiber that carries Light along its length In Telecommunications Free Space Optics (FSO is an Optical communication technology that uses light propagating in free space to transmit data between two points
- Toys and recreational sporting goods, such as the Flashflight
- Lumalive, a photonic textile
- In pulse oximeters for measuring oxygen saturation
- LED phototherapy for acne using blue or red LEDs has been proven to significantly reduce acne over a three-month period. A Flashflight is a Flying disc designed to be used for night-time disc games between two or more individuals Lumalive is a Photonic Textile, which uses cloth as a lighted graphic display medium A textile is a flexible material comprised of a network of natural or artificial Fibres often referred to as thread or Yarn. A pulse oximeter is a Medical device that indirectly measures the oxygen saturation of a patient's Blood (as opposed to measuring oxygen saturation directly through Oxygen saturation is a relative measure of the amount of Oxygen that is Dissolved or carried in a given medium Light therapy or phototherapy consists of exposure to daylight or to specific wavelengths of light using Lasers, LEDs Fluorescent lamps,
- Some flatbed scanners use an array of red, green, and blue 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.
- Computers, for hard drive activity and power on. Some custom computers feature LED accent lighting to draw attention to a given component. Many computer manufacturers use LEDs to tell the user its current state. One example would be the Mac, which tells its user when it is asleep by fading the LED activity lights in and out, in and out.
- Sterilization of water and other substances using UV light. Ultraviolet ( UV) light is Electromagnetic radiation with a Wavelength shorter than that of Visible light, but longer than X-rays [4]
Lighting
- Grow lights composed of LEDs are more efficient, both because LEDs produce more lumens per watt than other alternatives, and also because they can be tuned to the specific wavelengths plants can make the most use of. A grow light is an Electric lamp designed to promote plant growth by emitting an Electromagnetic spectrum appropriate for Photosynthesis.
- Light bulbs
- Lanterns
- Streetlights
- Large scale video displays
- Architectural lighting
- Light source for machine vision systems, requiring bright, focused, homogeneous and possibly strobed illumination. Machine vision (MV System is the application of Computer vision to industry and manufacturing
- Motorcycle and Bicycle lights
- Flashlights, including some mechanically powered models. MotorCycle is the title of a 1993 album by Rock band Daniel Amos, released on BAI Records. Bicycle lighting has two purposes seeing and being seen There are many types of Bicycle lights available each with its own advantages and disadvantages A flashlight (also called an electric torch or torch) is a portable electric spotlight which emits light from a small incandescent Lightbulb A mechanically powered flashlight is one of several varieties of Flashlight that are powered by electricity generated by the muscle power of the user so they do not need
- Emergency vehicle lighting
- Backlighting for LCD televisions and displays. Emergency vehicle lighting refers to any of several visual warning devices which may be known as light bars or beacons fitted to a vehicle and used when the driver wishes to convey to The availability of LEDs in specific colors (RGB) enables a full-spectrum light source which expands the color gamut by as much as 45%. A spectrum (plural spectra or spectrums) is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. In color reproduction including Computer graphics and Photography, the gamut, or color gamut (pronounced /ˈgæmət/ is a certain complete
- Stage lights using banks of LED's as replacement for incandescent bulbs. LED's produce less heat so LED stage lighting is cheaper to operate and reduces the risk of fire considerably.
- LED-based Christmas lights have been available since 2002, but are only now beginning to gain in popularity and acceptance due to their higher initial purchase cost when compared to similar incandescent-based Christmas lights. Christmas lights (also sometimes called fairy lights, twinkle lights or holiday lights in the United States are strands of Electric lights used For example, as of 2006, a set of 50 incandescent lights might cost US$2, while a similar set of 50 LED lights might cost US$10. The purchase cost can be even higher for single-color sets of LED lights with rare or recently-introduced colors, such as purple, pink or white. Regardless of the higher initial purchase price, the total cost of ownership for LED Christmas lights would eventually be lower than the TCO for similar incandescent Christmas lights since the LED requires much less power to output the same amount of light as a similar incandescent bulb. Total cost of ownership (TCO is a financial estimate designed to help consumers and enterprise managers assess direct and indirect costs More to the point, LEDs have practically unlimited life and are hard-wired rather than using unreliable sockets as do replaceable bulbs. So a set of LED lights can be expected to outlive many incandescent sets, and without any maintenance.
Indicators and signs
- Status indicators on a variety of equipment
- Traffic lights and signals
- Exit signs
- Railroad crossing signals
- Continuity indicators
- Elevator push-button Lighting
- Thin, lightweight message displays at airports and railway stations, and as destination displays for trains, buses, trams, and ferries. The traffic light, also known as traffic signal, stop light, traffic lamp, stop-and-go lights, robot or semaphore, An exit sign is a device in a public facility (such as a building airplane or boat that displays where the Emergency exit is The term level crossing (also called a railroad crossing, road through railroad, railway crossing, train crossing or grade crossing
- 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.
- Red, yellow, green, and blue LEDs can be used for model railroading applications
- In dot matrix arrangements for displaying messages. Model railroading (US or Railway modelling (UK Australia and Canada is a Hobby in which Rail transport systems are modeled at a reduced scale
- Because of their long life and fast switching times, LEDs have been used for automotive high-mounted brake lights and truck and bus brake lights and turn signals for some time, but many high-end vehicles are now starting to use LEDs for their entire rear light clusters. Besides the gain in reliability, this has styling advantages because LEDs are capable of forming much thinner lights than incandescent lamps with parabolic reflectors. A parabolic reflector (or dish or mirror) is a Parabola -shaped reflective device used to collect or distribute Energy such as The significant improvement in the time taken to light up (perhaps 0. 5s faster than an incandescent bulb) improves safety by giving drivers more time to react. It has been reported that at normal highway speeds this equals one car length increased reaction time for the car behind. White LED headlamps are beginning to make an appearance.
- As a medium quality voltage reference in electronic circuits. A voltage reference is an electronic device (circuit or component that produces a fixed (constant voltage irrespective of the loading on the device power supply variation and temperature 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. A Zener diode is a type of Diode that permits current in the forward direction like a normal diode but also in the reverse direction if the voltage is larger Although LED forward voltage is much more current-dependent than a good Zener, Zener diodes are not available below voltages of about 3 V.
Optoisolators and optocouplers
The LED may be combined with a photodiode or phototransistor in a single electronic device to provide a signal path with electrical isolation between two circuits. In Electronics, an opto-isolator (or optical isolator, optocoupler, photocoupler, or photoMOS) A photodiode is a type of Photodetector capable of converting Light into either current or Voltage, depending upon the mode of operation A photodiode is a type of Photodetector capable of converting Light into either current or Voltage, depending upon the mode of operation An optoisolator will have typical breakdown voltages between the input and output circuits of typically 500–3000 V. 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. A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument An optoisolator also allows information to be transferred between circuits not sharing a common ground potential. An optocoupler may not have such high breakdown voltages and may even share a ground between input and output, but both types are useful in preventing electrical noise, particularly common mode electrical noise, on a sensor circuit from being transferred to the receiving circuit (where it may adversely affect the operation or durability of various components) and/or transferring a noisy signal. In Telecommunication, the term common-mode interference has the following meanings Interference that appears on both signal leads (signal and circuit Optoisolators are also used in the feedback circuit of a DC to DC converter, allowing power to be transferred while retaining electrical isolation between the input and output. Direct current ( DC) is the unidirectional flow of Electric charge.
Light sources for machine vision systems
Machine vision systems often require bright and homogeneous illumination, so features of interest are easier to process. Machine vision (MV System is the application of Computer vision to industry and manufacturing LEDs are often used to this purpose, and this field of application is likely to remain one of the major application areas until price drops low enough to make signaling and illumination applications more widespread. LEDs constitute a nearly ideal light source for machine vision systems for several main reasons:
- Size of illuminated field is usually comparatively small and Vision systems or smart camera are quite expensive, so cost of LEDs is usually a minor concern, compared to signaling applications. Machine vision (MV System is the application of Computer vision to industry and manufacturing A smart camera is an integrated Machine vision system which in addition to image capture circuitry includes a processor which can extract information from images without need
- 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.
- LEDs often have or can be used with small, inexpensive lenses and diffusers, helping to achieve high light densities and very good lighting control and homogeneity.
- LEDs can be easily strobed (in the microsecond range and below) and synchronized; their power also has reached high enough levels that sufficiently high intensity can be obtained, allowing well lit images even with very short light pulses: this is often used in order to obtain crisp and sharp “still” images of quickly-moving parts.
- LEDs come in several different colors and wavelengths, easily allowing to use the best color for each application, 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 effect of ambient light.
- LEDs usually operate at comparatively low working temperatures, simplifying heat management and dissipation, therefore allowing plastic lenses, filters and diffusers to be used. Waterproof units can also easily be designed, allowing for use in harsh or wet environments (food, beverage, oil industries).
- LED sources can be shaped in several main 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).
- Very compact designs are possible, allowing for small LED illuminators to be integrated within smart cameras and vision sensors. A smart camera is an integrated Machine vision system which in addition to image capture circuitry includes a processor which can extract information from images without need
Power sources
LEDs have very low dynamic resistance, with the same voltage drop for widely varying currents. Consequently they cannot connect directly to most power sources without self destruction. A current control ballast is normally used, which is sometimes constant current.
Indicator LEDs
Miniature indicator LEDs are normally driven from low voltage DC via a current limiting resistor. |- align = "center"| |width = "25"| | |- align = "center"| || Potentiometer |- align = "center"| | | |- align = "center"| Resistor| | Currents of 2 mA, 10 mA and 20 mA are common. Some low current indicators are only rated to 2 mA, and should not be driven at higher current.
Sub-mA indicators may be made by driving ultrabright LEDs at very low current. Efficacy tends to reduce at low currents, but indicators running on 100 μA are still practical. The cost of ultrabrights is higher than 2 mA indicator LEDs.
LEDs have a low max repeat reverse voltage rating, ranging from approximately 2 V to 5 V, and this can be a problem in some applications. Back to back LEDs are immune to this problem. These are available in single color as well as bicolor types. There are various strategies for reverse voltage handling.
In niche applications such as IR therapy, LEDs are often driven at far above rated current. This causes high failure rate and occasional LED explosions. Thus many parallel strings are used, and a safety screen and ongoing maintenance are required.
Alphanumeric LEDs
These use the same drive strategy as indicator LEDs, the only difference being the larger number of channels, each with its own resistor. Seven-segment and starburst LED arrays are available in both common-anode or common-cathode form.
Lighting LEDs on mains
A CR dropper followed by full wave rectification is the usual ballast with series-parallel LED clusters.
A single series string minimises dropper losses, while paralleled strings increase reliability. In practice usually three strings or more are used.
Operation on square wave and modified sine wave (MSW) sources, such as many invertors, causes heavily increased resistor dissipation in CR droppers, and LED ballasts designed for sine wave use tend to burn on non-sine waveforms. In Physics, dissipation embodies the concept of a Dynamical system where important mechanical modes such as Waves or Oscillations lose Energy The non-sine waveform also causes high peak LED currents, heavily shortening LED life. An inductor & rectifier makes a more suitable ballast for such use, and other options are also possible.
Lighting LEDs on low voltage
LEDs are normally operated in parallel strings of series LEDs, with the total LED voltage typically adding up to around two-thirds of the supply voltage, with resistor current control for each string. |- align = "center"| |width = "25"| | |- align = "center"| || Potentiometer |- align = "center"| | | |- align = "center"| Resistor| |
LED current is proportional to power supply (PSU) voltage minus total LED string voltage. Where battery sources are used, the PSU voltage can vary widely, causing large changes in LED current and light output. In electronics a battery is a combination of two or more Electrochemical cells which store chemical Energy which can be converted into electrical energy For such applications, a constant current regulator is preferred to resistor control. Low drop-out (LDO) constant current regs also allow the total LED string voltage to be a higher percentage of PSU voltage, resulting in improved efficiency and reduced power use.
Torches run one or more lighting LEDs on a low voltage battery. These usually use a resistor ballast.
In disposable coin cell powered keyring type LED lights, the resistance of the cell itself is usually the only current limiting device. A watch battery, button cell, silver button cell, or coin cell is a small form-factor battery designed for use in wrist watches pocket The cell should not therefore be replaced with a lower resistance type, such as one using a different battery chemistry.
Finally, LEDs can be run from a single cell by use of a constant current switched mode invertor. The extra expense makes this option unpopular.
See also
- Nystagmus An eye condition in which sufferers have difficulty focusing on LED displays
- Photometry (optics) Main Photometry/Radiometry article—explains technical terms
- LED lamp—solid state lighting (SSL)
- Blinkies
- Throwies
- LED circuit
- Nixie tube
- Light Up the World Foundation
- Lumalive, a photonic textile
References
- Cited
- ^ LED. Nystagmus is a type of eye movement characterized by alternating slow phase movements in one direction and Saccade -like quick phases in the other direction This article deals with the usage of this term in Optics and Lighting. A LED lamp is a type of solid state lighting (SSL that utilizes Light-emitting diodes (LEDs as a source of illumination rather than Electrical filaments Solid-state lighting (SSL refers to a type of Lighting that utilizes Light-emitting diodes (LEDs Organic light-emitting diodes (OLED or Polymer Blinkys are small electronic devices that make very bright light (usually flashing using LEDs and small batteries An LED throwie is a small LED attached to a coin battery and a Rare earth magnet (usually with conductive Epoxy or Electrical tape In Electronics, the basic LED circuit is an electrical circuit used to power a Light-emitting diode (LED A nixie tube is an electronic device for displaying numerals or other information. The Light Up the World Foundation (LUTW is a non-profit humanitarian organization dedicated to providing lighting to poor people in remote areas who currently rely on Kerosene lamps Lumalive is a Photonic Textile, which uses cloth as a lighted graphic display medium A textile is a flexible material comprised of a network of natural or artificial Fibres often referred to as thread or Yarn. Retrieved on 2008-01-04. 2008 ( MMVIII) is the current year in accordance with the Gregorian calendar, a Leap year that started on Tuesday of the Common Events 46 BC - Titus Labienus defeats Julius Caesar in the Battle of Ruspina.
- ^ I. Moreno (2006). "LED Intensity Distribution". . International Optical Design, Technical Digest Retrieved on 2007-08-13. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 3114 BC - According to the Lounsbury correlation the start of the Maya calendar.
- ^ . . . (2008). "Modeling the radiation pattern of LEDs". . Optics Express Retrieved on 2008-01-25. 2008 ( MMVIII) is the current year in accordance with the Gregorian calendar, a Leap year that started on Tuesday of the Common Events 41 - After a night of negotiation Claudius is accepted as Roman Emperor by the Senate
- ^ a b c Development of a new water sterilization device with a 365 nm UV-LED, Medical and Biological Engineering and Computing, Volume 45, Number 12 / December, 2007
- ^ a b Zheludev, N. (2007). "The life and times of the LED — a 100-year history." (PDF). Nature Photonics 1 (4): 189–192. doi:. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.
- ^ Braunstein, Rubin (1955). "Radiative Transitions in Semiconductors," Physical Review 99 1892-3. DOI 10. 1103/PhysRev. 99. 1892. Braunstein observed infrared emission generated by simple diode structures using GaSb, GaAs, InP, and Ge-Si alloys cooled by liquid nitrogen to 77 K.
- ^ The first LEDs were infrared (invisible). The Quartz Watch. The Lemelson Center. Retrieved on 2007-08-13. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 3114 BC - According to the Lounsbury correlation the start of the Maya calendar.
- ^ Nick Holonyak, Jr. 2004 Lemelson-MIT Prize Winner. Lemenson-MIT Program. Retrieved on 2007-08-13. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 3114 BC - According to the Lounsbury correlation the start of the Maya calendar.
- ^ Wolinsky, Howard. "U. of I.'s Holonyak out to take some of Edison's luster", Chicago Sun-Times, February 5, 2005. Events 1576 - Henry of Navarre converts to Roman Catholicism in order to ensure his right to the throne of France. Year 2005 ( MMV) was a Common year starting on Saturday (link displays full calendar of the Gregorian calendar. Retrieved on 2007-07-29. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 1014 - Byzantine-Bulgarian Wars: Battle of Kleidion: Byzantine emperor Basil II inflicts a decisive defeat
- ^ Brief Biography – Holonyak, Craford, Dupuis (PDF). Technology Administration. Retrieved on 2007-05-30. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 1416 - The Council of Constance, called by the Emperor Sigismund a supporter of Antipope John XXIII burns Jerome of Prague following
- ^ 2006 Millennium technolgy prize awarded to UCSB's Shuji Nakamura. Retrieved on 2007-05-30. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 1416 - The Council of Constance, called by the Emperor Sigismund a supporter of Antipope John XXIII burns Jerome of Prague following
- ^ Margolin J. The Road to the Transistor, with references, including quote from Round's 1907 letter. Retrieved on 2007-05-30. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 1416 - The Council of Constance, called by the Emperor Sigismund a supporter of Antipope John XXIII burns Jerome of Prague following
- ^ "LEDs move into the ultraviolet", physicsworld. com, May 17, 2006. Events 1521 - Edward Stafford 3rd Duke of Buckingham, is executed for Treason. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Retrieved on 2007-08-13. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 3114 BC - According to the Lounsbury correlation the start of the Maya calendar.
- ^ Color distribution from multicolor LED arrays (PDF). Optics Express (2007). Retrieved on 2007-05-30. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 1416 - The Council of Constance, called by the Emperor Sigismund a supporter of Antipope John XXIII burns Jerome of Prague following
- ^ Alumni society honors four leaders in engineering and technology. Berkeley Engineering News (2000-09-04). 2000 ( MM) was a Leap year that started on Saturday of the Common Era, in accordance with the Gregorian calendar. Events 476 - Romulus Augustus, last emperor of the Western Roman Empire, is deposed when Odoacer proclaims himself Retrieved on 2007-01-23. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 393 - Roman Emperor Theodosius I proclaims his nine year old son Honorius co-emperor
- ^ GaN-based blue light emitting device development by Akasaki and Amano (pdf). Takeda Award 2002 Achievement Facts Sheet. The Takeda Foundation (2002-04-05). See also 2002 (disambiguation Year 2002 ( MMII) was a Common year starting on Tuesday of the Gregorian calendar. Events 456 - St Patrick returns to Ireland as a missionary bishop Retrieved on 2007-11-28. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. For the town in Argentina, see 28 de Noviembre. Events
- ^ United States Patent No. 5,578,839 (Nakamura et al.). United States Patent and Trademark Office (filed 1993-11-17). The United States Patent and Trademark Office ( PTO or USPTO) is an agency in the United States Department of Commerce that issues Patents to Year 1993 ( MCMXCIII) was a Common year starting on Friday (link will display full 1993 Gregorian calendar) Events 284 - Diocletian is proclaimed emperor by his soldiers Retrieved on 2007-01-23. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 393 - Roman Emperor Theodosius I proclaims his nine year old son Honorius co-emperor
- ^ Sensor Electronic Technology, Inc.: Nitride Products Manufacturer
- ^ "Jan Henrik Wold and Arne Valberg", 2001. Year 2001 ( MMI) was a Common year starting on Monday according to the Gregorian calendar.
- ^ "D. L. MacAdam and Ed. ", 1993. Year 1993 ( MCMXCIII) was a Common year starting on Friday (link will display full 1993 Gregorian calendar)
- ^ "E. Fred Schubert and Jong Kyu Kim", 2005. Year 2005 ( MMV) was a Common year starting on Saturday (link displays full calendar of the Gregorian calendar.
- ^ "Accidental Invention Points to End of Light Bulbs", LiveScience. com, October 21, 2005. Events 1512 - Martin Luther joins the theological faculty of the University of Wittenberg. Year 2005 ( MMV) was a Common year starting on Saturday (link displays full calendar of the Gregorian calendar. Retrieved on 2007-01-24. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 41 - Gaius Caesar (Caligula, known for his eccentricity and cruel Despotism, is Assassinated by his disgruntled
- ^ Quantum-dot LED may be screen of choice for future electronics Massachusetts Institute of Technology News Office,December 18, 2002
- ^ "Seoul Semiconductor squeezes 240 lumens into "brightest" LED", engadget, December 12, 2006. Events 627 - Battle of Nineveh: A Byzantine army under Emperor Heraclius defeats Emperor Khosrau II 's Persian Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Retrieved on 2007-08-13. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 3114 BC - According to the Lounsbury correlation the start of the Maya calendar.
- ^ "Nichia Unveils White LED with 150 lm/W Luminous Efficiency", Tech-On!, December 21, 2006. Events 69 - The end of the Year of the four emperors: Following Galba, Otho and Vitellius, Vespasian Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Retrieved on 2007-08-13. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 3114 BC - According to the Lounsbury correlation the start of the Maya calendar.
- ^ Electrical properties of GaN LEDs & Parallel connections (PDF). Application Note. Nichia. Retrieved on 2007-08-13. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 3114 BC - According to the Lounsbury correlation the start of the Maya calendar.
- ^ Solid-State Lighting: Comparing LEDs to Traditional Light Sources.
- ^ http://www.netl.doe.gov/ssl/PDFs/lifetimeWhiteLEDs_aug16_r1.pdf
- ^ Solid-State Lighting: Lumen Depreciation.
- ^ New efficient bulb sees the light (Web). A new type of super-efficient household light bulb is being developed which could spell the end of regular bulbs. . BBC News (Friday, 28 December 2007). Events 1065 - Westminster Abbey is Consecrated. 1308 - The reign of Emperor Hanazono, Emperor of Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Retrieved on 2008-01-01. 2008 ( MMVIII) is the current year in accordance with the Gregorian calendar, a Leap year that started on Tuesday of the Common New Year See also New Year The Ancient Romans began their consular year on January 1st since 153 BC
- ^ The Led Museum
- ^ James A. Worthey. How White Light Works. LRO Lighting Research Symposium, Light and Color. Retrieved on 2007-10-06. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 105 BC - Battle of Arausio: The Cimbri inflict the heaviest defeat on the Roman army of Gnaeus Mallius Maximus
- ^ Hecht, E: "Optics", Fourth Edition, page 591. Addison Wesley, 2002.
- ^ "Blue LEDs: A health hazard?", texyt. com, January 15, 2007. Events 588 BC - Nebuchadrezzar II of Babylon lays siege to Jerusalem under Zedekiah 's reign Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Retrieved on 2007-09-03. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 36 BC - In the Battle of Naulochus, Marcus Vipsanius Agrippa, Admiral of Octavian, defeats Sextus Pompeius
- ^ Light Impacts: Science News Online, May 27, 2006. Events 927 - Simeon the Great, Tsar of Bulgaria, dies 1120 - Richard III of Capua is anointed Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. 071214 sciencenews. org
- ^ Seoul Semiconductor launches AC LED lighting source Acriche. LEDS Magazine. Retrieved on 2008-02-17. 2008 ( MMVIII) is the current year in accordance with the Gregorian calendar, a Leap year that started on Tuesday of the Common Events 1500 - Battle of Hemmingstedt. 1600 - Philosopher Giordano Bruno is burned alive at Campo de' Fiori
- General
- Shuji Nakamura, Gerhard Fasol, Stephen J Pearton The Blue Laser Diode: The Complete Story, Springer Verlag, 2nd Edition (October 2, 2000)
- Mills, Evan (2005). Events 1187 - Siege of Jerusalem: Saladin captures Jerusalem after 88 years of Crusader rule 2000 ( MM) was a Leap year that started on Saturday of the Common Era, in accordance with the Gregorian calendar. "The Specter of Fuel-Based Lighting". Science 308: 1263–1264.
- Moreno, I. , "Spatial distribution of LED radiation," in The International Optical Design Conference, Proc. SPIE vol. 6342, 634216:1-7 (2006).
- Salisbury, David F. (October 20, 2005). Events 1740 - Maria Theresa takes the throne of Austria. France, Prussia, Bavaria and Saxony Year 2005 ( MMV) was a Common year starting on Saturday (link displays full calendar of the Gregorian calendar. Quantum dots that produce white light could be the light bulb’s successor. Exploration—The Online Research Journal of Vanderbilt University. (More details regarding the use of quantum dots as a phosphor for white LEDs. )
External links
- Solid State Lighting program at U.S. DOE
- Variations of light characteristics of white LEDs
- Photonics Sources Group, Tyndall National Institute GaN and other photonics research at the Tyndall National Institute, Ireland.
- Solid State Lighting, Michael Shur - Rensselaer Polytechnic
- Applications notes about Discrete LEDs including basic driver circuits
Dictionary
light-emitting diode
-noun
- (physics) a rectifying semiconductor device which converts electrical energy into electromagnetic radiation of a wavelength in or near the visible spectrum of light.
Dapyx Software network: MP3 Explorer | Ebook Manager | Zenithic