led light History
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DiscoveryThe first known report of a light-emitting solid-state diode was made in 1907 by the British experimenter H. J. Round. However, no practical use was made of the discovery for several decades.[20] 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).[2] Losev’s work languished for decades.
led light led light History.led lightsled light led light History.led lightsLight sources for machine vision systems
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Machine vision systems often require bright and homogeneous illumination, so features of interest are easier to process. 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 signalling and illumination applications more widespread. LEDs constitute a nearly ideal light source for machine vision systems for several main reasons:
for led light light machine sources systems visionfor led light light machine sources systems visionIllumination applications
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LEDs used as a replacement for incandescent light bulbs and fluorescent lamps are known as solid-state lighting (SSL)—packaged as a cluster of white LEDs grouped together to form a light source. LEDs are moderately efficient; the average commercial SSL currently outputs 32 lumens per watt (lm/W), and new technologies promise to deliver up to 80 lm/W. The long lifetime of LEDs make SSL very attractive. They are also more
Illumination applications led lightIllumination applications led lightLED applications
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List of LED applications
Some of these applications are further elaborated upon in the following text.
Architectural lighting
Status indicators on all sorts of equipment
Traffic lights and signals
Light source for machine vision systems, requiring bright, focused, homogeneous and possibly strobed illumination.
Disadvantages of using LEDs
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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.
led light led lights leds.lampsled light led lights leds.lampsAdvantages of using LEDs
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LEDs produce more light per watt than do incandescent bulbs; this is useful in battery powered or energy-saving devices.
LEDs can emit light of an intended color without the use of color filters that traditional lighting methods require. This is more efficient and can lower initial costs.
The solid package of an LED can be designed to focus its light. Incandescent and 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 have an extremely long life span. One manufacturer has calculated the ETTF (Estimated Time To Failure) for their LEDs to be between 100,000 and 1,000,000 hours.[17] Fluorescent tubes typically are rated at about 30,000 hours, and incandescent light bulbs at 1,000-2,000 hours.
LEDs mostly fail by dimming over time, rather than the abrupt burn-out of incandescent bulbs.
LEDs light up very quickly. A typical red indicator LED will achieve full brightness in microseconds; 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, while compact fluorescent lamps do.
light bulbs
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Unlike incandescent light bulbs, which light up regardless of the electrical polarity, LEDs will only light with positive electrical polarity. When the voltage across the p-n junction is in the 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. 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.
led light led light bulbs light bulbsled light led light bulbs light bulbsConsiderations in use
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=== Advantages of using LEDs ===
{{Unreferenced|date=November 2006}}
[[Image:LED symbol.svg|right|250px|thumb|LED schematic symbol]]
* LEDs produce more light per watt than do incandescent bulbs; this is useful in battery powered or energy-saving devices.
* LEDs can emit light of an intended color without the use of color filters that traditional lighting methods require. This is more efficient and can lower initial costs.
Failure modes
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The most common way for LEDs (and diode lasers) to fail is the gradual lowering of light output and loss of efficiency. 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. Gallium arsenide and aluminum gallium arsenide are more susceptible to this mechanism than gallium arsenide phosphide and indium phosphide. 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. Ionizing radiation can lead to the creation of such defects as well, which leads to issues with radiation hardening of circuits containing LEDs (e.g., in optoisolators). Early red LEDs were notable for their short lifetime.
White LEDs often use one or more phosphors. The phosphors tend to degrade with heat and age, losing efficiency and causing changes in 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. In some cases, especially with GaN/InGaN diodes, a barrier metal layer is used to hinder the electromigration effects. Mechanical stresses, high currents, and corrosive environment can lead to formation of whiskers, causing short circuits.
High-power LEDs are susceptible to current crowding, nonhomogenous distribution of the current density over the junction. This may lead to creation of localized hot spots, which poses risk of thermal runaway. 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. 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.
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. 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. LEDs and lasers grown on sapphire substrate are more susceptible to ESD damage.
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Most typical LEDs are designed to operate with no more than 30–60 milliwatts of electrical power. Around 1999, Philips Lumileds introduced power LEDs capable of continuous use at one watt. These LEDs used much larger semiconductor die sizes to handle the large power input. Also, the semiconductor dies were mounted to metal slugs to allow for heat removal from the LED die.
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