A Light Emitting Diode (LED) is a semiconductor light source. LEDsare used as indicator lamps in many devices and are increasingly usedfor other lighting. Introduced as a practical electronic component in1962, early LEDs emitted low-intensity red light, but modern versionsare available across the visible, ultraviolet, and infrared wavelengths,with very high brightness.Light-emitting diodes are used inapplications as diverse as aviationlighting, automotivelighting, advertising, generallighting, and traffic signals. LEDs haveallowed new text, video displays, andsensors to be developed, while their highswitching rates are also useful inadvanced communications technology.Infrared LEDs are also used of many commercial products includingin the remote control unitstelevisions, DVD players, and other domestic appliances.
Russian Oleg Vladimirovich Losev reportedcreation of the first LED in 1927. His research wasdistributed in Russian, German and Britishscientific journals, but no practical use was madeof the discovery for several decades. Hewlett Packard (HP) introduced LEDs in 1968. The technology proved to have major uses for alphanumeric displays and was integrated into HPs early handheld calculators. In the 1970s commercially successful LED devices at less than five cents each were produced by Fairchild Optoelectronics.
The first commercial LEDs werecommonly used as replacementsfor incandescent and neon indicatorlamps, and in seven-segmentdisplays, first in expensive equipmentsuch as laboratory and electronicstest equipment, then later in suchappliances as TVs, radios, telephones,calculators, and even watches. Thesered LEDs were bright enough only foruse as Indicators, as the light outputwas not enough to illuminate an area.
The LED consists of a chip of semiconducting material doped with impurities to create a p-n junction. As in other diodes, current flows easily from the p-side, or anode, to the n-side, or cathode, but not in the reverse direction. Charge-carriers electrons and holes—flow into the junction from electrodes with different voltages. When an electron meets a hole, it falls into a lower energy level, and releases energy in the form of a photon.Most materials used for LEDproduction have veryhigh refractive indices. This meansthat much light will be reflectedback into the material at thematerial/air surface interface.
In an organic light-emitting diode(OLED), the electroluminescent materialcomprising the emissive layer of thediode is an organic compound. The organicmaterial is electrically conductive due tothe delocalization of pi electrons causedby conjugation over all or part of themolecule, and the material thereforefunctions as an organicsemiconductor. The organic materials canbe small organic molecules ina crystalline phase, or polymers.
LEDs are produced in a variety of shapes and sizes.The colour of the plastic lens is often the same asthe actual colour of light emitted, but not always. Forinstance, purple plastic is often usedfor infrared LEDs, and most blue devices have clearhousings. Modern high power LEDs such as thoseused for lighting and backlighting are generally foundin surface-mount technology (SMT) packages
These are mostly single-die LEDs used asindicators, and they come in various sizes from2 mm to 8 mm, through-hole and surfacemount packages. They usually do not use aseparate heat sink. Typical current ratingsranges from around 1 mA to above 20 mA. Thesmall size sets a natural upper boundary onpower consumption due to heat caused by thehigh current density and need for a heat sink.
Many LED semiconductor chips are potted in clear or colour mouldedplastic shells. The plastic shell has three purposes:1. Mounting the semiconductor chip in devices is easier to accomplish.2. The tiny fragile electrical wiring is physically supported and protected from damage.3. The plastic acts as a refractive intermediary between the relatively high-index semiconductor and low-index open air.The third feature helps to boost the light emission from thesemiconductor by acting as a diffusing lens, allowing light to be emittedat a much higher angle of incidence from the light cone than the barechip is able to emit alone.
Solid-state devices such as LEDs are subject to very limited wear andtear if operated at low currents and at low temperatures. Many of theLEDs made in the 1970s and 1980s are still in service today. Typicallifetimes quoted are 25,000 to 100,000 hours, but heat and currentsettings can extend or shorten this time significantly. The mostcommon symptom of LED (and diode laser) failure is the graduallowering of light output and loss of efficiency. Sudden failures,although rare, can occur as well. Early red LEDs were notable for theirshort lifetime.
Typical indicator LEDs are designed to operate with no more than 30–60 milliwatts (mW) of electrical power. Around 1999, PhilipsLumileds introduced power LEDs capable of continuous use at one watt.These LEDs used much larger semiconductor die sizes to handle thelarge power inputs. Also, the semiconductor dies were mounted ontometal slugs to allow for heat removal from the LED die. Practicalgeneral lighting needs high-power LEDs, of one watt or more. Typicaloperating currents for such devices begin at 350 mA. Idealized example of light emission cones in a LED
LEDs emit more light per watt than incandescent lightbulbs. Their efficiency is not affected by shape andsize, unlike fluorescent light bulbs or tubes.LEDs can emit light of an intended color without usingany color filters as traditional lighting methods need.This is more efficient and can lower initial costs.LEDs can be very small (smaller than 2 mm2) and areeasily populated onto printed circuit boards.
LEDs light up very quickly. A typical red indicatorLED will achieve full brightness in under amicrosecond. LEDs used in communicationsdevices can have even faster response times.LEDs are ideal for uses subject to frequent on-off cycling, unlike fluorescent lamps that failfaster when cycled often, or HID lamps thatrequire a long time before restarting.LEDs can have a relatively long useful life. Onereport estimates 35,000 to 50,000 hours ofuseful life, though time to complete failure maybe longer. Fluorescent tubes typically are ratedat about 10,000 to 15,000 hours, dependingpartly on the conditions of use, andincandescent light bulbs at 1,000–2,000 hours.
In general, all the LED products can be divided into twomajor parts, the public lighting and indoor lighting. LEDuses fall into four major categories: Visual signals where light goes more or less directly from the source to the human eye, to convey a message or meaning. Illumination where light is reflected from objects to give visual response of these objects. Measuring and interacting with processes involving no human vision. Narrow band light sensors where LEDs operate in a reverse-bias mode and respond to incident light, instead of emitting light.
High-power LEDs (HPLED) can be driven at currents from hundredsof mA to more than an ampere, compared with the tens of mA forother LEDs. Some can emit over a thousand lumens.[Since overheatingis destructive, the HPLEDs must be mounted on a heat sink to allowfor heat dissipation. If the heat from a HPLED is not removed, thedevice will fail in seconds. One HPLED can often replace anincandescent bulb in a flashlight, or be set in an array to form apowerful LED lamp. High-power light-emitting diodes (Luxeon,Lumileds)
The vast majority of devices containing LEDs are "safeunder all conditions of normal use", and so are classifiedas "Class 1 LED product"/"LED Klasse 1". At present,only a few LEDs—extremely bright LEDs that also havea tightly focused viewing angle of 8° or less—could, intheory, cause temporary blindness, and so are classifiedas "Class 2". In general, laser safety regulations—andthe "Class 1", "Class 2", etc. system—also apply to LEDs.While LEDs have the advantage over fluorescentlamps that they do not contain mercury, they maycontain other hazardous metals suchas lead and arsenic.
Efficient lighting is needed for sustainablearchitecture. In 2009, a typical 13-watt LED lampemitted 450 to 650 lumens, which is equivalent toa standard 40-watt incandescent bulb. In 2011,LEDs have become more efficient, so that a 6-watt LED can easily achieve the same results. Astandard 40-watt incandescent bulb has anexpected lifespan of 1,000 hours, whereas an LEDcan continue to operate with reduced efficiencyfor more than 50,000 hours, 50 times longer thanthe incandescent bulb.