A LED produces a Rayonnement incoherent Monochromatique starting from an energy conversion. This radiation has a continuous emission spectrum. It belongs to the family of the components Optoélectronique S.
HistoryNick Holonyak Jr. (born in 1928) is the first to have created a LED with Visible spectrum in 1962. For a long time, the researchers believed to have to limit itself to the three colors: red, yellow and green the blue diode was developed in the Années 1990, followed by the white diode, starting point of new major applications: lighting, screens of television sets and computers.
Various familiesThere exist several manners of classifying the LED between them.
Classification according to the powerThe first is a classification by power:
- LED of low power < 1 [[Watt W]]. They are most known of the general public because they are present in our daily newspaper since years. It is they which play the part of luminous indicator on the apparatuses electric household appliances for example.
- the LED of strong power > 1 W. Often ignored general public, they are however in full rise and their applications are often ignored: flashes of cellphones and even general lighting. The principle of operation is certainly the same one, but important differences exist between the two families. It results from it that they have each one their own field of application.
Classification according to the emission spectrumAnother manner of classifying them is the nature of their spectrum. The reason of the distinction lies in the fact that the white LED can be used to light, which is one of the headlights applications of the future (near):
- LED of color: their spectrum is almost monochromatic since very end (it with the shape of one peak to the Analyseur of spectrum).
- white LED: their spectrum consists of several wavelengths.
Mechanism of emissionIt is at the time of the recombination of a electron and a hole in a semiconductor that there is emission of a Photon. Indeed, the transition from an electron between the band of conduction and the valence band can be done with the conservation of the Vecteur of wave . It is then radiative (emissive) and it is accompanied by the emission of a photon. In an emissive transition, the energy of the photon created is given by the difference of the energy levels before (Ei) and after (Ef) the transition:
A electroluminescent Diode is a junction PN which must be polarized in direct direction when one wants to emit light. The majority of the recombinations are radiative. The transmitting face of the LED is the zone P because it is radiative.
Techniques of manufactureThe Wavelength of the Rayonnement emitted is determined by the prohibited bandwidth and thus depends on material used. All the values of the luminous Specter can be reached with current materials. To obtain Infrarouge, the adapted material is the Gallium arsenide (GaAs) with like doping agent of the If or the Zn. The manufacturers propose many types of diodes to different specificities. One can quote the most widespread type: the diodes in GaAs, they are most economic and have a general use. Although they require a higher tension direct, the diodes in GaAlAs offer more a great power of exit, have a shorter wavelength (< 950 Nm, which corresponds to the maximum of sensitivity of the detectors to silicon) and have a good linearity up to 1.5 A. Enfin, the diodes with double GaAlAs (DH) heterojunction offer the advantages of the two preceding techniques (weak direct tension) by having very short switching times (duration necessary so that a current grows of 10% to 90% of its end value or to decrease from 90% to 10%), which allows flows of data very high in the numerical data transmissions by fibers optics. The switching times depend on the capacity of the junction in the diode.
Watt (lm/W). This parameter makes it possible to compare the effectiveness of the transformation energy into visible light of the various sources of light. It is sometimes wrongly called Apparent brightness.
According to the types of LED, the luminous output is included/understood between 15 and more than 100 lm/W. A great disparity in the performances is present according to the color and the power of the LED. The blue ones and the white ones do not exceed the 40 lm/W whereas the green ones can have an output much more raised owing to the fact that the eye discriminates little the wavelengths close to the green the theoretical limit of a source which would completely transform all electrical energy into visible light is of 683 lm/W. For that, it would be necessary that it has a monochromatic spectrum wavelength 555 Nm. A LED can approach this case because of its smoothness but, at present, we are seldom beyond 100 lm/W.
If the luminous output of the white LED is relatively average: superior with that of the lamps incandescent but lower than that of the fluorescent lamps or the gas-discharge lamps, the Apparent brightness is on the other hand very high: the spectrum of the emitted light is almost completely contained in the field of the visible one (the wavelengths lie between 400 Nm with 700 Nm). Contrary to the incandescent lamps and the gas-discharge lamps, the LED almost do not emit a Infrarouge. The LED of color generally have a better luminous output but do not enter in competition with the white traditional sources.
The luminous output depends on the design of the LED. To leave the device (semiconductor then external envelope in epoxy), the photons must cross (without being absorptive) the semiconductor, of the junction to surface, then to cross the surface of the semiconductor without undergoing reflection and, in particular not to undergo the total reflection interns which represents the large majority of the cases. Once arrived in the external epoxy resin envelope (sometimes tinted for practical reasons and not for optical reasons), the light crosses the interfaces towards the air to incidence close to the normal as the shape of dome with a diameter much larger allows it than the chip (3 to 5 mm instead of 300 [[Micrometer µm]]). In the LED of last generation, in particular for lighting, this plastic dome is the subject of an special attention, because the chips are rather millimetre-length in this case and the chart send-out must be of good quality. Contrary, for gadgets, one finds LED almost without dome.
FormThis component can be encapsulated in various forms intended to channel the flow of emitted light in a precise way: cylindrical with end rounded into 3,5,8 and 10 mm diameter, cylindrical with end flat, rectangular, on bent support, in crossing technology or to go up on the surface (CMS). The LED of power have, they, of the more homogeneous forms: the luxeon 1W opposite is rather representative.
- the general luminous intensity of the LED is rather weak, but sufficient for indication on table. The building of NASDAQ, with New York has an animated luminous frontage entirely carried out in LED (a few tens of thousands).
- Of the LED known as super luminous was born at the end of the 20th century.
The color of a LED can be generated various manners:
- coloring due to the wavelength of the semiconductor (transparent cap)
- coloring modified by the cap of diode (blue emission or UV + coating containing phosphorus)
- coloring by several emissions different wavelength: the polychromatic LED . They in particular make it possible to propose a vast color range.
Here some colorings according to the semiconductor used:
For the white, one does not speak but temperature wavelength of color (Tc). That of the LED is rather variable according to the model.
Strong points and weaknesses(see also Lamp with diode électroluminescente#Points strong and weaknesses)
- Weak with very weak electricity consumption due to a very good output (a few tens of milliwatts).
- Lifespan much longer than a traditional Lamp incandescent and an end which is declared by a progressive fall of output and not by a abrupt breakdown .
- Size much smaller than the traditional lamps. By assembling several LED, one can carry out lightings with innovative forms.
- Operation in Very low tension (TBT), pledge of safety and ease of transport. There exists for the campers of the flashlights with LED actuated by a simple dynamo with hand (“crank lamp”) of slow movement.
- considerable Asset as regards safety, compared to the traditional luminous systems, them luminous inertia is almost null. They ignite and die out in a very short time.
- Considering their power, the traditional LED 5 mm almost do not heat and do not burn the fingers. For the assemblies of power higher than 1 W, it is necessary to envisage a dissipation of heat without what the LED will be strongly damaged even destroyed because of heating. Indeed, a LED converts approximately 20% of electrical energy into light, the remainder being released in the form of heat.
- the LED known as white are generally blue LED covered with phosphorus, generally of the YAG: This (Yttrium Garnet Aluminum doped with Cerium). This white is generally cold and has a bad Indice of returned color (IRC).
- Disadvantages specific to the LED of strong power:
- the luminous output is weaker.
- the lifespan is weaker.
- the blue LED as well as the white LED contain a blue spectrum of strong dangerous intensity for the retina if their radiation enters the field of view, even peripheral. The problem arises for example with the flashes containing LED.
- the light blue, even of low intensity, present in a room to be slept during the night (for example, apparatus day before or radio-alarm) disturbs the cycle of the sleep by decreasing the synthesis of the Mélatonine.
- In 2006, the price with the purchase of the LED remains of two to four times higher than that of the traditional lamps, with equal Luminosité.
- Indication of state of apparatuses various (indicator lamps opposite before or on the circuit, instrument panels of cars)
- alphabetical or numerical Posting of measuring devices, computers, clock
- Postings of levels of measurements (levels of tanks, Modulation meter S)
- static or dynamic Posting of messages (luminous newspapers)
- Optocoupleur S
- transmissions of signals by Fiberoptic S
- remote controls (infra-red LED)
- electric eyes (infra-red LED)
- laser beam for the measuring devices
- laser beam for the reading and engraving of CD' S and DVD 'S
- lighting invisible for surveillance cameras (in the infra-red)
White LEDThe improvement of the output of the LED makes it possible to employ them to replace fluorescence or incandescent lamps, with the proviso of assembling them of sufficient number:
- LED drowned in the Bitumen for matérialisation of the tracks the night or per time of Fog.
- individual portable Indication (pedestrian, cyclist).
- Emergency lighting
- Lighting of short range transportable.
- Traffic lights automobile or motorcylcist (Flickering, night lights, Side-lights).
- In 2007, Audi and Lexus should profit from exemptions of the European commission to market models provided with front fires containing LED.
- Several cities replace their public lighting by LED in the goal to decrease their invoice of electricity and the luminous Pollution of the sky (lighting directional downwards). The recourse to the LED is also current in the tricolor fires. The example of Grenoble is generally quoted: the city carried out its return on investment in three years only. Indeed, the LED allow energy saving, but in fact especially the costs of maintenance drop, because of their robustness.
- Flashlights to piles or accumulating with Generating of refill built-in.
- Lamps of beaconing of the gardens supplied with Solar panel.
- Since 2006, the American group Graffiti Research Lab launched a movement named Led throwies (or thrown LED) which consists in brightening the public places by adding color on magnetic surfaces. For this, one combines a LED, a Pile with lithium and a Aimant, and one launches the unit on a magnetic surface.
- Recently, the LED are used to produce video screens of very big size (plates TV living room in large halls, stage…)
- screens of laptops lower than 13.3 inches of diagonal. The rétroéclairage of the screen by LED makes it possible to manufacture finer, more luminous screens and more treasurers that his predecessor LCD with rétroéclairage by fluorescent tube (technology CCFL). To note that the manufacturers remain rather fuzzy on the fact that the LED release more heat.
ConnectionThe electroluminescent diodes are polarized: account of the polarity will be taken (diagram in top of the page). It is always necessary to take account of the maximum intensity (typical: 10 to 30 my for a LED of indication) supported by the diode and thus to intercalate a resistance in series, calculated according to the supply voltage (Law of Ohm). For the applications of lighting, one will be able to gather several diodes in a diagram parallel series: it will be necessary in this case to take account of the voltage drop caused by the diodes in series to calculate resistance in series: the more there will be diodes in series, stronger will be the voltage drop; what will make it possible to decrease resistance in series and thus to increase the output of the device. The acceptable maximum current, as for him, will be multiplied by the number of diodes in parallel.
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