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3.7.1 Introduction:

The term LED stands for Light Emitting Diode. Modern electronics relies heavily upon LED light bulbs. For instance, LED‘s transmit information from remote controls, are used in traffic lights, digital LED clocks, flashlights, and to form images on jumbo television screens.

These low-power, smaller-sized light emitting diode (LED) devices are based on the company's existing standard and high brightness silicon carbide (sic) product technology. These new devices consume 50% the power and represent cost savings over the current standard and high brightness blue and green LED‘s.

These devices are available in production quantities and are currently shipping into high volume consumer applications. Target applications for these new devices include cellular phones, high-resolution video boards and segmented LED displays.

Physical function:

A LED is a special type of semiconductor diode. Like a normal diode, it consists of a chip of semi conducting material impregnated, or doped, with impurities to create a structure called 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 as it does so.

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 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. The materials used for an LED have a direct band gap with energies corresponding to near-infrared, visible or near-ultraviolet light.

LED development began with infrared and red devices made with gallium arsenide.

Advances in materials science have made possible the production of devices with ever-shorter wavelengths, producing light in a variety of colors.

61 3.7.2 Advantages and Disadvantages of LED’s:

Advantages:

 LED‘s are capable of emitting light of an intended color without the use of color filters that traditional lighting methods require.

 The shape of the LED package allows light to be focused. Incandescent and fluorescent sources often require an external reflector to collect light and direct it in a useable manner.

 LED‘s are insensitive to vibration and shocks, unlike incandescent and discharge sources.

 LED‘s are built inside solid cases that protect them, making them hard to break and extremely durable.

 LED‘s have an extremely long life span: typically ten years, twice as long as the best fluorescent bulbs and twenty times longer than the best incandescent bulbs.

 Further LED‘s fail by dimming over time, rather than the abrupt burnout of incandescent bulbs.

 LED‘s give off less heat than incandescent light bulbs with similar light output.

 LED‘s light up very quickly. An illumination LED will achieve full brightness in approximately 0.01 seconds, 10 times faster than an incandescent light bulb (0.1 second), and many times faster than a compact fluorescent lamp, which starts to come on after 0.5 seconds or 1 second, but does not achieve full brightness for 30 seconds or more. A typical red indicator LED will achieve full brightness in microseconds, or possibly less if it's used for communication devices.

Disadvantages:

 LED‘s are currently more expensive than more conventional lighting technologies.

The additional expense partially stems from the relatively low lumen output (requiring more light sources) and drive circuitry/power supplies needed. A good measure to compare lighting technologies is lumen/dollar.

 LED performance largely depends on the ambient temperature of the operating environment. "Driving" an LED 'hard' in high ambient temperatures may result in overheating of the LED package, eventually to device failure. Adequate heat sinking

62 is required to maintain long life. This is especially important when considering automotive/military applications where the device must operate over a large range of temperatures, with government-regulated output.

3.7.3 LED applications:

.LED‘s are used as informative indicators in various types of embedded systems:

 Status indicators, e.g. on/off lights on professional instruments and consumers audio/video equipment.

 In toys, especially as light up "eyes" of robot toys.

 Seven segment displays, in calculators and measurement instruments, although now mostly replaced by liquid crystal displays.

 Thin, lightweight message displays, e.g. in public information signs (at airports and railway stations and as destination displays for trains, buses, trams and ferries).

 Red or yellow LED‘s are used in indicator and [alpha] numeric 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.

 LED‘s may also be used to transmit digital information:

 Remote controls for TVs, VCRs, etc, using Infrared LED‘s.

 In fiber optic communications.

 In dot matrix arrangements for displaying messages.

 LED‘s find further application in safety devices, where high brightness and reliability are critical:

 In traffic signals, LED clusters are replacing colored incandescent bulbs.

 In level crossing lights, red LED‘s have been used to replace incandescent bulbs.

 In car brake and indicator lights, where the quick-on characteristic of LED‘s enhances safety.

 In bicycle lighting; also for pedestrians to be seen by car traffic.

 Signaling and emergency beacons or strobes.

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 Navigation lights on boats, which are red, green, and white and shine in specific directions. Boats use direct current batteries to power their lights, so not only does that match the requirements of LED‘s, but the efficiency of colored LED‘s is a big advantage.

LED’s are also used for illumination:

 In photographic darkrooms, red or yellow LED‘s are also used for providing lighting, which does not lead to unwanted exposure of the film.

 In flashlights (US) / torches (UK), and backlights for LCD screens.

 As a replacement for incandescent and fluorescent bulbs in home and office lighting, an application known as Solid State Lighting (SSL).

 In projectors. LED projectors are smaller, lighter, and produce much less heat than incandescent technology.