What is LED?

"Light emitting diodes, commonly called LEDs, are real unsung heroes in the electronics world. They do dozens of different jobs and are found in all kinds of devices. Among other things, they form the numbers on digital clocks, transmit information from remote controls, light up watches and tell you when your appliances are turned on. Collected together, they can form images on a jumbo television screen or illuminate a traffic light."
SOURCE: http://electronics.howstuffworks.com/led.htm

"A light-emitting diode (LED) is a semiconductor device that emits incoherent narrow-spectrum light when electrically biased in the forward direction. This effect is a form of electro luminescence. LEDs are small extended sources with extra optics added to the chip, which emit a complex intensity spatial distribution. The color of the emitted light depends on the composition and condition of the semi conducting material used, and can be infrared, visible or near-ultraviolet. Rubin Braunstein of the Radio Corporation of America first reported on infrared emission from gallium arsenide (GaAs) and other semiconductor alloys in 1955. Experimenters at Texas Instruments, Bob Biard and Gary Pittman, found in 1961 that gallium arsenide gave off infrared (invisible) light when electric current was applied. Biard and Pittman were able to establish the priority of their work and received the patent for the infrared light-emitting diode. Nick Holonyak Jr. of the General Electric Company developed the first practical visible-spectrum LED in 1962."
SOURCE: http://en.wikipedia.org/wiki/Light-emitting_diode

Advantages of LED

• 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 with occupancy sensors, since they are unaffected by frequent on-off cycling, unlike fluorescent lamps that burn out more quickly when cycled frequently.
• LEDs are built inside solid cases that protect them, unlike incandescent and discharge sources, making them extremely durable.
• LEDs have an extremely long life span when conservatively run: upwards of 100 000 hours, twice as long as the best fluorescent bulbs and twenty times longer than the best incandescent bulbs. (Incandescent bulbs can also be made to last an extremely long time by running at lower than normal voltage, but only at a huge cost in efficiency; LEDs have a long life when operated at their rated power.) LEDs ran at higher currents have a reduced life span. Further, 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 are produced in an array of shapes and sizes.(fig. 1) The 5mm cylindrical package (red, fifth from the left) is the most common, estimated at 80% of world production. The color of the plastic lens is often the same as the actual color of light emitted, but not always. For instance, purple plastic is often used for infrared LEDs, and most blue devices have clear housings. There are also LEDs in extremely tiny packages, such as those found on blinkers (not shown).

Common Applications of LED

• Architectural lighting
• Status indicators on all sorts of equipment
• Traffic lights and signals
• Exit signs
• Motorcycle and Bicycle lights
• Toys and recreational sporting goods, such as the Flashlight
• Railroad crossing signals
• Continuity indicators
• Flashlights. Some models that do not even use batteries are of this type.
• Light bars on emergency vehicles.
• Elevator Push Button Lighting
• Thin, lightweight message displays at airports and railway stations and as destination displays for trains, buses, trams and ferries.
• 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
• Remote controls, such as for TVs and VCRs, often use infrared LEDs.
• In optical fiber and Free Space Optics communications.
• In dot matrix arrangements for displaying messages.
• Glow lights, as a more expensive but longer lasting and reusable alternative to Glow sticks.
• Movement sensors, for example in optical computer mice
• 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. 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.
• Backlighting for LCD televisions and displays. The availability of LEDs in specific colors (RGB) enables a full-spectrum light source which expands the color gamut by as much as 45%.
• New stage lighting equipment is being developed with LED sources in primary red-green-blue arrangements.
• Lumalive, a photonic textile
• 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. For example, as of 2006, a set of 50 incandescent lights might cost $2.00 USD, while a similar set of 50 LED lights might cost $10.00 USD. 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 an LED requires much less power to output the same amount of light as a similar incandescent bulb.
• As a medium quality voltage reference in electronic circuits. The forward voltage drop (e.g. about 1.7V for a normal red LED) can be used instead of a Zener diode in low-voltage regulators. Although LED forward voltage is much more current-dependent than a good Zener, Zener diodes are not available below voltages of about 3V.
• Computers, for hard drive activity and power on. Some custom computers feature LED accent lighting to draw attention to a given component. Macintosh computers use LEDs to tell the user that it is asleep, by fading in and out. This fading is produced by either a triangular wave or a sine wave generator.