A light-emitting diode (LED) is a semiconductor device that emits incoherent narrow-spectrum light when a p-n junction is forward electrically biased. This effect is a form of electroluminescence. LEDs are a very efficient form of energy production and have already come down in cost far enough to be competitive for a great number of applications such as traffic lights. There are also white LEDs that can be used in place of incandescent light bulbs.
Efficiency
LEDs are an extremely efficient form of lighting. They produce far more light per watt than do incandescent bulbs - this is useful for energy conservation, but also extending the life of battery powered or energy-saving devices.
Lifespan
LEDs also have an extremely long life span. Philips has calculated the ETTF (Estimated Time To Failure) for their LEDs to be between 100,000 and 1,000,000 hours. Fluorescent tubes typically are rated at about 30,000 hours, and incandescent light bulbs at 1,000-2,000 hours.
Polarity
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.
Other Advantages
LEDs have several other advantages:
* 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. * 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 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 CFLs do.