LED: Light-emitting diode

light emitting-diode (LED) is a semiconductor diode that emits light when an electric current is applied in the forward direction of the device, as in the simple LED circuit. The effect is a form of electroluminescence where incoherent and narrow-spectrum light is emitted from the p-n junction.

LEDs are widely used as indicator lights on electronic devices and increasingly in higher power applications such as flashlights and area lighting. An LED is usually a small area (less than 1 mm2) light source, often with optics added to the chip to shape its radiation pattern and assist in reflection. The color of the emitted light depends on the composition and condition of the semiconducting material used, and can be infrared, visible, or ultraviolet. Besides lighting, interesting applications include using UV-LEDs for sterilization of water and disinfection of devices, and as a grow light to enhance photosynthesis in plants.

Color	Wavelength (nm)	Voltage (V)
Infrared	λ > 760	ΔV < 1.9
Red	610 < λ < 760	1.63 < ΔV < 2.03
Orange	590 < λ < 610	2.03 < ΔV < 2.10
Yellow	570 < λ < 590	2.10 < ΔV < 2.18
Gree	500 < λ < 570	2.18 < ΔV < 4.0
Blue	450 < λ < 500	2.48 < ΔV < 3.7
Purple	multiple types	2.48 < ΔV < 3.7
Violet	400 < λ < 450	2.76 < ΔV < 4.0
Ultraviolet	λ < 400	3.1 < ΔV < 4.4
White	Broad spectrum	ΔV = 3.5

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LED applications

Indicators and signs: Outdoor 4 x 3 m large LED screen used as clock.

Car lighting: LED daytime running lights for cars.

Flashlights and lanterns: the ones that utilize white LEDs are becoming increasingly popular because of their durability and longer battery life.

Advantages of using LEDs

Efficiency: LEDs produce more light per watt than incandescent bulbs; this is useful in battery powered or energy-saving devices.
Color: 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.
Size: LEDs can be very small (>2 mm2) and are easily populated onto printed circuit boards.
On/Off time: 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.
Cycling: 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.
Dimming: LEDs can very easily be dimmed either by Pulse-width modulation or lowering the forward current.
Slow failure: LEDs mostly fail by dimming over time, rather than the abrupt burn-out of incandescent bulbs.
Lifetime: LEDs can have a relatively long useful life. One report estimates 35,000 to 50,000 hours of useful life, though time to complete failure may be longer.
Fluorescent tubes typically are rated at about 30,000 hours, and incandescent light bulbs at 1,000–2,000 hours.
Shock resistance: LEDs, being solid state components, are difficult to damage with external shock, unlike fluorescent and incandescent bulbs which are fragile.
Focus: The solid package of the 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
Toxicity: LEDs do not contain mercury, unlike fluorescent lamps.

Types of LED

The main types of LEDs are miniature, high power devices and custom designs such as alphanumeric or multi-color.

Five- and twelve-volt LEDs

These are ordinary miniature LEDs that incorporate a suitable series resistor for direct connection to a 5 V or 12 V supply.

Flashing LEDs

Flashing LEDs are used as attention seeking indicators where it is desired to avoid the complexity of external electronics. Flashing LEDs resemble standard LEDs but they contain an integrated multivibrator circuit inside which causes the LED to flash with a typical period of one second. In diffused lens LEDs this is visible as a small black dot. Most flashing LEDs emit light of a single color, but more sophisticated devices can flash between multiple colors and even fade through a color sequence using RGB color mixing.

High power LEDs

High power LEDs (HPLED) can be driven at hundreds of mA (vs. tens of mA for other LEDs), some with more than one ampere of current, and give out large amounts of light. Since overheating is destructive, the HPLEDs must be highly efficient to minimize excess heat; furthermore, they are often mounted on a heat sink to allow for heat dissipation. If the heat from a HPLED is not removed, the device will burn out in seconds.
A single HPLED can often replace an incandescent bulb in a flashlight, or be set in an array to form a powerful LED lamp.
LEDs have been developed that can run directly from mains power without the need for a DC converter. For each half cycle part of the LED emits light and part is dark, and this is reversed during the next half cycle. The efficiency of HPLEDs is typically 40 lm/W[]. As of November 2008 some HPLEDs manufactured by Cree, Inc exceed 95 lm/W [] (e.g. the XLamp MC-E LED chip emitting Cool White light) and are being sold in lamps intended to replace incandescent, halogen, and even fluorescent style lights as LEDs become more cost competitive.

Multi-color LEDs

A “bi-color LED” is actually two different LEDs in one case. It consists of two dies connected to the same two leads but in opposite directions. Current flow in one direction produces one color, and current in the opposite direction produces the other color. Alternating the two colors with sufficient frequency causes the appearance of a blended third color. For example, a red/green LED operated in this fashion will color blend to produce a yellow appearance.
A “tri-color LED” is also two LEDs in one case, but the two LEDs are connected to separate leads so that the two LEDs can be controlled independently and lit simultaneously. A three-lead arrangement is typical with one commmon lead (anode or cathode).
RGB LEDs contain red, green and blue emitters, generally using a four-wire connection with one common lead (anode or cathode).
The Taiwanese LED manufacturer Everlight has introduced a 3 watt RGB package capable of driving each die at 1 watt.

Alphanumeric LEDs

LED displays are available in seven-segment and starburst format. Seven-segment displays handle all numbers and a limited set of letters. Starburst displays can display all letters.
Seven-segment LED displays were in widespread use in the 1970s and 1980s, but increasing use of liquid crystal displays, with their lower power consumption and greater display flexibility, has reduced the popularity of numeric and alphanumeric LED displays.

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