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Light-emitting diode

A light-emitting diode (LED) is a semiconductor device that emits incoherent monochromatic light when electrically biased in the forward direction. This effect is a form of electroluminescence. The color depends on the semiconducting material used, and can be near-ultraviolet, visible or infrared. Nick Holonyak Jr (1928 - ) developed the first practical visible-spectrum LED in 1962.[1]

Light-emitting diodes

Table of contents
1 LED Technology
2 LED Applications

LED Technology

A LED is a special type of semiconductor diode. Like a normal diode, it consists of a chip of semiconducting material impregnated, or doped, with impurities to create a structure called a pn junction. Charge-carriers (electrons and holess) are created by an electric current passing through the junction, and release energy in the form of photons as they recombine. The wavelength of the light, and therefore its colour, depends on the bandgap energy of the materials forming the pn junction. A normal diode, typically made of silicon or germanium, emits invisible far-infrared light, but the materials used for a LED have bandgap energies corresponding to near-infrared, visible or near-ultraviolet light.

Unlike incandescent bulbs, which can operate with either AC or DC, LEDs require a DC supply of the correct polarity. When the voltage across the pn junction is in the correct direction, a significant current flows and the device is said to be forward biased. The voltage across the LED in this case is fixed for a given LED and is proportional to the energy of the emitted photons. 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.

Conventional LEDs are made of inorganic minerals such as:

LED development began with infrared and red devices, and technological advances have made possible the production of devices with ever shorter wavelengths.

Blue LEDs became available in the late 1990s. They can be added to existing red and green LEDs to produce white light. Zinc selenide (ZnSe) LEDs can produce white light by emitting blue light from the pn junction which is then mixed with red-to-green light created by photoluminescence in the ZnSe.

The most recent innovation in LED technology is a device that can emit ultraviolet light. When ultraviolet light illuminates certain materials, these materials will fluoresce or give off visible light. White light LEDs have been produced by building ultraviolet elements inside material that fluoresces to produce white light.

Ultraviolet and blue LEDs are relatively expensive compared to the more common reds, greens, yellows and infrareds and are thus less commonly used in commercial applications.

The semiconducting chip is encased in a solid plastic lens, which is much tougher than the glass envelope of a traditional light bulb or tube. The plastic may be coloured, but this is only for cosmetic reasons and does not affect the colour of the light emitted.

Most typical LEDs are designed to operate with no more than 30-60 milliwatts of electrical power. Around 1999, commercial LEDs capable of continuous use at one watt of input power were introduced. These LEDs used much larger semiconductor die sizes to handle the large power input. As well, the semiconductor die were mounted to metal slugs to allow for heat removal from the LED die. In 2002, 5 watt LEDs were available with efficiencies of 18-22 lumens per watt. It is projected that by 2005, 10 watt units will be available with efficiencies of 60 lumens per watt. These devices will produce about as much light as a common 50 watt incandescent bulb, and will facilitate use of LEDs for general illumination needs.

In the last few years (up to 2003) there has been much research into organic LEDs or OLEDs, which are made of semiconducting organic polymers. The best efficiency of an OLED so far is about 10%. These promise to be much cheaper to fabricate than inorganic LEDs, and large arrays of them can be deposited on a screen using simple printing methods to create a colour graphic display.

LED Applications

LEDs offer benefits in terms of maintenance and safety. The typical working lifetime of a device, including the bulb, is ten years, which is much longer than the lifetimes of most other light sources. LEDs give off less heat than incandescent light bulbs and are less fragile than fluorescent lamps. Since an individual device is smaller than a centimeter in length, LED-based light sources used for illumination and outdoor signals are built using clusters of tens of devices.

Incandescent light bulbs for traffic signals and pedestrian crosswalks are gradually being replaced by LED clusters.

Lighting systems using incandescent bulbs are cheap to buy but inefficient, generating from about 16 lumen per watt for a domestic tungsten bulb to 22 lm/W for a halogen bulb. Fluorescent tubes are more efficient, from 50 to 100 lm/W for domestic tubes, allowing large energy savings, but are bulky and fragile and require starter circuits. LEDs are robust and moderately efficient, up to 32 lumen per watt, but are expensive, although their cost is falling. The technologies for LED production are developing rapidly.

The largest LED display in the world is 36 m high, at Times Square, Manhattan.

Conventional and SMD

There are two types of LED panels: conventional, using discrete LEDs, and SMD (Surface Mount Device) panels. Most outdoor screens and some indoor screens are built around discrete LEDs, also known as individually mounted LEDs. A cluster of red, green, and blue diodes is driven together to form a full-color pixel, usually square in shape. These pixels are spaced evenly apart and are measured from center to center for absolute pixel resolution.

Most indoor screens on the market are built using SMD technology — a trend that is now extending to the outdoor market. An SMD pixel consists of red, green, and blue diodes mounted on a chipset, which is then mounted on the driver PC board. The individual diodes are smaller than a pin and are set very close together. The difference is that minimum viewing distance is reduced by 25% from the discrete diode screen with the same resolution.

Indoor use generally requires a screen that is based on SMD technology and has a minimum brightness of 600 nits (a standard unit of luminance — candelas per square meter). This will usually be more than sufficient for corporate and retail applications, but under high ambient-brightness conditions, you may need more punch to compete. Fashion and auto shows are two examples of high-brightness stage lighting that may require a higher LED brightness. Conversely, when your screen may be in a shot on a television show, the requirement will often be for lower brightness levels with lower color temperatures.

For outdoor use, you need at least 2,000 nits for most situations, whereas higher brightness types of up to 5,000 nits cope even better with direct sunlight on the screen. Until recently, only discrete diode screens could achieve that brightness level. (The brightness of LED panels also can be turned down.)

For specific projects, you need to take into account factors such as sight lines, local authority planning requirements (if the installation is to become semi-permanent), vehicular access (trucks carrying the screen, truck-mounted screens, or cranes), cable runs for power and video (accounting for both distance and health and safety requirements), power, suitability of the ground for the location of the screen (check to make sure there are no pipes, shallow drains, caves, or tunnels that may not be able to support heavy loads), and overhead obstructions.