The LED was first invented in Russia in the 1920s, and introduced in America as a practical electronic component in 1962. Oleg Vladimirovich Losev was a radio technician who noticed that diodes used in radio receivers emitted light when current was passed through them. In 1927, he published details in a Russian journal of the first ever LED.

All early devices emitted low-intensity red light, but modern LED are available across the visible, ultraviolet and infra red wavelengths, with very high brightness.

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LED are based on the semiconductor diode. When the diode is forward biased (switched on), electrons are able to recombine with holes and energy is released in the form of light. This effect is called electroluminescence and the color of the light is determined by the energy gap of the semiconductor. The LED is usually small in area (less than 1 mm2) with integrated optical components to shape its radiation pattern and assist in reflection.

LED present many advantages over traditional light sources including lower energy consumption, longer lifetime, improved robustness, smaller size and faster switching. However, they are relatively expensive and require more precise current and heat management than traditional light sources.

Applications of LED are diverse. They are used as low-energy indicators but also for replacements for traditional light sources in general lighting and automotive lighting. The compact size of LED has allowed new text and video displays and sensors to be developed, while their high switching rates are useful in communications technology.

LED: Practical use

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The first commercial LED were commonly used as replacements for incandescent indicators, and in seven-segment displays, first in expensive equipment such as laboratory and electronics test equipment, then later in such appliances as TVs, radios, telephones, calculators, and even watches (see list of signal applications). These red LED were bright enough only for use as indicators, as the light output was not enough to illuminate an area. Later, other colors became widely available and also appeared in appliances and equipment. As the LED materials technology became more advanced, the light output was increased, while maintaining the efficiency and the reliability to an acceptable level. The invention and development of the high power white light LED led to use for illumination. Most LED were made in the very common 5 mm T1¾ and 3 mm T1 packages, but with increasing power output, it has become increasingly necessary to shed excess heat in order to maintain reliability, so more complex packages have been adapted for efficient heat dissipation. Packages for state-of-the-art high power LED bear little resemblance to early LED.

LED: Advantages

• Efficiency: LED produce more light per watt than incandescent bulbs.
• Color: LED 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: LED can be very small (smaller than 2 mm2) and are easily populated onto printed circuit boards.
• On/Off time: LED light up very quickly. A typical red indicator LED will achieve full brightness in microseconds. LED used in communications devices can have even faster response times.
• Cycling: LED 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: LED can very easily be dimmed either by Pulse-width modulation or lowering the forward current.
• Cool light: In contrast to most light sources, LED radiate very little heat in the form of IR that can cause damage to sensitive objects or fabrics. Wasted energy is dispersed as heat through the base of the LED.
• Slow failure: LED mostly fail by dimming over time, rather than the abrupt burn-out of incandescent bulbs.
• Lifetime: LED 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 10,000 to 15,000 hours, depending partly on the conditions of use, and incandescent light bulbs at 1,000–2,000 hours.
• Shock resistance: LED, 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: LED do not contain mercury, unlike fluorescent lamps.

LED: Disadvantages