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.
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
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
High initial price: LED are currently more expensive, price per lumen, on an initial capital cost basis, than most conventional lighting technologies. The additional expense partially stems from the relatively low lumen output and the drive circuitry and power supplies needed. However, when considering the total cost of ownership (including energy and maintenance costs), LED far surpass incandescent or halogen sources and begin to threaten compact fluorescent lamps. Temperature dependence: LED performance largely depends on the ambient temperature of the operating environment. Over-driving the LED in high ambient temperatures may result in overheating of the LED package, eventually leading to device failure. Adequate heat-sinking is required to maintain long life. This is especially important when considering automotive, medical, and military applications where the device must operate over a large range of temperatures, and is required to have a low failure rate. Voltage sensitivity: LED must be supplied with the voltage above the threshold and a current below the rating. This can involve series resistors or current-regulated power supplies. Light quality: Most cool-white LED have spectra that differ significantly from a black body radiator like the sun or an incandescent light. The spike at 460 nm and dip at 500 nm can cause the color of objects to be perceived differently under cool-white LED illumination than sunlight or incandescent sources, due to metamerism, red surfaces being rendered particularly badly by typical phosphor based cool-white LED. However, the color rendering properties of common fluorescent lamps are often inferior to what is now available in state-of-art white LED. Area light source: LED do not approximate a "point source" of light, but rather a lambertian distribution. So LED are difficult to use in applications requiring a spherical light field. LED are not capable of providing divergence below a few degrees. This is contrasted with lasers, which can produce beams with divergences of 0.2 degrees or less. Blue Hazard: There is increasing concern that blue LED and cool-white LED are now capable of exceeding safe limits of the so-called blue-light hazard as defined in eye safety specifications such as ANSI/IESNA RP-27.1-05: Recommended Practice for Photobiological Safety for Lamp and Lamp Systems. Blue pollution: Because cool-white LED (i.e., LED with high color temperature) emit much more blue light than conventional outdoor light sources such as high-pressure sodium lamps, the strong wavelength dependence of Rayleigh scattering means that cool-white LED can cause more light pollution than other light sources. It is therefore very important that cool-white LED are fully shielded when used outdoors. Compared to low-pressure sodium lamps, which emit at 589.3 nm, the 460 nm emission spike of cool-white and blue LED is scattered about 2.7 times more by the Earth's atmosphere. Cool-white LED should not be used for outdoor lighting near astronomical observatories. 
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