also interesting something about LED: Benefits Of LED

Commercial development [ edit source | edit beta ]

The first commercial LEDs were commonly used as replacements for incandescent and neon indicator lamps, and in seven-segment displays , ^{[ 24 ]} 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 uses ). Until 1968, visible and infrared LEDs were extremely costly, in the order of US$ 200 per unit, and so had little practical use. ^{[ 6 ]} The Monsanto Company was the first organization to mass-produce visible LEDs, using gallium arsenide phosphide (GaAsP) in 1968 to produce red LEDs suitable for indicators. ^{[ 6 ]} Hewlett Packard (HP) introduced LEDs in 1968, initially using GaAsP supplied by Monsanto. These red LEDs were bright enough only for use as indicators, as the light output was not enough to illuminate an area. Readouts in calculators were so small that plastic lenses were built over each digit to make them legible. Later, other colors became widely available and appeared in appliances and equipment. In the 1970s commercially successful LED devices at less than five cents each were produced by Fairchild Optoelectronics. These devices employed compound semiconductor chips fabricated with the planar process invented by Dr. Jean Hoerni at Fairchild Semiconductor . ^{[ 25 ] [ 26 ]} The combination of planar processing for chip fabrication and innovative packaging methods enabled the team at Fairchild led by optoelectronics pioneer Thomas Brandt to achieve the needed cost reductions. ^{[ 27 ]} These methods continue to be used by LED producers. ^{[ 28 ]}

LED display of a TI-30 scientific calculator (ca. 1978), which uses plastic lenses to increase the visible digit size

As LED materials technology grew more advanced, light output rose, while maintaining efficiency and reliability at acceptable levels. The invention and development of the high-power white-light LED led to use for illumination, and is slowly replacing incandescent and fluorescent lighting ^{[ 29 ] [ 30 ]} (see list of illumination applications ).

Most LEDs were made in the very common 5 mm T1¾ and 3 mm T1 packages, but with rising power output, it has grown increasingly necessary to shed excess heat to maintain reliability, ^{[ 31 ]} so more complex packages have been adapted for efficient heat dissipation. Packages for state-of-the-art high-power LEDs bear little resemblance to early LEDs.

Efficiency and operational parameters [ edit source | edit beta ]

Typical indicator LEDs are designed to operate with no more than 30–60 milliwatts (mW) of electrical power. Around 1999, Philips Lumileds introduced power LEDs capable of continuous use at one watt . These LEDs used much larger semiconductor die sizes to handle the large power inputs. Also, the semiconductor dies were mounted onto metal slugs to allow for heat removal from the LED die. LED power densities up to 300 W/cm2 have been achieved. ^{[ 46 ]}

One of the key advantages of LED-based lighting sources is high luminous efficacy . White LEDs quickly matched and overtook the efficacy of standard incandescent lighting systems. In 2002, Lumileds made five-watt LEDs available with a luminous efficacy of 18–22 lumens per watt (lm/W). For comparison, a conventional incandescent light bulb of 60–100 watts emits around 15 lm/W, and standard fluorescent lights emit up to 100 lm/W. A recurring problem is that efficacy falls sharply with rising current. This effect is known as droop and effectively limits the light output of a given LED, raising heating more than light output for higher current. ^{[ 47 ] [ 48 ] [ 49 ]}

The mechanism behind droop efficiency loss was identified in 2013 as Auger recombination . ^{[ 50 ]}

As of 2012, the Lumiled catalog gives the following as the best efficacy for each color: ^{[ 51 ]}

In September 2003, a new type of blue LED was demonstrated by the company Cree Inc. to provide 24 mW at 20 milliamperes (mA). This produced a commercially packaged white light giving 65 lm/W at 20 mA, becoming the brightest white LED commercially available at the time, and more than four times as efficient as standard incandescents. In 2006, they demonstrated a prototype with a record white LED luminous efficacy of 131 lm/W at 20 mA. Nichia Corporation has developed a white LED with luminous efficacy of 150 lm/W at a forward current of 20 mA. ^{[ 52 ]} Cree's XLamp XM-L LEDs, commercially available in 2011, produce 100 lm/W at their full power of 10 W, and up to 160 lm/W at around 2 W input power. In 2012, Cree announced a white LED giving 254 lm/W. ^{[ 53 ]}

Practical general lighting needs high-power LEDs, of one watt or more. Typical operating currents for such devices begin at 350 mA.

Note that these efficiencies are for the LED chip only, held at low temperature in a lab. Lighting works at higher temperature and with drive circuit losses, so efficiencies are much lower. United States Department of Energy (DOE) testing of commercial LED lamps designed to replace incandescent lamps or CFLs showed that average efficacy was still about 46 lm/W in 2009 (tested performance ranged from 17 lm/W to 79 lm/W). ^{[ 54 ]}

Cree issued a press release on February 3, 2010 about a laboratory prototype LED achieving 208 lm/W at room temperature. The correlated color temperature was reported to be 4579 K. ^{[ 55 ]} In December 2012 Cree issued another press release announcing commercial availability of 200 lm/W LED at room temperature. ^{[ 56 ]}

Lifetime and failure [ edit source | edit beta ]

Solid-state devices such as LEDs are subject to very limited wear and tear if operated at low currents and at low temperatures. Many of the LEDs made in the 1970s and 1980s are still in service in the early 21st century. Typical lifetimes quoted are 25,000 to 100,000 hours, but heat and current settings can extend or shorten this time significantly. ^{[ 57 ]}

The most common symptom of LED (and diode laser ) failure is the gradual lowering of light output and loss of efficiency. Sudden failures, although rare, can occur as well. Early red LEDs were notable for their short service life. With the development of high-power LEDs the devices are subjected to higher junction temperatures and higher current densities than traditional devices. This causes stress on the material and may cause early light-output degradation. To quantitatively classify useful lifetime in a standardized manner it has been suggested to use the terms L70 and L50, which is the time it will take a given LED to reach 70% and 50% light output respectively. ^{[ 58 ]}

LED performance is temperature dependent. Most manufacturers' published ratings of LEDs are for an operating temperature of 25 °C. LEDs used outdoors, such as traffic signals or in-pavement signal lights, and that are utilized in climates where the temperature within the light fixture gets very hot, could result in low signal intensities or even failure. ^{[ 59 ]}

LED light output rises at lower temperatures, leveling off, depending on type, at around ?30 °C. ^{[ citation needed ]} Thus, LED technology may be a good replacement in uses such as supermarket freezer lighting ^{[ 60 ] [ 61 ] [ 62 ]} and will last longer than other technologies. Because LEDs emit less heat than incandescent bulbs, they are an energy-efficient technology for uses such as in freezers and refrigerators. However, because they emit little heat, ice and snow may build up on the LED light fixture in colder climates. ^{[ 59 ]} Similarly, this lack of waste heat generation has been observed to sometimes cause significant problems with street traffic signals and airport runway lighting in snow-prone areas. In response to this problem, some LED lighting systems have been designed with an added heating circuit at the expense of reduced overall electrical efficiency of the system; additionally, research has been done to develop heat sink technologies that will transfer heat produced within the junction to appropriate areas of the light fixture. <sup>[ 63 ]


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