Light-emitting diode (LED) technology has been the focus of attention in recent years as the future of lighting technology. Used for years in applications such as consumer electronics and exit signs; LEDs are now competing in all areas of lighting. While still on the cutting edge of research and development, the light-emitting quality of diodes was first discovered in 1907 and modern LED technology was initially developed in the 1960s. Despite its long history and the current hype, there is often confusion about what LEDs actually are and how they compare to traditional lighting technologies. This article will help to illuminate these issues and provide some insight as to where LED technology may be headed in the future.
An LED is a semiconductor diode, which is a small chip of silicon that has been treated with chemical elements that create a positive-negative (p-n) interaction. The positive side contains electron holes, while the negative side contains free electrons. When connected to a power source, the holes and electrons are forced together, releasing energy in the form of a light photon.
The white light common in traditional lamps is necessary for many general lighting applications. However, LEDs are not inherently white light sources. White light is created using two primary methods, phosphor conversion and RGB. In phosphor conversion, blue LED chips are coated with a yellow phosphor to produce white light. The RGB approach produces white light by mixing the three primary colors—red, green and blue.
LEDs have distinct characteristics that offer users a number of benefits and present researchers with challenges to improve the technology for the next generation.
While conventional lamps radiate in a 360° area, the light output of LEDs is directional. This can create challenges in area lighting, but it also increases lighting performance in many applications by putting light exactly where it is needed. Also, LEDs may not perform well in dimming applications if legacy controls are used.
How do LEDs compare with more conventional lighting technologies, such as incandescent and fluorescent? The leading indicators of lighting performance include rated life, efficacy (measured in lumens per watt) and color rendering index (CRI), which indicates how well a light source renders colors of objects. Another important measure for a light source is lumen maintenance, which compares initial light output to the amount of light produced at a specific future time.
|Type||Rated Life (hours)||Lumens Per Watt||CRI||Lumen Maintenance|
|LED||50,000||45-100||70-90||95 to 98 percent*|
|Fluorescent||10,000-20,000||60-100||80-90||90 to 95 percent*|
|*After 8,000 hours of use.|
As the table above indicates, LED lighting compares remarkably well in many of these categories. While it does not match the color rendering index of incandescent lamps, the rated life hours and efficacy of LEDs are significantly higher. LEDs compare favorably to fluorescent lamps in color rendering and they have begun to match them in efficacy. The directional nature of LEDs can actually increase their lighting efficiency in some applications.
LED technology is evolving rapidly, with new products constantly appearing on the market. One important new area of development is organic LEDs (OLED). These devices are made from plastic or other carbon-based materials. They are a diffused light source and are inherently thin and flexible. Currently, they are not as efficient in producing light as conventional LEDs and they are highly sensitive to moisture.
While LEDs are commercially available and used in a variety of applications, technological issues in the manufacturing process must be overcome before they can be mass-produced in a cost-effective manner. Standards and test procedures for LED products are maturing. While the future of LED technology looks bright, the relatively high purchase cost remains a major barrier to widespread use.