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An introduction to Solid State Lighting

Solid-state lighting (SSL) is the new lighting technology to emerge for many years: it uses semi-conducting materials to convert electricity into light. The term “solid state” refers to the fact that light in an LED is emitted from a solid object (a block of semiconductor) rather than from a vacuum or gas tube, as is the case in traditional incandescent light bulbs and fluorescent lamps. SSL covers different types of technologies including light-emitting diodes (LEDs) and organic light-emitting diodes (OLEDs). LEDs are the more mature technology compared to OLEDs, particularly for white-light general illumination applications, but both technologies are evolving rapidly.
White LEDs for general illumination

An LED is a very small (dot-sized) electrical device that produces light through the semi-conducting properties of its metal alloys. LEDs have been around since the 1960s, but were used mainly as simple indicator lamps in electronics and equipment.
White LEDs are now approaching performance levels that make them attractive for use in automobiles, aircraft, elevators, and some task light applications, and a variety of white LED products are available on the market. However, for most illumination applications, white LEDs cannot yet compete with traditional light sources on the basis of performance or cost.
The best white LEDs are similar in efficiency to compact fluorescent lamps (CFLs), but most of the white LEDs currently available in consumer products are only marginally more efficient than incandescent lamps. The best white LEDs available today can produce about 45-50 lpw (lpw for lumens per watt is the measure of how efficiently the light source is converting electricity into usable light.). For comparison, incandescent lamps typically produce 12-15 lpw; CFLs produce at least 50 lpw. Many LED products may appear energy efficient because they use only a small amount of energy, but they often have very low light output. True energy efficiency means using the most efficient light source or system that is capable of providing the amount and quality of light needed. On-going research and development efforts are making steady progress in improving the performance of white LEDs to levels suitable for general lighting applications.
White LEDs light output

To approach the total light output of a typical incandescent or CFL, some LEDs must be grouped together and the “high-brightness” white LEDs typically come in just 1-watt to 3-watt sizes. However, for some applications, LEDs can provide enough light on the task, even though the total light output is lower than comparable incandescent or fluorescent sources. Indeed, the light emitted from an LED is directional in nature, and in some applications, less light is lost in the fixture than with traditional light sources. For such applications, it is helpful to know how much total light the LED product provides and compare it to competing products using traditional light sources.
White LEDs life time

There is not yet an official industry standard defining “life” of an LED since LEDs don’t “burn out;” but simply get dimmer over time. The leading manufacturers define it as the point at which light output has decreased to 70% of initial light output. Using that definition, the best white LEDs have been found to have a useful life of around 35,000 hours of continuous operation. For comparison, a 75-watt incandescent light bulb lasts about 1,000 hours and a comparable CFL lasts 8,000 to 10,000 hours.

LED lifetime depends greatly on operating temperature. An increase in operating temperature of 10 °C can cut the useful life of an LED in half. So to evaluating LED product life claims, one should ask about the assumed operating temperature and any measures to mitigate heat in the device.
White LEDs cost

For the moment, white LEDs cost significantly more than traditional light sources. The combination of high price and low light output may make them a poor replacement for current technology in most general illumination applications. For comparison, today’s white LEDs cost more than $50 per thousand lumens, a typical 75-watt incandescent light bulb, costs about $1.00 per thousand lumens and a comparable CFL costs less than $5.00. The technology targets to achieve are $20/klm by 2007, less than $5/klm by 2012, and less than $2/klm by 2020. However, for some applications, the extra durability of LEDs is worth a higher purchase price. Outdoor pathway and step lighting is an example of a sensible application for today’s white LEDs. They provide a small amount of light right where it’s needed, avoid frequent bulb changes in fixtures that are difficult to access, and can be powered by solar cells, eliminating the need for running wire outdoors. LEDs are also good for applications where vibration often leads to early failure of conventional light sources. Being a solid-state device, LEDs are highly resistant to damage caused by vibration.

Source: DOE (US Department of Energy) publication “Energy-Efficient Lighting and Light-Emitting Diodes”

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