A light-emitting diode (LED) is a semiconductor device that emits incoherent narrow-spectrum light when a p-n junction is forward electrically biased. This effect is a form of electroluminescence. LEDs are a very efficient form of energy production and have already come down in cost far enough to be competitive for a great number of applications such as traffic lights. There are also white LEDs that can be used in place of incandescent light bulbs.
LEDs are an extremely efficient form of lighting. They produce far more light per watt than do incandescent bulbs - this is useful for energy conservation, but also extending the life of battery powered or energy-saving devices.
LEDs also have an extremely long life span. Philips has calculated the ETTF (Estimated Time To Failure) for their LEDs to be between 100,000 and 1,000,000 hours. However, most manufacturers rate their LED lifespan at 50,000 hours and some take a more conservative figure of 35,000 hours. This is due to the fact that after a certain number of active hours there is a fall-off in efficiency of the light, both in terms of lumen and color temperature, and while the light will still continue to operate, a user would probably want to replace them at that time.
Fluorescent tubes typically are rated at about 30,000 hours, CFL's or Compact Florescent Lights between 7,500 hours and 15,000 hours and incandescent light bulbs at 1,000-2,000 hours.
 LED Retrofits
LEDs can retrofit incandescent lighting both in the home and on the street. Converting traffic or pedestrian signals from costly and inefficient incandescent light bulbs to LED modules is a simple, environmentally smart solution for reducing energy costs and use, resulting in:
- Up to a 90% reduction in electricity usage and energy costs
- Significant cost savings to taxpayers through both reduced energy costs but also through reduced maintenance costs (because of their long lifetime LEDs do not need to be changed nearly as much as standard bulbs).
 Service Learning and LEDs
LED retrofitting has been used in service learning projects in order to decrease the carbon footprint of a municipality, and also to teach students about sustainability. Students at Clarion University of Pennsylvania used Appropedia to coordinate research into the environmental impacts of switching from standard incandescent light bulbs to LEDs around Pennsylvania. open access
Unlike incandescent light bulbs, which light up regardless of the electrical polarity, LEDs will only light with positive electrical polarity. When the voltage across the p-n junction is in the correct direction, a significant current flows and the device is said to be forward-biased. If the voltage is of the wrong polarity, the device is said to be reverse biased, very little current flows, and no light is emitted. LEDs can be operated on an alternating current voltage, but they will only light with positive voltage, causing the LED to turn on and off at the frequency of the AC supply.
 Other Advantages
LEDs have several other advantages:
- LEDs 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.
- When used in applications where dimming is required, LEDs do not change their color tint as the current passing through them is lowered, unlike incandescent lamps, which turn yellow.
- LEDs 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.
- LEDs, being solid state components, are difficult to damage with external shock. Fluorescent and incandescent bulbs are easily broken if dropped on the ground.
- LEDs mostly fail by dimming over time, rather than the abrupt burn-out of incandescent bulbs.
- LEDs light up very quickly. A typical red indicator LED will achieve full brightness in microseconds; LEDs used in communications devices can have even faster response times.
- LEDs can be very small and are easily populated onto printed circuit boards.
- LEDs do not contain mercury, while compact fluorescent lamps CFLs do.
- LEDs work in silence. The days of humming bulbs came to an end with the creation of LEDs, there's no ticking or pinging like regular light bulbs.
- LEDs are incredibly safe. With so little energy lost to heat and so little energy used overall, LED light bulbs run cool, which means no burnt fingers.
- When LEDs are used as a replacement for older stronger incandescent bulb, designers can choose cheaper materials in lamp shades and make lamp shades smaller and closer to the LED bulb, not having to worry about eventual overheating, burning or setting fire to the lamp shades material, thanks to the very low heat created.
 Light up the World Foundation
Light Up The World Foundation (LUTW) is the first humanitarian organization to utilize renewable energy and solid-state lighting technologies to bring affordable, safe, healthy, efficient, and environmentally responsible illumination to people who do not have access to power for adequate lighting.
 LED Manufacturers and Retailers
- Eco Lighting Products
- Eco Electrical Services
- LED Flashlights
- LED light manufacturer
- LED Bulb
- LED Downlight
- LED Strip
- LED Light
- LED Light Supplier
 LED Traffic Lights
For frequently asked questions about LED traffic light retrofits see the LED traffic light FAQ.
 LED Display Lighting
 LED Refrigerated Display
 More Information about LEDs
LEDs Magazine - LEDs Magazine is a bimonthly publication available by FREE subscription as an electronic (PDF) download. The magazine contains technical articles, case studies, application notes, product information, business and financial news, and a wide variety of other information relevant to the LED industry. A print version of the publication is available by paid subscription.
Green Lighting: Filled with step-by-step instructions, Green Lighting shows you how to save money and energy with light-emitting diodes (LEDs), compact fluorescent lighting (CFL), solar lights, windows, skylights, fixtures, controls, and other bright ideas.
- Joshua M. Pearce, “Appropedia as a Tool for Service Learning in Sustainable Development”, Journal of Education for Sustainable Development, 3(1), pp.47-55, 2009.
 See also
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