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 and house lights. There are also white LEDs that can be used in place of incandescent light bulbs.

## Efficiency

LEDs are an extremely efficient form of lighting. They produce far more light per watt than incandescent bulbs - this is useful for energy conservation, but also extending the life of battery powered or energy-saving devices.

## Lifespan

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 most likely 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. This is because of LEDs long lifetime do not need to be changed nearly as much as standard bulbs. The Victorian Government of Australia under the Victorian Energy Upgrades Program offers incentives if a Victorian household or business upgrade to LED lights.

### Cheap LED retrofit from CFL

If you are low on budget but want to change out your CFL to a LED, it is recommended to look into Philips InstantFit LED lamps. The Philips T8 bulb is the same tube shape as your typical CFL which allows installation of it to be instantly snapped into your fixture. Philip's InstantFit LED lamps claims they will deliver 40% energy savings and a flicker free performance. Based off of Home Depot's website, 4 foot 17 watt T8 bulbs cost \$6.97 each.

### 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.[1] open access

## Polarity

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.

## Calculations behind LED

The wavelength (and color) of light emitted in a radiative recombination of carriers injected across a p-n junction is determined by the difference in energy between the recombining electron-hole pair of the valence and conduction bands. The approximate energies of the carriers correspond to the upper energy level of the valence band and the lowest energy of the conduction band, due to the tendency of the electrons and holes to equilibrate at these levels. Consequently, the wavelength (l) of an emitted photon is approximated by the following expression:

l = hc/Ebg


where h represents Planck's constant, c is the velocity of light, and Ebg is the band gap energy. In order to change the wavelength of emitted radiation, the band gap of the semiconducting material utilized to fabricate the LED must be changed. Gallium arsenide is a common diode material, and may be used as an example illustrating the manner in which a semiconductor's band structure can be altered to vary the emission wavelength of the device.[1] To see pictures of what LED lamps look like, look here You can see how each individual LED light fits into the shape of a lamp.

• LEDs can emit light of an intended color by choosing the type of temperature of the output, in Kelvin units, without the use of color filters that traditional lighting methods require. This is more efficient and can lower initial costs. CRI (Color Rendering Index) is what determines the quality of the LED's in regards to intended color.
• 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.Although this isn't always the case. Some cheaper LED bulbs can take time to warm up in order to reach their full color potential.
• 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.

• LED lights are expensive because the market for them is not big enough to be mass producing them at a cheaper cost. Therefore, many shy away at switching their existing structures for new ones.
• phosphors are used for there abilities to emit a broader spectrum of light. Unfortunately, there have been problems with the inability to function properly.
• Another hindrance of a worldwide switch to LED lamps or lighting is the technological switch that households will have to make to accommodate to the lights. For the lights to be energy efficient, the fixtures would need to be switched.
• A common complaint with LED street lighting[2] is that it's not "easy" on the eyes from the seemingly harsh light they output. But, this is typically caused by improper installation and color temperature choosing.

## 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

### LED traffic lights

For frequently asked questions about LED traffic light retrofits see the LED traffic light FAQ.