Thursday, February 3, 2011

   A Light Emitting Diode (LED) is a semiconductor device which converts electricity into light. LED lighting has been around since the 1960s, but is just now beginning to appear in the residential market for space lighting. At first white LEDs were only possible by "rainbow" groups of three LEDs -- red, green, and blue -- by controlling the current to each to yield an overall white light. This changed in 1993 when Nichia created a blue indium gallium chip with a phosphor coating that is used to create the wave shift necessary to emit white light from a single diode. This process is much less expensive for the amount of light generated.

Each diode is about 1/4 inch in diameter and uses about ten milliamps to operate at about a tenth of a watt. LEDs are small in size, but can be grouped together for higher intensity applications. LED fixtures require a driver which is analogous to the ballast in fluorescent fixtures. The drivers are typically built into the fixture (like fluorescent ballasts) or they are a plug transformer for portable (plug-in) fixtures. The plug-in transformers allow the fixture to run on standard 120 volt alternating current (AC), with a modest (about 15 to 20 percent) power loss

The efficacy of a typical residential application LED is approximately 20 lumens per watt (LPW), though efficacies of up to 100 LPW have been created in laboratory settings. Incandescent bulbs have an efficacy of about 15 LPW and ENERGY STAR® qualified compact fluorescents are about 60 LPW, depending on the wattage and lamp type. Some manufacturers claim efficacies much higher than 20 LPW; make sure to examine system efficacy, which accounts for the power use of all components. In December 2006, the U.S. Department of Energy studied the efficacy of four luminaries. All four fell short of the manufacturers’ claims; the study implies that manufacturers are relying on measurements of how much light an isolated LED produces, rather than how much light an LED luminaire actually delivers. 

LEDs are better at placing light in a single direction than incandescent or fluorescent bulbs. Because of their directional output, they have unique design features that can be exploited by clever designs. LED strip lights can be installed under counters, in hallways, and in staircases; concentrated arrays can be used for room lighting. Waterproof, outdoor fixtures are also available. Some manufacturers consider applications such as gardens, walkways, and decorative fixtures outside garage doors to be the most cost-efficient. 

LED lights are more rugged and damage-resistant than compact fluorescents and incandescent bulbs. LED lights don't flicker. They are very heat sensitive; excessive heat or inappropriate applications dramatically reduce both light output and lifetime. Uses include:
  • Task and reading lamps
  • Linear strip lighting (under kitchen cabinets)
  • Recessed lighting/ceiling cans
  • Porch/outdoor/landscaping lighting
  • Art lighting
  • Night lights
  • Stair and walkway lighting
  • Pendants and overhead
  • Retrofit bulbs for lamps
Definitions and Terms
Term Definition Units How to interpret
Color Temperature Color of light Kelvin (K) Sunlight at sunrise is 1800K
100W Incandescent light bulb is 2850K
Overcast Sky is 6500K
Color Rendering Index (CRI) Light’s effect on color Scale of 0 to 100 with sunlight at 100 The higher the number, the more “true” the color will look in that light
Brightness The intensity of the light. Lumens The higher the lumens, the brighter the light
Power Amount of electrical energy consumed Watts Lower the watts, the lower the energy consumed
Efficacy The efficiency of the bulb to convert electricity into light Lumens per Watt More efficient bulbs provide more light using less energy

Individual LEDs are considerably more efficient; however, the lamp or fixture design is reduced by the driver and electronics. In addition, LEDs do not produce heat like incandescent bulbs.

LEDs last considerably longer than incandescent or fluorescent lighting. LEDs don't typically burn out like traditional lighting, but rather gradually decrease in light output. Their "useful life" is defined by the Alliance for Solid-State Illumination Systems and Technologies (ASSIST) as the time it takes until 70% of initial light output is reached, often 50,000 hours. They are resistant to thermal and vibrational shocks and perform well when subjected to frequent on-off cycling.
LED lamps have many advantages over traditional lighting methods.   These include:
  • Low energy consumption – retrofit bulbs range from 0.83 to 7.3 Watts
  • Long service life – LED bulbs can last up to 50,000 hours
  • Durable – LED bulbs are resistant to thermal and vibrational shocks and turn on instantly from -40C° to 185C°, making them ideal for applications subject to frequent on-off cycling, such as garages and basements
  • Directional distribution of light – good for interior task lighting
  • No infrared or ultraviolet radiation – excellent for outdoor use because UV light attracts bugs
  • Safety and environmentally conscious – LEDs contain no mercury and remain cool to the touch
  • Fully dimmable – LEDs do not change their color tint when dimmed unlike incandescent lamps that turn yellow
  • No frequency interference – no ballast to interfere with radio and television signals
  • Range of color – LEDs can be manufactured produce all colors of the spectrum without filters, they can also produce white light in a variety of color temperatures

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