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LED street light Article From Wikipedia
LED street light
LED street light in United Kingdom
|Type||LED, Street Light|
2006 of March Europe orSouth America
2010 North America
Design and style
An LED street light is an integrated light that uses light emitting diodes (LED) as its light source. These are considered integrated lights because, in most cases, the luminaire and the fixture are not separate parts (except LEDGine-based luminaires). New in manufacturing, the LED light cluster is sealed on a panel and then assembled to the LED panel with a heat sink to become an integrated lighting fixture.
Different designs have been created that incorporate various types of LEDs into a light fixture. The current trend is to use high power 1 watt LEDs. However, some companies use low power LEDs in their products, including several low power LEDs packed together to perform the same purpose as a single high power LED. The shape of the LED street light depends on several factors, including LED configuration, the heat sink used with the LEDs and aesthetic design preference.
Heat sinks for LED street lights are similar in design to heat sinks used to cool other electronics such as computers. Heat sinks tend to have as many grooves as possible to facilitate the flow of hot air away from the LEDs. The area of heat exchange directly affects the lifespan of the LED street light.
The lifespan of an LED street light is determined by its light output compared to its original design specification. Once its brightness decreases by 30 percent, an LED street light is considered to be at the end of its life.
Most LED street lights have a lens on the LED panel, which is designed to cast its light in a rectangular pattern, an advantage compared to traditional street lights, which typically have a reflector on the back side of a high-pressure sodium lamp. In this case, much of the luminance of the light is lost and produces light pollution in the air and surrounding environment. Such street lights can also cause glare for drivers and pedestrians.
A drawback of LED focus panels is that most light is directed to the road, and less light to the footpaths and other areas. This can be addressed by the use of specialised lens design and adjustable mounting spigots.
The primary appeal of LED street lighting is energy efficiency compared to conventional street lighting fixture technologies such as high pressure sodium (HPS) and metal halide (MH). Research continues to improve the efficiency of newer models. One such advance can be found in a street light product created by Lighting Science Group Corporation. One model of LED street lights produced by this group is up to 60 percent more efficient than previous models, lasting for 12 years.
An LED street light based on a 901 milliwatt output LED can normally produce the same amount of (or higher) luminance as a traditional light, but requires only half of the power consumption. LED lighting does not typically fail, but instead decreases in output until it needs to be replaced.
As the LED lighting fixtures normally produce less lumen/Watt illumination  it is very crucial to have a well distributed illumination pattern in order to do the same job as higher lumen/Watt conventional fixtures. So a good design of LED street lights is to point different LEDs in one fixture to different target points.
Health concerns about LED streetlights
- Exposure to the light of white LED bulbs suppresses melatonin by up to five times more than exposure to the light of pressure sodium bulbs.The fact that white light, emitting at wavelengths of 400-500 nanometers suppresses the production of melatonin produced by the pineal gland is known. The effect is disruption of a human being’s biological clock resulting in poor sleeping and rest periods.
- Research at the University of Madrid Complutense University has claimed that long term exposure to LED Street-lighting can cause irreparable harm to the retina of the human eye. The Madrid study said this was caused by the high level of radiation in the’’blue band’’
- Artificial night-time lighting has various effects on humans (not to mention wildlife) and that exposure to optical radiation affects human physiology and behavior, both directly and indirectly. Many areas are not well understood, and a position statement from the Illumination Engineering Society (IES) emphasizes mainly the need for further research.
- There is a main risk from glare. A French Government report published in 2013 agreed that a luminance level higher than 10,000 cd/m2 causes visual discomfort whatever the position of the lighting unit in the field of vision. As the emission surfaces of LEDs are highly concentrated point sources, the luminance of each individual source can be 1000 times higher than the discomfort level. The level of direct radiation from this type of source can therefore easily exceed the level of visual discomfort
Advantages of LED street lights
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- Low energy consumption: The much lower energy usage of LED lighting can dramatically reduce operating costs.
- Long and predictable lifetime: The lifetime of LED street lights is usually 10 to 15 years, three times the life of current technologies adopted. The much less frequent need to service or replace LEDs means lower maintenance cost.
- More accurate color rendering: The color rendering index is the ability of a light source to correctly reproduce the colors of the objects in comparison to an ideal light source. Improved color rendering makes it easier for drivers to recognize potential road hazards.
- Quick turn on and off: Unlike fluorescent lamps, which take time to heat up once switched on, LEDs come on with full brightness instantly. Unlike mercury vapor, metal halide and sodium vapor lamps (commonly used in street lighting), LEDs do not have a problem restarting immediately (hot ignition) following a brief power failure or inadvertent turn off.
- RoHS compliance: LEDs don‘t contain mercury or lead, and don‘t release poisonous gases if damaged.
- Less attractive to nocturnal insects: Nocturnal insects are attracted to ultraviolet, blue and green light emitted by conventional light sources.
- Fewer electrical losses: All other types of lighting (except incandescent) require ballasts, additional electronic and/or electromagnetic components, in which some power is consumed.
- Optically efficient lighting equipment: Other types of street lights use a reflector to capture the light emitted upwards from the lamp. Even under the best of conditions, the reflector absorbs some of the light. Also for fluorescent lamps and other lamps with phosphor coated bulbs, the bulb itself absorbs some of the light directed back down by the reflector. The glass cover, called a refractor, helps project the light down on the street in a desired pattern but some light is wasted by being directed up to the sky (light pollution). LED lamp assemblies (panels) do not require reflectors and can be designed to provide the desired coverage without a refractor.
- Reduced glare: Directing the light downward onto the roadway reduces the amount of light that is directed into driver‘s eyes.
- Higher light output even at low temperatures: While fluorescent lights are comparably energy efficient, on average they tend to have lesser light output at winter temperatures.
Disadvantages of LED street lights
- There is a main risk from glare. A French Government report published in 2013 agreed that a luminance level higher than 10,000 cd/m2 causes visual discomfort whatever the position of the lighting unit in the field of vision.
- The initial cost of LED street lighting is high and as a consequence it takes several years for the savings on energy to pay for that. The high cost derives in part from the material used since LEDs are often made on sapphire or other expensive substrates.
- LED street lights may make light pollution significantly worse in some areas as they are brighter than the lights that are being replaced and thus are increasing light pollution. According to one America study they may well exacerbate known and possible unknown effects of light pollution on human health
- The replacement of HPS street lighting with LED street lighting is leading to a major change in the colour of the urban sky glow. The higher blue content in LED lighting is likely to increase sky glow considerably affecting bird migration and disrupting diurnal animals.
- There is progressive wear of layers of phosphor in white LEDs, that with time lead to devices being moved from one photobiological risk group to a higher one.
LED street lights have been installed or announced for installation in several large cities as well as smaller cities throughout the world. Some cities in the US have decided to convert to LED lights The United States Department of Energy has available a number of reports on the results of many pilot projects for municipal outdoor lighting. Cities that have announced their intention to install LED street lights includeBoston, Worcester, and Cleveland.
Cities that have installed at least some LED street lights include: