High-intensity discharge lamp
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High-intensity discharge (HID) lamps include the types of electrical lights: mercury vapor, metal halide, high-pressure sodium, low-pressure sodium and less common, xenon short-arc lamps. The light-producing element of these lamp types is a well-stabilized arc discharge contained within a refractory envelope (arc tube) with wall loading in excess of 3 W/cm² (19.4 W/in.²).
Compared to fluorescent and incandescent lamps, HID lamps produce a much larger quantity of light in a relatively small package.
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Construction
HID lamps produce light by striking an electrical arc across tungsten electrodes housed inside a specially designed inner fused quartz or fused alumina tube. This tube is filled with both gas and metals. The gas aids in the starting of the lamps. Then, the metals produce the light once they are heated to a point of evaporation. Like fluorescent lamps, HID lamps require a ballast to start and maintain their operation.
Types of HID lamps include mercury vapor (CRI range 15-55), metal halide (CRI range 65-80), and high-pressure sodium (CRI range 22-75). Mercury vapor lamps, which originally produced a bluish-green light, were the first commercially available HID lamps. Today, they are also available in a color corrected, whiter light. But they are still often being replaced by the newer, more efficient high-pressure sodium and metal halide lamps. Standard low-pressure sodium lamps have the highest efficiency of all HID lamps, but they produce a yellowish light. High-pressure sodium lamps that produce a whiter light are now available, but efficiency is somewhat sacrificed. Metal halide lamps are less efficient but produce an even whiter, more natural light. Colored metal halide lamps are also available.
Applications
HID lamps are typically used when high levels of light over large areas are required, and when energy efficiency and/or long life are desired. These areas include gymnasiums, large public areas, warehouses, outdoor activity areas, roadways, parking lots, and pathways. More recently, however, HID sources, especially metal halide, have been used in small retail and residential environments. HID lamps have also made indoor gardening practical, especially for plants that require a good deal of high intensity sunlight, like vegetables and flowers.
Some HID lamps such as Mercury Vapor Discharge produce large amounts of UV radiation and therefore need diffusers to block that radiation. In the last few years there have been several cases of faulty diffusers, causing people to suffer severe sunburn and Arc eye.
Recently, HID lamps have gained use in motor-vehicle headlamps. This application has met with mixed responses from motorists, mainly in response to the amount of glare that HID lights can cause. However, many motorists still use these lights as they emit a clearer, more natural appearing light than normal headlamps. In the united states, HID lights are only legal as OEM installations. However, many people install automotive HID retrofit kits that simply place an HID capsule in a lamp designed for an incandescent light source. Without proper projectors, the light from the HID bulb is not distributed properly, ultimately resulting in less visibility for the driver. Additionally, HID lights draw extra electric current, and as a result thicker gauge wiring is required to prevent electrical hazards such as melted wires or electrical fire.
HID lamps are also being used for all external lights on the Airbus A380 superjumbo airliner.
See also
References
- {{cite book
| first = John | last = Waymouth | year = 1971 | title = Electric Discharge Lamps | publisher = The M.I.T. Press | location = Cambridge, MA | id = ISBN 0262230488 }}
- {{cite web
| author=National Highway Traffic Safety Administration | year= | url=http://www.nhtsa.gov/cars/rules/rulings/glare.html | title=Glare from Headlamps and other Front Mounted Lamps
Federal Motor Vehicle Safety Standard...
| work= | publisher=US Department of Transportation | accessdate=2006-01-23 | accessyear= }}