HID or High Intensity Discharge Lamps
1.What is a HID or High Intensity Discharge Lamp?
A high-intensity discharge (HID) lamp is a type of electrical lamp which produces light by means of an electric arc between tungsten electrodes housed inside a translucent or transparent fused quartz or fused alumina arc tube. This tube is filled with both gas and metal salts. The gas facilitates the arc's initial strike. Once the arc is started, it heats and evaporates the metal salts forming a plasma, which greatly increases the intensity of light produced by the arc and reduces its power consumption. High-intensity discharge lamps are a type of arc lamp.
2. HID and fluorescent and incandescent lamps?
Compared with fluorescent and incandescent lamps, HID lamps have higher luminous efficacy since a greater proportion of their radiation is in visible light as opposed to heat. Their overall luminous efficacy is also much higher: they give a greater amount of light output per watt of electricity input.
3. Where and when are the High Intensity Discharge Lamps used?
HID lamps are typically used when high levels of light over large areas are required, and when energy efficiency and/or light intensity are desired. You can find this lamps used at the: gymnasiums, large public areas, warehouses, movie theaters, football stadiums, outdoor activity areas, roadways, road ilumination, parking lots, and pathways.
More recently, HID lamps, especially metal halide, have been used in small retail and residential environments. HID lamps have made indoor gardening practical, particularly for plants that require a good deal of high intensity sunlight; HID lamps are a common choice of light source for marijuana growing operations. They are also used to reproduce tropical intensity sunlight for indoor aquariums. Ultra-High Performance (UHP) HID lamps are used in LCD or DLP projection TV sets or projection displays.
Most HID lamps produce significant UV radiation, and require UV-blocking filters to prevent UV-induced degradation of lamp fixture components and fading of dyed items illuminated by the lamp. Exposure to HID lamps operating with faulty or absent UV-blocking filters causes injury to humans and animals, such as sunburn and arc eye. Many HID lamps are designed so as to quickly extinguish if their outer UV-shielding glass envelope is broken.
Beginning in the early 1990s, HID lamps have been employed in motor vehicle headlamps. This application has met with mixed responses from motorists, who appreciate the improved nighttime visibility from HID headlamps but object to the glare they can cause. Internationalized European vehicle regulations require such headlamps to be equipped with lens cleaners and an automatic self-leveling system to keep the beams aimed correctly regardless of vehicle load and altitude, but no such devices are required in North America, where inherently more glaring beam patterns are also permitted. Retrofitting HID bulbs in headlamps not originally designed to accept them results in extremely high levels of glare, and is illegal throughout most of the world.
3. High Intensity Discharge Lamp Life:
At the end of life, many types of high-intensity discharge lamps exhibit a phenomenon known as cycling. These lamps can be started at a relatively low voltage. As they heat up during operation, however, the internal gas pressure within the arc tube rises and a higher voltage is required to maintain the arc discharge. As a lamp gets older, the voltage necessary to maintain the arc eventually rises to exceed the voltage provided by the electrical ballast. As the lamp heats to this point, the arc fails and the lamp goes out. Eventually, with the arc extinguished, the lamp cools down again, the gas pressure in the arc tube is reduced, and the ballast can once again cause the arc to strike. The effect of this is that the lamp glows for a while and then goes out, repeatedly.
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