High-intensity discharge lamps (HID lamps) are a type of electrical gas-discharge 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 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.
High-intensity discharge lamps make more visible light per unit of electric power consumed than fluorescent and incandescent lamps since a greater proportion of their radiation is visible light in contrast to infrared.
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. These areas include gymnasiums, large public areas, warehouses, movie theaters, football stadiums,outdoor activity areas, roadways, parking lots, and pathways. More recently, HID lamps have been used in small retail and even residential environments because of advances in reduced lumen bulbs. Ultra-High Performance (UHP) HID lamps are used in LCD or DLP projection TV sets or projection displays as well.
HID lamps have made indoor gardening practical, particularly for plants that require high levels of direct sunlight in their natural habitat; HID lamps, specifically metal halide and high-pressure sodium, are a common light source for indoor gardens. They are also used to reproduce tropical intensity sunlight for indoor aquaria.
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 seen applications in automotive headlamps. Xenon or High-Intensity Discharge (HID) lighting provides brighter headlights and increases visibility of many peripheral objects (i.e., street signs and pedestrians) left in the shadows by standard halogen lighting.
HID lamps are used in high-performance bicycle headlamps as well as flashlights and other portable lights, because they produce a great amount of light per unit of power. As the HID lights use less than half the power of an equivalent tungsten-halogen light, a significantly smaller and lighter-weight power supply can be used.
HID lamps are also used in lamps for underwater diving. The higher efficacy of HID lamps compared to halogen units means longer burn times for a given battery size and light output.
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