High-intensity discharge (HID) lamps include these types of electrical lamps: mercury vapor, metal halide (also HQI), high-pressure sodium (Son), low-pressure sodium (Sox) 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/cmÃâ€Å¡Ãƒâ€šÃ‚² (19.4 W/in.Ãâ€Å¡Ãƒâ€šÃ‚²).

Compared with fluorescent and incandescent lamps, HID lamps produce a far higher quantity of light per unit area of lamp package.

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, forming a plasma.

Applications

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, outdoor activity areas, roadways, 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, especially for plants that require a good deal of high intensity sunlight, like vegetables and flowers. They are also used to reproduce tropical intensity sunlight for indoor Aquariums.

Some HID lamps such as Mercury Vapor Discharge produce large amounts of UV radiation and therefore need UV-filters to block that radiation. In the last few years there have been several cases of faulty UV-filters, causing people to suffer severe sunburn and Arc eye. Regulations may now require guarded lamps or lamps which will quickly burn out if their outer envelope is broken.

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. Internationalised European vehicle regulations require such headlamps to be equipped with lens cleaners and an automatic self-levelling system to keep the beams aimed correctly regardless of vehicle load and attitude, but no such devices are required in North America, where inherently more glaring beam patterns are also permitted.

HID lamps are used in high-end bicycle lighting as front headlamps. They are desirable because they produce much more light than a halogen lamp of the same wattage. Halogen lights appear somewhat yellow in color; HID bicycle lights look faintly blue-violet. As the HID lights use less than half the wattage of an equivalent halogen light, it allows much lighter batteries to be used.

HID lamps are also being used on many general aviation aircraft for landing and taxi lights.