High-intensity discharge lamp

Encyclopedia : H : HI : HIG : High-intensity discharge lamp

15 kW Xenon short-arc lamp used in IMAX projectors

High-intensity discharge (HID) lamps include these types of electrical lamps: mercury vapor, metal halide (also HQI), 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.

Contents

1 Construction
2 Auxiliary devices
3 Applications
4 See also
5 References
6 External links

Construction

Diagram of a high pressure sodium lamp.

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.

Types of HID lamps include:

Mercury vapor (CRI range 15-55)
Metal halide (CRI range 65-80, ceramic MH can go to 90's)
Low-pressure sodium (CRI 0 owing to their monochromatic light)
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.

Auxiliary devices

Like fluorescent lamps, HID lamps require a ballast to start and maintain their arcs. The method used to initially strike the arc varies: mercury vapor lamps and some metal halide lamps are usually started using a third electrode near one of the main electrodes while other lamp styles are usually started using pulses of high voltage.

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, 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 aquaria.

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. 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. However, many motorists still prefer these lights as they emit a clearer, brighter, more natural appearing light than normal headlamps.

HID lamps are also being used for all external lights on the Airbus A380 superjumbo airliner.

References

External links

[Teachers burned by UV light from cracked halide lamp]

L i g h t i n g and L a m p s
Incandescent:	Incandescent light bulb>Conventional - Halogen - Parabolic Aluminized Reflector (PAR)
Fluorescent:	Compact fluorescent lamp>Compact Fluorescent (CFL) - Linear fluorescent - Induction lamp
Gas discharge:	High-intensity discharge lamp>High-Intensity Discharge (HID) - Mercury-vapor - Metal-halide - Neon - Sodium vapor
Electric arc:	Arc lamp - Hydrargyrum medium-arc iodide>HMI - Xenon arc - Yablochkov candle
Combustion:	Carbide lamp>Acetylene/Carbide - Candle - Gas lighting - Kerosene lamp - Limelight - Oil lamp - Safety lamp
Other types:	Sulfur lamp - Light-emitting diode>Light-Emitting Diode (LED) - Optical fiber - Plasma