High-intensity discharge lamp

High-intensity discharge (HID) lamps include these types of electrical lamps: mercury vapor, metal halide (also HQI), high-pressure sodium (HPS) 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 with fluorescent and incandescent lamps, HID lamps produce a far higher quantity of light per unit area of lamp package.

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, forming a plasma.

Types of HID lamps include:

• Mercury vapor (CRI range 15-55)
• Metal halide (CRI range 65-80, ceramic MH can go to 90s)
• 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. 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, 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 also being used on many general aviation aircraft for landing and taxi lights.

End of 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 but as they heat up during operation, the internal gas pressure within the arc tube rises and more and more voltage is required to maintain the arc discharge. As a lamp gets older, the maintaining voltage for 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.

More sophisticated ballast designs detect cycling and give up attempting to start the lamp after a few cycles. If power is removed and reapplied, the ballast will make a new series of startup attempts.