Metal-halide lamp

thumb|Metal halide lamp with arc tube shield, for open fixtures

thumb|A common [[spectrum of American metal halide lamps, which contains sodium iodide and scandium iodide.]]

thumb|Metal halide floodlights at a baseball field

thumb|Metal halide lamp for automotive headlights

thumb|Metal halide lamps were invented by [[Charles Proteus Steinmetz in 1912 and are now used in almost every city in the world.]]

A metal-halide lamp is an electrical lamp that produces light by an electric arc through a gaseous mixture of vaporized mercury and metal halides (compounds of metals with bromine or iodine). It is a type of high-intensity discharge (HID) gas discharge lamp. As one of the most efficient sources of high CRI white light, metal halides were the fastest growing segment of the lighting industry. However LEDs have almost entirely replaced metal halide in both applications, with just a handful of purists still holding on.

Operation

Like other gas-discharge lamps such as the very-similar mercury-vapor lamps, metal-halide lamps produce light by ionizing a mixture of gases in an electric arc. In a metal-halide lamp, the compact arc tube contains a mixture of argon, neon or xenon, mercury, and a variety of metal halides, such as sodium iodide and scandium iodide. The particular mixture of metal halides influences the correlated color temperature and intensity (making the light more blue or red, for example). When started, the argon gas in the lamp is ionized first, which helps to maintain the arc across the two electrodes with the applied starting voltage. The heat generated by the arc and electrodes then ionizes the mercury and metal halides into a plasma, which produces an increasingly brighter white light as the temperature and pressure increases to operating conditions.

The arc-tube operates at anywhere from 5–50 atm or more (70–700 psi or 500–5000 kPa) and 1000–3000 °C. Like all other gas-discharge lamps, metal-halide lamps have negative resistance (with the rare exception of self-ballasted lamps), and so require a ballast to provide proper starting and operating voltages while regulating the current flow through the lamp. About 24% of the energy used by metal-halide lamps produces light (an efficacy of 65–115 lm/W),

Pulse-start metal-halide bulbs don't contain a starting electrode which strikes the arc, and require an ignitor to generate a high-voltage (1–5 kV on cold strike, over 30 kV on hot restrike) pulse to start the arc. Electronic ballasts include the igniter circuit in one package. American National Standards Institute (ANSI) lamp-ballast system standards establish parameters for all metal-halide components (with the exception of some newer products).

Color temperature

thumb|alt=Line graph of relative intensity vs wavelength| Output spectrum of a typical metal-halide lamp showing peaks at 385nm, 422nm, 497nm, 540nm, 564nm, 583nm (highest), 630nm, and 674nm.

Because of the whiter and more natural light generated, metal-halide lamps were initially preferred to the bluish mercury vapor lamps. With the introduction of specialized metal-halide mixtures, metal-halide lamps are now available with a correlated color temperature from 3,000 K to over 20,000 K. Color temperature can vary slightly from lamp to lamp, and this effect is noticeable in places where many lamps are used. Because the lamp's color characteristics tend to change during its life, color is measured after the bulb has been burned for 100 hours (seasoned) according to ANSI standards. Pulse start metal halide lamps have improved color rendering and provided a more controlled kelvin variance (±100 to 200 kelvins) because of better arctube shapes compared to probe start metal halide lamps, which do not require a starting electrode to be present and allow higher pressure and temperature of the halides.

The color temperature of a metal-halide lamp can also be affected by the electrical characteristics of the electrical system powering the bulb and manufacturing variances in the bulb itself. If a metal-halide bulb is underpowered, because of the lower operating temperature, its light output will be bluish because of the evaporation of mercury alone. This phenomenon can be seen during warmup, when the arc tube has not yet reached full operating temperature and the halides have not fully vaporized. It is also very apparent with dimming ballasts. The inverse is true for an overpowered bulb, but this condition can be hazardous, leading possibly to arc-tube explosion because of overheating and overpressure.

Starting and warm up

thumb|alt=Short horizontal glass tube with flattened ends, with light coming from the center, the surroundings are relatively dark|400 W metal-halide lamp shortly after powering up

A cold metal-halide lamp cannot immediately begin producing its full light capacity because the temperature and pressure in the inner arc chamber require time to reach full operating levels. Starting the initial argon arc (or xenon in automotive) sometimes takes a few seconds, and the warm up period can be as long as five minutes (depending upon lamp type). During this time the lamp exhibits different colors as the various metal halides vaporize in the arc chamber.

If power is interrupted, the lamp's arc will extinguish, and the high pressure that exists in the hot arc tube will prevent restriking the arc; with a normal ignitor a cool-down period of 5–10 minutes will be required before the lamp can be restarted, but with special ignitors and specially designed lamps, the arc can be immediately re-established. On fixtures without instant restrike capability, a momentary loss of power can mean no light for several minutes. For safety reasons, some metal-halide fixtures have a backup tungsten-halogen incandescent lamp that operates during cool-down and restrike. Once the metal halide restrikes and warms up, the incandescent safety light is switched off. A warm lamp also tends to take more time to reach its full brightness than a lamp that is started completely cold.

Most hanging ceiling lamps tend to be passively cooled, with a combined ballast and lamp fixture.

End of life behaviour

thumb|Old metal halide lamp|alt=

Metal halide lamps, usually lose their output or change color due to the loss of halides and arctube blackening. They stop working at the end of life that is similar to mercury lamps. In rare cases, they can also cycle on/off. Some can exhibit major color shift, and in rare cases, explode.

Risk of lamp explosion

alt=Metal Halide Bulb Shattered|thumb|A metal halide bulb that has shattered as a result of an arc tube explosion due to uninterrupted prolonged use.

All metal halide arc tubes deteriorate in strength over their lifetime due to chemical attack, thermal stress and mechanical vibration. As the lamp ages the arc tube becomes discolored (often obtaining a dark grey shade), absorbing light and getting hotter. The tube will continue to become weaker until it eventually fails, causing the breakup of the tube.

Early failure of the arc tube may occur due to manufacturing defects. Manufacturers may "season" new lamps to check for such defects before sale.

Since a metal-halide lamp contains gases at a significant high pressure (up to 3.4 atmospheres), failure of the arc tube is inevitably a violent event. Fragments of arc tube will break the outer bulb, and hot glass fragments may fall on people or objects below. Hot fragments may present a fire hazard. Fixtures are designed to contain hot fragments with a hard glass cover, or may be designed for lamps with a quartz tube surrounding the arc tube to prevent breakage.

Shattering of the arc tube may be avoided by replacing the lamp if there is an excessive blackening of the arc tube, the arc tube begins to swell, there is a sudden changing of the light color, or the lamp begins to cycle on and off.

Gallery

File:MetalHalideHighBay.jpg|A low-bay light fixture using a high-wattage metal-halide lamp, of the type used in factories and warehouses

File:HK CWB Tung Lo Wan 聖馬利亞堂 Saint Mary's Church electric light lamp May-2013.JPG|Metal halide floodlight

File:150 Watt Metal Halide.jpg|A Philips MHN-TD 150W/842 (150 watts, 4200 K) linear/tubular metal-halide lamp

File:Closeup Metal Halide Lamp.jpg|A Philips MHN-TD 150W/842 linear/tubular metal-halide lamp lit up at half power

File:Skybeamer-uniqema-640.jpg|A light source using a broad-spectrum metal-halide lamp pointing upward towards the sky

File:Night-Game.jpg|A metal-halide light bank at a softball field

File:Metal-Halide lamp 70 Watt screw fixture.jpg|A ceramic metal-halide lamp 70 Watt - screw fixture (aquarium)

</gallery>thumb|upright|[[Pictograms on the packaging of a 35 W lamp. The multitude of instructions is due to the higher complexity of use in comparison to other lamp types.]]