Metal-halide lamps, a member of the high-intensity discharge (HID) family of lamps, produce high light output for their size, making them a compact, powerful, and efficient light source. By adding rare earth metal salts to the mercury vapor lamp, improved luminous efficacy and light color is obtained. Originally created in the late 1960s for industrial use, metal-halide lamps are now available in numerous sizes and configurations for commercial and residential applications.
Like most HID lamps, metal halide lamps operate under high pressure and temperature, and require special fixtures to operate safely.
Since the lamp is small compared to a fluorescent or incandescent lamp of the same light level, relatively small reflective luminaires can be used to direct the light for different applications (flood lighting outdoors, or lighting for warehouses or industrial buildings).
Metal-halide lamps are used both for general lighting purposes, and for very specific applications that require specific UV or blue-frequency light.
Because of their wide spectrum, they are used for indoor growing applications, in athletic facilities and are quite popular with reef aquarists, who need a high intensity light source for their corals.
Another widespread use for such lamps is in professional lighting fixtures, where they are commonly known as MSD lamps and are generally used in 150, 250, 400, 575 and 1,200 watt ratings, especially intelligent lighting.
Like other gas-discharge lamps such as the very similar mercury-vapor lamps, metal-halide lamps produce light by making an electric arc in a mixture of gases. In a metal-halide lamp, the compact arc tube contains a high-pressure mixture of argon, mercury, and a variety of metal halides. The mixture of halides will affect the nature of light produced, influencing the correlated color temperature and intensity (making the light bluer, or redder, for example). The argon gas in the lamp is easily ionized, and facilitates striking the arc across the two electrodes when voltage is first applied to the lamp. The heat generated by the arc then vaporizes the mercury and metal halides, which produce light as the temperature and pressure increases.
Common operating conditions inside the arc tube are 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 require auxiliary equipment to provide proper starting and operating voltages and regulate the current flow in the lamp. About 24% of the energy used by metal-halide lamps produces light (65–115 lm/W), making them substantially more efficient than incandescent bulbs.
Metal-halide lamps consist of an arc tube with electrodes, an outer bulb, and a base.
Inside the fused quartz arc tube two tungsten electrodes doped with thorium, are sealed into each end and current is passed to them by molybdenum foil seals in the fused silica. It is within the arc tube that the light is actually created.
Besides the mercury vapor, the lamp contains iodides or sometimes bromides of different metals. Scandium and sodium are used in some types, thallium, indium and sodium in European Tri-Salt models, and more recent types use dysprosium for high colour temperature, tin for lower colour temperature. Holmium and thulium are used in some very high power movie lighting models. Gallium or lead is used in special high UV-A models for printing purposes. The mixture of the metals used defines the color of the lamp. Some types for festive or theatrical effect use almost pure iodides of thallium, for green lamps, and indium, for blue lamps. An alkali metal, (sodium or potassium), is almost always added to reduce the arc impedance, allowing the arc tube to be made sufficiently long and simple electrical ballasts to be used. A noble gas, usually argon, is cold filled into the arc tube at a pressure of about 2 kPa to facilitate starting of the discharge.
The ends of the arc tube are often externally coated with white infrared reflective zirconium silicate or zirconium oxide to reflect heat back onto the electrodes to keep them hot and thermionically emitting. Some bulbs have a phosphor coating on the inner side of the outer bulb to improve the spectrum and diffuse the light.
In the mid-1980s a new type of metal-halide lamp was developed, which, instead of a quartz (fused silica) arc tube as used in mercury vapor lamps and previous metal-halide lamp designs, use a sintered alumina arc tube similar to what has been used in the high pressure sodium lamp. This development reduces the effects of ion creep that plagues fused silica arc tubes. During their life, because of high UV radiation and gas ionization, sodium and other elements tends to migrate into the quartz tube, resulting in depletion of light emitting material and so, cycling. The sintered alumina arc tube does not allow the ions to creep through, maintaining a more constant colour over the life of the lamp. These are usually referred as ceramic metal-halide lamps or CMH lamps.
Most types are fitted with an outer glass bulb to protect the inner components and prevent heat loss. The outer bulb can also be used to block some or all of the UV light generated by the mercury vapor discharge, and can be composed of specially doped "UV stop" fused silica. Ultraviolet protection is commonly employed in single ended (single base) models and double ended models that provide illumination for nearby human use. Some high powered models, particularly the Lead-Gallium UV printing models and models used for some types of sports stadium lighting do not have an outer bulb. The use of a bare arc tube can allow transmission of UV or precise positioning within the optical system of a luminaire. The cover glass of the luminaire can be used to block the UV, and can also protect people or equipment if the lamp should fail by exploding.
Some types have an Edison screw metal base, for various power ratings between 10 and 18,000 watts. Other types are double-ended, as depicted above, with R7s-24 bases composed of ceramic, along with metal connections between the interior of the arc tube and the exterior. These are made of various alloys (such as iron-cobalt-nickel) that have a thermal coefficient of expansion that matches that of the arc tube.
Metal-halide lamps require electrical ballasts to regulate the arc current and deliver the proper voltage to the arc. Like high-pressure mercury vapour lamps, some metal-halide bulbs contain a third electrode to initiate the arc when the lamp is first lit (which generates a slight flicker when the lamp is first turned on). Pulse-start metal-halide lamps don't contain a starting electrode, but they 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. American National Standards Institute (ANSI) lamp-ballast system standards establish parameters for all metal-halide components (with the exception of some newer products).
Electronic ballasts include ignitor and ballast into a single package. These ballasts use high-frequency to drive the lamps. Because they have less loss than a line-frequency "iron" ballast, they are more energy efficient. High-frequency operation does not increase lamp efficacy as for fluorescent lamps.
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 places where many lamps are used. Because the lamp's color characteristics tend to change during lamp's life, color is measured after the bulb has been burned for 100 hours (seasoned) according to ANSI standards. Newer metal-halide technology, referred to as "pulse start," has improved color rendering and a more controlled kelvin variance (±100 to 200 kelvins).
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. 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
A "cold" (below operating temperature) 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 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, even briefly, 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 re-started, but with special ignitors with 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, many 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; immediately restoring power to a hot lamp before it has re-struck can make it take even longer to relight, because of power consumption and heating of the passively cooled lamp ballast that is attempting to relight the lamp.
End of life behaviour
At the end of life, metal-halide 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. This causes the lamp to glow for a while and then goes out, repeatedly. In rare occurrences the lamp explodes at the end of its useful life.
Modern electronic 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.
Risk of lamp explosion
All HID arc tubes deteriorate in strength over their lifetime because of various factors, such as chemical attack, thermal stress and mechanical vibration. As the lamp ages the arc tube becomes discoloured, absorbing light and getting hotter. The tube will continue to become weaker until it eventually fails, causing the breakup of the tube.
Although such failure is associated with end of life, an arc tube can fail at any time even when new, because of unseen manufacturing faults such as microscopic cracks. However, this is quite rare. Manufacturers typically "season" new lamps to check for manufacturing defects before the lamps leave the manufacturer's premises.
Since a metal-halide lamp contains gases at a significant high pressure, failure of the arc tube is inevitably a violent event. Fragments of arc tube are launched, at high velocity, in all directions, striking the outer bulb of the lamp with enough force to cause it to break. If the fixture has no secondary containment (e.g. a lens, bowl or shield) then the extremely hot pieces of debris will fall down onto people and property below the light, likely resulting in serious injury, damage, and possibly causing a major building fire if flammable material is present.
The risk of a "nonpassive failure" of an arc tube is very small. According to information gathered by the National Electrical Manufacturers Association (www.nema.org), there are approximately 40 million metal-halide systems in North America alone, and only a very few instances of nonpassive failures have occurred. Although it is not possible to predict, or eliminate the risk, of a metal-halide lamp exploding, there are several precautions that can be taken to reduce the risk:
- Using only well designed lamps from reputable manufacturers and avoiding lamps of unknown origin.
- Inspecting lamps before installing to check for any faults such as cracks in the tube or outer bulb.
- Replacing lamps before they reach their end of life (i.e. when they have been burning for the number of hours that the manufacturer has stated as the lamp's rated life).
- For continuously operating lamps, allowing a 15 minute shutdown for every 7 days of continuous operation.
- Relamp fixtures as a group. Spot relamping is not recommended.
Also, there are measures that can be taken to reduce the damage caused should a lamp fail violently:
- Ensuring that the fixture includes a piece of strengthened glass or polymeric materials between the lamp and the area it is illuminating. This could be incorporated into the bowl or lens assembly of the fixture.
- Using lamps that have a reinforced glass shield around the arc tube to absorb the impact of flying arc tube debris, preventing it from shattering the outer bulb. Such lamps are safe to use in 'open' fixtures. These lamps carry an "O" designation on the packaging reflective of American National Standards Institute (ANSI) standards.
Other safety concerns
Although an excellent source of lighting for the reef aquarium, there has been concern voiced by some aquarists over the potential ill-effects of close-range contact with metal-halide lighting that is demanded by the hobby. Some individuals have noticed temporary blurred vision even after very brief exposure to metal-halide lighting. This blurring of vision could be linked to photokeratitis (snow blindness) – the result of unprotected exposure to ultraviolet (UV) radiation.
Broken and unshielded high-intensity metal-halide bulbs have been known to cause eye and skin injuries, particularly in school gymnasiums. See the following article from the FDA: Ultraviolet Radiation Burns from High Intensity Metal Halide and Mercury Vapor Lighting Remain a Public Health Concern. Also see: Teachers battle dangerous lighting conditions and Photokeratitis and UV-Radiation Burns Associated With Damaged Metal Halide Lamps.
- Waymouth, John (1971). Electric Discharge Lamps. Cambridge, MA: The M.I.T. Press. ISBN 0-262-23048-8.
- ^ US patent 4171498, Dietrich Fromm et al., "High pressure electric discharge lamp containing metal halides", issued 1979-10-16
- ^ US patent 3234421, Gilbert H. Reiling, "Metallic halide electric discharge lamps", issued 1966-02-08
- ^ High Intensity Discharge Lamps (NASA)
- Raymond Kane, Heinz Sell Revolution in lamps: a chronicle of 50 years of progress (2nd ed.), The Fairmont Press, Inc. 2001 ISBN 0881733784
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