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. Developed in the 1960s, they are similar to mercury vapor lamps, but contain additional metal halide compounds in the quartz arc tube, which improve the efficiency and color rendition of the light. The most common metal halide compound used is sodium iodide. Once the arc tube reaches its running temperature, the sodium dissociates from the iodine, adding orange and reds to the lamp's spectrum from the sodium D line as the metal ionizes. As a result, metal-halide lamps have high luminous efficacy of around 75–100 lumens per watt, which is about twice that of mercury vapor lights and 3 to 5 times that of incandescent lights and produce an intense white light. Lamp life is 6,000 to 15,000 hours. As one of the most efficient sources of high CRI white light, metal halides as of 2005[update] were the fastest growing segment of the lighting industry. They are used for wide area overhead lighting of commercial, industrial, and public spaces, such as parking lots, sports arenas, factories, and retail stores, as well as residential security lighting and automotive headlamps (xenon headlights). The lamps consist of a small fused quartz or ceramic arc tube which contains the gases and the arc, enclosed inside a larger glass bulb which has a coating to filter out the ultraviolet light produced. They operate at a pressure between 4 and 20 atmospheres, and require special fixtures to operate safely, as well as an electrical ballast. Metal atoms produce most of the light output. They require a warm-up period of several minutes to reach full light output.
1 Uses 2 Operation 3 Components
3.1 Arc tube 3.2 Outer bulb 3.3 Base 3.4 Ballasts
3.4.1 Self-ballasted lamps
4 Color temperature 5 Starting and warm up 6 End of life behaviour
6.1 Risk of lamp explosion
Uses Metal-halide lamps are used for general lighting purposes both indoors and outdoors, such as commercial, industrial, and public spaces, parking lots, sports arenas, factories, and retail stores, as well as residential security lighting; automotive and specialty applications are further fields of usage. Metal-halide lamps are used in automobile headlights, where they are commonly known as "xenon headlamps" due to the use of xenon gas in the bulb instead of the argon typically used in other halide lamps. Another widespread use for such lamps is in photographic lighting and stage 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. Because of their wide spectrum and good efficiency, they are used for indoor growing applications and are quite popular with reef aquarists, who need a high intensity light source for their corals. 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 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 harsh 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 with a filament), 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), making them substantially more efficient than incandescent bulbs, which typically have efficiencies in the range 2–4%. Components
150 watt metal-halide bulb in fixture, about halfway through warmup
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 an AC voltage is applied to them
through molybdenum foil seals fused in silica. It is the arc between
the two electrodes where the light is actually created.
Besides mercury vapor, the lamp contains iodides or bromides of
Electronic ballast for 35 W metal halide light bulbs
The electric arc in metal-halide lamps, as in all gas discharge lamps has a negative resistance property; meaning that as the current through the bulb increases, the voltage across it decreases. If the bulb is powered from a constant voltage source such as directly from the AC wiring, the current will increase until the bulb destroys itself; therefore, halide bulbs require electrical ballasts to limit the arc's current. There are two types:
Inductive ballast - Many fixtures use an inductive ballast, also known as a magnetic ballast, similar to those used with fluorescent lamps. This consists of an iron-core inductor. The inductor presents an impedance to AC current. If the current through the lamp increases, the inductor reduces the voltage to keep the current limited. Electronic ballast - These are lighter and more compact. They consist of an electronic oscillator which generates a high frequency current to drive the lamp. Because they have lower resistive losses than an inductive ballast, they are more energy efficient. However, high-frequency operation does not increase lamp efficiency as for fluorescent lamps.
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). Self-ballasted lamps As of 2012, several companies have started to offer self-ballasted metal-halide lamps as a direct replacement for incandescent and self-ballasted mercury-vapor lamps. These lamps include an arc tube with a starting electrode as well as a tubular halogen lamp which is connected in series and used to regulate the current in the arc tube. A resistor provides the current limiting for the starting electrode. Like self-ballasted mercury-vapor lamps, self-ballasted metal-halide lamps are connected directly to mains power and do not require an external ballast. In contrast to the former, these lamps usually have a clear outer bulb without a coating, making the arc tube and the halogen lamp tube clearly visible from the outside. Color temperature
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.
400 W metal-halide lamp shortly after powering 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 (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, 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 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, 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
Old HMI lamp
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 once again causes 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
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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 such defects before the lamps leave the manufacturer's premises. Since a metal-halide lamp contains gases at a significant high pressure (up to 50 psi), 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 (such as 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" (explosion) of an arc tube is very small. According to information gathered by the National Electrical Manufacturers Association, 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 impossible to predict or eliminate the risk of a metal-halide lamp exploding, there are several precautions that can 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 seven days of continuous operation. Relamping fixtures as a group. Spot relamping is not recommended.
Also, there are measures that can be taken to reduce the damage caused by a lamp failure violently:
Ensuring that the fixture includes a piece of strengthened glass or
polymeric materials between the lamp and the area it is illuminating.
This can be incorporated into the bowl or lens assembly of the
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
American National Standards Institute
Lamps that require an enclosed fixture are rated "/E". Lamps that do not require an enclosed fixture are rated "/O" (for open). Sockets for "/O" rated fixtures are deeper. "/E" rated bulbs flare at the base, preventing them from fully screwing into a "/O" socket. "/O" bulbs are narrow at the base allowing them to fully screw in. "/O" bulbs will also fit in an "/E" fixture. Gallery
A low-bay light fixture using a high-wattage metal-halide lamp, of the type used in factories and warehouses
A light source using a broad-spectrum metal-halide lamp pointing upward towards the sky
A metal-halide light bank at a softball field
A metal-halide high voltage lamp 70
70W M98, M139, M143
100W M90, M140
150W M102, M142
175W M57, M137
250W M58, M138, M153
320W M132, M154
350W M131, M171
400W M59, M135, M155
1000W M47, M141
Hydrargyrum medium-arc iodide lamp
^ a b c d e f g h Hordeski, Michael F. (2005). Dictionary of energy efficiency technologies. USA: CRC Press. pp. 175–176. ISBN 0-8247-4810-7. ^ a b c d e Grondzik, Walter T.; Alison G. Kwok; Benjamin Stein; John S. Reynolds (2009). Mechanical and Electrical Equipment for Buildings, 11th Ed. USA: John Wiley & Sons. pp. 555–556. ISBN 0-470-57778-9. ^ a b Light Right: A practising engineer's manual on energy-efficient lighting. TERI Press. 2004. pp. 19–20. ISBN 81-7993-044-0. ^ a b "Metal Halide". Venture Lighting. Retrieved 2012-12-14. ^ Flesch, Peter (2006). Light and light sources: high-intensity discharge lamps. Springer. pp. 45–46. ISBN 3-540-32684-7. ^ 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) Archived 2010-01-13 at the Wayback Machine.
Waymouth, John (1971). Electric Discharge Lamps. Cambridge, MA: The M.I.T. Press. ISBN 0-262-23048-8. Raymond Kane, Heinz Sell Revolution in lamps: a chronicle of 50 years of progress (2nd ed.), The Fairmont Press, Inc. 2001 ISBN 0-88173-378-4
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