Metal halide lamp
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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. 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. They are also considered a "point" light source, so reflective luminaires are often required to concentrate the light for purposes of the lighting application.
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[edit] Uses
Metal halide lamps are used both for general industrial purposes, and for very specific applications which require specific UV or blue-frequency light. They are used for indoor growing applications, because they can provide the spectrum and temperature of light which encourage general plant growth. They are most often used in athletic facilities. Metal Halide lights are quite popular with reef aquarists, who need a high intensity light source for their corals. Another widespread use for such lamps is in higher-end professional lighting fixtures, especially intelligent lighting. In this application they are commonly known as MSD lamps, and are generally used in 150, 250, 575 and 1200 watt ratings.
[edit] Operation
Like other gas-discharge lamps such as the very-similar mercury-vapor lamps, metal halide lamps produce light by passing an electric arc through 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 70-90 psi (480-620 kPa) and 2000 °F (1090 °C)[citation needed].
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[1]), making them generally more efficient than fluorescent lamps, and substantially more efficient than incandescent bulbs.
[edit] Components
Metal halide lamps consist of the following main components. Some types have a metal base, E26, E27, E39 or E40, (Edison screw, followed by diameter in milimetres), for various power ratings between 50 and 3500 watts and other types are double-ended, as depicted above, with R7s-24 bases composed of ceramic and various FerNiCo iron-cobalt-nickel alloys that allow an electrical connection. Most types are fitted with an outer glass bulb to protect the inner components, support frame and arc tube from oxidation, heat loss and provide a shield to prevent the short wavelength UV light generated by the mercury vapor discharge, which is transmitted by the fused silica inner bulb or arc tube from escaping as it is blocked by the soda or borosilicate glass bulbs used on older single ended, (single base) models or by specially doped "U.V. stop" fused silica outer bulbs of more contemporary single ended and most double ended models. 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 in some types, Thallium, Indium and Sodium in European "Tri-Salt" models, and more recent types using Dysprosium for high colour temperature, Tin for lower colour temperature, Holmium and Thulium in some very high power rated Movie lighting models and Gallium and/or Lead in special high U.V.-A models for printing purposes). A noble gas, usually Argon, is cold filled into the arc tube at a pressure of about 15Torr to facilitate starting of the discharge. The mixture of the metals used defines the color of the lamp, and some types for festive or theatrical effect use almost pure iodides of Thallium, for Green lamps, and Indium, for blue lamps. An alkali metal, usually Sodium and sometimes Potassium, are almost always added to reduce the arc impedance, allowing the arc tube to be made sufficiently long and simple control gear, (Ballasts), to be used. The ends of the arc tube are often externally coated with white infra red reflective Zirconium silicate or Zirconium Oxide to reflect heat back onto the electrodes to keep them hot and thermionically emitting. Some high powered models, particularly the Lead-Gallium U.V. printing models and models used for some types of sports stadia lighting do not possess an outer bulb and consist of just the bare arc tube, this allows passage of U.V. or precise positioning within the optical system of a luminaire, the cover glass of which blocks the U.V. and protects people below, should the lamp explosively fail.
Developed in the mid 1980's was a new type of metal halide lamp which instead of a "quartz", (fused silica) arc tube as used in mercury vapor lamps and previous metal halide lamp designs, possess a sintered alumina arc tube similar to what has been used in the high pressure sodium lamp since the 1960s. This development reduces the effects of "ion creep" which plagues fused silica arc tubed lamps. (The intense U.V. flux inside the outer bulb of a fused silica arc tube lamp impinges upon the "pass wire" which conducts electricity from the base-outer seal end of the lamp to the crown end of the arc tube. Thus releases photoelectrons by the einstein effect, particularly when the pass wire is negative during that polarity of the applied A.C. cycle. These photoelectrons are attracted to the outer surface of the arc tube where thy accumulate, like the static charge on a C.R.T. television screen. This sets up a strong electric field across the arc tube wall which gradually, during the life of the lamp, draws metal ions, usually Sodium, from the discharge column inside the arc tube through the red hot silica where they are discharged by the electrons on the outside as they emerge and instantly evaporate off the hot surface as the metallic element to condense in the outer bulb. This gradually increases the impedance of the lamp as well as changing the colour, usually to the bluer raw mercury discharge. This effect is quite noticable in large installations of lamps, such as shopping concourses. Eventually the lamps "pulse", by running up to full power, but because of the increased impedance their full power arc voltage is greater than that of the supply, so they extinguish to cool and re-strike to repeat the cycle.) The sintered alumina arc tube does not allow the ions to creep through maintaining a much more constant colour over the life of the lamp and allowing much more precise definition of the colour the lamp will emit. These are usually referred as ceramic metal halide lamps or CMH lamps.
Some bulbs have a phosphor coating on the inner side of the outer bulb to improve the spectrum and diffuse the light.
[edit] Ballasts
Metal halide lamps require electrical ballasts to regulate the arc current flow and deliver the proper voltage to the arc. Probe start metal halide bulbs contain a special 'starting' electrode within the lamp 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 do not require a starting electrode, and instead use a special starting circuit referred to as an ignitor to generate a high-voltage pulse to the operating electrodes. American National Standards Institute (ANSI) lamp-ballast system standards establish parameters for all metal halide components (with the exception of some newer products).
A few electronic ballasts are now available for metal halide lamps. The benefit of these ballasts is more precise management of the lamp's wattage, which provides more consistent color and longer lamp life. In some cases, electronic ballasts are reported to increase efficiency (i.e. reduce electrical usage). However with few exceptions, high-frequency operation does not increase lamp efficiency as in the case of high-output (HO) or very high-output (VHO) fluorescent bulbs. High frequency electronic operation does however allow for specially designed dimming metal halide ballast systems.
[edit] Color temperature
Metal halide lamps were initially preferred to mercury vapor lamps in instances where natural light was desired because of the whiter light generated (mercury vapor lamps generating light that was much bluer). However the distinction today is not as great. Some metal halide lamps can deliver very clean "white" light that has a color-rendering index (CRI) in the '80s. With the introduction of specialized metal halide mixtures, metal halide lamps are now available that can have a correlated color temperature as low as 3000 K (very yellow) to 20,000 K (very blue). Some specialized lamps have been created for the spectral absorption needs of plants (indoor gardening) or animals (indoor aquariums). Due to tolerances in the manufacturing process, color temperature can vary slightly from lamp to lamp, and the color properties of metal halide bulbs cannot be predicted with 100% accuracy. Moreover, per ANSI standards the color specifications of metal halide bulbs are measured after the bulb has been burned for 100 hours (seasoned). The color characteristics of a metal halide lamp will not conform to specifications until the bulb has been properly seasoned. Color temperature variance is seen greatest in "probe start" technology lamps (±300 kelvins). 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 it will have a lower physical temperature and its light output will be 'cooler' (more blue, or very similar to that of a mercury vapor lamp). This is because the lower arc temperature will not completely vaporize and ionize the halide salts which are primarily responsible for the warmer colors (reds, yellows), thus the more-readily ionized mercury will dominate the light output. This phenomenon is also 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 rupture due to overheating and overpressure. Moreover, the color properties of metal halide lamps often change over the lifetime of the bulb. Often, in large installations of MH lamps, particularly of the quartz arc-tube variety, it will be seen that no two are exactly alike in color.
[edit] 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 re-striking the arc; a cool-down period of 5–10 minutes will be required before the lamp can be re-started. A warm lamp also tends to take more time to reach its full brightness than a lamp which 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, due to power consumption and heating of the passively cooled lamp ballast that is attempting to relight the lamp.
This is a major concern in some lighting applications where prolonged lighting interruption could create manufacturing shut-down or a safety issue. A few metal halide lamps are made with "instant restrike" capabilities where the lamp, ballast and socket are built to withstand the 30,000 volt re-ignition pulse supplied via a separate anode wire.
[edit] End of life
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. 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.
A common example of this is the street lights you see in the evening turning off and reigniting as soon as they have "cooled off".
[edit] Dangers and effects on humans
[edit] Eyes
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 which 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.
[edit] FDA cautions
Broken and unshielded high intensity metal halide bulbs could 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...
[edit] Explosion hazard
Metal halide lamps of the quartz arc tube variety are susceptible to explosion at the end of their rated life due to the corrosive effects of the halide salts on the quartz. Burning universal position lamps horizontally also increases risk of explosion due to higher arc wall temperature. Mechanical shock such as vibration may also cause explosion, as may leaving the lamp on all day. As such, metal halide fixtures should be turned off at least 15 minutes each week to reduce explosion hazard, and a quartz shroud may also be added for further protection and to reduce halide ions migrating through the arc tube wall (quartz metal halide lamps only).
[edit] See also
[edit] References
- Waymouth, John (1971). Electric Discharge Lamps. Cambridge, MA: The M.I.T. Press. ISBN 0-262-23048-8.
