Editing Metal-halide lamp (section)

==Components==
[[Image:Closeup Metal Halide Lamp.jpg|thumb|150&nbsp;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.

===Arc tube===
Inside the [[fused quartz]] ''arc tube'', two [[tungsten]] electrodes doped with [[thorium]] are sealed into each end and an [[alternating current|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 [[iodide]]s or [[bromide]]s of different metals. [[Iodine]] and [[bromine]] are of the halogen group of the periodic table, and so are termed "halides" when ionized. [[Scandium]] and [[sodium]] are also used in some types, with [[thallium]], [[indium]], and [[sodium]] in European ''Tri-Salt'' models. [[Dysprosium]] used for high [[color temperature]] and [[tin]] for lower color temperature.  [[Holmium]] and [[thulium]] are used in very high power movie lighting models and in daylight colored metal halide lamps for area floodlighting, compact low wattage metal halide lamps, as well as stadium lighting in Europe.  [[Gallium]] or [[lead]] are 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 [[electrical impedance|impedance]], allowing the arc tube to be made sufficiently long and simple [[electrical ballast]]s to be used. A [[noble gas]], usually [[argon]], is cold filled into the arc tube at a pressure of about 2&nbsp;kPa to facilitate starting of the discharge. Argon filled lamps are typically quite slow to start up, taking several minutes to reach full light intensity; xenon fill, as used in automotive headlamps, start up relatively faster.

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 [[thermionic]]ally 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 [[Sintering|sintered]] [[alumina]] arc tube similar to those used in the [[high pressure sodium lamp]]. This development reduces the effects of ion creep that plagues fused silica arc tubes. During their life, sodium and other elements tend to migrate into the quartz tube and because of high UV radiation and gas ionization, will result in erosion of the electrodes therefore causing cycling of the lamp. The sintered alumina arc tube does not allow the ions to creep through, maintaining a more constant color over the life of the lamp. These are usually referred as [[ceramic metal-halide lamp]]s or CMH lamps.

The concept of adding metallic iodides for spectral modification (specifically: sodium - yellow, lithium - red, indium - blue, potassium and rubidium - deep red, and thallium - green) of a mercury arc discharge to create the first metal-halide lamp can be traced to patent US1025932 in 1912 by [[Charles Proteus Steinmetz]], the "Wizard of General Electric".

The amount of mercury used has lessened over years of progress.

===Outer bulb===
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 [[ultraviolet|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.

===Base===
Some types have an [[Edison screw]]  <!-- No Mogul bases? Edison screw seems awfully small for 18 kW! --> 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.

===Ballasts===
[[File:Osram POWERTRONIC PTi 35.JPG|thumb|upright=0.6|Electronic ballast for 35 W metal halide light bulbs]]
The electric arc in metal-halide lamps, as in all [[gas discharge lamp]]s 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 ballast]]s 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 lamp]]s. 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 high frequency which is then converted to a low frequency square wave 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 lamp]]s. It may cause [[acoustic resonance]] in the arc, shortening the lamp life.<ref>{{Cite book |last1=Weibin |first1=Cheng |chapter-url=https://www.infona.pl//resource/bwmeta1.element.ieee-art-000001711902 |chapter=Suppressing Acoustic Resonance in HID Lamp with Combined Frequency Modulation |last2=Yanru |first2=Zhong |last3=Shun |first3=Jin |title=2006 37th IEEE Power Electronics Specialists Conference |date=2006 |pages=1–5 |doi=10.1109/PESC.2006.1711902 |isbn=978-0-7803-9716-3 |language=English}}</ref>

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&nbsp;kV on cold strike, over 30&nbsp;kV<ref name=ecm>{{cite web|url=https://www.ecmweb.com/content/minimize-hid-lighting-system-downtime |title=Minimize HID Lighting System Downtime |website=Electrical Construction & Maintenance |access-date=20 September 2018 |quote=Instant restrike and quick restart systems eliminate the delay in restarting MH lamps after a sag or interruption. They use specially wired CWA ballasts and high-voltage ignitors that produce a high voltage (8kV to 40kV) to restart special lamps.|date=September 1998 }}</ref> 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).