Editing Metal-halide lamp (section)

===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.