Editing Metal-halide lamp (section)

==Operation==
Like other [[gas-discharge lamp]]s such as the very-similar [[mercury-vapor lamp]]s, 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 (element)|mercury]], and a variety of metal [[halide]]s, such as sodium iodide and scandium iodide.<ref name="Flesch">{{cite book   
  | last =  Flesch
  | first = Peter
  | title = Light and light sources: high-intensity discharge lamps
  | publisher = Springer
  | year = 2006
  | pages = 45–46
  | url = https://books.google.com/books?id=fWHQbhgxpAkC&q=%22metal+halide+lamp%22+gas+discharge+mercury&pg=PA45
  | isbn = 978-3-540-32684-7}}</ref> The particular mixture of metal halides influences the [[Color temperature#Correlated color temperature|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 (physics)|plasma]], which produces an increasingly brighter white light as the temperature and pressure increases to operating conditions.

The arc-tube operates at anywhere from 5–50 atm or more<ref>{{US patent reference | number = 4171498 | y = 1979 | m = 10 | d = 16 | inventor = Dietrich Fromm et al. | title = High pressure electric discharge lamp containing metal halides}}</ref> (70–700&nbsp;[[pound-force per square inch|psi]] or 500–5000&nbsp;[[kPa]]) and 1000–3000&nbsp;°C.<ref>{{US patent reference | number = 3234421  | y = 1966 | m = 02 | d = 08 | inventor = Gilbert H. Reiling | title = Metallic halide electric discharge lamps}}</ref>  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 (electrical)|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&nbsp;[[lumen (unit)|lm]]/[[Watt|W]]),<ref name="venturelighting"/> making them substantially more efficient than [[incandescent light bulb|incandescent bulbs]], which typically have efficiencies in the range 2–4%.