Metal-halide lamp
[1] Developed in the 1960s, they are similar to mercury vapor lamps,[1] but contain additional metal halide compounds in the quartz arc tube, which improve the efficiency and color rendition of the light.
[1] They are used for wide area overhead lighting[2] of commercial, industrial, and public places, such as parking lots, sports arenas, factories, and retail stores,[1] as well as residential security lighting, automotive headlamps (Often generically known as "xenon headlights") and indoor cannabis grow operations.
Because of their wide spectrum and good efficiency they were used for indoor growing applications, specifically cannabis, and were quite popular with reef aquarists who needed a high intensity light source for their corals.
[7] The particular mixture of metal halides influences the correlated color temperature and intensity (making the light more blue or red, for example).
A noble gas, usually argon, is cold filled into the arc tube at a pressure of about 2 kPa to facilitate starting of the discharge.
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.
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 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.
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.
If a metal-halide bulb is underpowered, because of the lower operating temperature, its light output will be bluish because of the evaporation of mercury alone.
This phenomenon can be 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 explosion because of overheating and overpressure.
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).
For safety reasons, some metal-halide fixtures have a backup tungsten-halogen incandescent lamp that operates during cool-down and restrike.
[12] All metal halide arc tubes deteriorate in strength over their lifetime due to chemical attack, thermal stress and mechanical vibration.
As the lamp ages the arc tube becomes discolored (often obtaining a dark grey shade), absorbing light and getting hotter.
Since a metal-halide lamp contains gases at a significant high pressure (up to 3.4 atmospheres), failure of the arc tube is inevitably a violent event.
