Description of the Lamp / Bulb

This sort of lamp consists of a tubular outer bulb approx. 10 mm (.4 inch) in diameter which contains the arc tube (inner bulb). The outer bulb is made of special quartz such as cerium-doped quartz which blocks most ultraviolet, especially the more dangerous short and medium wavelengths as well as much of the 365-366 nM longwave mercury line cluster.
The arc tube or inner bulb is made of plain fused quartz and has tungsten electrodes with the distance between the tips approx. 4.2, maybe 5 millimeters (approx. or slightly under .2 inch). Its construction resembles that of a miniaturized short arc lamp, but true short arc lamps have a much more concentrated arc.
The arc tube has xenon gas in it at a couple of atmospheres to maybe a few atmospheres when cold and a few to maybe several atmospheres when hot. There is also mercury in the bulb, and when it is vaporized the mercury adds at least 20 atmospheres of pressure for a total pressure of around or maybe even over 30 atmospheres.
Metal halides - salts - are also in the arc tube. The formulation in automotive HID lamps includes sodium and scandium halides (probably iodides) and maybe traces of others such as lithium and thallium halides.
More ordinary metal halide lamps do not have high pressure xenon but have low pressure argon instead. The high pressure xenon is used to obtain some usable light output during warmup before the other ingredients have vaporized.