Xenon Arc Lamps April 21 2015
All modern xenon short-arc lamps use a fused quartz envelope with thoriated tungsten electrodes. Fused quartz is the only economically feasible material currently available that can withstand the high pressure (25 atmospheres for an IMAX bulb) and high temperature present in an operating lamp, while still being optically clear. The thorium dopant in the electrodes greatly enhances their electron emission characteristics. Because tungsten and quartz have different coefficients of thermal expansion, the tungsten electrodes are welded to strips of puremolybdenum metal or Invar alloy, which are then melted into the quartz to form the envelope seal.
Because of the very high power levels involved, large lamps are water-cooled. In those used in IMAX projectors, the electrode bodies are made from solid Invar and tipped with thoriated tungsten. An O-ring seals off the tube, so that the naked electrodes do not contact the water. In low power applications the electrodes are too cold for efficient electron emission and are not cooled; in high power applications an additional water cooling circuit for each electrode is necessary. To save costs, the water circuits are often not separated and the water needs to bedeionized to make it electrically non-conductive, which, in turn, lets the quartz or some laser media dissolve into the water.
In order to achieve maximum efficiency, the xenon gas inside short-arc lamps is maintained at an extremely high pressure — up to 30 atmospheres (440 psi / 3040 kPa) — which poses safety concerns. If a lamp is dropped, or ruptures while in service, pieces of the lamp envelope can be thrown at high speed. To mitigate this, large xenon short-arc lamps are normally shipped in protective shields, which will contain the envelope fragments should breakage occur. Normally, the shield is removed once the lamp is installed in the lamp housing. When the lamp reaches the end of its useful life, the protective shield is put back on the lamp, and the spent lamp is then removed from the equipment and discarded. As lamps age, the risk of failure increases, so bulbs being replaced are at the greatest risk of explosion. Because of the safety concerns, lamp manufacturers recommend the use of eye protection when handling xenon short-arc lamps. Because of the danger, some lamps, especially those used in IMAX projectors, require the use of full-body protective clothing.