||In late 2001 GE expanded its ceramic metal halide family with a tiny 20 watt rating aimed specifically at superseding low voltage halogen sources. Prior to this, Toshiba and Ushio had been selling 20W ceramic metal halides in Japan but they did not achieve widespread success due to the very high system cost. GE appears to be enjoying considerably better success with this European equivalent.
From the technological standpoint the GE arc tube has a very elegant design, making use of the company's innovative second generation 3-piece arc tube. This employs injection-moulded ceramic end plugs having a large pocket in which the electrode is located. When sintered into the arc tube body the effect is to increase the wall thickness of the ceramic at the tube ends. This is the region where molten metal halide salts generally reside during lamp operation, and where corrosion of the ceramic wall is greatest. Traditionally the arc tube wall must be thick so that during life, corrosion is never able to penetrate all the way through. The thicker tube does absorb some light though, and takes a long time to run up and cool down. The innovative GE design affords adequate protection in the corrosion area, but permits more light to pass through the thin central portion and a run-up time of just 72 seconds.
The X-ray photo here shows how the corrosion of metal halide salts has attacked the arc tube ends in a standard and new GE design of arc tube. Additionally it can be seen that the wall thickness near the centre has been reduced and this is what allows GE lamps to operate at higher luminous efficacy. Corrosion can also be reduced by employing a shaped elliptical arc tube with better heat uniformity, but that is an expensive solution which add to the system costs.