The possibility of employing ceramic arc tubes for metal halide lamps is not a new idea - indeed their use was proposed in the earliest patents on metal halide lamps back in the 1960's.  Their potential advantages were recognised even at that time, in that they are more resistant than quartz to the corrosive nature of the metal halide salts within the arc tube.  In addition they can operate at temperatures beyond which quartz begins to soften.  Ceramics thus offer the possibility to increase the loading in the arc tube, bringing about an increase in wall temperature, which in turn increases colour rendering properties and luminous efficacy.

However while the ceramic arc tube itself is more resistant to halide corrosion, the same cannot be said of the end seals where the metal lead wires pass through the ceramic.  The end seals must be kept at low enough temperatures to reduce the rate of corrosion, and traditional metals known for ceramic to metal seals cannot be exposed to the halides.

The first practical solution to this problem was developed by Thorn Lighting in England during the early 1980s, and in 1981 that company exhibited the world's first Ceramic Metal Halide lamp at the Hannover World Light Fair.  This so-called 'TSH' Tin Sodium Halide lamp is illustrated below, and employs electrically conductive cermet caps to close the ends of the arc tube and make a halide-resistant seal.  The unique molecular spectrum of the tin chloride dose also resulted in rather good colour properties for its time.  But the marketing division did not take the idea further, since this lamp required a special kind ballast that was not available.

The commercial introduction of ceramic lamps had to wait until 1994 when Philips unveiled its 'CDM' range having the 'Protruding Plug' kind of end seal.  In this approach the seal is displaced some distance away from the heat of the arc tube, thus bringing its temperature down to safe limits.  An ingenious arrangement of different metallic conductors is contained within the seal both to ensure halide resistance and prevent cracking of these delicate seals.  All subsequent lamps are based on similar seals.