||The ceramic metal halide lamp which is so popular today has actually been in existence many years longer than its place on the market would suggest. Developments on the technology started with Thorn Lighting's CMH project in the early 1970's, and in 1981 the company was ready to present the technology and came very close to marketing this lamp. The company's stand at the 1981 Hanover World Light Show was illuminated with this lamp and it was expected that it would become available later that year.
Unfortunately the lamp was never to be seen again after that time. The company's reason was that this design had a very high arc tube voltage, roughly double that of other HID lamps, and there was no control gear available for it. In addition, owing to the small voltage drop across the ballast for this lamp, when operated on the 240V mains supply, the system was rather inefficient despite the good performance of the lamp itself. The company therefore decided to suspend a market launch until a lamp could be developed that operated on conventional ballasts. Research continued ten years until the time when Thorn was taken over by GE Lighting, which axed the CMH project. It was a further four years before GE re-entered the CMH arena once Philips had announced its solution of the many technical problems.
Of particular interest are the arc tube seals in this lamp. Thorn, which made its own 'Stellox' PCA ceramic tubing, also pioneered the "Cermet" seal for its CMH and SON arc tubes. The black end plugs are made of electrically conductive ceramic-molybdenum composites and no wire penetrates the seal. It forms a seal highly resistant to corrosion by halides - unfortunately the frit seal was not so resistant.