||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. Serious developments on the technology started with Thorn Lighting's CMH project in the early 1970's, and by 1981 the company was ready to present the technology and came very close to marketing its Tin Sodium Halide lamp. The Thorn Lighting stand at the 1981 Hanover World Light Show was illuminated with this lamp and it was expected that it would become available before the end of the year.
The version here is a special type made up for analytical purposes, having a transparent discharge tube of single crystal sapphire. The end seals are very critical in this kind of lamp. The molten reservoir of tin chloride and sodium iodide dose remains in the end corners of the arc tube, which must be hot enough to ensure that sufficient halide is vaporised, but not so hot that the seals are chemically attacked. This state of affairs is easier to control with a shaped end to the tube. Since sapphire tubing could, back in the 1980s, only be supplied in tubular form, it was necessary to shape the end sections with a plug of Stellox material (polycrystalline alumina) , which has a similar coefficient of thermal expansion and can be sealed to sapphire relatively easily.
The electrical feedthroughs take the form of Thorn's unique molybdenum cermet materials, an electrically conductive composite plug of ceramic and molybdenum. The tungsten electrode shank, and molybdenum lead wire are co-sintered into this plug (see diagram at left) thus it performs the dual function of sealing the arc tube and acting as the electrical feedthrough.