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The first British metal halide lamp was the Mazda Metalarc 400W from British Lighting Industries, a Thorn-AEI joint company. It was unusually based on the American sodium-thorium-scandium chemistry, in view of the fact that Thorn had a strong technical agreement with Sylvania. In 1968 it was replaced by this Kolorarc model, following Sylvania's lead of eliminating the thorium to attain improved life.
Sodium-Scandium lamps have the advantage of higher luminous flux than competing tri-band and rare earth chemistries, but were not developed elsewhere in Europe because of the challenges associated with running them on simple choke-type ballasts. However Thorn took the lead in solving many of these problems. The principal difficulty is that scandium iodide reacts with hot quartz to form stable scandium oxide + silicon iodide. The latter reacts with the tungsten electrodes, in which the silicon dissolves, releasing elemental iodine. Iodine is strongly electronegative, and the gradual increase in its concentration limits lamp life by its effect on increasing the reignition peaks of the arc voltage.
Thorn's first lamps required a special autotransformer ballast to deliver a high open circuit voltage, but this requirement was lifted in 1972 by the development of exceptionally clean lampmaking processes. These included polishing of arc tubes with ammonium bifluoride and hydrofluoric acid, and the world's first use of anhydrous metal halide pellets - a process developed jointly by Thorn and APL Materials. Thorn was also unusual for sandblasting its arc tubes to improve colour homogeneity in precision optical systems. This made lamps such as this Base Down model the favoured light source in demanding applications such as Lighthouses. |