||Following the perfection of the Edge-Defined Film-Fed Growth technique of manufacturing complex shape single crystal ceramics by H.H. LaBelle and Gerald Meiling at Corning, there was much interest in employing single crystal sapphire as an arc tube for high pressure sodium lamps. The material was marketed by Corning for several years under the trade name Corstar and Westinghouse was first to market with Corstar lamps, in 1976. The traditional arc tube material is polycrystalline alumina, the principal difference is that single crystal tubing is transparent, whereas polycrystalline alumina is translucent. Its multiple boundaries between the tiny individual crystals scatter the light and reduce transparency.
Single crystal sapphire not only has better light transmission which instantly delivers improved luminous efficacy, it also offers reduced sodium loss at higher temperatures. Thus two new lamps were investigated with this material - one having higher luminous efficacy, the other being a 'deluxe' lamp with better colour rendering which was brought about by an increase in wall loading. Regrettably, single crystal sapphire is not only rather expensive, it also has different coefficients of thermal expansion parallel and perpendicular to the tube axis. In view of the fact that it actually only delivered some 3% better efficacy than the best PCA, and that seals to metal conductors were very tricky to make even experimentally, lamps having sapphire arc tubes were only made and sold for a brief period only by Thorn, Osram-GEC and Westinghouse.
This lamp employs cup-type niobium-zirconium end seals which have been sealed to the arctube with a eutectic oxide braze. Also included is a small disc of tungsten behind each electrode to help keep the seals cool and prevent failures.