||This is one of the earliest styles of high pressure sodium lamp made in the UK. Dating to 1974 it represents the first commercial design adopted by British Lighting Industries, as the Thorn / AEI group of companies were known during their mergers over the years 1958-1967.
The arc tube is made of "Stellox", a polycrystalline alumina doped with magnesia and yttria to produce the unique bi-phase grain structure invented at the Rugby works of AEI Lamp & Lighting Co. The properties of Stellox are similar to GE's Lucalox, the first translucent polycrystalline alumina, but its different grain structure was of crucial importance in circumventing the GE patent.
The arc tube ends are closed with pressed cups of niobium. These are brazed to the Stellox using a reactive alloy of titanium, zirconium and vanadium. Ti is the element forming the reactive seal to the ceramic, however its melting point is inconveniently high at around 1800-1850°C. The addition of the other metals forms an alloy that can be fused at around 1400°C which is much more desirable for seal fabrication. Typically, the seal of this type was made by sandwiching three thin washers of each metal over the end of the arc tube and capping them with the niobium cup, before heating in a vacuum furnace. The titanium washer is always the one in contact with the alumina, to form the most reactive seal. One end is blind, while the other is extended into a niobium exhaust tube. This has been pinched-off by cold fusion to permanently seal the sodium amalgam and xenon gas within the arc tube. The outer bulb is coated with a diffusing layer of ground glass in an effort to reduce glare, and enable use in luminaires for mercury fluorescent lamps.