||This is one of the earliest HPS lamps made in the UK. Dating to 1969 it represents the first commercial design adopted by British Lighting Industries, as the Thorn / AEI group was 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 a bi-modal grain structure having two distinct particle size distributions. Stellox was developed at AEI Leicester and has similar proeries to GE's Lucalox, the first translucent alumina, but its different grain structure was of crucial importance in circumventing the GE patent. The greater thickness of this arc tube coupled with its rough crystalline surface makes it appear far more opaque than modern tubes.
The arc tube ends are closed with pressed cups of niobium, brazed to the Stellox using an 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 lamp production. 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 electrodes are of primitive design and employ Thorn's braided cathode, originally developed for fluorescent lamps.