Stellox High Pressure Sodium

This is one of the earliest styles of high pressure sodium lamp made in the UK. Dating to 1969 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 Leicester 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 greater thickness of this arc tube coupled with its rough crystalline surface makes it appear far more opaque than modern tube.

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.
Manufacturer: British Lighting Industries Ltd.
Lamp Power: 250 Watts
Lamp Current: 3.0 Amps
Lamp Voltage: 100 ±15 Volts
Cap Type: E40s/45 Ni plated brass + vitrite
Bulb Type: TD-51 TD-58 in eighths/inch
Bulb Finish: Clear Borosilicate glass
Electrodes: Backwound Tungsten BCT Emitter
Arc Length:
Atmosphere: Inner: Na,Hg | Xe Outer: Hard Vacuum
Luminous Flux: 25,000 lm (@ 100 hrs)
Luminous Efficacy: 100 lm/W (@ 100 hrs)
Colour Temperature & CRI: CCT: 2300K CRI: Ra 12
Chromaticity Co-ordinates: CCx: 0.534 CCy: 0.431
Rated Lifetime: 5,000 hours 70% survival
Warm-up & Re-strike Time: 3-4 minutes 1 minute
Burning Position: Universal
Overall Length: 257 mm 101/8 inches
Light Centre Length: 160 mm 61/4 inches
Factory: Leicester England
Date of Manufacture: March 1969 Date Code: P3904
Original & Present Value: £14.18 (1971) £179.00 (2014)
 
References: 1) British Lighting Industries Comprehensive Catalogue 1969/70.
2) Thorn Lighting Comprehensive Price List, UK, 19th April 1972.