Atlas KolorSON 400W

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.
Manufacturer: Thorn Lighting Ltd.
Lamp Power: 400 Watts
Lamp Current: 4.4 Amps
Lamp Voltage: 105 ± 15 Volts
Cap Type: E40s/45 Ni plated brass + vitrite
Bulb Type: ED-120 ED-37 in eighths/inch
Bulb Finish: Diffuse coated Borosilicate glass
Electrodes: Backwound Tungsten BCT Emitter
Arc Length:
Atmosphere: Inner: Na,Hg | Xe Outer: Hard Vacuum
Luminous Flux: 44,000 lm (@ 100 hrs) 40,000 lm (average)
Luminous Efficacy: 110.0 lm/W (@ 100 hrs)
Colour Temperature & CRI: CCT: 2100K CRI: Ra 12
Chromaticity Co-ordinates: CCx: 0.534 CCy: 0.431
Lifetime: Not rated
Warm-up & Re-strike Time: 3-4 minutes 1 minute
Burning Position: Universal
Overall Length: 280±6 mm 11 inches
Light Centre Length:
Factory: Leicester England
Date of Manufacture: December 1974 Date Code: 12
Original & Present Value: £18.38 (1972)
References: 1) Thorn Lighting Comprehensive Catalogue, UK, 1973-74.
2) Thorn Lighting Comprehensive Price List, UK, 19th April 1972.