Thorn EMI 35W SOX 2nd Generation

During the 1950s and 60s Thorn and its predecessors fell behind their competitors in the field of single-ended low pressure sodium lamps. The earliest SO/H types were of course made by BTH and AEI-Mazda, but when the Integral SOI and the IR-Coated SOX ranges were introduced in 1955 and 1964 respectively, the Thorn/Mazda did not follow the lead of their competitors at Philips and Osram-GEC. Possibly this is because the company pioneered its own low pressure sodium concept in the form of the SLI Linear lamps in 1959, and believed in the superiority of that lamp type to the exclusion of the traditional single-ended ranges.

By the late 1960s it became clear that this was a mistake, and Thorn sourced in both SOI and SOX lamps from its competitors at Osram and Philips. It was not until 1972 that Thorn finally realised that it could not afford to ignore the single-ended concept, and set up its own SOX production.

This lamp is representative of Thorn's 2nd generation SOX. Like GEC's first SOX lamps, Thorn copied the Philips design with sodium-retaining dimples. After a few years a cheaper and more robust GEC dimple-free lamp was developed, thanks to a graded film thickness along lamp length. Thorn appreciated the superiority of this concept and also changed to the dimple-free design with graded film thickness. The discharge tube is supported at the U-bend by a small steatite ceramic tube held in a steel support. This avoids electrical contact between the discharge tube and the electrically conductive IR film, which could lead to sodium loss. The other end of the discharge tube is supported by a mica disc. The cap is made by Thorn's subsidiary Lamp Caps Ltd and is made from a phenolic resin with a brass shell.
Manufacturer: Thorn Electrical & Musical Industries
Lamp Power: 35 Watts
Lamp Current: 0.6 Amps 0.54A Starting Current
Lamp Voltage: 70 Volts 450V ignition
Cap Type: BY22d Phenolic Resin & Brass
Bulb Type: T-50 T-16 in eighths/inch
Bulb Finish: Clear Soda-lime glass
Electrodes: CCC-4 Beehive Black W + BCT emitter
Arc Length:
Atmosphere: Na | 99% Ne, 1% Ar Outer: Hard Vacuum
Luminous Flux: 4,600 lm (@ 100 hrs) 4,500 lm (@ 2000 hrs)
Luminous Efficacy: 131.4 lm/W (@ 100 hrs)
Colour Temperature & CRI: CCT: 1700K CRI: Ra -44
Chromaticity Co-ordinates: CCx: 0.574 CCy: 0.425
Rated Lifetime: 16,000 hours 50% survival & 80% maintenance
Warm-up & Re-strike time: Approx. 15 minutes Instantaneous
Burning Position: Vertical cap up ± 110°
Overall Length: 311 mm 12¼ inches
Light Centre Length: 190 mm 7½ inches
Factory: Leicester England
Date of Manufacture: February 1986 Date Code: 50
Original / Present Value: GB £15.75 (1988) GB £37.72 (2014)
References: 1) Thorn Lighting Comprehensive Catalogue, UK, 1987/88
2) Thorn UK Price List, 7th November 1988
3) Thorn EMI Lamp Datasheet 4:96:21, SOX Lamps, April 1986