Philips SO-X 40W with Tin Oxide Film

In 1964 Philips announced a major breakthrough in the efficacy of the low pressure sodium lamp, thanks to an improved thin-film coating to improve thermal insulation. It was a natural development of Osram's 1961 introduction of the gold-coated lamp which employed a metallic heat-reflection film. The Philips innovation was to shift from metallic to semiconductor coatings, which offer similarly high infrared reflectivity but with lower light absorption.

This lamp is representative of the very first model to have been made with a semiconductor heat-reflection film. It is a prototype model from 1963, made a few months before production commenced. The coating material is tin oxide and its thickness is approximately 0.32 microns, at which point the trade-offs between infrared reflection and the blocking of visible light are optimised for the highest lamp efficacy. Infrared reflectivity is about 80%, and transmission of the sodium yellow light is approximately 88%. Lamps having tin oxide coatings can be recognised by the orange-reddish colour reflections from the coated glass surface, which is evident in some of the photographs.

The tin-oxide coated lamps were introduced under the name "SOX" in ratings of 40, 60, 100 and 150W to replace the former "SOI" types 60, 85, 140 and 200W - offering both an energy saving and simultaneous increase in luminous flux. There was no replacement for the 45W SOI, and a new 200W SOX joined the range. Tin-oxide SOX lamps were manufactured by both Philips and Osram-GEC, but they were produced for only a few years before being superseded in 1967-68 by still more efficient models having a superior indium-tin oxide coating.
Manufacturer: Philips Gloeilampenfabrieken
Lamp Power: 40 Watts
Lamp Current: 0.6 Amps 0.54A Starting Current
Lamp Voltage: 75 Volts 450V ignition
Cap Type: BY22d Porcelain Insulator
Bulb Type: T-50 T-16 in eighths/inch
Bulb Finish: Tin oxide film Soda-lime glass
Electrodes: CCC-4 Beehive BCT emitter
Arc Length:
Atmosphere: Na | 99% Ne, 1% Ar Outer: Hard Vacuum
Luminous Flux: 4,300 lm (@ 100 hrs) 4,085 lm (mean to 4000hrs)
Luminous Efficacy: 107.5 lm/W (@ 100 hrs)
Colour Temperature & CRI: CCT: 1700K CRI: Ra -44
Chromaticity Co-ordinates: CCx: 0.574 CCy: 0.425
Rated Lifetime: 6,000 hours 50% survival
Warm-up & Re-strike time: 15 minutes Instantaneous
Burning Position: Vertical cap up ± 110°
Overall Length: 312 mm 12¼ inches
Light Centre Length: 190 mm 7½ inches
Factory: Eindhoven Netherlands
Date of Manufacture: December 1963
Original / Present Value: £3 6s 2d (1964) £59.87 (2014)
References: 1) Philips UK Lamp Catalogue, 1964.
2) Recent Improvements in Sodium Lamps, M.H.A. van de Weijer, Philips Technical Review V23 No.8/9, 1961/62, pp.246-257.
3) Low Pressure Sodium Lamps with Indium Oxide Filter, H.J.J. van Boort & R. Groth, Philips Technical Review V29 No.1, 1968, pp.17-18.