||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.
The coating material is tin oxide and its thickness is approximately 0.32 microns, at which point the trade-offs between infrared reflection and blocking of visible light are optimised for 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 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. Osram-GEC was quick to follow suit with the launch of its own range, represented by the lamp on this page. Unusually for that company, the discharge tube feature Philips-style sodium-retaining dimples, which were employed for a few years. The discharge tube bend is suspended between small mica discs held by a tulip-shaped metal spring clip, and a mica disc at the lower end. By 1968 these tin-oxide lamps had been replaced by another Philips development, in which the coating was changed to a superior indium-tin oxide film.