||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SOX lamps were manufactured by both Philips and Osram-GEC, but they were produced for only a few years before being superseded in 1968 by still more efficient models having a superior indium-tin oxide coating.