||Less than five years after Osram-GEC introduced its Golden Linear 60W and 175W lamps having a heat-reflection film of gold, the coating was replaced by a semiconductor film of tin oxide. The new material had been pioneered in 1964 with the advent of the Philips SOX range, a technology that its competitor was swift to copy for the GEC SOX lamps. In 1967-68 the film was improved again with the slightly superior indium-tin SOX.
It is not known precisely when GEC first applied these coatings to linear sodium, however private communication with former staff estimates the date at around 1964-66. Certainly by 1969 the company's literature confirms its use in all three ratings of 60W, 160W and 200W. Thorn had also launched its semiconductor-coated SLI 140W in 1966 - which may have pre-dated the GEC's use of this material.
The new semiconductor film must have allowed a considerable reduction in manufacturing cost and complexity vs the former coatings of vacuum-sputtered gold stripes, and for the 60W model allowed a small increase in luminous efficacy. The original 60W golden linear was rated 5,700lm initially whereas the semiconductor coated version featured here attains 6,000lm.
Thorn later offered a superior 60W clover-leaf version to attain still higher efficacy and increased the flux to 6,500lm, but was not successful and abandoned its 60W product in the 1970s. The market for the 60W lamps was rather small and despite its better performance, Thorn was not able to compete with GEC which had been active for many years previously in the 60W SLI/H business.