||In 1961 Osram-GEC put the first lamp on the market which made use of thin-film optical coatings for the purpose of heat reflection. Known as the Golden Linear, it offered a notable improvement in performance beyond AEI-Mazda's linear sodium lamps that employed glass sleeves as a heat reflector. The GEC technology applied a 50 nanometre thick film of gold sputtered onto the inside of the outer jacket, probably with an antireflection top coat of lead sulphide.
Although costly, this raised efficacy by about 10% because gold is a better heat reflector than glass. Absorption of sodium light by the coating is considerable, and earlier tests with gold films on SOI lamps had shown that it was not practical because it caused too much reduction in luminous flux. However the asymmetric light distribution of the grooved SLI discharge tube allowed this problem to be overcome, by leaving two clear bands coinciding with the direction in which the majority of the light is radiated.
The GEC Golden Linear lamps were produced in two ratings of 60W and 175W. This 175W model impressively matches the 20,000lm flux of the original AEI 200W lamp thanks to a reduction in current density and a more efficient discharge. This was made possible by the superior thermal insulation which permits a greater diameter discharge tube, combined with a reduction in operating current. This made the GEC lamp electrically incompatible with the 200W rating, but it had the advantage of functioning on standard 0.9-amp ballasts. Gold lamps were produced only until about 1966 when they were superseded by the tin oxide film that had been applied a few years earlier on the SOX range. This 175W model was replaced by the GEC 160W SLI/H.