||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 equally innovative Linear Sodium lamps that had been introduced a few years earlier. Whereas the AEI linear sodium lamps employed a glass sleeve around the discharge tube plus an evacuated outer envelope to provide the necessary thermal insulation, the GEC technology applied a 50 nanometre thick film of pure gold, sputtered onto the inside of the outer jacket.
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, intended to replace the AEI 60W and 200W models. Gold-coated lamps were produced only until 1966 when they were superseded by more efficient models having a tin-oxide semiconductor film. This particular lamp is an experimental version that was not sold by GEC. It has a discharge tube shape similar to that employed by German Osram on its 220W linear sodium lamp, with much shorter indents than were normally applied. It is not known whether or not this brought any performance advantage.