||Just a year after Osram-GEC launched the world's first twin-filament incandescent lamps for use in television studios, its competitor, Thorn Lighting, announced on 3rd March 1968 the first halogen versions. Thanks to changing from the GEC's large hard glass bulbs to a compact quartz envelope, the Thorn lamps doubled the lifetime to 200 hours. This was made possible due to the greater mechanical strength of the quartz bulb, which can be filled with higher gas pressure so as to retard the rate of filament evaporation. Additionally it featured a small increase in luminous flux, which was maintained during life thanks to the halogen cycle.
A year later GEC caught up with its own improved lamp, opting for a glass halogen version instead of quartz. The use of glass limited the gas pressure needed for longer life, but did offer improved lumen maintenance as well as a lamp that was much cheaper than Thorn's quartz designs, and as a result it was still able to compete well.
The quartz lamp is of particularly unusual design. Due to the difficulty of forming a pinch-seal with four conductors, there are two separate seal tubes, each pinch-sealed around two conductors. These are fused to a flat quartz plate, whose outer rim has been fused to a length of large diameter quartz tube which forms the lamp envelope. Four small tubes of quartz attached to the dome of the bulb provide support for the filament mount assembly. The cap takes the form of metal sheet, tightly clamped around the seal tubes with asbestos gauze in between, and crimped around a ceramic insert which carries the four contact pins. The bulb is filled with bromine so as to avoid a shift in colour due to the purple vapour of the usual iodine fills of the time.