||This is one of the first halogen retrofits developed as a direct replacement for GLS incandescent lamps. It relies on one of Thorn's major innovations, the so-called Backbone support, which permitted the extension of mains voltage halogen technology to lower wattages and allowed universal burning. Previously, linear halogen lamps had their filaments supported by molybdenum spiral rings in contact with the quartz bulb. That technique could only be applied to filaments sufficiently robust to carry the support, i.e. down to about 300W, and lamps had to be burned horizontally. Thorn's solution was to add strength by supporting the filament supports on a rigidly mounted quartz spine.
Having achieved a mains voltage halogen capsule of similar luminous flux as ordinary incandescent lamps, it was logical to equip it with an ordinary screw cap and market it as an upgrade over the GLS lamp. Thorn promoted the concept as a long-life design offering 4000 hours life with no decrease in luminous flux vs incandescent. Ordinarily such long life is rather difficult to achieve in a halogen lamp having such fine filament wire due to the effects of the bromine halogen fill attacking the cooler filament tails, but Thorn was able to achieve it by a novel bromine-iodine mixture to reduce the rate of the halogen cycle - note the feint purple hue of iodine vapour in the detail photograph.
The quartz tube of the capsule is extended below the pinch-seal into a ceramic base to which it is cemented. For safety reasons the whole is enclosed within an outer glass bulb, but it must still be used in closed fixtures owing to the capsule explosion risk. In the late 1980s this design was superseded by a full glass lamp to achieve a cost reduction.