||Following the 1958 launch of the first halogen lamps by GE, their compact dimensions and stable output soon attracted the interest of photographers and studio engineers. The company's Photo Lamp Department was quick to roll out modified versions employing shorter coiled-coil filaments, however they all shared the same basic format as the first halogen lamps, in that they were linear double-ended sources with cylindrical filaments. While their axially symmetric radiation pattern is ideal for some optical systems, they were not suitable for Fresnel-lens spotlights, of the type used for studio and theatre illumination. For several years more, that market had no alternative but to continue using incandescent lamps with flat grid filaments.
In March 1966 GE's engineers made a breakthrough by launching this Q750T20/4CL lamp, the first halogen source to feature a planar filament. It was intended to replace the earlier 750T24 studio spotlight lamps, types DVH and EDG, with which it is both physically and electrically interchangeable but offers four times the life, 4.5 times the lumen-hours, plus constant output and colour temperature.
This lamp pre-dates the advent of single ended capsules - note the unusual double ended quartz inner capsule. This has been sealed into the bulb of a conventional medium bi-post lamp to achieve the necessary physical compatibility. Because these compact filament arrays are liable to heavy arcing at end of life, a fuse is enclosed in the ceramic tube at the top of the outer jacket. This unusual lamp was produced for only a few years before being superseded by the type EGR, which dispensed with the former double jacket design by employing a single ended quartz capsule cemented directly into an extended ceramic base.