||The halogen lamp illustrated here is representative of the first type developed for Stage and Theatre lighting. It marks a significant departure from the original halogen sources which employed an elongated filament of small diameter. When used in conjunction with a reflector they produce a highly asymmetric beam, whose application in theatre lighting is restricted to large area floodlighting of the Cyclorama. With such a long filament it was impossible for the new halogen sources to achieve sufficient optical control to enter the more important Spotlighting applications.
GE attacked the problem by introducing a compact coiled-coil filament, which reduced the aspect ratio of the 750W linear halogen filament from about 30:1 down to a mere 5:1. To reduce problems of filament sag, it is supported at its centre by a cranked tungsten wire which is sealed into the quartz bulb at one pinch as well as in the tip-off area.
This new compact source was capable of efficiently producing a narrow spot beam of light in conjunction with luminaires employing ellipsoidal reflector optics, and it marked the first use of halogen lamps in theatre spotlighting. The Q750T12/CL gained rapid international popularity even in 240V countries, where it was standard practice to wire up two lanterns in series. By the mid 1970s this lamp was superseded by the type EGG, which dispensed with the former double jacket design by employing a single ended quartz capsule cemented directly into the lamp base.
Incidentally this lamp is based on the original tungsten-iodine chemistry, and when in operation the capsule is filled with the characteristic purple vapour of elemental iodine.