1971 Tungsram #50609 (5000 W / 220 V)

The invention in the late 1950s of the tungsten-iodine lamp by Elmer G. Fridrich and Emmet H. Wiley at GE (USA) brought forth an entirely new class of compact incandescent lamps with a virtually 100 % flux maintenance through life. This revolution arose from the discovery that adding a small amount of a halogen and traces of oxygen (the importance of the latter was discovered only later) to the atmosphere of a filament lamp results, with the right operating conditions, in most of the evaporated tungsten material being returned to the hot filament. The key mechanism at play here is the combination of evaporated tungsten atoms with the additives, forming volatile tungsten oxyiodide molecules which do not condense on the bulb wall provided it is maintained at a high temperature. This thus enabled the design of much smaller incandescent lamps whose radiator could be operated at higher temperatures, resulting in higher source luminances, in greater lumen efficacies and in higher light color temperatures.

This breakthrough proved very beneficial to certain applications such as photography and optics as much smaller and far more effective light sources could be designed. Within a few years a series of compact tungsten halogen versions of the bulbous high-wattage studio lamps were released on the market and by the early 1970s GE's technology had been widely adopted by other lamp manufacturers across the globe. The rate of product introduction depended to a large extent on how fast those lampmakers could develop the specialized manufacturing tools required for the new lamps.

Interestingly, for its largest studio lamps (5 and 10 kW), Tungsram of Hungary used manufacturing techniques borrowed from its XHP xenon short-arc lamp production. The 1971 5 kW model 50609 featured here shows a design which is unlike anything else produced by other lampmakers. Instead of using a standard single-ended construction, Tungsram just replaced the central body of an XHP by a large quartz vessel in which a biplane tungsten filament is held by quartz hangers. The whole lamp was then filled with an argon-nitrogen atmosphere and some solid iodine, with the trace amount of oxygen required to make the tungsten-iodine cycle work coming from residual impurities. Typical of studio incandescent lamp, the filament operates at a high temperature so as to emit light with a 3200 K color temperature, ideally suited for "tungsten"-balanced films used in the motion picture industry.

There is no doubt that a great deal of manual work from skilled glassblowers went into the assembly of such a lamp. Even the electrical feedthroughs at both extremities are of the complex graded glass kind used in xenon arc lamps. All in all, those high-wattage tungsten halogen lamps must have been far more expensive than their standard non-halogen equivalent, or than the single-ended versions produced by competitors. Tungsram eventually phased out those complex halogen lamps during the 1970s and adopted a more classical and cheaper tubular lamp design with a straight coiled filament and mechanically-pressed moly end seals. Even the fill was changed to Philips's superior bromine chemistry which does not absorb visible light and does not segregate the way iodine vapor does in its buffer atmosphere.