Metalarc with Flat Frame Arc Tube Mount

Dating to around 1965, this is one of the first examples of the Metalarc metal halide lamps created by Sylvania in USA. During the 1960's practically every lamp manufacturer of significance experimented with metal halide technology, all in an effort to deliver better efficacy and better colour rendering than is possible from a pure mercury discharge.

The 400-watt arc tube is of a rather advanced design for its age, with well formed pinch seals and end chambers around the electrodes to prevent halide condensation here. Coatings of alumina serve as heat reflectors to maintain high temperature behind the electrodes. The arc tube chemistry is based on the sodium-scandium system which was pioneered by Sylvania, and delivers considerably higher initial lumens and better colour control than was available in GE and Westinghouse's chemistries at the time.

Curiously, the arc tube is mounted on Sylvania's famous flat mount frame, developed to increase the ruggedness of mercury lamps. This was standard for the very first metal halide lamps. But owing to the close proximity between the arc tube and the charged metal frame, the small sodium ions within the arc tube could be physically drawn into the quartz wall and out of the lamp, resulting in colour changes. In 1966 this problem was overcome by Sylvania's invention of the so-called "frameless" mount assembly. There was no frame alongside the arctube, instead a short upper frame was clamped to one end of the arc tube, and a similar short section between the lower end and the base. The electric current carried to the upper frame was conducted through a thin molybdenum fly-wire spaced as far away from the arctube as possible to minimise the electric field strength.
Manufacturer: Sylvania Electric Products Inc.
Lamp Power: 400 Watts
Lamp Current: 3.3 Amps
Lamp Voltage: 133 Volts
Cap Type: E39s/41 Ni plated brass + vitrite
Bulb Type: BT-117 BT-37 (in eighths/inch)
Bulb Finish: Clear Lead Borosilicate glass
Electrodes: Backwound coil Thoriated tungsten
Arc Length:
Atmosphere: (Na,Sc)Ix, Hg | Ar Outer: Nitrogen
Luminous Flux: 34,000 lm @ 100h 25,600 lm average
Luminous Efficacy: 97.5 lm/W @ 100h
Colour Temperature & CRI: 4500K Ra 65
Chromaticity Co-ordinates: CCx: 0.362 CCy: 0.371
Rated Lifetime: 10,000 hours to 50% survival
Warm-up & Re-strike Time: 3-4 minutes 5 minutes
Burning Position: Vertical ± 15°
Overall Length: 292 mm 11/2 inches
Light Centre Length: 178 mm 7 inches
Factory: Manchester, NH U.S.A.
Date of Manufacture: Approx. 1965
Original Value: Unknown
References: 1) Sylvania USA Engineering Bulletin 0-344 (July 1979).
2) Smithsonian Institution Website "Lighting a Revolution"