Metalarc M400/BU with Na-Sc-Th Chemistry

Dating to 1965, this is one of the first examples of the MetalArc metal halide lamps created by Sylvania. Its arc tube chemistry differs from the sodium-indium-thallium tri-band fill exploited by all other manufacturers at the time - Sylvania instead opted for a multi-line radiator consisting of the iodides of Thorium and Scandium.

These generate light with improved efficacy, but at high colour temperature and cause severe arc constriction which leads to instability and flickering. Sodium iodide is therefore added as an arc-fattening agent which improves stability, also lowering colour temperature and further increasing efficacy. The shape of the arc tube is rather advanced for its time, with well formed pinch seals and end chambers around the electrodes to maximise halide vapour pressures. Coatings of zirconia serve as heat reflectors to maintain high temperature behind the electrodes. Sylvania was also first to change metal halide lamps from vacuum to nitrogen-filled outer bulbs, to reduce end of life arcing risks.

This early lamp had many problems. Notably, the arc tube is mounted on Sylvania's famous 'girder frame', developed to increase the ruggedness of mercury lamps. Owing to the proximity of this metal frame to the arc tube in a gas-filled bulb, photoelectric effects led to sodium loss from the arc tube, bringing colour changes and very short life. In 1966 Sylvania invented the so-called "frameless" mount, which increased life to 7,500 hours. The Thorium fill was also found to limit lamp life, due to is rapid chemical reaction with the quartz and electrodes. In 1968 MetalArc lamps were redesigned with sodium-scandium-iodide chemistry, increasing life again to about 10,000 hours.
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,Th,Sc)Ix,Hg | Ar Outer: Nitrogen
Luminous Flux: 32,000 lm @ 100h 25,600 lm @ 3000h
Luminous Efficacy: 80.0 lm/W @ 100h
Colour Temperature & CRI: 4500K Ra 65
Chromaticity Co-ordinates: CCx: 0.365 CCy: 0.392
Rated Lifetime: 1000-1500 hours
Warm-up & Re-strike Time: 3-4 minutes 20 minutes
Burning Position: Vertical ± 15°
Overall Length: 292 mm 111/2 inches
Light Centre Length: 178 mm 7 inches
Factory: Manchester, NH U.S.A.
Date of Manufacture: March/April 1965 Date Code: 51 16
Original Value: US $23.25 (1968)
References: 1) Sylvania Lamp Catalogue, France, 1966
2) Sylvania Lamp Catalogue, USA, November 1968
3) Electric Discharge Lamps, J.F. Waymouth, The MIT Press, 1971
4) Smithsonian Institution Website "Lighting a Revolution"