Westinghouse Metal Halide MH400/BU/4

Westinghouse launched its first metal halide lamp in 1964, the so-called MH400 B.O.C. - the name referring to its Better Output & Colour vs mercury lamps. Its chemistry was similar German Osram lamps, based on the iodides of dysprosium and thallium, but with the addition of indium. The arc tube was enclosed in an evacuated outer bulb and produdced a daylight colour with excellent fidelity, but modest efficacy. Following Sylvania's 1966 invention of the sodium-scandium lamp chemistry during 1964-66, that was quickly copied in view of the excellent colour stability and higher efficacy.

The life of these early lamps was short, owing to sodium loss from the arc tube. This was identified by Sylvania as being due to photomission from the metal support frame, which released free electrons that accumulated near the the arc tube surface and pulled sodium ions into the wall. The Sylvania solution, also copied by GE, was to develop the so-called frameless support in which the electrically conducting frame is replaced by a thin flywire as far away from the arc tube as possible. Westinghouse implemented a different solution, illustrated here. The metal frame is retained for its mechanical robustness, but alongside the arc tube this has been sleeved with a quartz tube which minimises the photoemission effect. The outer envelope is nitrogen filled.

Wesintghouse's first lamp of this design was rated only for vertical burning, but this later model may also be used horizontally - the lighted photos show that the arc is relatively stable. The third auxiliary ignition electrode is unusual in this lamp in that it is not short-circuited against the main electrode when hot, instead it is disconnected completely via a bi-metal switch.
Manufacturer: Westinghouse
Lamp Power: 400 Watts
Lamp Current: 3.3 Amps
Lamp Voltage: 135 Volts
Cap Type: E39s/41 Brass + glass insulator
Bulb Type: BT-117 BT-37 in eighths/inch
Bulb Finish: Clear Lead borosilicate glass
Electrodes: Backwound Coiled Thoriated Tungsten
Arc Length:
Atmosphere: (Na,Sc)Ix,Hg | Ar Outer: Nitrogen
Luminous Flux: 34,000 lm (@ 100 hrs) 25,600 lm (average)
Luminous Efficacy: 85.0 lm/W (@ 100 hrs)
Colour Temperature & CRI: 4500K Ra 65
Chromaticity Co-ordinates: CCx: 0.365 CCy: 0.392
Rated Lifetime: 20,000 hours 70% flux & 50% survival
Warm-up & Re-strike Time: 3-4 minutes 20 minutes
Burning Position: Base Up to 15° below Horizontal
Overall Length: 292 mm 111/2 inches
Light Centre Length: 178 mm 7 inches
Factory: Bath, NH U.S.A.
Date of Manufacture: February 1980 Date Code: 20
Original Value: US $47.00 (1980)
References: 1) Westinghouse Lamp Catalog, USA, January 1980