Metalarc Fluorescent M400/C BUH
Following the launch of the original clear Metalarc lamps, they were joined by fluorescent versions. Although the metal halide arc radiates less UV than a mercury lamp, there is still sufficient to excite the phosphor, improving colour rendering while providing a warmer light. The light distribution pattern is also a closer match with coated mercury lamps.

Field testing soon revealed the surprising phenomenon that coated lamps suffered rather short life, which was attributed to a more rapid rate in the increase of lamp ignition and reignition voltages. The cause was traced to the evolution of hydroxyl contaminants from the phosphor. Hydrogen would diffuse through the hot wall of the arc tube, forming hydrogen iodide which has an inconveniently high vapour pressure and strongly absorbs free electrons.

Although hydrogen getters existed, they also gettered trace oxygen impurities in the outer bulb and produced a reducing atmosphere that led to phosphor degradation. For several years, the only solution was to excercise extreme cleanliness in the lamp manufacturing. In 1972 the problem was solved with Sylvania's invention of the barium peroxide getter, which sorbs hydrogen but not oxygen. The powder is somewhat sensitive to degradation by UV light and is loaded into a metal can behind a porous copper gauze, and is positioned facing outwards in the area of the bulb crown.

Another ignition aid, also introduced in 1973, was to multiply the voltage applied to the auxiliary ignition electrode via a pair of resistors and a zener diode. The diode can be seen in the photo of the lamp neck, behind a nickel heat-reflecting disc which serves to prevent it from overheating.
Manufacturer: GTE Sylvania
Lamp Power: 400 Watts
Lamp Current: 3.2 Amps
Lamp Voltage: 133 Volts
Cap Type: E39s/41 Ni plated brass + vitrite
Bulb Type: BT-117 BT-37 (in eighths/inch)
Bulb Finish: Yttrium Vanadade Lead Borosilicate glass
Electrodes: Backwound coil Thoriated tungsten
Arc Length: 46mm 17/8 inch
Atmosphere: (Na,Sc)Ix, Hg | Ar Outer: Nitrogen
Luminous Flux: 34,000 lm @ 100h 24,600 lm average
Luminous Efficacy: 85.0 lm/W @ 100h
Colour Temperature & CRI: 3700K Ra 70
Chromaticity Co-ordinates: CCx: 0.390 CCy: 0.365
Rated Lifetime: 15,000 hours to 50% survival
Warm-up & Re-strike Time: 2 minutes 10 minutes
Burning Position: Base Up to Horizontal
Overall Length: 292 mm 111/2 inches
Light Centre Length: 178 mm 7 inches
Factory: Manchester, NH U.S.A.
Date of Manufacture: May 1979 Date Code: 101
Original Value: US $37.25 (1979)
 
References: 1) GTE Sylvania Lamp Catalogue, USA 1979, p.34
2) GTE Sylvania Engineering Bulletin, MetalArc Lamps, 0-344, July 1979
3) GTE Sylvania Product Information Guide, Metalarc Lamps, MA-143, c. 1980
4) GTE Sylvania Brochure, Metalarc Lamps, MA-130R, c. 1980
5) US Patent 3737710, MH Lamp having Barium Peroxide & Copper Getter, J.F.Waymouth, W.M.Keeffe, C.Gungle (GTE Sylvania), 1972
6) US Patent 3900761, MH Lamp having Voltage-Multiplying Diode and Resistor Pair, R.W.Freese, R.C.Lekebusch, P.W.Ulcickas (GTE Sylvania), 1973