Philips Self-Starting Metal Halide HPI-BUS 400W
Philips has perhaps invested more effort than any other manufacturer in simplifying the ignition requirements of metal halide lamps. Almost from the outset it switched from argon to a neon-argon Penning fill, reducing ignition voltage from several kilovolts to about 700V. At the end of the 1970s a further improvement was made with this HPI-BUS (BU = Base Up, S = Starterless) - the first metal halide to strike at mains voltages without any ignitor. As such it can function as a direct retrofit for ordinary mercury lamps.

This feat was possible thanks to improvements in arc tube chemical purity, combined with the addition of a third auxiliary electrode and 20k ohm series resistor, similar to the arrangement in a mercury lamp. In a metal halide lamp this can lead to premature failure by explosion of the arc tube - the quartz pinch-seal being weakened by electrolysis between the auxiliary and adjacent main electrodes. Philips overcame that problem by short-circuiting the auxiliary to the adjacent main electrode via a vacuum thermal switch, which closes after the lamp has warmed up. The glass capsule of the switch is visible in the photo of the lamp neck.

Whereas Philips HPI lamps were originally designed for horizontal operation, this model was intended to replace mercury lamps, which are invariably burned base-up. This required a redesign of the arc tube for vertical operation. The arc tube is not visible here, but its shape is substantially the same as in the HPI-T 400W/BU with the addition of a heat-reflective zirconia coating on the lower side of the arc tube. A 250W HPI-BUS was later developed but withdrawn owing to starting difficulties - a sufficiently pure arc tube chemistry is very difficult to achieve in smaller sizes.
Manufacturer: Philips Lighting
Lamp Power: 400 Watts
Lamp Voltage: 125 Volts
Lamp Current: 3.4 Amperes
Cap Type: E40/45 Nickel Plated Brass
Bulb Type: ED-120 ED-37 in eighths/inch
Bulb Finish: YVO4:Eu,Tb,Gd Borosilicate
Electrodes: Single Coil ThO2 emitter
Arc Length: 38 mm 1 1/2 inches
Atmosphere: NeAr½% | Hg(Na,In,Tl)Ix Outer: Ne:N2 60:40
Luminous Flux: 31,500 lm @ 100h 28,300 lm @ 2000h
Luminous Efficacy: 78.8 lm/W @ 100h 70.8 lm/W @ 2000h
Colour Temperature & CRI: CCT: 4000K CRI: Ra 65
Chromaticity Co-ordinates: CCx: CCy:
Rated Lifetime: 24,000 hrs to 50% survival
Warm-up & Re-strike Time: 4 minutes 15 minutes
Burning Position: VBU ± 15°  
Overall Length: 282±8 mm 11 1/8 inches
Light Centre Length: 175±5 mm 7 inches
Factory: Turnhout Belgium
Date of Manufacture: October 1987 K7
Original / Present Value: £57.97 (1988) £112.00 (2009)
 
References: 1) Philips Lighting UK Catalogue 1988.
2) Philips Lighting Brochure "Metal Halide Gas Discharge Lamps", Dec. 1981
3) Philips Lighting UK Price List, August 1988.
4) Philips Training Manual MTO Turnhoutlampen - Metaalhalogenidelampen, F. Oostvogels, D. v.Pijkeren & G. Rouweler, 1985.