Osram SOI/H Integral Low Pressure Sodium
This SOI lamp makes a very interesting addition to the GEC's family of advanced integral low pressure sodium lamps.  Rated at a phenomenal 280 watts, it achieves this impressive power by making use of two 140-watt discharge tubes within the same outer envelope.  However lamp efficacy is somewhat low, because there is insufficient room to include the usual heat-insulating glass sleeves in this monster. Introduced in 1958, this lamp was conceived for three seperate markets but never achieved great success and it was withdrawn just a few years later.

The primary reason for its development was to offer streetlighting authorities the opportunity to reduce electricity consumption as road traffic volumes eased late into the night, by simply switching off one discharge tube and therefore halving the power consumption.  A second reason for its inception was to reduce the size of the bulky lanterns which had been developed around Philips' 5-foot long 300W SOI lamp.  Thirdly, if both discharge tubes are connected in parallel, only one will ever strike up, and it was possible to double the lamp maintenance period by effectively equipping each lantern with an auxiliary discharge tube.

Perhaps because of the expense of this lamp, or the fact that there was insufficient room to incorporate any heat-reflecting glass sleeves to boost efficiency, or simply because of its complexity, fragility and considerable weight, this lamp never achieved wide acceptance.  In addition its requirement for two ballasts and duplicated wiring would have added to installation expense, and the GEC decided to cease manufacture of this innovative product around 1960.  This lamp is one of only two known examples still in existence.
Manufacturer: The General Electric Company of England PLC
Lamp Power: 280 Watts
Lamp Current: 2x 0.90 Amps 0.81A Starting current
Lamp Voltage: 2x 155-190 Volts 200-250V Mains voltage
Cap Type: UX4 4-Pin British Valve Base
Bulb Finish: Clear Soft glass arc tube
Bulb Type: Inner: 2x TU-20 x 48 Outer: T-65
Overall Length: 519 mm 20½ inches
Light Centre Length: 280 mm 11¼ inches
Arc Length:
Electrodes: CCC-4 Black Tungsten BCT emitter
Atmosphere: Na | 99% Ne, 1% Ar Outer: Hard Vacuum
Luminous Flux: 16,800 lm (average through life)
Luminous Efficacy: 60.0 lm/W (average through life)
Colour Temperature & CRI: CCT: 1700K CRI: Ra-44
Chromaticity Co-ordinates: CCx: 0.574 CCy: 0.425
Burning Position: Horizontal ± 15°
Rated Lifetime: Not Published
Warm-up & Re-strike Time: 8 minutes Instantaneous
Factory: Shaw, Oldham England
Date of Manufacture: October 1958 Date Code PL
Original & Present Value: £7 18s 0d (1959) £123.00 (2005)
 
References: 1) Osram-GEC Lamp Catalogue, 1959-1960.