GEC SOI/H Integral Low Pressure Sodium

Invented by GEC in 1955, the 'Integral' SOI lamp replaced the former detatchable dewar jackets with a single piece outer bulb. This resulted in simpler lamps, and prevented the 'breathing' of the earlier design in which outside air was drawn around the discharge tube each time it cooled, carrying in dust and moisture. The Integral lamps stayed clean for longer and delivered improved lumen maintenance, while also starting better due to elimination of troublesome conductive moisture films on the discharge tube.

After 1956 the GEC lamps were improved again by placing an extra sleeve of glass around each limb of the discharge tube. This reflected about 50% of the radiated heat back to the discharge tube, while absorbing only some 3% light. The better insulation allowed a larger diameter discharge tube to be used, which lowered the discharge current density and boosted luminous efficacy by about 10%. Ignition was further improved by adding pair of nickel wires spiralled around each limb and connected to the opposite electrode.

A drawback of the integral design was that the turbulent air flow present within the former dewar jackets was no longer present. This used to help create a uniform temperature along the lamp, but the vacuum atmosphere of the SOI lamps causes the electrode ends to tun hotter than the U-bend. The result is that during life, sodium is distilled towards the bend and older lamps suffered sodium depletion at the cap end. This resulted in faster lumen depreciation of the SOI design. It was partly overcome by adjusting the gas fill pressure such that sodium-devoid regions operated cooler, encouraging sodium to migrate back, but was not fully solved until the introduction of SOX lamps in 1964.
Manufacturer: The General Electric Company of England PLC
Lamp Power: 85 Watts
Lamp Current: 0.68 Amps
Lamp Voltage: 144 Volts 450V ignition
Cap Type: By22d Mycalex & Brass
Bulb Type: T-50 T-16 in eighths/inch
Bulb Finish: Clear Soda-lime glass
Electrodes: CCC-4 Beehive Black W + BCT emitter
Arc Length:
Atmosphere: Inner: Na | Ne Ar1% Outer: Hard Vacuum
Luminous Flux: 7,500 lm (initial) 6,400 lm (average)
Luminous Efficacy: 88.2 lm/W (initial)
Colour Temperature & CRI: CCT: 1700K CRI: Ra -44
Chromaticity Co-ordinates: CCx: 0.574 CCy: 0.425
Rated Lifetime: 6,000 hours 70% survival
Warm-up & Re-strike time: Approx. 15 minutes Instantaneous
Burning Position: Vertical cap up ± 110°
Overall Length: 415 ± 10 mm 165/16 inches
Light Centre Length: 230 ± 10 mm 6¾ inches
Factory: Shaw, Oldham England
Date of Manufacture: April 1973 Date Code ED
Original / Present Value: £4.60 (1976) £29.57 (2014)
References: 1) Osram-GEC Lamp Catalogue, UK, 1960-61, p.23
2) Osram-GEC Low Pressure Sodium Lamps Brochure, 1972
3) Osram-GEC Low Pressure Sodium Lamps Brochure, 1976
4) Osram-GEC Technical Data, Low Pressure Sodium Lamps L4/HT, October 1969
5) A New Sodium Lamp, E.H. Nelson & S.A.R. Rigden, Light & Lighting August 1956 pp.217-220
6) UK Patent 801,482, Improvements in or Relating to Sodium Vapour Electric Discharge Lamps, S.A.R. Rigden, Filed Jan. 27th 1955