GEC SOI/H Integral Low Pressure Sodium

Invented by GEC in 1955, the 'Integral' SOI lamp marked an important progression from the technology of earlier SO sodium lamps. The dewar jacket on the former design served only to keep the discharge tube warm by reducing conducted and convected heat losses, but was not particularly efficient and did little to reduce radiation losses.

The most significant design change in the SOI lamp was that the dewar jacket was dispensed with, and replaced with a single piece outer glass tube in which the discharge tube was sealed under high vacuum. Additionally the efficacy was improved by placing an extra sleeve of glass around each limb of the discharge tube. This served to reflect about 50% of the radiated heat back to the discharge tube wall, while absorbing only some 3% light. The better insulation allowed a larger diameter discharge tube to be used, which offered lower current density and boosted luminous efficacy by about 10%. For easy ignition a pair of nickel wires spiralled around each limb and connected to the electrode of the opposite limb was also introduced.

A drawback of the integral design was, however, that the turbulent air flow present around SO discharge tubes was no longer present. This used to help create a uniform temperature along the discharge tube, but the improved insulation of the SOI design meant that the electrode ends ran hotter than the U-bend. As a result sodium could frequently be distilled down to the bend end and many lamps suffered sodium depletion in parts of the discharge tube. This often resulted in rapid lumen depreciation of the SOI design, a problem which was not adequately overcome until the introduction of SOX type lamps in 1964.
Manufacturer: The General Electric Company of England PLC
Lamp Power: 140 Watts
Lamp Current: 0.9 Amps 0.81A Starting Current
Lamp Voltage: 160 Volts 450V ignition
Cap Type: By22d Mycalex & Brass
Bulb Type: T-65 T-20½ 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: 12,400 lm (initial) 10,600 lm (average)
Luminous Efficacy: 88.6 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: Horizontal ± 20°
Overall Length: 518 ± 10 mm 20¾ inches
Light Centre Length: 280 ± 10 mm 11 inches
Factory: Shaw, Oldham England
Date of Manufacture: July 1972 Date Code DG
Original / Present Value: £5.25 (1976) £33.75 (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