Osram SO/H Sodium Vapour with Dewar Jacket

This SO/H sodium vapour lamp was manufactured by the General Electric Company of England, under its Osram brand name in 1949. It represents one of the later designs of SO/H lamp, after it had been subjected to many improvements in glass technology and production engineering.

The discharge tube lining is manufactured from GEC X91 (formerly Na10) barium-aluminoborate glass, produced at the Osram Lemington Glassworks. This glass is characterised by a lower susceptibility to staining from sodium attack, but has a high rate of argon gas adsorption and a poor adhesion to liquid sodium. As a result, lamp life is determined by the duration before which the glass cleans up too much of the argon component of the neon-argon gas filling, which raises the striking voltage above what the ballast can supply. In addition, if the lamp is not kept absolutely level when alight, the sodium can run down to one end where it forms large light-blocking mirrors and leaves the other end devoid of sodium vapour which compromises lamp efficacy. The high stresses associated with X91 glass also often resulted in cracking during manufacture.

Despite these drawbacks, no effort has been spared in assembling the lamp to a level of very high quality. It also exhibits the later design of GEC electrode construction, in which a small magnesia ceramic tube is sealed into the glass around each wire leading to the electrodes. This prevents liquid sodium from coming into contact with the weak point at the glass-to-metal seal and reduces the risk of the seal failing in service. The etch of the dewar jacket has been placed on the inner glass vessel, protecting it from fading or being accidentally wiped off when the lamp is cleaned.
Manufacturer: The General Electric Company of England PLC
Lamp Power: 140 Watts
Lamp Current: 0.9 Amps 0.81A Starting current
Lamp Voltage: 165 Volts 450V ignition
Cap Type: By22d Porcelain
Bulb Type: Inner: TU-16 (TU-5") Dewar: TA-65 (TA-20½")
Bulb Finish: Inner: Borate Soft Glass Dewar: Clear Soft Glass
Electrodes: CCC-4 Beehive Black W + BCT emitter
Arc Length:
Atmosphere: Inner: Na | Ne99% Ar1% Dewar: Vacuum
Luminous Flux: 9,000 lm (average)
Luminous Efficacy: 64.3 lm/W (average)
Colour Temperature & CRI: CCT: 1700K CRI: Ra -44
Chromaticity Co-ordinates: CCx: 0.574 CCy: 0.425
Rated Lifetime: Not Published
Warm-up & Re-strike time: Approx. 15 minutes Instantaneous
Burning Position: Horizontal +20° -5°
Overall Length: 518 ± 10 mm 20¾ inches
Light Centre Length: 280 mm 11 inches
Factory: Hammersmith England
Date of Manufacture: January 1949 Date Code 85
Original / Present Value: £4 15s 6d (1951) £134.00 (2014)
References: 1) Osram-GEC Lamp Catalogue, UK, 1951, p.39
2) Osram-GEC Technical Data, Low Pressure Sodium Lamps L4/HT, October 1969