Osram SOI/H Integral Sodium - Experimental
Illustrated here is a special experimental sodium lamp made at Osram's Shaw lamp works, which was never offered for sale.  A significant shortcoming of the SOI type lamp was that because the discharge tube was enclosed by high vacuum, the electrodes end of the discharge tube would run hotter than the U-bend end.  This often result in sodium being distilled from hot areas to cold, thus hotter regions of the discharge tube became depleted of sodium and the luminous efficacy fell at these points.

Philips attacked this problem by blowing small dimples into the sides of the discharge tube, which naturally ran at a cooler temperature.  Sodium would condense in the dimples and remain here for much of the lamp life, ensuring that there was always sufficient sodium vapour present throughout the discharge.

However, dimples could not be used with the GEC's design in which a close-fitting glass insulation sleeve was slipped over each limb of the discharge tube. Philips lamps sacrificed efficacy by employing one large sleeve over the whole discharge tube, large enough to accommodate the dimples.  To create a similar cooling effect without loss of efficacy, this sample was tested in which the sleeves have holes in the side through which some heat is able to escape.  It is, however, evident from the sodium position in this sample that the technique was not successful - presumably the cooling effect was not great enough to encourage sodium condensation.  Consequently this test lamp was not marketed, and GEC's SOI lamps were prone to sodium migration until the problem was solved with the newer generation of SOX-type lamps having graded film thickness.
Manufacturer: The General Electric Company of England PLC
Lamp Power: 140 Watts
Lamp Current: 0.90 Amps 0.81A Starting current
Lamp Voltage: 160 Volts 200-250V Mains voltage
Cap Type: BY22d Mycalex Insulator
Bulb Finish: Clear Soft glass arc tube
Bulb Type: Inner: TU-20 x 48 Outer: T-65
Overall Length: 525 mm 20¾ inches
Light Centre Length: 290 mm 11½ inches
Arc Length:
Electrodes: CCC-4 Black Tungsten BCT emitter
Atmosphere: Na | 99% Ne, 1% Ar Outer: Hard Vacuum
Luminous Flux: 9,800 lm (average through life)
Luminous Efficacy: 75.7 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: 11 minutes Instantaneous
Factory: Shaw, Oldham England
Date of Manufacture: July 1958 Date Code PG
Original & Present Value: N/A (Engineering Sample)
 
References: 1) Osram-GEC Lamp Catalogue, 1959-1960.
2) Private Communication with Osram Lamp Works, Shaw, UK.