Philora SO500 Self-Starting Low Pressure Sodium

One drawback of early low pressure sodium lamps was that they required fairly complex electrical control gear for their operation. The first designs required not only means for stabilising the discharge current, but also for providing a separate supply to heat the electrodes to their optimal operating temperature. That was soon dispensed with but then presented a problem that to strike the discharge, a brief high voltage pulse was necessary. Additionally, all models required a separate dewar jacket to provide the necessary thermal insulation. During the mid 1930s a number of developments were made to overcome both of these limiations, but none were particularly successful and they remained in production for only a few years.

This lamp is a type developed by German Osram around 1936. It was the first model to make use of an integral vacuum jacket, and employs several features to enable it to strike at ordinary mains voltages. Firstly it employs a shorter discharge tube of larger diameter than normal. Secondly it contains two auxiliary electrodes to strike the discharge. These are connected via a pair of electrical resistances in the outer jacket. When energised a small discharge strikes at one end of the tube, between the main and first starting electrode. Then a second discharge strikes between the two probes at the U-bend. Finally the discharge strikes across the whole lamp. Further details of the construction can be found in the LPS Technical Section of this site.

Owing to the fact that both the lifetime and efficacy of this lamp type was quite considerably less than the competing SO/H design, it proved not to be a commercial success. It is believed that by 1940 it had been discontinued.
Manufacturer: NV Philips Gloeilampenfabrieken
Lamp Power: 80 Watts
Lamp Current: 1.2 Amps
Lamp Voltage: 75 Volts Breakdown 190V max.
Cap Type: E40s/45 Solid Copper
Bulb Finish: Clear Soft glass arc tube
Bulb Type: Inner: TU-24 Outer: T-70
Overall Length: 340 mm
Light Centre Length:
Discharge Length: 350 mm
Electrodes: C-8 Tungsten BaCO3 emitter
Atmosphere: Na | 99% Ne, 1% Ar Outer: Hard Vacuum
Luminous Flux: 5,000 lm (@ 100 hrs) (5,500 Hefner Lumens)
Luminous Efficacy: 62.5 lm/W (@ 100 hrs) (68.8 Hlm/W)
Colour Temperature & CRI: CCT: 1700K CRI: Ra-44
Chromaticity Co-ordinates: CCx: 0.574 CCy: 0.425
Burning Position: Vertical Base Up ± 95°
Rated Lifetime: Not Published
Warm-up & Re-strike Time: 20 minutes Instantaneous
Factory: Eindhoven The Netherlands
Date of Manufacture: Circa 1935-1940
Original & Present Value: Unknown
References: 1) Osram Brochure "Fabrik und Werkstatt Beleuchtung durch Osram Dampflampen", September 1937.
2) Osram Brochure "Natriumdampflampen", November 1936.
3) Elektrische Gasentladungslampen, W. Uyterhoeven, Julius Springer Berlin, 1938.
4) Technisch-wissenschaftliche Abhandlungen aus dem Osram-Konzern, V4 1936 pp.15-19.
5) Entladungslampen, P.J. Oranje, Philips Technische Bibliothek, 1943, pp.63-106.