Philips 'Philora' SO500 Low Pressure Sodium
Illustrated here is a most unusual early low pressure sodium lamp manufactured by Philips under their Philora brand name, often used on the company's oldest discharge lamps.  Named the Philora SO500, it is rated 80 Watts and has a number of unusual constructional features. The discharge tube has a hand-formed U-bend and is dimensionally similar to a 45W SO type but with larger bore tubing.  The outside of the bend is coated with platinum paint for heat reflection purposes, to keep the bend warm.  Inside the discharge tube are two thinner glass tubes each having a fine coiled filament sealed inside.  These are wired in parallel with the discharge via a C-shaped ceramic clad resistor just above the lamp cap.

Most of the discharge tube is enclosed within an oval-section glass sleeve for heat reflection purposes.  This is sealed onto the inner crown of the outer bulb.  Circular lines in the outer glass indicate that this was an early moulded glass bulb and it was not made from tubing.  The cap is a heavy-walled E40s screw base, curiously made from copper.

It is believed that this lamp was made to operate from the mains electricity supply with a simple choke coil only, obviating the need for a bulky high-voltage leakage reactance autotransformer.  When energised the filaments would heat up first, causing some sodium to vaporise.  This would lower the striking voltage of the arc tube and when it reduced to a suitable level the discharge would strike automatically, the filaments then extinguishing as the resistor limited the current

The lamp appears to be a design first produced by Osram who marketed it for general lighting until 1937 when the conventional dewar-type SO lamps began to supersede it.
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-17 x 40 Outer: T-65
Overall Length:
Light Centre Length:
Arc Length:
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: Emmasingel, 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.