Philips SO-X with Tin Oxide Film
This lamp is among the first types ever made which employs a thin film for heat reflection purposes, the coating in this case being of tin oxide approximately 0.32 microns thick.  Lamps having stannic oxide films like this can easily be identified from the colour of light reflections seen in the coating.  In the main photograph here, the characteristic orange colour of the reflections is quite evident.

This material has an excellent infrared reflectivity of 80% while also transmitting some 89% of the sodium yellow light, and these two factors are responsible for the high luminous efficacy of this lamp.  By superseding the former SOI design which used glass tubes for thermal insulation, the luminous efficacy of 140W U-shaped sodium lamps was enhanced from 104 to 125 lm/W and the lamp was re-rated to 100W.

The introduction of tin oxide lamps marked the first use of the new name SO-X, which was applied to the new eXtra high efficiency coated range.  They were put on the market by Philips in 1966 and the technology was quickly adopted by Osram as well.  However the tin lamp was manufactured for less than two years before it was superseded, making it one of the rarest sodium lamps in existence today.  By 1968 it had been taken over by a new range of re-rated SOX lamps having films based on the more efficient indium oxide.

This lamp employs sodium retention dimples in the discharge tube in an effort to maintain good sodium distribution, and the cap is of the porcelain type with a nickel plated collar.  The top spring at the U-bend end is of a particularly complex design which was soon simplified with the introduction of indium SOX lamps in 1968.
Manufacturer: NV Philips Gloeilampenfabrieken
Lamp Power: 40 Watts
Lamp Current: 0.60 Amps
Lamp Voltage: 75 Volts
Cap Type: BY22d Porcelain Insulator
Bulb Finish: Tin Oxide Film Soda-lime glass
Bulb Type: Inner: TU-14 x 32 Outer: T-50
Overall Length: 310 mm 12¼ inches
Light Centre Length: 183 mm 7¼ inches
Arc Length:
Electrodes: CCC-4 Black Tungsten BCT emitter
Atmosphere: Na | 99% Ne, 1% Ar Outer: Hard Vacuum
Luminous Flux: 4,300 lm (@ 100 hrs) 4,085 lm (mean thro' life)
Luminous Efficacy: 107.5 lm/W (@ 100 hrs) 102.1 lm/W (mean thro' life)
Colour Temperature & CRI: CCT: 1700K CRI: Ra-44
Chromaticity Co-ordinates: CCx: 0.574 CCy: 0.425
Burning Position: Verical Base Up ± 95°
Rated Lifetime: 6,000 hours (Guaranteed to 4,000 hrs)
Warm-up & Re-strike Time: 10 minutes Instantaneous
Factory: Emmasingel, Eindhoven The Netherlands
Date of Manufacture: Circa 1964
Original & Present Value: £3 6s 0d (1965) £42.75 (2005)
 
References: 1) Philips UK Lamp Catalogue, 1965.