Philips SO-X with Indium Oxide Film
The tin oxide coated SOX lamp had only been on the market for two years before it was superseded by an enhanced range of lamps having reflective coatings substantially based on indium oxide.  The new lamps were first sold during 1968 and owing to the greater thermal insulation efficiency afforded by the superior film, lamp wattage was reduced again to deliver a new range having greater efficacy but similar luminous flux.  The new line of lamps was known as re-rated SOX for the few years following their introduction and they were offered in 35W, 55W, 90W and 135W sizes.

The new coating, another development of the Philips laboratories, is substantially based on indium oxide but still contains a percentage of tin.  Tin improves the mobility of electrons in the semiconducting film, which not only makes it a better electrical conductor it also enhances the infra-red reflectivity of the film.

The tin-doped indium oxide coating introduced at this time is almost identical to the material still employed in modern standard SOX lamps.  Indium oxide coated lamps are easy to distinguish from the earlier tin-based films by the colour of light reflections seen in the coating.  Tin lamps show a characteristic yellow/orange colour reflection, whereas the 0.32 micron indium films imparts a greenish tinge to reflected light colour.  This lamp employs sodium retention dimples in the discharge tube in an effort to maintain good sodium distribution, and the cap is of the more modern Alkyde polymer style with a nickel plated collar.  The top spring at the U-bend end has been much simplified in this design, and eliminates the sharp corners on the earlier top support which had led to breakage of the outer bulb before.
Manufacturer: NV Philips Gloeilampenfabrieken
Lamp Power: 35 Watts
Lamp Current: 0.60 Amps
Lamp Voltage: 70 Volts
Cap Type: BY22d Alkyde Insulator
Bulb Finish: Indium 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 (Lighting Design Lumens)
Luminous Efficacy: 122.9 lm/W (Lighting Design Efficacy)
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: December 1971 Date Code 1M
Original & Present Value: Unknown
 
References: 1) Philips UK Brochure "SOX Lamps" PL8260/3, September 1969.