GEC SOX/H 90W with Dimples

Within three years of the 1964 introduction of the Philips SOX range (and the equivalent Osram-GEC SOX) with tin-oxide coatings, they were superseded by a still more efficient range employing a film of indium-tin oxide. The new material showed higher mobility of free electrons, evidenced by its lower electrical resistance, which made it a better IR reflector without compromising light transmission. Once again Osram-GEC followed suit, this lamp being an early prototype from 1967, just before the launch.

Owing to the greater thermal insulation efficiency afforded by the indium 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 initially offered in 35, 55, 90, 135 and 180W sizes.

The discharge tube of this early lamp features Philips-style sodium-retaining dimples, including at the side of the bend. Whereas the Philips film coating process was optimised to achieve a uniform thickness along the tube, and the dimples were essential to avoid sodium migration, the GEC process produced a coating having graded thickness, slightly thicker around the U-bend than the electrodes. This can be seen by the gradient in colour of reflected light in the film. It was found that this obviated the requirement for the dimples, which were abandoned by Osram-GEC after a few years.

The rest of the lamp construction is typical of GEC's early SOX production, employing a tulip-shaped clip to support the U-bend of the discharge tube between a pair of small mica discs, and a bayonet cap Mycalex insulator.
Manufacturer: The General Electric Company of England PLC
Lamp Power: 90 Watts
Lamp Current: 0.95 Amps
Lamp Voltage: 112 Volts
Cap Type: BY22d Mycalex Insulator
Bulb Type: T-65 T-20½ in eighths/inch
Bulb Finish: Indium-tin oxide film Soda-lime glass
Electrodes: CC-4 Beehive Black W + BCT emitter
Arc Length:
Atmosphere: Na | 99% Ne, 1% Ar Outer: Hard Vacuum
Luminous Flux: 12,750 lm (@ 100 hrs) 12,500 lm (average to 4,000h)
Luminous Efficacy: 141.7 lm/W (@ 100 hrs)
Colour Temperature & CRI: CCT: 1700K CRI: Ra -44
Chromaticity Co-ordinates: CCx: 0.574 CCy: 0.425
Rated Lifetime: >6,000 hours 50% survival
Warm-up & Re-strike time: 15 minutes Instantaneous
Burning Position: Vertical Cap Up ± 110°
Overall Length: 518 ± 10 mm 20½ inches
Light Centre Length: 280 mm 11 inches
Factory: Shaw, Oldham England
Date of Manufacture: November 1967
Original / Present Value: Unknown
References: 1) Osram-GEC Lamp Catalogue, Australia, 1969, p.47
2) Osram-GEC Technical Data, Low Pressure Sodium Lamps L4/HT, October 1969
3) Osram-GEC Low Pressure Sodium Lamps Brochure, 1972