Philips SOX Compact 18W

Following the development of the first Philips Mini-SOX 18W lamp of 1977, and the even smaller Osram-GEC SOX 10W a few years later, PHilips began a project to maximise the efficacy of miniature low pressure sodium lamps. The sample featured here is one of the resulting prototypes.

A key feature is its very narrow diameter discharge tube, measuring just 7mm internally. This allowed a reduction in lamp current, and hence discharge current density, with a knock-on benefit for luminous efficacy. To achieve a given power dissipation with such low current density, it was of course necessary to increase the length of the discharge tube. Keeping such a large discharge tube of low power dissipation warm enough for the sodium vapour pressure to build up to the level required for optimum efficacy meant that thermal insulation of this lamp had to be maximised.

That has been achieved by a specially enhanced indium-tin oxide coating having an electrical resistance of about 5 ohms per square, similar to that employed on SOX-E lamps. A nickel plated steel heat-reflection disc provides additional insulation at the U-bend, and this supports a small platinum catalyst pellet which breaks down hyrdocarbon vapours in the outer bulb such that they can be removed by the barium getter near the cap, thereby limiting thermal conduction and convection losses.

The lamp is unusually equipped with an E27 screw cap. Philips experimented with various designs having powers from about 8W to 25W, but the concept was never commercialised - perhaps because of the relatively energy saving vs traditional SOX lamps.
Manufacturer: Philips Gloeilampenfabrieken
Lamp Power: 18 Watts
Lamp Current: 0.25 Amps
Lamp Voltage: 75 Volts 300V ignition
Cap Type: E27s Brass + Vitrite
Bulb Type: TD-38 TD-12 in eighths/inch
Bulb Finish: Indium-tin oxide film Soda-lime glass
Electrodes: CCC-4 Beehive Black W + BCT emitter
Arc Length:
Atmosphere: Na | 99% Ne, 1% Ar Outer: Hard Vacuum
Luminous Flux: 2,160 lm (@ 100 hrs)
Luminous Efficacy: 120 lm/W (@ 100 hrs)
Colour Temperature & CRI: CCT: 1700K CRI: Ra -44
Chromaticity Co-ordinates: CCx: 0.574 CCy: 0.425
Rated Lifetime: Not Published
Warm-up & Re-strike time: 15 minutes Instantaneous
Burning Position: Not Published
Overall Length:
Light Centre Length:
Factory: Eindhoven Netherlands
Date of Manufacture: Approx 1985
Original / Present Value: Engineering Sample No commercial value
References: 1) US Patent 4,441,045 "Low Pressure Sodium Vapour Lamp", 1984.