Thorn SOX Miniature Low Pressure Sodium

Following Osram-GEC's launch of the miniature 10W SOX lamp, there was much interest in potential applications for such small low pressure sodium light sources. Domestic and commercial exterior security lighting was the principal application of high efficiency small sodium lamps, for instance the 10W SOX offered considerable energy savings over the 75W incandescent lamp it was designed to replace.

Thorn EMI and Philips produced prototype 10W lamps compatible with GEC's, but theirs were never sold. Thorn also wished to create this smaller 6W lamp because it is so short it can be fitted into ordinary luminaires designed for GLS incandescent lamps with no modifications or special lanterns being required. However the luminous efficacy of SOX lamps decreases as the wattage is reduced, and it was difficult to make a small lamp sufficiently efficient. The GEC 10W lamp managed to achieve 100lm/W, but this fell to 80lm/W for the 6W size, a figure not significantly greater than the white-light compact fluorescent lamps beginning to appear at the same time. Because of this, SOX lamps smaller than 10W never appeared for sale on the market. Philips did manage to achieve 100lm/W for its unusual 8W screwbase lamp, but it was still not marketed.

It is interesting to note that the indium film on this lamp is applied to an inner sleeve of cut-down SLI Linear Sodium outer jacket, placed inside an evacuated 40mm diameter bulb. To maintain high temperature at the bend the glass here is constricted, and an aluminised mica disc serves as an excellent heat reflector. Electrodes are of the braided type to maximise lamp life and reduce the rate of end blackening which would be very detrimental for such a small lamp.
Manufacturer: Thorn EMI Lighting Ltd.
Lamp Power: 6 Watts
Lamp Current: ~0.20 Amps (provisional)
Lamp Voltage: ~30 Volts (provisional)
Cap Type: B22d/36x26 Ni Brass, Vitrite Insulator
Bulb Finish: Indium Tin Oxide Soda-lime glass
Bulb Type: Inner: TU-12 x 28 Outer: TD-38
Overall Length: 120 mm 4¾ inches
Light Centre Length: 80 mm 3¼ inches
Arc Length:
Electrodes: CCC-6 Black Tungsten BCT emitter
Atmosphere: Na | 98.5% Ne, 1.5% Ar Outer: Hard Vacuum
Luminous Flux: 500 lm (@ 100 hrs)
Luminous Efficacy: 83.3 lm/W (@ 100 hrs)
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: Not Published
Warm-up & Re-strike Time: 10 minutes Instantaneous
Factory: Leicester England
Date of Manufacture: 1985
Original & Present Value: Engineering Sample - No Commercial Value