Atlas SLI/H 60W Linear Low Pressure Sodium

Following AEI's 1959 invention of the linear sodium concept, the next major change arrived with the 1966 introduction of another novel form of discharge tube which pushed the luminous efficacy higher again. In parallel semiconductor IR coatings were applied to improve thermal insulation.

This lamp employs a discharge tube whose cross-section takes the shape of a 4-leaf clover. It offers an improvement over the original crescent shape by further increasing light extraction and decreasing current density. Maintenance of sodium position over life is ensured by 32 grooves known as 'greenhouses' running along the edge of each 'leaf'.

A spinoff advantage is a reduction in striking voltage thanks to the unobscured line-of-sight between the electrodes, enabling the use of a low pressure neon-argon-xenon gas filling to bring a further efficacy increase without loss of life. Following Thorn Lighting's takeover of AEI, the new lamps switched to Thorn's Braided cathodes, formed from seven tungsten wires woven into a hollow tube, which contain more emissive material and bring another life extension. The heat-reflective coating is of tin-doped indium oxide.

These improvements brought a modest gain of about 10% in efficacy and earned Thorn the top place in the SLI lamps market, but curiously the company was not successful in the 60W category. Possibly this is due to the fact that Thorn is not believed to have produced lanterns for the 60W lamp. The 60W was a development of its competitor Osram-GEC who did offer a number of lanterns, and even though Thorn had the better lamp, the replacements market continued to be dominated by the GEC SLI/H 60W alternative.
Manufacturer: British Lighting Industries Ltd.
Lamp Power: 60 Watts
Lamp Current: 0.83 Amps
Lamp Voltage: 80 Volts
Cap Type: 2x GY13 Aluminium + Paxolin
Bulb Type: T-38 T-12 in eighths/inch
Bulb Finish: Indium-tin oxide film Soda-lime glass
Electrodes: BC-6 Braided Black W + BCT emitter
Arc Length: 279 mm 11 inches
Atmosphere: Na | 99%Ne 0.8%Ar 0.2%Xe Outer: Hard Vacuum
Luminous Flux: 6,500 lm (@ 100 hrs) 6,300 lm (average)
Luminous Efficacy: 108.3 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 70% survival, 100% flux
Warm-up & Re-strike time: Approx. 15 minutes Instantaneous
Burning Position: Horizontal ± 20°
Overall Length: 426.2 mm 16¾ inches
Insertion Length: 419.1 mm max 16½ inches
Factory: Leicester England
Date of Manufacture: September 1973 Date Code: B1493
Original & Present Value: £3.99 (1972) £50.37 (2014)
References: 1) Thorn Lighting manufacturing specs for SLI/H 60W Lamps, 1972.
2) AEI Mazda Lamp Data Sheet 96-201, September 1961.
3) AEI Mazda Lamp Catalogue, 1963.
4) Thorn Lighting Comprehensive Price List, UK, 19th April 1972.