SLI/H 200W 'HO' Linear Low Pressure Sodium
Following AEI's 1959 invention of the linear sodium concept, the next major change arrived with the 1966 introduction of discharge tubes having the shape of a 4-leaf clover. In parallel a semiconductor IR coating of tin oxide was applied to improve thermal insulation. This allowed the 20,000lm output of the original 200W lamp to be boosted 25% to 25,000lm. A spinoff advantage is a lower striking voltage thanks to the unobscured line-of-sight between electrodes, which enabled the use of a low pressure Ne-Ar-Xe gas filling to bring a further efficacy increase without loss of life.

This so-called HO or High Output model was introduced around 1971 and marks yet another advance in efficacy. The ratio of surface area to cross-sectional area of the discharge tube has been increased by switching to a 5-leaf shape, which allows still greater light extraction. In parallel the IR coating is the superior indium-tin oxide composition, whose better reflectivity allows a wider discharge tube - the decrease in current density bringing the output to 27,500lm. Rather inconveniently the larger inner required the outer jacket to be increased to the non-standard T12½ diameter.

The electrode design was also shifted at this time to employ the more efficient and robust beehive SOX design. Since these heavier cathodes require some 6 Amps to be brought to incandescence, greater than the short-circuit current available from the SLI 200W ballast, the new HO lamp was no longer compatible with the original 200W circuits having a thermal starter-switch. It could only be installed in new luminaires having leakage reactance autotransformer ballasts. Because of this, the regular 200W lamp had to be continued in production alongside the new HO model.
Manufacturer: Thorn Electrical Industries
Lamp Power: 200 Watts 190W Initial
Lamp Current: 1.55 Amps
Lamp Voltage: 145 Volts
Cap Type: 2x GY13 Aluminium + Paxolin
Bulb Type: T-39 T-12½ in eighths/inch
Bulb Finish: Indium Tin oxide film Soda-lime glass
Electrodes: CCC-4 Beehive Black W + BCT emitter
Arc Length: 753 mm 29 5/8 inches
Atmosphere: Na | 99%Ne 0.8%Ar 0.2%Xe Outer: Hard Vacuum
Luminous Flux: 28,050 lm (@ 100 hrs) 27,500 lm (average)
Luminous Efficacy: 147.6 lm/W (@ 100 hrs) 137.5 lm/W (average)
Colour Temperature & CRI: CCT: 1700K CRI: Ra -44
Chromaticity Co-ordinates: CCx: 0.574 CCy: 0.425
Rated Lifetime: 6,000 hours 80% survival, 100% flux
Warm-up & Re-strike time: 7 minutes Instantaneous
Burning Position: Horizontal ± 20°
Overall Length: 908.8 mm max 35 3/4 inches
Insertion Length: 895.0-901.7 mm max 35 3/4 inches
Factory: Leicester England
Date of Manufacture: September 1980 Date Code 81
Original & Present Value: £7.51 (1972) £94.80 (2014)
 
References: 1) Thorn Lighting Comprehensive Catalogue, UK, 1980, p.228
2) Thorn Lighting Comprehensive Price List, UK, 19th April 1972