Thorn SONXL-T 70W with Cermet Seals

Despite its small size Thorn became a world leader in HPS technology, perhaps facilitated by the fact that its Leicester site was the only place in the world to design and make both the lamps as well as their ceramic arc tube materials.

This lamp represents a particularly novel idea, employing Cermet seals similar to those created for the company's equally pioneering invention of the world's first ceramic metal halide lamp. Cermets are composites of CERamic and METal, in this case consisting of pressed slugs of alumina-molybdenum powder, into which the tungsten electrode and a molybdenum leadwire have been co-sintered. The use of cermets permitted the introduction of an equally innovative radio frequency induction heating process to seal them to the arc tube using a special frit glass. By eliminating metal parts from the interfaces, expansion coefficients are better matched and leaks are reduced. The frit is also displaced to a cooler region, slowing sodium reactions and increasing life.

Thorn developed a Cermet SON lamp in the early 1980s, but it was not sold to problems of hydrogen entrapment. That gas, which hinders starting and raises arc voltage, is normally able to escape via niobium parts which are porous to hydrogen but in a cermet lamp it is trapped. The problem was solved by developing an arc tube getter, consisting of a short coil of Barium-titanium alloy or Zirconium coated Titanium wire behind one electrode, visible in the X-Ray. These highly advanced Cermet lamps were launched only in the 50W and 70W SON-XL models in 1991. Sadly in 1992 GE-Thorn's HPS operations were closed and relocated to GE-Tungsram's factory in Budapest, where the Cermet seal was most regrettably not further exploited.
Manufacturer: GE-Thorn Lamps Ltd.
Lamp Power: 70 Watts
Lamp Current: 0.98 Amps
Lamp Voltage: 90 Volts 1.8-2.3kV x 2μs/cycle ignition
Cap Type: E27s/27 Ni plated brass + vitrite
Bulb Type: T-38 T-12 in eighths/inch
Bulb Finish: Clear Soda-lime glass
Electrodes: Backwound Tungsten BCT Emitter
Arc Length: 44.6 mm 1.756 inches
Atmosphere: Na20:Hg80 | Xe 220 torr Outer: Hard Vacuum
Luminous Flux: 6,500 lm (@ 100h) 6,400 lm (@ 2000h)
Luminous Efficacy: 92.9 lm/W (@ 100h)
Colour Temperature & CRI: CCT: 2000K CRI: Ra 25
Chromaticity Co-ordinates: CCx: 0.530 CCy: 0.430
Rated Lifetime: 28,500 hours 50% survival, 78% flux
Warm-up & Re-strike Time: 3 minutes <1 minute
Burning Position: Universal
Overall Length: 154 mm 6 inches
Light Centre Length: 100 mm 4 inches
Factory: Leicester England
Date of Manufacture: Approx. 1991
Original Value: GB £33.08 (1992)
References: 1) Thorn Lighting News, Staff Newspaper, 1990.
2) Thorn Lighting Product Data Sheet 70W SON-T & SONXL-T 96-2037AA, 10-10-1989.
3) Thorn Lighting SON Manufacturing Specifications.
4) GE-Thorn UK Price List, March 1992.