Thorn SON with Cermet Arc Tube Seals

Thorn developed a unique range of cermet seals for its experimental Ceramic Metal Halide lamps during the 1980's, and following the company's decision not to market that lamp it investigated the use of this seal in the high pressure sodium lamp.

Cermets are composite materials of ceramic and metal, in this case consisting of pressed slugs of alumina doped with tungsten powder.  Molybdenum wires are co-sintered into the cermet, to which the electrode coils are attached and electrical connections are made.  Because a metal tube or wire does not penetrate the seal in these arc tubes, they have the capability to offer far greater reliability by eliminating seal leaks.

The use of cermet seals did pose one problem though.  In the standard niobium-sealed lamps, any hydrogen impurities within the arc tube are able to diffuse out through the metal itself and it does not pose a major problem.  Cermets are not permeable to hydrogen though and the first lamps were made with a cermet seal at one end only, to allow hydrogen to diffuse out of the other end.  In this final production model which was sold for a few years, both ends are cermet sealed.  The hydrogen problem was overcome by incorporating a small coil of barium-titanium wire over the end of one of the electrode shanks.  This acts as a superb getter and effectively removes the hydrogen impurities from the arc tube.

Regrettably when GE took over Thorn Lighting in 1992, this innovative cermet sealing technology was abandoned in favour of cheaper standard production in Hungary.
Manufacturer: GE-Thorn Lamps Ltd.
Lamp Power: 70 Watts
Lamp Current: 0.98 Amps
Lamp Voltage: 90 Volts
Cap Type: E327s/27 Ni plated brass + vitrite
Bulb Finish: Clear Soda-lime glass
Bulb Type: T-38 T-12 in eighths/inch
Overall Length: 154 mm 6 inches
Light Centre Length: 86 mm 3 3/8 inches
Arc Length:
Electrodes: Backwound Tungsten Barium Calcium Tungstate
Atmosphere: Na,Hg | Xe Outer: Hard Vacuum
Luminous Flux: 6,000 lm @ 100 hours
Luminous Efficacy: 85.7 lm/W @ 100 hours
Colour Temperature & CRI: CCT: 1900K CRI: Ra 25
Chromaticity Co-ordinates: CCx: 0.542 CCy: 0.415
Burning Position: Universal
Rated Lifetime: 24,000 hours to 50% survival
Warm-up & Re-strike Time: 8 minutes 5 minutes max
Factory: Leicester England
Date of Manufacture: 1991
Original Value: Laboratory sample - no commercial value