Thorn Experimental Sapphire Metal Halide

This lamp is believed to be an early attempt to create either a ceramic metal halide lamp, or a high pressure sodium lamp having whiter light and improved colour rendering. The construction is based on Thorn's standard 250W SON lamp, with a Corstar single crystal sapphire arc tube. The electrodes are similar to HPS types, although perhaps without any emitter coating since that would be rapidly attacked by metal halides. They are probably sealed with titanium by electron beam brazing into niobium tubes, which in turn are sealed to polycrystalline alumina end plugs and to the sapphire arc tube with an intermediate frit glass.

When lit, this lamp radiates the intense green spectrum of thallium plus very weak sodium and mercury lines. There is no metallic sodium visible, although if it was intended as a ceramic metal halide lamp it perhaps contains sodium, indium and thallium halides. However HPS arc tubes have a rather cool area behind the electrodes which may not be hot enough to fully vaporise salts other than thallium iodide, which has an unusually high vapour pressure.

In the 1970s-80s there were efforts to create a mixed metal vapour lamp, based on an HPS arc tube containing other metals such as thallium and cadmium. This was not successful due to the need for higher temperatures to vaporise the other metals, which led to short life. Thorn may have attempted to overcome this by adding other metals as halides, which have higher vapour pressures. However the life of such a lamp is also expected to be short, since the halides were found to attack niobium and the frit seals. That problem was overcome when Thorn developed the Cermet seals that facilitated its TSH lamp in 1981.
´╗┐Manufacturer: Thorn Electrical Industries
Lamp Power: 250 Watts
Lamp Current: 1.3 Amps
Lamp Voltage: 210 Volts 3 kV ignition pulse
Cap Type: E40/45 Nickel plated brass
Bulb Type: TD-46 TD-15 in eighths/inch
Bulb Finish: Clear Borosilicate
Electrodes: Backwound Coil No emitter
Arc Length:
Atmosphere: Xe|Hg(Tl,Na,?)Ix Outer: Hard Vacuum
Luminous Flux: 11,750 lm
Luminous Efficacy: 47.0 lm/W
Colour Temperature & CRI: CCT:6300K CRI: Ra31
Chromaticity Co-ordinates: CCx:0.289 CCy: 0.554
Lifetime: Short
Warm-up & Re-strike Time: 3 minutes 5 minutes
Burning Position: Universal
Nominal Length: 257 mm 10 inches
Light Centre Length: 158 mm 61/4 inches
Factory: Leicester England
Date of Manufacture: Circa 1975-1980
Original Value: N/A
References: 1) Private Communication, Ken Downton, GE-Thorn Research & Engineering Laboratories, Leicester, 1996.