Thorn SON with 'Corstar' Sapphire Arc Tube
Following the perfection of the Edge-Defined Film-Fed Growth technique of manufacturing complex shape single crystal ceramics by Corning Glassworks, there was much interest in using "Corstar" single crystal sapphire as an arc tube for high pressure sodium lamps.  The traditional arc tube material is polycrystalline aluminium oxide, which is in fact chemically virtually identical to single crystal sapphire.  The principal difference is that single crystal sapphire tubing is transparent, whereas polycrystalline alumina is not.  Its multiple boundaries between the tiny individual crystals scatter the light and reduce its transparency.

Single Crystal sapphire not only has better light transmission which instantly delivers improved luminous efficacy, it can also withstand higher operating temperatures and pressures.  Thus two new sodium lamps were investigated with this material - one having higher luminous efficacy, the other being a 'deluxe' lamp having better colour rendering which was facilitated by operating the sapphire arc tube at higher temperatures.

Regrettably, single crystal sapphire is not only rather expensive, it also has different coefficients of thermal expansion parallel and perpendicular to the arc tube axis. In view of the fact that this material actually only delivered some 3% better efficacy for its high price and that seals to metal conductors were very tricky to make even experimentally, lamps having sapphire arc tubes were only manufactured and sold for a brief period by Thorn, Osram-GEC and Westinghouse. Perhaps in the future, we may see this material appear again for ceramic metal halide lamps where its enhanced corrosion resistance properties may be of value to deliver lamps can burn at higher temperatures to deliver even better colour rendering properties.
Manufacturer: Thorn EMI PLC
Lamp Power: 70 Watts
Lamp Current: 0.98 Amps
Lamp Voltage: 90 Volts
Cap Type: E27s/27 Ni plated brass + vitrite
Bulb Finish: Clear Soda-lime glass
Bulb Type: E-70 E-22 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: Not published
Warm-up & Re-strike Time: 5 minutes 15 minutes max.
Factory: Leicester England
Date of Manufacture: Circa 1983
Original & Present Value: Engineering sample - no commercial value