SON-E 70W with 'Corstar' Sapphire Arc Tube
Following the perfection of the Edge-Defined Film-Fed Growth technique of making shaped 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 delivers improved luminous efficacy, it can also withstand higher 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, achieved by operating the sapphire arc tube at higher temperatures.

Regrettably, single crystal sapphire is not only expensive, it also has different coefficients of thermal expansion parallel and perpendicular to the tube axis. In view of the fact that this material actually only delivered some 3% better efficacy and that seals to metal conductors were very tricky to make even experimentally, lamps having sapphire arc tubes were only produced for a brief period by 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: 0.93 Amps
Lamp Voltage: 95 Volts
Cap Type: E27s/27 Ni plated brass + vitrite
Bulb Type: E-70 E-22 in eighths/inch
Bulb Finish: Clear Soda-lime glass
Electrodes: Backwound Tungsten BCT Emitter
Arc Length:
Atmosphere: Na20:Hg80 | Xe 25 torr Outer: Hard Vacuum
Luminous Flux: 5,800 lm (@ 100h) 5,300 lm (@ 2000h)
Luminous Efficacy: 82.9 lm/W (@ 100h)
Colour Temperature & CRI: CCT: 2000K CRI: Ra 35
Chromaticity Co-ordinates: CCx: 0.542 CCy: 0.415
Lifetime: 10,000 hours 90% flux & 85% survival
Warm-up & Re-strike Time: 5 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: Circa 1983
Original Value: Laboratory sample - no commercial value
 
References: 1) GE-Thorn UK Lamp Catalogue 1991