Westinghouse Ceramalux Corstar Sapphire HPS

Following the perfection of the Edge-Defined Film-Fed Growth technique of making shaped single crystal ceramics by H.H. LaBelle and Gerald Meiling at Corning, there was much interest in employing single crystal sapphire as an arc tube for high pressure sodium lamps. The material was marketed by Corning for several years under the trade name Corstar, and in 1976 Westinghouse was first to launch lamps with this material. The principal difference vs traditional polycrystalline alumina tubes is that Corstar is transparent, whereas PCA is translucent. PCA's multiple crystalline boundaries scatter light and reduce transparency.

Single crystal sapphire not only has better light transmission which instantly delivers improved luminous efficacy, it also offers reduced sodium loss at higher temperatures. Thus two new lamps were investigated with this material - one having higher luminous efficacy, the other being a 'deluxe' lamp with better colour rendering which was brought about by an increase in wall loading. Regrettably, single crystal sapphire is not only rather expensive, it also has different coefficients of thermal expansion parallel and perpendicular to the tube axis. In view of the fact that it actually only delivered some 3% better efficacy than the best PCA, and that seals to metal conductors were very tricky to make even experimentally, lamps having sapphire arc tubes were only made and sold for a brief period only by Thorn, Osram-GEC and Westinghouse.

This lamp employs cup-type niobium-zirconium end seals which have been sealed to the arctube with a eutectic oxide braze. Also included is a small disc of tungsten behind each electrode to help keep the seals cool and prevent failures.

Manufacturer:	Westinghouse Electric Company
Lamp Power:	400 Watts
Lamp Current:	4.6 Amps	6.9A Starting Current
Lamp Voltage:	100 Volts	2-4kV Ignition Voltage
Cap Type:	E39s/41	Brass with vitrite
Bulb Finish:	Clear	Leaded Hard glass
Bulb Type:	ET-57	ET-18 in eighths/inch
Overall Length:	248 mm	9¾ inches
Light Centre Length:	146 mm	5¾ inches
Arc Length:
Electrodes:	Backwound Tungsten	Barium Calcium Tungstate
Atmosphere:	Na,Hg \| Xe	Outer: Hard Vacuum
Luminous Flux:	50,000 lm	(@ 100 hrs)
Luminous Efficacy:	125.0 lm/W	(@ 100 hrs)
Colour Temperature & CRI:	CCT: 2000K	CRI: Ra 25
Chromaticity Co-ordinates:	CCx: 0.530	CCy: 0.430
Burning Position:	Universal
Rated Lifetime:	Not Published
Warm-up & Re-strike Time:	4 mins	15 minutes
Factory:	Bath, NH	U.S.A.
Date of Manufacture:	January 1977	Date Code 17
Original & Present Value:	Unknown


References:	1) Smithsonian Institution Website "Lighting a Revolution" http://americanhistory.si.edu/lighting/scripts/s20t.htm.