GB1056817A - Improvements in and relating to electric discharge lamps - Google Patents

Abstract

1,056,817. Metal-vapour lamps. GENERAL ELECTRIC CO. July 1, 1964 [July 1, 1963], No. 27187/64. Heading H1D. An electric discharge lamp comprises a tubular elongated envelope having an internal diameter lying in the range 3 to 15 mm. of material resistant to the attack of caesium vapour at least up to a temperature of approximately 900‹ C., a pair of electrodes sealed respectively into opposite ends of the envelope, and an ionizable medium within the envelope comprising caesium and mercury, the caesium exerting a partial pressure in the range of 15 to 350 mm. Hg and the mercury exerting a partial pressure in the range of 0À15 to 15 atmospheres in normal operation of the lamp. The ceramic envelope 2 of the lamp shown is of sintered transparent polycrystalline alumina preferably containing more than 99À5% of Al 2 O 3 having titanium metallized ends to which caps 3 and 4 of a nickel chromium iron alloy are brazed. Strain equalizing rings 5 and 6 of alumina are similarly brazed to the caps 3 and 4. A double wound tungsten coil cathode 8 is supported by a tube 7 of stainless steel or iron nickel chromium alloy sealed through a centrally perforated boss of cap 3 and provided with a filling aperture 12 and a sealing pinch or weld 13. The projecting length of the tube 7 may be adjusted to obtain a desired caesium saturation vapour pressure, but the lamp is operated with unsaturated caesium vapour if a mercury pressure of greater than 5 atmospheres is required. A similar electrode 10 is supported by a metal tube 11 from cap 4. The electrodes 8 and 10 may alternatively be tungsten rods, preferably thoriated. The envelope 2 may. have an internal diameter of 6 mm. with an inter-electrode spacing of about 60 mm. A starting gas of neon, argon, krypton, xenon, or mixture thereof at a filling pressure of 5 to 50 mm. Hg is provided. Luminous efficiency is plotted against caesium pressure (Fig. 2; not shown).