GB998464A - Improvements in gaseous electric discharge lamps - Google Patents

Abstract

998,464. Discharge lamps. GENERAL ELECTRIC CO. Jan. 23, 1962 [Jan. 23, 1961], No. 2402/62. Heading H1D. A discharge lamp comprises a sealed envelope containing mercury in an amount which is completely evaporated during operation to provide a pressure of between 1 and approximately 15 atmospheres and a metallic halide other than a fluoride in an amount to provide under operating conditions a partial pressure of approximately 10<SP>-3</SP> to 10<SP>3</SP> mms. of vaporized halide, the walls of the envelope being spaced with respect to the electrodes so that during operation the coldest portion of the interior wall of the envelope is maintained at a temperature in excess of approximately 600‹ C. The envelope can contain a quantity of free halogen in an amount sufficient to provide a partial pressure of halogen vapour of approximately 1 to 200 mms. The envelope can also contain a noble gas as starting gas, for example argon at 15 mms. pressure. The halide pressure preferably does not exceed one atmosphere and the halide can comprise a plurality of iodides in an amount to provide independent partial pressures of approximately 1 to 200 mms. for each iodide. The iodides preferably comprise sodium and thallous with or without indium iodide. Other halides referred to are the iodides, bromides and chlorides of Li, Na, Cs, Ca, Cd, Ba, Hg, Ga, In, Tl, Ge, Sn, Th, Se, Te and Zn. The lamp comprises a sealed quartz envelope 2 supported within an evacuable outer envelope 1, means being provided within the space between the inner and outer envelopes for minimizing the transmission of heat through the space. The means can comprise a hard vacuum of 1 micron or less within the space, a low pressure filling of a high molecular weight gas such as xenon, heat shields provided about the end regions of the inner envelope such as heat reflecting shields 23 comprising metallic layers evaporated on the end surfaces of the inner envelope, or a material such as quartz wool which is substantially transparent to visible light but able to block infra-red radiation. A starting electrode 5 is connected through a resistance 16 to input lead 14. The electrodes 3, 4 comprise a tungsten rod and a sliver of thorium with a tungsten coil wrapped therearound. In operation all the mercury is vaporized, the arc temperature is maintained at approximately 3000‹ to 6000‹ C. and the inner surface of the envelope 2 is maintained at a temperature of approximately 600‹ to 1200‹ C.