Abstract: A discharge tube is comprised of a glass bulb, a cathode, and an anode. Impregnating porous tungsten with barium makes the cathode tip portion. A clearance between the end face of a lead rod and the end face of the cathode tip portion is filled with a molybdenum-ruthenium brazing filler metal.

Abstract: A discharge tube in which a cathode with a cathode tip portion being fixed to a lead rod and an anode opposed to the cathode tip portion are encapsulated in a discharge gas atmosphere to effect arc discharge, wherein the cathode tip portion comprises: a metal substrate of an impregnated type in which a porous, refractory metal is impregnated with an electron-emissive material or a sintered type in which a refractory metal containing an electron-emissive material is sintered; and a coating of a refractory metal covering a predetermined portion in a surface of the metal substrate and having a thickness of not less than 0.02 &mgr;m nor more than 5 &mgr;m, wherein the metal substrate has a cusp pointed toward the anode, and wherein a tip portion of the cusp of the metal substrate is exposed without being covered by the coating.

Abstract: In order to meet various requirements for discharge tubes for use as light sources, including easiness of production, stability of discharge, long lifetime, etc. without any contradiction between them, a discharge tube of the present invention involves an improvement in the discharge tube for use as a light source constructed to effect arc discharge in a discharge gas atmosphere between a cathode 2, having a cathode tip portion 22 fixed to a lead rod 21, and an anode 3 opposed thereto while keeping the cathode and anode enclosed in the gas. The cathode tip portion has a cusp pointed toward the anode and consists of a refractory metal substrate 221 of an impregnated type or a sintered type containing an electron-emissive material, and a refractory metal coating 222 covering the surface except at the tip portion of the cusp.

Abstract: A discharge tube in which a cathode with a cathode tip portion being fixed to a lead rod and an anode opposed to the cathode tip portion are encapsulated in a discharge gas atmosphere to effect arc discharge, wherein the cathode tip portion comprises: a metal substrate of an impregnated type in which a porous, refractory metal is impregnated with an electron-emissive material or a sintered type in which a refractory metal containing an electron-emissive material is sintered; and a coating of a refractory metal covering a predetermined portion in a surface of the metal substrate and having a thickness of not less than 0.02 &mgr;m nor more than 5 &mgr;m, wherein the metal substrate has a cusp pointed toward the anode, and wherein a tip portion of the cusp of the metal substrate is exposed without being covered by the coating.

Abstract: A discharge tube is comprised of a glass bulb, a cathode, and an anode. Impregnating porous tungsten with barium makes the cathode tip portion. A clearance between the end face of a lead rod and the end face of the cathode tip portion is filled with a molybdenum-ruthenium brazing filler metal.

Abstract: A discharge tube is comprised of a glass bulb, a cathode, and an anode. The cathode consists of a cathode tip portion of a bullet shape made by impregnating porous tungsten with barium, and a cylindrical lead rod of molybdenum having a recess. Apart of a base of the cathode tip portion is placed in the recess of the lead rod and a clearance between the internal surface of the recess of the lead rod and the side face of the insert part of the cathode tip portion is sealed with a molybdenum-ruthenium brazing filler metal.