Discharge tube with substantially exposed cathode tip portion for use as a light source

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. At the tip portion of the cusp in the cathode tip portion, the metal substrate is exposed without being covered by the coating, or the coating is made so thin or porous as to substantially expose the metal substrate.

Dependencies are inferred from the claim text.