Mercury-free arc tube for a discharge lamp

摘要

A mercury-free arc tube for a discharge lamp has a closed chamber filled with rare gas and a metal halide containing at least Na halide or Sc halide and electrodes disposed on both end portions of the closed chamber so as to be opposed to each other, wherein the electrode rod is stepped shape which satisfies relationship of 1.1<A 1 /A 2 <7.3, whereas A 1 is a cross sectional area of the electrode in a top-end side area protruded into the closed chamber and A 2 is a cross sectional area of the electrode in a base-end side area fixed to the end portion of the closed chamber.