Metal halide discharge lamp with electrodes having unequal thoria contents

摘要

A metal halide discharge lamp includes an arc tube having a chemical fill, and a pair of electrodes sealed in the arc tube. The electrodes are tungsten and have differing thoria contents. The electrodes with differing thoria contents provide faster starting times than electrodes with equal low thoria contents and eliminate the electrode meltback, voltage increase and lumen loss that is characteristic of electrodes with equal but higher thoria contents. Preferably, one electrode has a thoria content of about 1.5 to 2 weight percent, and the other electrode has a thoria content of about one weight percent or less.