Electrode for metal halide discharge lamp

Claim 1

A metal halide discharge lamp in a power range of about 5 watts to 40 watts and having an efficacy exceeding 35 lumens per watt that comprises a quartz tube envelope of the double-ended type having a first neck and a second neck axially arranged on opposite ends of a bulb having a bulb wall that defines an arc chamber of a predetermined volume, predetermined quantities of mercury and a metal halide salt within said chamber, and first and second elongated electrodes of a refractory metal each extending axially through a respective one of the necks into said arc chamber, each of said first and second electrodes having axial tips spaced apart to define an arc gap therebetween; wherein the improvement comprises each of said first and second electrodes having a lead-in wire of diameter 0.007 inches or less formed of said refractory metal supported in the quartz of the associated neck and entering the chamber and a post member composed of said refractory metal and supported on said lead-in wire out of contact with said neck and said bulb wall, each said post member being of a diameter up to about 0.014 inches and larger than its associated lead-in wire, the larger diameter post portions having a relatively large surface area in contact with the mercury and metal halide vapors in the lamp so that heat conducted away from the tips of said electrodes is predominately transferred to the vapors in the chamber, while the smaller diameter lead in wires conduct only a small amount of the heat into the respective neck portions, thereby limiting heat flow from the chamber at or near the neck portions.