Bulb geometry for low power metal halide lamp

Claim 1

A metal halide discharge lamp that includes a tube envelope of the double-ended type having a first neck and a second neck axially arranged on opposite ends of a bulb and joining first and second shanks to the bulb which has a bulb wall that defines an arc chamber of a predetermined volume, said first and second necks being pinched in to restrict heat flow from the bulb wall to the respective shanks, 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 said necks into said arc chamber, the electrodes having axial tips spaced apart to define an arc gap therebetween, said lamp having a rated power that depends on said chamber volume, the quantities of mercury and salt in the chamber, and the arc gap; and wherein the bulb wall has a thickness that increases gradually from a mid-chamber plane, midway between said necks and normal to the axis of the lamp, to the respective first and second necks; and said bulb wall has first and second quarter-chamber planes normal to said axis and midway between said mid-chamber plane and the respective first and second necks, said wall having respective first and second quarter-chamber cross sectional areas at said first and second quarter-chamber planes, respectively, wherein said lamp has a rated quarter-chamber loading factor equal to the rated power of the lamp divided by the sum of the first and second quarter chamber cross sectional area, said rated quarter-chamber loading factor being in the range of 70 to 350 watts per square centimeter.