Universal burn metal halide lamp

A universal burn metal halide arc discharge lamp rated for power input of no more than 150 watts, comprising a vitreous, light transmissive outer envelope with an inert gas present in the space between said arc chamber and the interior surface of said envelope, said chamber having a wall loading of 17-23 watts/cm.sup.2 during operation and having an ellipsoidal shape including a volume no greater than 1 cm.sup.3 with an aspect ratio ranging between 1.6-2.3, said chamber being coated at both ends with a heat reflective coating having a coating length of from 12-16% of the arc chamber length, and enclosing within a pair of spaced apart electrodes each extending into said chamber a distance no greater than 15% of said chamber length, said chamber further containing (i) inert starting gas, (ii) metal halide comprising a halide of sodium and at least one additional metal in an amount sufficient to achieve metal halide density of from 1.5 to 4 mg/cc, and (iii) mercury in an amount sufficient to achieve desired operating voltage of no more than 100 volts during operation of said lamp, and wherein said arc discharge lamp exhibits a difference in lumen output between vertical and horizontal orientation which is less than 10% in variance to one another.