an arc discharge metal halide lamp for use in selected lighting fixtures having a discharge chamber with light permeable ceramic walls of a selected shape about a discharge region of a selected volume . a pair of electrodes are supported in the discharge region separated from one another by a separation length . the separation length is in a ratio to the effective inner diameter that is greater than four . ionizable materials are provided in the discharge region comprising a quantity of mercury in a ratio to the discharge region volume that is less than 4 mg / cm 3 , a noble gas , praseodymium halide , and sodium halide .