A lamp includes a discharge vessel and electrodes extending into the discharge vessel. The lamp further includes an ionizable fill sealed within the vessel. The fill includes an inert gas and a halide component. The halide component includes a sodium halide, a thallium halide, at least one of a calcium halide and a strontium halide, and at least one of a rare earth halide. The rare earth (RE) halide is selected from the group consisting of lanthanum, cerium, praseodymium, samarium and neodymium, and combinations thereof. The fill establishes a Na/RE ratio of 8≦Na/RE≦48.