Abstract: An improved microwave-excited electrodeless discharge bulb and a lamp system using the bulb are designed to produce a nearly white light at an increased luminous efficacy. The bulb encloses a filling containing a mixture of a rare earth halide, a sodium halide, mercury, and a rare gas. The rare earth halide is at least one selected from a group consisting of a cerium halide and praseodymium halide. With the inclusion of cerium halide and/or praseodymium halide in combination with the mercury, the bulb can produce nearly white light at superior luminous efficacy, when energized by the microwave.