Patent ID: 7138766

Claim:
A magnesium-free metal halide lamp mounted for use in an illumination device for dimmed lighting that changes a luminous flux of the lamp by changing lamp power, comprising: an arc tube including a translucent ceramic discharge vessel and two electrodes held in a magnesium-free discharge space that exists within the discharge vessel and has a plurality of halides enclosed therein, wherein the halides include sodium halide, cerium halide, thallium halide, and at least one halide selected from a group consisting of dysprosium halide, holmium halide, thulium halide, gadolinium halide, and erbium halide; and a base that feeds power to the electrodes from a maximum lamp power Wmax to a minimum lamp power Wmin whereby a dimmable operation of different luminous flux is realized from the lamp, wherein a surface area S [cm 2 ] of an inner surface of the discharge vessel satisfies Wmax/60≦S≦Wmin/20.