| abstract |
A metal halide discharge lamp (10) operated from a D.C. power source (28), which is suitable for an optical light source (5), has a quartz arc tube (16). Anode (18a) and cathode (18b) electrodes are disposed in the arc tube (16). A gap length (L) between electrodes (18a,18b) is so short that the metal halide discharge lamp (10) is capable to function as a point light source. The arc tube (16) contains an inert gas, mercury and metal halide additives including metal bromine. By utilizing the cataphoresis effects, the transformation of silica (SiO₂) of the arc tube in cristobalite or the formation of an opaque substance on the arc tube is prevented. Atomic ratio of bromine to halogen contained in the arc tube is about 60% to 90% and the wall load developed in the arc tube is substantially greater than 40 W/cm². |