Patent ID: 7872420

Claim:
A metal halide lamp comprising: a translucent ceramic arc tube enclosure including: a main tube inside which a discharge space is formed; and two narrow tubes having smaller diameter than the main tube, each connected to either end of the main tube; two electrodes; and a metal halide provided inside the arc tube enclosure, wherein one of the two electrodes is disposed so that it protrudes inside the main tube from inside of one of the two narrow tubes, wherein the other one of the two electrodes is disposed so that it protrudes inside the main tube from the other one of the two narrow tubes, wherein when the rated lamp wattage is denoted by W (watt), an inside diameter of the main tube by D (mm), an electrode protruding length which is the distance from boundary between the main tube and the narrow tubes to an end of the electrode by L (mm), and the distance between ends of the two electrodes by E (mm), wherein a bulb wall loading G (watt/cm 2 ) represented by G=W/(3.14×D×E×0.01) falls within the range of 15≦G≦40, wherein a relationship 0.0001×W+0.405≦L/D≦0.0003×W+0.465 is established, and wherein the metal halide lamp has a rated lamp wattage between 450 W and 1500 W.