Patent ID: 7038385

Claim:
A mercury-free arc tube for a discharge lamp unit comprising: a closed glass bulb held between pinch seal portions located at opposite ends of said closed glass bulb; and a pair of electrode rods provided in said closed glass bulb so as to be opposite to each other, said closed glass bulb including main light emitting metal halide and starting rare gas enclosed in said closed glass bulb, and said pair of electrode rods respectively having an outer diameter d which is constant along the entire length of the electrode rods; wherein an outer diameter d of each of said electrode rods and a tube current I supplied to said electrode rods satisfy a relationship of 1.0≦I/d≦5.0, wherein “I” is in amperes and “d” is in millimeters, wherein each of said electrode rods has the same outer diameter d, wherein a charged pressure P of said starting rare gas enclosed in said closed glass bulb and the outer diameter d of each of said electrode rods satisfy a relationship of 2.0≦P·d≦6.0, wherein “P” is in atmosphere, and said pair of electrode rods include portions that extend into the closed glass bulb, wherein a volume V of each of said portions of said electrode rods that extends into said closed glass bulb and a tube current I supplied to said electrode rods satisfy a relationship of 1.0≦I/V≦10.0, wherein “V” is in cubic millimeters.