An arc tube, which is capable of prolonging the life thereof by preventing occurrence
of a leak caused from a crack of an arc-tube body, has the average surface roughness
of each of outer surfaces 26Aa and 26Ba of tungsten electrodes 26A
and 26B pinch-sealed to pinch seal portions 20b1 and
20b2 on the two sides of a light-emission tube 20a of
an arc-tube body 20 is set to be 3 m or smaller. Thus, the state
of pinch-sealing of the tungsten electrodes 26A and 26B to the pinch
seal portions 20b1 and 20b2 is brought
to a state in which the two elements are engaged with small pits and projections.
Therefore, great compressive stress is not left in the region adjacent to the joint
surface between the pinch seal portions 20b1 and 20b2
and the tungsten electrodes 26A and 26B as distinct from the
conventional structure. In a case where a crack of the arc-tube body 20 is
formed owing to the residual compressive stress, the crack is limited to a local
portion which is the region adjacent to the joint surface. As a result, the crack
is not enlarged to reach the surface of the arc-tube body 20.
