Patent ID: 7196462

Claim:
An arc tube comprising: a pair of electrodes, each electrode having a bead mount configuration; and a glass tube having pinch-sealed ends each provided with a different one of the pair of electrodes, the glass tube being turned at a substantially middle thereof and wound around an axis from the middle to both ends thereof, to have a double-spiral structure; wherein a pitch of (a) a spiral part in a vicinity of one of the ends and (b) an adjacent spiral part in a direction of the axis is set larger than a pitch of other adjacent spiral parts to widen a gap between the one end and the adjacent spiral part, the glass tube has a bent area provided between (a) a position thereof corresponding to a top of the electrode sealed at the one end and (b) a position thereof away from an end face of the one end by ½ of one spiral formed around the axis, the glass tube being bent at the bent area in the direction of the axis so that a gap between the spiral part in the vicinity of the one end and the adjacent spiral part widens gradually from the bent area toward the one end and a gap between adjacent spiral parts of the glass tube in the direction of the axis, between (a) a position at which the glass tube is turned and (b) a position of next adjacent the bent area, is in a range of 0.5 mm or more and less than 3 mm, and the gap between the one end and the adjacent spiral part is in a range of 3 mm to 12 mm inclusive.