Patent ID: 7423370
Filing Date: 2008-09-09
Classification: H01J

Abstract:
1. An arc tube comprising: a glass tube that is turned at a substantially middle thereof and wound around an axis from the middle to both ends thereof, to have a double-spiral structure; and a pair of electrodes sealed at both the ends of the glass tube, 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 first gap between the one end and the adjacent spiral part, the first gap between the one end and the adjacent spiral part is in a range of 3 mm to 12 mm inclusive, each end of the glass tube at which the electrode is sealed has a pinched and sealed end part, and a second 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 glass tube corresponding to where the second gap starts to widen to become the first gap, is in a range of 0.5 mm or more and less than 3 mm.