| 摘要 |
PROBLEM TO BE SOLVED: To provide a gas discharge tube capable of emitting light of high brightness constantly. SOLUTION: An inner surface 262a of an arc ball housing concave part 262 which greatly affects to a shape of an arc ball in a gas discharge tube is formed circular in cross section and bulges to the outside. The arc ball is formed in front of a slit-shaped focusing opening 40 so as to be housed in the arc ball housing concave part 262 in a regular shape. The arc ball having uniform brightness distributions in a longitudinal direction of the slit-shaped focusing opening 40 can be securely formed within the arc ball housing concave part 262.
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