US Patent 7569993 Gas discharge tube with discharge path limiting means

In a gas discharge tube there is carried out narrowing of the discharge path with cooperation of the first opening 20 and second opening 12 in order to obtain higher luminance of light. Further, in order to maintain excellent starting-properties of a lamp even if the discharge path is narrowed, a predetermined voltage is applied to a second discharge path limit portion 11 externally. Thereby, a positive or active starting discharge is produced in such a manner as to pass through the first opening 20. Further, the second opening 12 is comprised of not only a straight section 13 extending in a direction of an optical axis Y, but also a spread section 14 extending from an end portion of the straight section 13 toward the first opening 20.