| 摘要 |
PROBLEM TO BE SOLVED: To eliminate the possibility that the emission tube portion of an arc tube composed so that an arc tube is almost concentrically surrounded by a shroud tube will be fructured during lighting of the arc tube. SOLUTION: Clearance g1 between the uppermost point P1 on outer face 20e of emission tube portion 20a and inner wall face 18c of shroud tube 18 is set at g1 >=0.3 mm based on the prescribed experimental result. This prevents rapid creation of devitrification by heat accumulation phenomenon and dopant diffusion phenomenon to develop many micro-cracks at the upper end where temperature becomes the highest in the emission tube portion 20a during the lighting of the discharge valve. As a result, generation of an explosion of the emission tube portion 20a with the micro-cracks as the starting points can be prevented in advance.
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