| 摘要 |
In an ultrahigh pressure mercury lamp that encloses mercury is enclosed in an arc tube, an electrode has a head portion whose diameter is larger than that of an axis portion and a cylindrical portion formed to project from and extend, integrally with a back end face of the head portion, wherein an inner circumference face the cylindrical portion is apart from the axis portion so as to surround the axis portion. During an alternating current lighting, a relational expression of d/(1/f)×1/2≧3.8 is satisfied, wherein a frequency, which relates to an anode operation period that is the longest in the anode operation period during which one electrode serves as an anode, is represented as f, and a distance in an axial direction from the leading edge position of the head portion of the electrode to a boundary position between the head portion and the cylindrical portion is represented as d.
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