Patent ID: 8305003

Claim:
An ultrahigh pressure mercury lamp, comprising, an arc tube that comprises silica glass; a light emission section; sealing portions that continuously form from both ends of the light emission section, and a pair of electrodes, wherein each of the pair of electrodes has a rod shape axis portion whose base end portion is buried and held in the sealing portion, wherein the electrodes are arranged to face each other, wherein each of the pair of electrodes has a head portion whose diameter is larger than that of the axis portion and a cylindrical portion formed to project from and extend, integrally with a back end face of the head portion, such that an inner circumference face thereof is apart from the axis portion so as to surround the axis portion, wherein during an alternating current lighting, a relational expression of d/(1/f) 1/2 ≧3.8 is satisfied, wherein frequency relating to an anode operation period which is the longest in the anode operation period during which one electrode serves as an anode, is represented as f (Hz), and wherein 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 (mm).