Patent ID: 8198816

Claim:
An extra-high pressure mercury lamp, comprising: a quartz glass arc tube including an arc tube portion and sealing portions connected to the arc tube portion, the arc tube encloses 0.15 mg/mm 3 or more of mercury; and a pair of electrodes disposed face to face in the arc tube, each electrode comprises a rod portion and a base end portion, the base end portion is embedded in the sealing portion; characterized in that, one of the pair of electrodes serves as a cathode and further comprises a head portion and a tube-like portion, the head portion is larger than the rod portion in diameter, the tube-like portion is connected to a rear end portion of the head portion, the tube-like portion extends in an axial direction of the electrode and comprises an inner cylindrical surface separated from the rod portion forming an axially-extending annular gap between the tube-like portion and the rod portion; and the rod portion and the head portion are formed integrally.