Patent ID: 7759872

Claim:
Discharge lamp, comprising: an arc tube in which a discharge space is formed and in which a cathode and anode are arranged in opposition to each other, the cathode having a tapering part in the shaped of a truncated cone having a circumferential surface with a diameter which decreases in a direction to a tip end of the truncated cone, wherein at least one area of the tapering part is provided with concave-convex parts, the concave-convex parts being formed of a plurality of separate convex parts having adjoining end points which are connected to each other such that adjacent sides of the convex parts form concave parts, the concave-convex parts extending around the entire circumferential surface of the tapering part at a distance from the tip end of the truncated cone, and wherein the tapering part is provided with an annular groove, the annular groove being formed in part by the concave-convex parts, wherein the annular groove, in cross section, has a bottom surface that is one of flat, concavely curved and grooved, wherein a cross section of the tapering part which includes a center axis of the cathode is such that corner points of the convex parts of the concave-convex parts are located on an envelope curve that is convex with respect to the center axis of the cathode and that curves inward from the circumferential surface of the truncated cone in a direction towards the center axis of the tapering part and toward the bottom surface of said annular groove.