Patent ID: 7649319

Claim:
Ultra-high pressure mercury lamp, comprising: an arc tube, a pair of opposed electrodes opposite in the arc tube, and at least 0.15 mg/mm 3 mercury in the arc tube as an emission material, wherein at least one of the electrodes has an axial part which is made essentially of tungsten and a starting aid part which is made essentially of tungsten, wherein at least one of the axial part and the starting aid part comprises an electron emission material with a smaller electron affinity than tungsten such that the value of S/A in mm 2 /% by weight is in the range 1.9×10 1 ≦S/A≦1×10 4 , where S in mm 2 is the area of a region of the electrodes projecting into the arc tube that contains the electron emission material and A in % by weight is the concentration of the electron emission material in said region.