| 摘要 |
To devise a UV light source which has an increased UV irradiance in the short wavelength range, prevents blackening of the light emission tube and a decrease of the irradiance and has a long durability, a high pressure mercury lamp (1) of the direct current type is provided, comprising a light emission tube (10), a cathode (13) and an anode (14) arranged at a distance oppositely to each other inside said light emission tube, said cathode (13) having a shape of a truncated cone (16) with a flat part (18) at a tip end thereof, mercury filled inside said light emission tube in an amount of 0.05 to 0.10 mg/mm 3 , wherein the distance between the cathode (13) and the anode (14) is 1.4 to 1.8 mm, and a tip diameter of the flat part (18) at the tip end of the cathode (13) is 0.1 to 0.3 mm.
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