Rare gas discharge lamp and an original irradiation device

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PROBLEM TO BE SOLVED : To produce a stable discharge between external, electrodes, and to suppress a flicker of the light, by providing a pair of belt form external electrodes separately on the outer peripheral surface of the envelope of a rare gas discharge lamp, and forming strange form parts to the side ends of the external electrodes. SOLUTION : At least at the side edge of either one of the side edges 4a, 4b, 5a, and 5b in a pair of belt form external electrodes 4 and 5 which are provided on the outer peripheral surface of a rare gas discharge lamp 1, a strange form part 6 of a triangular form or the like having a perodicity is formed along almost the full length. Particularly in the external electrodes 4 and 5, it is preferable to arrange the strange parts 6 opposing to an envelope 2 in the shortest distance. The envelope 2 has a luminous layer at the inner surface. When this discharge lamp is started by a lighting device, the potential frequency at the tip part of the strange form part 6 is made higher than other parts, and the discharge is produced stably, and furthermore, the producing position is also stabilized. And, the correcting operation in a document irradiation device can be suppressed at the minimum, and a sufficient reading accuarcy can be obtained.