| 摘要 |
PROBLEM TO BE SOLVED: To elongate an effective light emission range to uniformly emit light and to maintain the effective light emission range over the entire life cycle. SOLUTION: An arc tube 12 having both ends sealed, having fluorescent substance 7 applied to the inner surface thereof and having light transmission property is provided. A pair of electrodes 4 fixed to the end of a pair of lead-in wires 3 that is introduced inside from the arc tube 2 and has conductivity is covered with a cap 5 only opening in the direction of the end of the arc tube 2. A discharge path forming part 9 extended from both ends of The arc tube 2 in the inner direction of the cap 5 and overlapped with the cap 5 by a predetermined amount is provided. When a glow discharge is generated in the arc tube 2, ultraviolet rays are generated over the entire inner length of the arc tube 2 because a Faraday dark space is formed inside the cap 5. By this, the light emission over the entire inner length of the arc tube 2 can be obtained and the effective light emission range to uniformly emit light can be elongated. In addition, since blackening due to spattering of the electrodes 4 stays inside the cap 5, the effective light emission range is maintained through the entire life cycle.
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