| 摘要 |
PROBLEM TO BE SOLVED: To improve light output without changing enclosure size or tube input with a relatively simple structure by setting an adhesive quantity of a fluorescent layer within a specific range. SOLUTION: A fluorescent layer 2A including one or two kinds of phosphors in an inner face of an enclosure 1 consisting of a glass bulb is formed, and an adhesive quantity of the fluorescent layer 2A is set to 5 to 30 mg per 1 cm<2> . An opening angleθ1 of a first opening 7 in external electrodes 5 and 6 is set to be greater than an opening angleθ2 of a second opening 8, and the opening angleθ1 is set within the range of 60 to 120 deg.. At an inner face part of the enclosure 1 substantially corresponding to a first opening 7 in the external electrodes 5 and 6, an aperture part 2a at which the fluorescent layer 2A is not formed is formed. Thereby, a light output emitted from the first opening via the aperture part 2a can be efficiently improved even if the enclosure size or tube input is changed.
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