| 摘要 |
PROBLEM TO BE SOLVED: To improve earthquake resistance and resistance to impacts irrespective of the length of a lamp by forming at least one recessed part recessed radially inwardly in a straight tube portion of an outer tube. SOLUTION: Recessed parts 8 in a straight tube portion of an outer tube 5 are formed to keep a vertical distanceμ, where 0<=μ<=0.05 mm, between the outer surface of an inner tube 1 and an inner surface of the outer tube 5. That is, the outer surface of the inner tube 1 and the inner surface of the outer tube 5 abut on each other or form a gap 6 of 0.05 m or less, in terms of a vertical distance. As another embodiment, gaps communicating with a gap 6 are formed between opposite-end sealed parts of the inner tube 1 and opposite-end sealed parts of the outer tube 5. While the load resisting strength of a double-tube type fluorescent lamp without recessed parts falls below 250G when it exceeds 150 mm in the length of lamp, results have been obtained that this lamp withstands 250G or higher. Shapes such as a crank shape, a U shape, and a W shape, or a longitudinal cross-sectional shape of a glass bulb are also applicable.
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