| 摘要 |
PURPOSE:To form a heat insulating film only on one end side of a luminous tube and save the material cost by arranging a filament ballast nearer toward the end direction of one luminous tube not provided with the heat insulating film by a fixed distance along a tube wall from the center section of the luminous tube. CONSTITUTION:A filament ballast 5 is arranged nearer toward the end section 3a of one luminous tube 3 not provided with a heat insulating film 6 by lmm along a tube wall from the center section of the luminous tube 3. The luminous tube 3 is constituted to satisfy the equation: 1.8<=r/R<=4.3, 1.5<=L/l<=3.5, where Lmm is the tip distance between a pair of electrodes 4A, 4B, Rmm is the radius of the luminous tube 3, and rmm is the distance from the center A of the luminous tube 3 to the filament ballast 5. Accordingly, the heat insulating film 6 may be formed only at one end side of the luminous tube 3, thus an inexpensive ballast built-in type discharge lamp which can save the material cost and omit the labor can be obtained.
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