To easily and accurately connect metal caps to a tubular glass bulb by forming a band-like light diffusion layer at a specified position of a surface of the bulb along the axis, and forming a specified groove on each cap.
A glass bulb 1 has a light diffusion layer 4 formed extending along the axis on the side of a half surface, and the other region is mainly transparent. A filament 2 is disposed in the bulb 1 along its axis. A first metal cap 5A is disposed at one end, and a groove 57 is formed on this cap 5A at the position of linear symmetry to the center line X-X dividing into two the light diffusion layer 4 formed on the outer circumferential surface of the bulb 1. This cap 5A has a power feeder terminal 51 electrically communicated with the filament 2. On the other end of the bulb 1, a second metal cap 5B having a power feeder terminal 51 electrically communicated with the filament 2 is formed without the provision of a groove on the circumferential surface.
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