A manufacturing method of an arc tube including a flat-spiral glass tube,
- the flat-spiral glass tube being made up of:
- a central portion;
- a first end portion;
- a second end portion;
- a first flat-spiral portion formed between the central portion and the first end portion;
- and a second flat-spiral portion formed between the central portion and the second end portion,
- the flat-spiral glass tube being made of a glass tube including a central-portion scheduled portion, a first-end-portion scheduled portion, a second-end-portion scheduled portion, a first-flat-spiral-portion scheduled portion, and a second-flat-spiral-portion scheduled portion,
- the manufacturing method comprising:
- a first step of forming the glass tube into a substantially circular cone shape, by holding the central-portion scheduled portion of the glass tube in softened state to a vertex of a jig having a substantially circular cone shape, and winding to a conical surface of the jig at least the first-flat-spiral-portion scheduled portion and the second-flat-spiral-portion scheduled portion;
- a second step of flattening the glass tube formed in the substantially circular cone shape into a substantially disc shape so that part of a tubular axis of the glass tube, which corresponds to the first-flat-spiral-portion scheduled portion and the second-flat-spiral-portion scheduled portion, is substantially included in a plane;
- a third step of, after the second step, providing the first end portion and the second end portion with a corresponding electrode;
- and
- a fourth step of, after the third step, a) bending a portion of the first flat-spiral portion including the first end portion so that the first end portion approaches the second flat-spiral portion, and b) bending a portion of the second flat-spiral portion including the second end portion so that the second end portion approaches the first flat-spiral portion, wherein
- a distance Gb1 is substantially uniform, where the distance Gb1 is measured in a parallel direction to the plane and is between adjacent exterior walls of the circular-cone-shaped glass tube respectively belonging to a first substantial part of the first-flat-spiral-portion scheduled portion and a second substantial part of the second-flat-spiral-portion scheduled portion,
- a distance Ge1 is longer than the distance Gb1, where the distance Ge1 is measured in the parallel direction to the plane and is between adjacent exterior walls of the circular-cone-shaped glass tube respectively belonging to the first-end-portion scheduled portion and the second-flat-spiral-portion scheduled portion;
- and between adjacent exterior walls of the circular-cone-shaped glass tube respectively belonging to the second-end-portion scheduled portion and the first-flat-spiral-portion scheduled portion, and
- after the fourth step, a distance Ge2 is substantially the same distance as a distance Gb2, where the distance Ge2 is measured in the parallel direction to the plane and is;
- between adjacent exterior walls of the flat-spiral glass tube respectively belonging to the first end portion and the second flat-spiral portion;
- and between adjacent exterior walls of the flat-spiral portion, and the distance Gb2 is measured in the parallel direction to the plane and is between adjacent exterior walls of the flat-spiral glass tube respectively belonging to the first flat-spiral portion and the second flat-spiral portion.