Patent ID: 8445857

Claim:
A flame sensor comprising: an electrical discharge tube including a tube portion having a tube with openings at both ends of the tube, a plug portion having an ultraviolet light-transparent glass that blocks one of the openings of the tube, a base portion blocking the other opening of the tube, and a pair of electrical discharge electrodes being sealed within the electrical discharge tube, which produces electrical discharge by incidence of ultraviolet light from a light source through the plug portion, the electrical discharge electrodes each having a disc-shape; and a focusing member having reflecting surface for reflecting the ultraviolet light from the light source to focus on the electrical discharge electrodes, wherein the reflecting surface has either a parabolic shape or a conical shape having a first opening becoming wider towards the light source and a second opening for allowing the ultraviolet light to reach the electrical discharge electrodes which are provided at a focus of the reflecting surface, the reflecting surface is formed to be more distant from a line towards the light source from the electrical discharge electrodes, the line running through the center of, and being normal to, the respective disc-shaped electrical discharge electrodes, and the plug portion is provided closer to the second opening than to the first opening of the reflecting surface of the focusing member.