| 摘要 |
PURPOSE:To increase an S/N ratio and improve detection ability by setting a run path for a light-transmittable film slantingly to an optical path which extends from a projecting element to a photodetecting element. CONSTITUTION:A small hole is formed in the light-transmittable film at a specific position and 10 the photodetecting element 8 and photodetecting element 9 are arranged across the run path for the film. The optical path 11 from the element 8 to the element 9 is slanted to the run passage 10 for the film 1 and light is incident at an angle theta to a perpendicular of the film 1. Consequently, the traveling distance D1 of the light in the film 1 becomes long and the same effect as an increase in the thickness of the film is obtained. Further, the surface and internal surface of the film increase in reflection factor and transmitted light is reduced. Therefore, the light transmissivity of the film decreases apparently and there is a great difference in the quantity of photodetection by the element 8 between the part where the film is present and the part of the same hole, thereby improving the S/N. |
