| 摘要 |
PURPOSE:To limit the change of distance between two photodetectors arranged in parallel to each other within a prescribed range regardless of the change of ambient temperature, by keeping the distance between the optical axes at a certain value by utilizing the difference of the linear expansion rate among different types of metals. CONSTITUTION:The upper plate 101 containing the lens 61 and the photodetector 71 is fixed to the middle plate 14 at an end with the screw 13 with the other end pressed onto the plate 14 with the plate spring 151 which is directly provided near the center of the plate 14. This spring 15 is attached on the plate 101, and a gap is provided between the spring 15 and the plate 101 to absorb the expansion and contraction caused by the heat expansion. At the same time, the spring 15 has a gap to absorb the expansion and contraction due to the heat expansion or vibration. The reading part 82 is provided in an exactly same way as the reading part 81 mentioned above. The upper plates 101 and 102 are made of the material having a larger coefficient of heat expansion than the plate 14, and the size of each reading part is set to satisfy the prescribed conditions. As a result, the distance between the optical axes is kept constant even though the reading part expands or contracts by the ambient temperatures. |
