| 摘要 |
PURPOSE:To prevent a projection spot from increasing in diameter and to eliminate defocusing due to the missing of a light receiving spot by varying the focal length of a projection lens optionally according to object distance. CONSTITUTION:The diameter of the projection spot is proportional to L/f, where L is the distance from the projection lens 2c to an object and (f) is the focal length of the projection lens 2c. Consequently, even if the object distance L increases, the projection spot is prevented from increasing diameter by increasing the focal length (f). For the purpose, a lens barrel body 6 which can turns around a shaft 5a is provided in front of the projection lens 2c and lenses 2a and 2b are stored therein and put on an optical axis when necessary. Therefore, even if the object distance varies, the lens focal length is varied, so the photodetection spot is prevented from increasing in diameter, thereby preventing defocusing due to the missing. |
