| 摘要 |
PURPOSE:To detect automatically the gap between an object lens and a disc with a simple structure by receiving the reflected light, which is on the outside of an incident luminous flux to an object lens, from the disc by a photodetector. CONSTITUTION:An incident luminous flux A of a laser passes an object lens 8 and forms a minute spot on a disc 9, and a reflected luminous flux B passes an optical system, and video information is read by a detector. The luminous flux B passes the same path as the luminous flux A if the lens 8 is focused on the disc 9, but the luminous flux B is spread conically and is detected by a photodetector 29 if the lens 8 is nearer to the disc than the regular position. Thus, a gap l between the lens 8 and the disc 9 is detected automatically by the easy method, and the collision between the lens 8 and the disc is prevented to improve the safety even if the lens 8 tries to approach the disc from the regular position because the in-focus state cannot be attained. |
