| 摘要 |
PURPOSE:To prevent unstable operation with a simplified circuit in an optical reader which is disposed with a photodetector having a photodetection surface divided to multiple parts on the optical axis of a focusing lens and is disposed with a knife edge for shielding a part of the light transmitting the focusing lens by projecting the knife edge up to the position inclusive of the optical axis. CONSTITUTION:A knife edge 11 is projected up to the position inclusive of the optical axis 12 of a focusing lens L. Then, the light (the light reflected by a disc 10 in the positions A, B, C) focusing on the optical axis is always partly shielded by the outside of the knife edge 11 and the light (the light reflected by the disc 10 in the position D) focusing between the edge 11 and the lens L is mostly shielded by the edge 11 so that the output difference of the photodetector 7 is made to 0 voltage only at A where the micro-spot light is formed and an S- shaped characteristic intersecting with the 0 voltage at a large gradient is obtd. A focusing position is therefore detected only by the zero cross detection circuit and a drawing control circuit is simplified. Since the edge 11 is required merely to be projected so as to include the optical axis, the assembly and adjustment are easy. |
