| 摘要 |
PURPOSE:To execute with a high accuracy a focus control by an inexpensive and light constitution by forming an optical means by placing correspondingly many absorbing parts for absorbing a light by shifting the position, on both faces of a parallel face plate for making a light transmit through. CONSTITUTION:A luminous flux is made incident on a disk 15 through an objective lens 14, and its reflected light is led to a photodetector 17 through an optical means 16. The optical means 16 is formed by placing an absorbing part 16a which does not make a light transmit through by shifting the position, on both faces of a transmitting part 16b of a parallel face plate for making a light transmit through, and a line for connecting the end parts of a pair of absorbing parts 16a is inclined by a prescribed angle against the optical axis. When a disk 15 is focused, parallel rays are made incident on the optical means 16, and when they approach the lens 14, they become a diffused light, a transmitting light is made incident mostly on the right half 17b of the photodetector, and when it goes away from the lens, it becomes a converged light and made incidently mostly on the left half 17a. By detecting this difference by an amplifier 18, a focused state is detected, and the detection accuracy can be raised by an inexpensive and light structure. |
