| 摘要 |
PURPOSE:To obtain a stable focal point detector having the reduced effect of diffraction even when a light spot crosses a guide groove for retrieval, etc., by providing a photodetector at an image forming position in the direction rectangular to the extending direction of the guide groove for convergent light passed through a cylindrical lens. CONSTITUTION:A flat/convex cylindrical lens 20 is set to receive convergent light 14 sent from an automatic focus detecting lens 13 and also set to focus a luminous flux in the direction parallel to the extending direction (X) of a guide groove 8. Then a concentric circle 2-split photodetector 16 is provided at an image forming position in the direction rectangular to the extending direction of the groove 8 for the convergent light passed through the lens 20, i.e., at a focal position 21. Thus the light distribution becomes linear. Then the outputs of photodeteting parts of both inner and outer sides 16a and 16b are supplied to a differential amplifier 18. Then the characteristics can be obtained for a sensor making use of the difference between the center part and both end parts of the light 14. |
