| 摘要 |
PURPOSE:To improve the detection sensitivity, by shielding a photodetector for focus detection from 60-95% reflected luminous flux for focus detection including the light on the optical axis in the focus detecting method of the knife edge system. CONSTITUTION:The reflected light from a disc 6 is transmitted through a 1/4 wavelength plate 4, a deflecting beam splitter 3, and a converging lens system 7 and is converged to a photodetector 8. A light shielding plate 9 is so provided that the photodetector 8 is shielded from 50% converged luminous flux. Photoelectric conversion signals SA and SB are taken out from light receiving parts 8A and 8B of the photodetector 8 in this state, and the control is so performed that SA-SB=0 is true to converge a laser light onto a disc 6 correctly; and in this focus detection of the knife edge system, the detection sensitivity is not always high. Therefore, the photodetector 8 is shielded from 60-95% luminous flux including the light on the optical axis by the light shielding plate 9 to improve the detection sensitivity. Thus, the light shielding rate is improved considerably up to 70-85%. |
