| 摘要 |
PURPOSE:To provide high sensitivity for the out of focus near the focusing point by deviating the optical axis of a lens of detection system with respect to the optical axis of an objective lens. CONSTITUTION:The reflected light from the recording layer or reflection layer 10 of a recording medium passes through an objective lens 11 and is made into the light parallel with a main beam 1 when the focus is matched. The parallel light are condensed through a lens 12 of detection system deviated from the optical axis on a photodetector 5. Then, alpha1=beta1 delta1=epsilon1 hold, the light is condensed on the blind body 9 between the two photodetection cells 3, 4 of the photodetector 5, resulting in the decreased detection sensitivity. When the recording layer or reflection layer 10 approaches slightly to an objective lens 11, nu2>zeta2=xsi2>mu2 holds, and when the layer parts therefrom, nu3<zeta3=xsi3<mu3 holds, thus the light spot spreads asymmetrically to the right and left, resulting in the increased detection sensitivity. The high detection sensitivity for the slight out of focus near the focusing point is thus obtd. |
