| 摘要 |
<p>An objective lens having chromatic aberration is disposed in an optical system for focusing recording and/or reproducing beams upon a surface of an optical disc, and two detecting light beams having different wavelengths are passed through the lens. When the surface of the optical disc is positioned at a focal point of one of the beams, the other beam is not focused on the optical disc. When the surface of the disc is positioned midway between the focal points of the two beams, the two light spots have equal intensity. The light intensity of the two spots is detected, so that focusing errors can be determined based on the difference between the detected intensities.</p> |
