| 摘要 |
PROBLEM TO BE SOLVED: To simplify the information reproduction of recording media varying in the thickness of transparent substrates by setting optical characteristics according to the thinness of the transparent substrates in an objective lens for condensing the light from a semiconductor laser as a light spot to a recording surface. SOLUTION: The luminous flux peripheral part of a return luminous flux and, more particularly, the part where an island-shaped part 2a occurs is shielded for a photodetector 8 for generating a reproduced signal by the part on the outer side of the opening 9A of an aperture means 9, by which the decrease in the amplitude of the intensity of the photodetecting signal by the 'out-of-focus' of the light spot SP is compensated. In such a case, the intensity of the photodetecting signal from the photodetector 8 when the aperture means 9 is used is effectively increased in its amplitude according to the positional relation between the center of the light spot SP and the mark and the sure detection of the mark is made possible. |
