| 摘要 |
PURPOSE:To reproduce and erase exactly high density recording information without always depending on a tracking signal, by changing a spot shape of an irradiated light. CONSTITUTION:A photodetector 61 is provided with four split photodetecting sensors F1-F4, and detects a spot shape of an optical beam which is irradiated to a position of a point P of an optical card 2. For instance, in case of a spot of a true circle, it can be discriminated by constituting it in advance so as to become equal to each photodetecting quantity of the photodetecting sensors F1-F4, and by providing furthermore plural pieces of photodetecting sensors, a complicated spot shape can also be detected. An optical spot discriminating circuit 62 discriminates a shape of an optical spot which is photodetected by the photodetector 61, by detecting photodetecting quantities F1p-F4p of the photodetecting sensors F1-F4, by which a condition of an irradiated spot of the point P of the optical card 2 is known, although it is indirect. Also, when it is detected that the irradiated spot of the point P is not a necessary shape, a spot shape control output 63 is generated, and the constitution of an objective lens 8 is varied so as to conform to an object condition. |
