| 摘要 |
PURPOSE:To obtain an inexpensive optical video disc device having excellent light use efficiency by tracking in accordance with the difference between output signals of two reflected diffraction light detectors and automatically focusing and positioning in accordance with the sum of output signals. CONSTITUTION:When a laser luminous flux 2 is condensed to an information track 6 by a condenser lens 4, the result becomes reflected diffraction lights 101 and 102 by the edge of the track and is converged to a convergence point 11. Photodetectors 91 and 92 are set in positions other than the convergence point 11 in the reflected laser luminous flux to detect the difference between output signals of detectors 91 and 92, and the condenser lens is moved in the direction of an arrow, thereby performing tracking. The laser luminous flux is converged correctly to the information track 6 in the position of a disc 5. In case of the position of a disc 51, the convergence point is shifted to a point 111, and the sum of output signals of photodetectors 91 and 92 is smaller; and in case of the position of a disc 52, the sum of output signals becomes larger. The condenser lens 4 is moved in the direction of the arrow with respect to the disc 5 in accordance with this change of the sum of output signals, thereby performing auto- focusing. |
