| 摘要 |
PURPOSE:To accurately perform the recording and reproducing even when there are remarkable variations in the scale of birefringence which belongs to a recording medium by providing two kinds of cells with a different light receiving area from each other to a photodetector and providing a changeover device to correspond to the operation and nonoperation of a focus control device for inputting an output of the above photodetector to a birefringence control device. CONSTITUTION:In the case of the nonoperation of the focus control, the light receiving area of the photodetector 8 is increased, whereas in the case of the operation of the focus control, the light receiving area is decreased, thus controlling a wavelength plate (metamorphous Berek type birefringent plate) 9 based on their respective light receiving quantities. Consequently, the control of the wavelength plate 9 is feasible even under the state of unoperability of the focus control without focusing. By this method, even for an optical card having variations in the quantity of birefringence, the necessary quantity of light for reproducing can be obtained, while the quantity of birefringence can be corrected, no matter whether the focus control is under operation or nonoperation. |
