| 摘要 |
PURPOSE:To eliminate unevenness in light quantity even when an object lens moves in a wide range and to make the readout of signals from tracks of a wide extent possible by means of the operation of an actuator only so as to make accessing time shorter by setting the diameter of the object lens at a value which is specific times as large as the movable distance of the object lens. CONSTITUTION:The diameter of an object lens 1 is set at a value which is larger than the movable distance of the lens 1 by 2-10 times. As a result, even when the object lens 1 moves leftward and rightward to a large extent, the light quantity does not change and a wide extent becomes accessible. The optimum value exists in the ratio of the diameter of the lens 1 to the movable distance of the lens and it is known from experiments carried out by using an actual device that, when the lens diameter is 2-10 times as large as the movable distance of the lens, excellent results are obtained. Therefore, the movable extent of the objective lens can be made wider with a simple constitution and the range of the tracks which are accessible by means of the operation of an actuator 2 only is widened. As a result, the accessing time of a driving device is shortened. |
