| 摘要 |
PURPOSE:To condense a light beam to the smallest spot at all times on the surface of a recording medium even when an offset is generated in a focus controlling system, by applying the signal proportional to the quantity of the light reflected from the recording medium to the focus controlling system. CONSTITUTION:The output signals from the two elements on the diagonal lines of quadrisected photosensors 29 among the signals detected with the photosensors 29 are added to each other by adders 30 and 31, and the differences between these signals are obtained with a differential amplifier 37, whereby a focus error signal is detected. All the four output signals of the sensors 29 are added by an adder 32, and the added signal is added to or subtracted from the focus error signal in an adder 35 via an amplifier 34. The total sum of the four signals is proportional to the intensity of the light reflected and returned from the medium surface. Therefore, even if the reflectance of the surface of a recording medium changes, the offset of the focus control system is eliminated. |
