| 摘要 |
PURPOSE:To satisfactorily detect a focus distortion due to displacement of the information recording surface, by using a light detector whose light receiving surface has been divided concentric circles. CONSTITUTION:The light receiving surface of a light detector 5 is divided into two concnetric circles, when light has been received, each divided surface generates a signal output depending upon the quantity of irradiated light, and the diameter of projected light is discriminated according to the output difference. And in case the focus is matched on the information recording surface, the differential output is adjusted to zero by projecting regular light onto the light detector 5. Thus the focus status which is always satisfactory can be obtained by moving the condensing lens 2 in its axis direction in accordance with the differential output depending upon distortion of the focus. |
