| 摘要 |
PURPOSE:To perform focus control for the purpose of keeping the interval between a video disc and a condenser lens constant by detecting the reflection light of a light beam, which is projected onto an information recording medium, by a photo detector including many minute photo detection elements. CONSTITUTION:When the interval between condenser lens 15 and disc 1 is optimum, an irradiation spot of light beam 16 converged optimumly onto disc 1 is generated. The size of the spot of reflection light 16 on photo detector 17 is changed according to change of the interval between disc (1) and condenser lens 15. The number of elements, which receive lights, out of many minute photo detection elements D1-D10 of photo detector 17 is changed by change of the focus state. Many envelope detection circuits 23 and 24 are provided correspondingly to many photo detection elements D1-D10, and these outputs drive moving coil 37 of condenser lens 15. By this constitution, accurate focus control can be performed. |
