| 摘要 |
PURPOSE:To increase the detected sensitivity and the SN ratio of a demodulated electric signal, by comparing the intensity of light between photodetectors and discriminating whether the image forming plane is front/rear focus or in focus for focusing. CONSTITUTION:A light emitted and converged from a condenser lens 15 is split into two at a luminous flux splitter 16, one light passing through the splitter 16 is received at a photodetector 17 provided in front of the focus of the lens 15, the other light reflected at the splitter 16 is received at a photodetector 18 provided at the back of the focus of the lens 15, the light including the optical information at the photodetectors 17, 18 is detected as the light intensity value and converted into the electric signal. The photodetecting plane receiving the light of the photodetectors 17, 18, i.e., the photodetection diameter is made small since the light received on the photodetectors 17, 18 is converged on the lens 15, then the detection sensitivity of the photodetectors 17, 18 is increased. The light intensity of the two photodetectors 17, 18 is compared and the objective lens 5 is moved for focusing so as to obtain equal light intensity of the photodetectors 17 and 18. |
