| 摘要 |
PURPOSE:To detect a focus accurately with simple constitution, by inserting an interference filter, which has the peak of transmittance for the wavelength of a light source, on the optical path between an object lens and a photodetector. CONSTITUTION:In the focusing state, a parallel light is incident to an interference filter 12 because the reflected light from a disc 6 is the parallel light, and the reflected light is not affected at all in this filter 12 and is incident to two photodetectors 10 and 11 equally. Consequently, the difference signal from two photodetectors 10 and 11 becomes zero, and it is detected that an object lens 5 is focused on the surface of the disc 6. If the disc 6 approaches the object lens 5 or gets away from the object lens 5, a difference is generated between outputs of two photodetectors 10 and 11. |
