| 摘要 |
An image is formed on a plane by light passing through a lens. The lens and the plane are movable relative to each other so as to focus the image. A two-dimensional diffraction grating, preferably of the phase grating type, is disposed in the optical path at a distance spaced away from the plane. A scanning mechanism is provided including a plate disposed between the diffraction grating and the plane, and having a slit therethrough. Light passing through the diffraction grating and through the slit impinges upon a photoelectric element, preferably a CdS cell exhibiting a dip effect, which provides an electrical signal used in a positioning mechanism that provides for maximizing the contrast of the image formed by the light passing through the diffraction grating and impinging on the photoelectric element so as to bring the image into focus. |
