| 摘要 |
PURPOSE:To detect a focus with a high precision without restrictions on detection width, by illuminating a recording face with a luminus flux whose section has a shape of a narrow-width ring and utilizing the fact that the reflected luminous flux is shifted to the outside or the inside of the incident luminous flux. CONSTITUTION:The luminous flux from a light source 1 passes through a transmission part of a mirror 2 having the transmisson part in the center and is reflected on a mirror having a transmission part in the center and is condensed onto a recording face 5 by an object lens 4. Mirrors 2 and 3 have elliptic transmission areas, whose projections to a plane vertical to the optical axis are round, in centers of specular surfaces. When the recording face 5 approaches the object lens 4, the section of the luminous flux is enlarged, and therefore, the luminous flux is incident to the specular surface part of the mirror 2 and is incident to a detector 6. When the recording face 5 receds from the object lens 4, the section of the reflected luminous flux is reduced, and the luminous flux is incident to a detector 7. Consequently, the focus is detected by outputs of detectors 6 and 7. |
