Patent GB-1468679-A (GB)
Title: Method of and apparatus for foccussing optical imaging means
IPC: G02B7/34
Filed: 1973-12-04
Abstract: 1468679 Determining focal point photoelectrically ERNEST LEITZ WETZLAR GmbH 4 Dec 1973 [11 Dec 1972] 56165/73 Heading G1A [Also in Divisions G3 and H4] A method of focusing optical imaging means having an optical axis and an entrance pupil, comprises: disposing image-splitting means to provide next adjacently to each other multiple images of the entrance pupil; arranging first and second photo-electric receiver means so that each receives components of light passing through a respective part of one of the images and through an analogously different part of the or each other of the images of the entrance pupil, to generate respective electrical signals corresponding thereto, comparing the electrical signals; and using the comparison to adjust the focusing of the imaging means in accordance with the magnitude and sign of the comparison signal. An optical focusing device comprises: optical imaging means provided with an optical axis and with an entrance pupil; image-splitting optical means to provide next adjacently to each other multiple images of the entrance pupil; first and second photo-electric receiver means arranged so that each receives components of light passing through a respective part of one of the images and through an analogously different part of the or each other of the images of the entrance pupil, to generate respective electrical signals corresponding thereto; means to generate a comparison signal; and means responsive to the comparison signal to adjust the focusing of the imaging means. An image-splitting prism 10, Fig. 1, having a corner 21 intersecting the optical axis of a lens 1" at a predetermined focal plane, so refracts light from the entrance pupil falling upon prism faces on opposite sides of the optical axis as to form two images, each having a respectively different part adjacent the optical axis and a complementary part remote therefrom. As illustrated when viewing a point object (not shown), a ray focused at a point  3 in front of the predetermined focal plane containing the edge 21 is refracted to pass close to the optical axis. On the other hand, a ray focused on a point 13 behind the predetermined plane is refracted so as to deviate from the axis. A beam-splitting cube 11, having complementary masks 14, 12 on its exit faces, directs rays focused at the point 3 or 14 to a respective photo-cell 20 or 19. The difference between the photo-cell signal outputs is used to determine the magnitude and sense of any required correction to the lens position 1", possibly applied by servo means. Where the object extends in a field of view, as with a camera, light focused at different regions of the image is directed by a plurality of segments of a triangular or square pyramid raster 10' Fig. 2. To permit visual observation the beam-splitter cube may take the form of a polarizing beam divider (11') Fig. 3 (not shown), the photo-cells (19, 20) viewing transversely, and the eye along, the optical axis. The central light stop is replaced by an outer reflector (12') in front of an eye-piece (25) and separated from the cube (11') by a #/4 plate (24). A differential receiver group 27, Fig. 5 comprises central photo-cell elements 28 each located behind a respective apex of a pyramid raster 10" and each surrounded by an outer photo-cell element 29. The combined outputs of elements 28 may be compared with those of elements 29.