Patent GB-1384485-A (GB)
Title: Method of and apparatus for locating the plane of sharp focussing of an image
IPC: G02B7/34
Filed: 1973-06-19
Abstract: 1384485 Utilizing photo-electric detectors ERNST LEITZ GmbH 19 June 1973 [19 June 1972] 28981/73 Heading G1A The plane of sharp focusing of an image formed by an imaging system 1 is located by employing a scattering element 3 arranged to scatter image radiation incident thereon, the scattering element being dimensioned to have at least one local frequency within a range corresponding to a range of local frequencies present in the image, generating electrical signals from at least part of the scattered image radiation, combining these signals to derive an output signal, and controlling the spacing between the imaging system and the scattering element so as to cause the output signal to adopt a maximum value. Light from a source (not shown) and carrying information from a desired object plane (not shown) passes through the system 1 to the scattering element 3 which is movable perpendicularly to the optical axis by a drive system 3'. Two photoelectric detectors 4, 5 disposed symmetrically about the optical axis provide the electrical signals which are combined in a comparator 6 to form signals indicative of their difference or quotient in such manner that an electrical output signal dependent upon the difference in focusing between the image and the scattering element is obtained. The sign of the output signal is obtained by the modulation of the combined signals from detectors 4, 5 with a reference signal derived from the drive means 3'. The output signal is applied to a servo element 2 which controls the spacing between the system 1 and the element 3. The scattering element may be moved in two co-ordinate directions perpendicular to each other, in which case further photo-electric detectors with comparators are provided. The beam components from the scattering element are either fed simultaneously to two detectors 4, 5, as shown, or alternately to a single detector.  Preferably the scattering element comprises a diffraction grating constructed to introduce phase differences between radiation components passing through different portions thereof. A frequency filter may be employed to suppress frequency components outside the spectral range of the local frequency components present in the image radiation. For a better separation of the electrical signals from interfering signal components and to obtain a sign of the deviation, the signals may be electrially cross-correlated with a further signal indicative of the sum of the signals obtained from the detectors 4, 5, a push-pull signal being derived by means of geometric or physical optical beam-splitters and further photo-electric detectors, and the difference between the sum signal and the push-pull signal being obtained. The system 1 may incorporate 1, 2 or 4.identical objectives, and a diaphragm having a plurality of pairs of eye-shaped crescent elements of equal size, the crescent elements being disposed symmetrically about the optical axis and the curved surfaces of the two elements of each pair lying against the periphery of a circular disc of the same radius as the curved surfaces. A read-out device for the distance measurement value of an object from a part of the apparatus may be connected with a component forming part of the servo device 2.