Patent GB-1324323-A (GB)
Title: Automatic focusing of an optical image
IPC: G02B27/40
Filed: 1972-01-27
Abstract: 1324323 Automatic focusing control IMAGE ANALYSING COMPUTERS Ltd 27 Jan 1972 [5 Feb 1971] 3994/71 Heading G1A A system for automatically obtaining a correctly focused image in an optical system, for example, a microscope, of a specimen mounted on a carrier, comprises a surface on the carrier to reflect the images of two objects, spaced by different amounts from the reflecting surface, onto a photo-sensitive device from which are obtained by line scanning two separate electrical signals whose magnitudes are respectively indicative of the sharpness of focus of the two object images, the said sharpness being determined by the spacing between the carrier and the optical system, the relative positions of the two objects and the photo-sensitive device being such that the two object images are equally de-focused when the spacing between the carrier and the optical system gives a correctly focused image of the specimen, circuit means for generating an electrical error signal whose magnitude is proportional to any difference between the magnitudes of said two electrical signals, and focus adjusting means operable in response to the error signal to alter the spacing between the carrier and the optical system in a manner to reduce the error signal magnitude. Two microscope arrangements are described, one for opaque specimens Fig. 1 (not shown) and one for translucent specimens Fig. 6 (not shown), each using a grating arrangement as in Fig. 2. A rectangular field stop 24, which is mounted to move with the optical system of the microscope, is arranged so that it is accurately focused on the reflective surface (for example, the specimen itself) when the microscope is correctly focused, and "gratings" 28 and 30 are provided on respective sides of the stop 24 such that their images are then equally out of focus at the specimen. Each "grating" 28, 30 comprises an aperture almost as large as that of stop 24 but having alternate slits 32 and gaps 33 mutually staggered along one edge such that slits 32 of grating 28 are visible through gaps 33 of grating 30, and vice-versa. Line scanning at the edge of the field can then, by suitable grating, produce electrical signals indicative of the state of focus of the respective slits 32. In the case of opaque specimens the whole field is scanned by a television camera 11 Fig. 4 whose output is gated at 23 and 25 to separate the scans of slits 32 of the two grating images from the picture content and from each other. The resultant outputs are fed to peak white detectors 27 and 29 which feed a difference amplifier 31 providing an error signal when its inputs differ, and this drives a focusing motor (not shown) by one small increment per frame scan. If the gratings are so far out of focus as to give a peak white level below a threshold I an out-of-range signal causes the focusing motor to scan rapidly over its entire range. When the difference signal is less than a threshold II, defining a dead band, the signal is cancelled and focus is held. If the error signal exceeds a threshold III an out-of-focus warning signal is generated and scanning stopped. In the case of translucent specimens separate light sources 50 and 48 Fig. 7 of different wavelengths are used for the transmitted illumination and for the formation of the grating images respectively. The grating images are reflected by a surface on the specimen carrier 42, preferably by a wavelength selective coating on a glass surface in contact with the specimen. Filters (52, 58) Fig. 6 (not shown) and/or a dichroic mirror (56) are used to separate the wavelengths, one of which may be infra-red or ultra-violet. As shown, separate scanning cameras 66 and 64 are used for picture content and focusing detection, respectively, and the output from camera 64 is merely the signals from the two grating images which are separated by gates 74 and 76 and their high-frequency contents compared to provide focus control. Peak white (or peak black) detectors, as 27, 29 of Fig. 4, may be used instead of high-pass filters 78, 80.