GB995277A - Improvements relating to methods and apparatus for determining image contrast - Google Patents

Abstract

995, 277. Photo-electric determination of image sharpness. LOGETRONICS Inc. April 5, 1962 [April 10, 1961], No.13222/62. Heading G1A. The invention relates to a method and apparatus for determining image sharpness in optical systems, in which an image is projected on the surface of a photoconductive element having a non- linear resistance/light intensity characteristic, the sharpness of the image is changed and the resulting change in the resistance of the element is sensed. In the embodiment shown a lens 18 used to form a real image of an object such as a photographic transparency 12 illuminated by a light source 10, or a real image of a distant object such as a star, on a plane 16, is mounted on a rack 24 which may be moved by a motor 30 or manually by a knob 32. A pair of non-linear responsive photo-cells such as cadmium sulphide cells are positioned so that one, 34, has its active surface above the plane 16 and the other 36 has its surface in the plane 16. The cells are connected in a bridge circuit formed by resistances 38, 40, a battery 42 and an amplifier 48 the output of which may be connected by means of a switch 56 either to the motor 30 or an ammeter 54. When the image is focused in a plane above both cells 34, 36 each will receive diffused light and on the deflection of the meter 54 will be zero as shown by point A on the graph (Fig. 2) the abscissa of which is the position of the plane of focus and the ordinate of which is deflection of the meter or output of the amplifier 48. When the image is focused on the surface of cell 34, cell 36 still receives diffused light and the meter deflection is shown at B. Point C represents the deflection when the image is focused in a plane between the cells so that each receives diffused light and point D is the deflection when the focus is on the plane 16; in this case the cell 34 receives diffused light. Point E is the deflection when the focus is below the plane 16. Thus from the direction and amount of deflection of the meter the direction in which knob 32 must be turned to move the lens 18 to produce a sharp image on the plane 16 or alternatively the output of the amplifier may, by switching it to the motor 30, be used to control the position of the lens. In further embodiments (Figs. 3 and 5 not shown) both cells have their active surfaces on the plane 16 but the photo-cell 36 is prevented from receiving a sharp image by providing a diffusing film or body between it and the light source 10, and in one of the embodiments (Fig. 5) the bridge circuit is modified for use with an A.C. supply. A single non-linear cell may be used to detect image sharpness but the above embodiments are to be preferred since a single cell will not discriminate between changes in focus and changes in light intensity. The apparatus may be used for focusing enlargers or for calibrating the autofocus mechanism of cameras. It may also be used for lens testing to measure the curvature of field by moving the cell through the plane of best focus and also by moving it laterally, taking measurements for a co-ordinate system and plotting them. The apparatus may also be used to detect coincidence between two similar images projected on the plane 16. This may be achieved by placing an opaque strip along a diameter of the lens so that when a condition of coincidental focus occurs two sharp images will coincide producing maximum deflection of the meter 54. The apparatus may be used with ultra violet or infra red light.