Patent GB-1448614-A (GB)
Title: Focus detecting apparatus
IPC: G02B7/36
Filed: 1974-07-15
Abstract: 1448614 Automatic focusing HONEYWELL Inc 15 July 1974 [18 July 1973 (2)] 31184/74 Addition to 1448424 Heading H4D In an automatic focusing system, Fig. 1, images of the scene to be focused are projected by lenses 25, 27 and 47 on to a first array of detectors 1, a second array of detectors 3 and the photographic film 49, the outputs of the dectors of the first array being compared in unit 34 with the outputs of the detectors of the second array on a sequential, successive basis, to give a signal F for causing a control 45 to move lens 25 along the array 1 until the signal F indicates that the images on the arrays are the same, the lens 47 being so mechanically coupled to the lens 25 for movement to and from the film 49 that the image becomes focused on the film when said equality of array images is achieved. The comparisons are switched by two antiphase squarewave signals 01, 02 from an oscillator 39. When for example signal 01 is at -V, detectors 13, 17, 21 will be coupled via forwardly biased diodes 55, 59, 63 to the outputs of detectors 5, 7 and 9. Since the detectors are all photoresistive, and the sides of the detectors in both arrays not connected to the diodes are connected to a +V source (array 1) and a -V source (array 3), the junction points 29, 31, 33 have potentials dependent on the relative resistances and illumination of the detectors. Assuming that the scene has a bright upper half and a dull lower half, the images on the arrays for an infocus condition will be as shown in Fig. 2A. Detector 5 has the same illumination as detector 13 and signal A (junction 29), Fig. 2B, is at zero potential. Similarly detector 9 has the same illumination as detector 21, giving signal C at zero potential. Detector 7 is more illuminated than detector 17 such that signal B has a net positive potential. Signal 02 now becomes - V so that the comparison via the forwardly biased idodes 57, 61, 65 is between detectors 5, 7, 9 and detectors 15, 19, 23 respectively. The signals A, and C are at zero potential as before, but now detector 19 has a lower resistance than detector 7 such that signal 3 is at a net negative potential. The signal A and C thus proceed at zero volts whereas signal B switches from equal positive to a negative voltage at the rate of the 01 signal. Unit 35 performs a full wave rectification of A, B, C and combines the results to give the signal C at a positive potential but having no ripple. Phase detector 37, referenced by the signal 01, thus gives a zero potential output F for the control signal whereby no focus change is effected. When the scene is too near, the image on array 1 moves up with respect to that on array 3 (Fig. 2C) such that detector 7 is now more than half illuminated. In the resultant comparison, signals A and C (Fig. 2D) are again at zero potential, but signal B has an average positive value. This results in the rectified sum signal D having a ripple in antiphase with the signal 01, yielding a control signal F having a positive potential, causing downwards movement of lens 25 and outward movement of lens 47. When the scene is too far, the image on array 1 moves down, and the signals of Fig. 2F are obtained. The diodes may be replaced by FETs and four detectors of the first array may be alternately compared with the 1st, 2nd, 3rd, 4th and the 2nd, 3rd, 4th, 5th detectors of the second array, Fig. 3 (not shown).