Patent GB-1472454-A (GB)
Title: Method for determining the position of a focussing plane
Inventor: LEITZ LUDWIG
IPC: G02B7/34
Filed: 1974-06-12
Abstract: 1472454 Photoelectric focus detection ERNST LEITZ WETZLAR GmbH 12 June 1974 [18 June 1973] 26026/74 Heading H4D To determine when an image produced by a lens 1, Fig. 1 is in a required plane, a special frequency filter 2 is positioned in and vibrated in the plane in a direction at right-angles to the divisions of the filter. The contributions 4a<SP>1</SP> and 4a<SP>111</SP> to the image by two spaced sections 1<SP>1</SP> and 1<SP>111</SP> of the lens are detected by respective photoelectric detectors 5. The movement of the filter 2 produces alternating outputs from the two detectors which are in phase if the image is in said plane. Due however to the lateral spacing of the lens sections 1<SP>1</SP> and 1<SP>111</SP>, a "parallax" effect is produced between the two part images falling on the detectors, such that when the image is not in said plane the outputs are out of phase, the sense of the plane difference being indicative of the sense of deviation of the image from the plane. To overcome spurious signals being generated by brightness variations across the image, the filter is in the form of a plurality of prisms having alternate faces 21, 22 which diffract to give two images 4a and 4b both modulated and both having an associated pair of photodetectors 5. Suitable sum and difference combinations Fig. 2 (not shown), of the outputs of the detectors produces an accentuation of the phase varying alternating signals and the cancellation of the spurious signals. A light stop may be incorporated in a prism arrangement in front of each pair of detectors to block off the light from the central strip section 1<SP>11</SP> of the lens as well as directing (refracting) the two part images on to the respective detectors, Fig. 3 (not shown). The lens 1 may provide a plurality of pairs of spaced sections, either linear or semiannular (Figs. 4 and 6, not shown), to give wide base line and narrow base line systems for simultaneous large and small distance measurements respectively. The central section of the lens may be used to give an object movement (parallel to the oscillatory direction of the filter) indication. A zone plate 9, Fig. 8 (details Fig. 9) may be placed in front of the lens 1 to co-operate with three lenses 3a, 3b, 3c in dividing the image into two outer sections 2a, 2b and a middle section 2b, the section 2b including a sub-section 2e. The middle section image 2b is focussed by lens 3b on to two pairs of detectors 5 corresponding to the two pairs of detectors in Fig. 1 and corresponding to an object at a near distance Y. The sub-section image 2e is detected by detectors 5c, 5b and gives velocity information at far distance X. Outer detectors with lenses 3a, 3b give focussing information corresponding to the object at distance X. An aid for a blind person, Fig. 15b incorporates a horizontally directed system using lens 1 with zone plate 9a to give detection of obstacles at two distances and a downwardly directed system using lens 1a and zone plate 9b for floor obstacles, e.g. steps, at two distances, a light source 16 being used for night use. The outputs of the systems are communicated to vibrators actuating buttons 14a-14g, for tactile finger sensing. The lens 1 may provide for spaced sections 1<SP>1</SP>-1<SP>v</SP>, Fig. 15 with the image being circularly scanned over a fixed pyramidal grating 12 by a rotating optical wedge 11. Four pairs of detectors can then be used giving increased accuracy.