Patent GB-585821-A (GB)
Title: Improvements in or relating to range finders
IPC: G01C3/16
Filed: 1944-04-06
Abstract: 585,821. Range - finders. INTERNATIONAL POLAROID' CORPORATION. April 6, 1944, No. 6517. Convention date, March 17, 1943. [Class 97 (i)] In a stereoscopic range-finder, means are provided for forming double images, one for each eye of the observer, of a single graticule. These images are superimposed on the field of view and means are provided for blocking from each eye the image intended to be seen by the other eye. In the form shown in Figs. 2 and 3, the graticule comprises a light source 30 consisting of a vertical line filament positioned at the focus of a collimating lens 32 of diameter at least as great as the actual interocular distance of the observer. In front of the graticule is mounted a birefrigent uniaxial double-image-forming element 34 such as a calcite crystal or a Wollaston or Rochon prism. Element 34 provides two oppositely polarized images and when rotated about the axis passing between the centre of the lens and its focus, the image formed by the ordinary ray remains stationary whilst that formed by the extraordinary ray rotates about the first image. The linearly polarized images are preferably converted into circularly polarized images, one clockwise and the other counterclockwise, by a quarter-wave retardation plate 35. Elements 34, 35 are mounted in a frame 36 for rotation together by the gearing shown in Fig. 3. In order that only the relative lateral movement of the images may be observed a mask 38 having a horizontal slot 40 of such depth that it is always filled by portions of the images is positioned between the frame 36 and the lens. The circularly polarized light is analysed by elements 42 each comprising a linearly polarizing element having bonded thereto a quarter-wave retardation element having a principal vibration direction at 45 degrees to the transmission axis of the linear polarizer, one blocking clockwise circularly polarized light and the other counter-clockwise. A single quarter-wave element positioned in the slot 40 may be used in place of the two quarter-wave elements bonded to the linear elements of the analysers 42. The images are then seen through the transparent mirrors 25 in the eyepieces 26 in which the field provided by the mirrors 22, 25 is seen. The pointer 54 associated with the gearing for rotating the frame 36 works over a range scale 55 which indicates a minimum when the images are most widely separated and a maximum when they are substantially superimposed. The range-finder is adjusted so that the stereoscopic image of the graticule is in line with the image of the object being ranged on ; the knob 50 is then rotated until the image appears to lie in the plane of the object being ranged on. In the form shown in Fig. 8, the graticule 62 comprises an opaque plate having an outline shape etched thereon. Elements 64, 65 correspond to elements 34, 35 of Fig. 2, whilst elements 66 and 68 comprise respectively a quarter-wave plate and calcite crystal so positioned with respect to their axes and the axes of elements 64, 65 that they resolve the two circularly polarized beams transmitted by element 65 into two linearly polarized beams of opposite polarization. A third quarter-wave plate 70 reconverts into circularly polarized beams and circular polarizers 74 are provided corresponding to elements 42. Elements 64, 65 are rotatable together whilst elements 66, 68, 70 are rotatable together but in the opposite direction. In Fig. 10, wedge prisms 85 are mounted for rotation in opposite directions and a septum 88 for ensuring that each eye sees only one image may be provided. The invention may be applied to a rangefinder not provided with means for increasing the human interocular distance.