Patent GB-723910-A (GB)
Title: Improvements relating to optical viewing instruments
IPC: G01S7/04
Filed: 1953-03-06
Abstract: 723,910. Optical viewing apparatus. BARR & STROUD, Ltd., DICKSON, J. H., and BUCKINGHAM, J. March 6, 1953 [March 14, 1952], No. 6751/52. Class 97 (1). [Also in Group XL (c)] Images of a chart and of a radar plan position indicator (P.P.I.) are viewed in superposition by two optical systems, that for viewing the P.P.I. being either a telescope or a simple system in which the image is viewed directly without lenses through a beam-splitting prism, and that for viewing the chart being a telescope of variable magnification for changing the magnification of the chart image whilst maintaining its position constant. As shown in Figs. 2 and 3 the cathode-ray tube 20 on which the P.P.I. display is formed is connected by a screen 21 to a casing 22 containing the optical system. Light from the tube 20 reaches the eyepiece 23 by way of objectives 29, prism 30, collector lens 31, projector lens 32, prism 33, semi-transparent reflecting surface 35 and field lens 34. The surface 35 is in the form of an interference coating. The chart with which the P.P.I. is to be compared is located below the casing and an image of it is formed in the eyepiece by way of objectives 54, 53 (not shown) focused by means of a helical slot 29a, a roof prism 36, collector lens 37, prism 38, projector lens 39 and field lens 34. A filter disc 56 is provided in this beam to bring the two images seen in the eyepiece to the same intensity. To adjust the scale of the chart image to be the same as that of the P.P.I. image the lens 39 is connected in a holder 39a carried on a rack 42 moved axially of the eyepiece by a knob 40, the image of the chart being maintained in focus by the simultaneous movement of the prism 38 effected by a cam 52 rotated through gearing 43 from the rack 42. A projector 63 is provided to project a spot of light on the chart at its intersection with the axis of the objectives 53 and 54. A head rest 24, Fig. 2, having apertures 24a, 24b which may register alternatively with the eyepiece is provided to allow the observer to view either with the left or right eye, a pivoted shutter 62 obscuring the view of the other eye. Alternatively the cathode-ray tube 20 may be located before apertures 70 or 71 in the casing, suitable prism systems being provided to direct the light to the lens 31. In the modification shown in Fig. 9 the P.P.I. tube 74 is viewed directly through the semireflecting surface of the prism 77 while light from the chart passes objectives 78 and 79 and is reflected into the eyepiece by a prism 80, lenses 82 and 83 and the prism 77. The lens 79 and the prism 80 are simultaneously moved by a suitable rack and cam arrangement to change the magnification of the chart image, the intensity of which is varied by rotation of a filter disc 81. Alternatively the reflectivity of the internal surface of the prism 77 may vary over its surface, the intensity of the chart image being adjusted by movement of the prism 77 to bring different portions of that surface into use. In a further modification the P.P.I. tube is located within the casing 22.