GB564754A - Improvements in distance or range finders - Google Patents

Abstract

564,754. Range-finders. EDEN, J. L. April 5, 1943, No. 5392. [Class 97 (i)] A short home-base range - finder gives two overlapping images of different magnification obtained from two spaced view - points. Since the images are of different sizes, they can coincide at one point only in the field of view, a double image being seen elsewhere. Owing to the relative shift between the two images produced by the parallax effect, the position in the field of view of the poiht at which the two images coincide corresponds to the distance of the object plane, so that observation of this point enables the range of the object to be determined. In the form shown, the images are formed by two spaced lenses 3, 6 of different powers and are viewed through a pinhole 12, that from the lens 3 being seen by reflection from a prism 2 and neutral-tinted glass plate 5, and that from the lens 6 by transmission through the plate 5 and also through a second glass plate 7. An image of a scale 8 is also seen in the field of view by reflection from the plate 7, a negative lens 10 being interposed between the plate and scale. The use of a pinhole eyepiece allows the two images and also the scale to be seen simultaneously in focus. The system is so arranged that the two images of a point or line on a distant object coincide when the instrument is directed so that they appear at one end of the scale image. For nearer object planes the two images will, owing to the parallax effect, undergo a relative shift to an extent dependent upon the nearness of the .object plane, so that, due to the different sizes of the images, the coincidence point will travel across the image field, and its position may be observed in relation to the image of the scale 8. This scale may be an ordinary millimetre scale, a comparative scale on the case of the instrument giving the ranges corresponding to the readings obtained on it. A second viewing pinhole 4 is provided in the end of casing, farther than the pinhole 12 from the corner to which the plane of the glass plate 5 is directed, so that the two images seen through the pinhole 4 are shifted relatively to one another as compared with those seen through the pinhole 12. This allows long and short range readings to be obtained by use of the pinholes 12, 4 respectively. In a modification, the lenses 3, 6 are of the same focal length, the observed images being of different sizes owing to the difference in the optical paths from them to the pinhole. The lens, 3 is arranged between the prism 2 and glass plate 5, and the plate 7 and lens 10 are not provided. The scale 8 is arranged between the lens 3 and prism 2 and may be inclined to the plane of the lens so that the graduations are increasingly magnified towards the end giving the maximum range readings. Alternatively, a mirror may replace the prism 2 and the scale may be engraved on it.