BerlinĬoincidence vs stereoscopic rangefinders WW1 Coincidence rangefinder at Atlantikwall Raversyde, Belgium. The second eyepiece showed the operator a range scale so the user could range and read the range scale simultaneously. Coincidence rangefinders made by Barr and Stroud used two eyepieces, and may be confused with stereoscopic units. The degree of rotation of the compensator determines the range to the target by simple triangulation. At this point the images are said to be in coincidence. Therefore, in one arm of the instrument a compensator is adjusted by the operator to tilt the beam until the two images match. Since either beam enters the instrument at a slightly different angle the resulting image, if unaltered, will appear blurry. This reflected beam first passes through an objective lens and is then merged with the beam of the opposing side with an ocular prism sub-assembly to form two images of the target which are viewed by the observer through the eyepiece. The optical bar is ideally made from a material with a low coefficient of thermal expansion so that optical path lengths do not change significantly with temperature. At either side the incident beam is reflected to the center of the optical bar by a pentaprism. Light from the target enters the rangefinder through two windows located at either end of the instrument. The coincidence rangefinder uses a single eyepiece. Usage Eyepiece image of a naval rangefinder, showing the displaced image when not yet adjusted for range Photographic rangefinders were initially accessories, from which the distance read off could be transferred to the camera's focusing mechanism later they were built into rangefinder cameras, so that the image was in focus when the images were made to coincide. Optical rangefinders using this principle, while applicable to several purposes, were widely used for military purposes-determining the range of a target-and for photographic use, determining the distance of a subject to photograph to allow focusing on it. By rotating the prisms in opposite directions using a differential gear, a degree of horizontal displacement of the image can be achieved. Two prism wedges which, when aligned result in no deviation of the light, are inserted into the light path of one of the two lenses. The device consists of a long tube with a forward-facing lens at each end and an operator eyepiece in the center. The two can normally be distinguished at a glance by the number of eyepieces.ĭesign Schematic diagram of a coincidence range finder They were also used in rangefinder cameras.Ī stereoscopic telemeter looks similar, but has two eyepieces and uses a different principle, based on binocular vision. Coincidence rangefinders were important elements of fire control systems for long-range naval guns and land-based coastal artillery circa 1890–1960. There are subtypes split-image telemeter, inverted image, or double-image telemeter with different principles how two images in a single ocular are compared. Optical ranging devices American soldiers using a coincidence rangefinder with its distinctive single eyepiece during army maneuvers in the 1940s.Ī coincidence rangefinder or coincidence telemeter is a type of rangefinder that uses mechanical and optical principles to allow an operator to determine the distance to a visible object.
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