A coincidence rangefinder or coincidence telemeter is a type of

rangefinder A rangefinder (also rangefinding telemeter, depending on the context) is a device used to Length measurement, measure distances to remote objects. Originally optical devices used in surveying, they soon found applications in other fields, suc ...

that uses the principle of

triangulation In trigonometry and geometry, triangulation is the process of determining the location of a point by forming triangles to the point from known points. Applications In surveying Specifically in surveying, triangulation involves only angle m ...

and an optical device to allow an operator to determine the distance to a visible object. There are subtypes split-image telemeter, inverted image, or double-image telemeter with different principles how two images in a single ocular are compared. Coincidence rangefinders were important elements of fire control systems for long-range

naval gun Naval artillery is artillery mounted on a warship, originally used only for naval warfare and then subsequently used for more specialized roles in surface warfare such as naval gunfire support (NGFS) and anti-aircraft warfare (AAW) engagements. T ...

s and land-based

coastal artillery Coastal artillery is the branch of the armed forces concerned with operating anti-ship artillery or fixed gun batteries in coastal fortifications. From the Middle Ages until World War II, coastal artillery and naval artillery in the form of ...

circa 1890–1960. They were also used in

rangefinder camera A rangefinder camera is a camera fitted with a rangefinder, typically a split-image rangefinder: a range-finding focusing mechanism allowing the photographer to measure the subject distance and take photographs that are in sharp focus. Most v ...

s. A

stereoscopic rangefinder A stereoscopic rangefinder or stereoscopic telemeter is an optical device that measures distance from the observer to a target, using the observer's capability of binocular vision. It looks similar to a coincidence rangefinder, which uses differe ...

looks similar, but has two eyepieces and uses a different principle, based on

binocular vision Binocular vision is seeing with two eyes. The Field_of_view, field of view that can be surveyed with two eyes is greater than with one eye. To the extent that the visual fields of the two eyes overlap, #Depth, binocular depth can be perceived. Th ...

. The two can normally be distinguished at a glance by the number of eyepieces.

Principle

Coincidence rangefinders work through the principle of triangulation. In the pictured example, triangulation can be used to determine the range of the ship ''𝑑''. The position of the lenses ''A'' and ''B'' are known, and the angle of the lenses ''α'' and/or ''β'' is set by the operator so that both are aimed at the target. Because the distance between ''A'' and ''B'' on a coincidence rangefinder is typically fixed, once the angle is set correctly the operator need only read the range from the scale.

Design

The device consists of a long tube with a forward-facing lens at each end and an operator eyepiece in the center. 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. By rotating the prisms in opposite directions using a differential gear, a degree of horizontal displacement of the image can be achieved.

Applications

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. 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

s, so that the image was in focus when the images were made to coincide.

Usage

The coincidence rangefinder uses a single eyepiece. Light from the target enters the rangefinder through two windows located at either end of the instrument. At either side the incident beam is reflected to the center of the optical bar by a

pentaprism A pentaprism is a five-sided reflecting prism (optics), prism used to deviate a beam of light by a constant 90°, even if the entry beam is not at 90° to the prism. The beam reflects inside the prism ''twice'', allowing the transmission of an i ...

. The optical bar is ideally made from a material with a low

coefficient of thermal expansion Thermal expansion is the tendency of matter to increase in length, area, or volume, changing its size and density, in response to an increase in temperature (usually excluding phase transitions). Substances usually contract with decreasing temp ...

so that optical path lengths do not change significantly with temperature. 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. Since either beam enters the instrument at a slightly different angle the resulting image, if unaltered, will appear blurry. Therefore, in one arm of the instrument a compensator is adjusted by the operator to tilt the beam until the two images match. At this point the images are said to be in coincidence. The degree of rotation of the compensator determines the range to the target by simple

. Coincidence rangefinders made by Barr and Stroud used two eyepieces, and may be confused with stereoscopic units. The second eyepiece showed the operator a range scale so the user could range and read the range scale simultaneously. File:Leica-I telemeter IMG 0254.jpg, Coincidence telemeter on a Leica I File:Polish destroyer's range-finder.JPG, Rangefinder of the Polish destroyer ORP Wicher File:Afstandsmeter Openluchtmuseum Atlantikwall.jpg, WW1 Coincidence rangefinder at Atlantikwall Raversyde, Belgium. Made by C.P. Goerz A.G. Berlin

Coincidence vs stereoscopic rangefinders

In November and December 1941, the United States National Defense Research Committee conducted extensive tests between the American Bausch and Lomb M1 stereoscopic rangefinder and the British Barr and Stroud FQ 25 and UB 7 coincidence rangefinders, and concluded "that the tests indicate no important difference in the precision obtainable from the two types of instrument — coincidence and stereoscopic. They do indicate, however, that the difference in performance between large and small instruments is by no means as great as would be anticipated from simple geometrical optics. The report concludes with the belief that stereoscopic and coincidence acuities are about equal. Under favourable conditions existing instruments of the two types perform about equally well, and the choice between them for any given purpose must be based on matters of convenience related to the particular conditions under which they are to be used."

References

External links

Survey article, ''Rangefinders'' from ''A Dictionary of Applied Physics''

* ttps://web.archive.org/web/20081201105849/http://www.admirals.org.uk/records/adm/adm186/adm186-259.pdf ''Progress in Gunnery Material, 1922–1923'', comparison of rangefinder designs.
''Naval Ordnance And Gunnery Volume 2'', Fire Control Chapter 16F, Optics.

Detailed data on Barr & Stroud Rangefinders, c1916
{{Stereoscopy Firearm sights Armoured fighting vehicle vision and sighting equipment Anti-aircraft weapons Military computers Length, distance, or range measuring devices