Arangefinder (alsorangefinding telemeter, depending on the context) is a device used tomeasure distances to remote objects. Originally optical devices used insurveying, they soon found applications in other fields, such as photography, the military, and space travel. They were especially useful for finding the range of a target, such as innaval gunnery andanti-aircraft artillery. The wordtelemeter is derived from Ancient Greek τῆλε (têle) 'distant, far away' and μέτρον (métron) 'something used to measure'.
The first rangefinder telemeter was invented byJames Watt in 1769 and put to use in 1771 in surveying canals. Watt called his instrument a micrometer, a term now used with a different meaning in engineering (themicrometer screw gauge). It consisted of two parallelhairs in the focal plane of a telescopeeyepiece crossing an upright hair. At the point to be measured, two sliding targets on a surveyor's rod were adjusted to align with the hairs in the telescope. The distance to the rod could then be determined from the distance between the targets on the rod by trigonometry.[1]
Several others have been credited with the invention of the rangefinder telemeter at one time or another. TheRoyal Society of Arts gave an award to W. Green for its invention in 1778, even though they were made aware of Watt's priority.[2]
In 1778,Georg Friedrich Brander invented thecoincidence rangefinder. Two mirrors set a distance apart horizontally in a long slim box, similar to asubtense bar but located at the measuring station, and forming two images. This rangefinder does not require a measuring rod at the target and could perhaps be considered the first true telemeter. In 1790Jesse Ramsden invented a half-image range finder. ThoughAlexander Selligue is often mistakenly credited with the invention, he did invent an improved rangefinder with fixed lenses in 1821 and is responsible for coining the term.[3][4]
In 1881, the BritishRoyal Artillery adopted thedepression range finder, which had been developed by Captain H.S.S. Watkin for use bycoastal artillery. It used the measurement of the angle of depression from the observer, sited on a high vantage point, to the waterline of the target vessel.[5]
In 1899,Carl Pulfrich atCarl Zeiss AG fabricated a practicalstereoscopic rangefinder,[6] based on a patent of Hector Alexander de Grousilliers.[7]
World War II-era rangefinders worked optically with two telescopes focused on the same target but a distance apart along a baseline. The range to the target is found by measuring the difference in bearing of the two telescopes and solving theskinny triangle. Solutions can be obtained automatically, using tables or, rarely, manual calculation. The greater the distance to the target, the longer the baseline needs to be for accurate measurement. Modern rangefinders use an electronic technology such aslasers orradar.
Applications includesurveying,navigation, to assist focusing inphotography, choosing agolf club according to distance, and correcting aim of a projectileweapon for distance.
Laser rangefinders are used in golf to measure the yardage of a particular shot but also to gauge slope and wind as well. There has been debate over whether they should be allowed in tournaments. While their use is banned on the professional level, they are becoming widely used on the amateur level.[8][9]
Rangefinders may be used by users offirearms over long distances, to measure the distance to a target to allow for projectile drop.[10] Until the development of electronic means of measuring range during theSecond World War, warships used very large optical rangefinders—with a baseline of many meters—to measure range for naval gunnery.
In recent decades, laser rangefinders have become popular inarchery, especially amongbowhunters.[11] Rangefinders marketed for archery use will have angle-compensation features, with an internal computer calculating the horizontal distance to the target. Bowhunters will often shoot fromtreestands or in steep terrain, and for relatively short shots (40 yards and under), an archer will need to aim using the horizontal distance instead of the line-of-sight distance.[12]
Rangefinders are used for surveying in forestry. Special devices with anti-leaf filters are used.
Since the 1990s, rangefinders have been used invirtual reality systems to detect operator movements and locate objects.[13]
