A rangefinder (also rangefinding telemeter, depending on the context) is a device used to
measure distances to remote objects. Originally optical devices used in
surveying, 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 in
naval gunnery and
anti-aircraft artillery. The word telemeter is derived from
Ancient Greekτῆλε (têle) 'distant, far away', and μέτρον (métron) 'something used to measure'.
Designs
The first rangefinder telemeter was invented by
James 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 (the
micrometer screw gauge). It consisted of two parallel
hairs in the focal plane of a telescope
eyepiece 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. The
Royal 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 the
coincidence rangefinder. Two mirrors set a distance apart horizontally in a long slim box, similar to a
subtense 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 1790
Jesse Ramsden invented a half-image range finder. Though
Alexander 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 British
Royal Artillery adopted the
depression range finder, which had been developed by Captain H.S.S. Watkin for use by
coastal 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]
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 the
skinny 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 as
lasers or
radar.
A
laser rangefinder, also known as a laser telemeter, is a
rangefinder that uses a
laser beam to determine the
distance to an object. The most common form of laser rangefinder operates on the
time of flight principle by sending a laser pulse in a narrow beam towards the object and measuring the
time taken by the pulse to be reflected off the target and returned to the sender. Due to the high
speed of light, this technique is not appropriate for high precision sub-millimeter measurements, where
triangulation and other techniques are often used. It is a type of scannerless
lidar.
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]
Ballistics
Rangefinders may be used by users of
firearms 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 the
Second World War, warships used very large optical rangefinders—with a baseline of many meters—to measure range for naval gunnery.
Forestry
Rangefinders are used for surveying in forestry. Special devices with anti-leaf filters are used.
Virtual reality
Since the 1990s, rangefinders have been used in
virtual reality systems to detect operator movements and locate objects.[11]
See also
GfK-Telemeter, a device to determine the viewing rate
^Farey, Pat and Spicer, Mark (2009) Sniping: An Illustrated History Zenith Press, Grand Rapids, Michigan,
page 7,
ISBN978-0-7603-3717-2
^Kidd, Cory D. et al. (1999) "The aware home: A living laboratory for ubiquitous computing research" Lecture Notes in Computer Science 1670: pp. 191–198,
doi:
10.1007/10705432_17
Dickinson, H. W. (2010), James Watt: Craftsman and Engineer, Cambridge University Press,
ISBN978-1108012232
Further reading
"Range Finder (instrument)." Encyclopædia Britannica Online. Encyclopædia Britannica,
Army Test and Evaluation Command, Aberdeen Proving Ground Maryland (1969) Laser Rangefinders Ft. Belvoir
Defense Technical Information Center, U.S. Army, Ft. Belvoir, Virginia,
OCLC227620848 (early history of the use of lasers in rangefinders)
Photographic and Imaging Manufacturers Association (1999) American national standard for photography (optics) : rangefinders and other focusing aids – performance specifications (revision and redesignation of "ANSI PH3.619-1988" as "ANSI/PIMA IT3.619-1998") American National Standards Institute, New York,
OCLC41501265
Hicks, Roger and Schultz, Frances (2003) Rangefinder: Equipment, History, Techniques Guild of Master Craftsman, Lewes, UK,
ISBN1-86108-330-0