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## About Range Finders

A rangefinder is a device that measures distance from the observer to a target, for the purposes of surveying, determining focus in photography, or accurately aiming a weapon. Some devices use active methods to measure (such as sonar, laser, or radar); others measure distance using trigonometry (stadiametric rangefinders and parallax, or coincidence rangefinders). These methodologies use a set of known information, usually distances or target sizes, to make the measurement, and have been in regular use since the eighteenth century.

An **active rangefinder** transmits a beam, mostly a laser beam, which is reflected back to the rangefinder by the object. The known velocity of the beam and the time of travel is used to establish the range. The strength and concentration of the laser beam influence the rangefinding capabilities of the instrument to a great extent. Because light coloured homogenous objects reflect the above mentioned beams better than dark unhomogenous objects, the rangefinding capabilities depend also on the object characteristics.

**Stadiametric rangefinding**, or the stadia method is a technique of measuring distances with a telescopic instrument. The term stadia comes from a Greek unit of length. Stadiametric rangefinding is used for surveying and in the telescopic sights of firearms, artillery pieces or tank guns, as well as some binoculars and other optics. It is still used in some light equipment, but in many professional applications it has been replaced with microwave, infrared, or laser range finding methods.

The stadia method is based upon the principle that in similar triangles homologous sides are proportional. This means that, for a right triangle with a given angle, the ratio of adjacent side length to opposite side length (see tangent) is constant. By using a reticle with marks of a known angular spacing, the principle of similar triangles can be used to find either the distance to objects of known size or the size of objects at a known distance. In either case, the known parameter is used, in conjunction with the angular measurement, to derive the length of the other side.

Since a radian is defined as the angle formed when the length of a circular arc equals the radius of the circle, a milliradian (sometimes called a mil), is the angle formed when the length of a circular arc equals 1/1000 of the radius of the circle. An object 5 meters high, for example, will cover 1 mrad at 5000 meters, or 5 mrad at 1000 meters, or 25 mrad at 200 meters. Since the radian expresses a ratio, it is independent of the units used; an object 6 feet high covering 1 mrad will be 6000 feet distant.