A rangefinder is an instrument used to measure long distances. It is mostly use by surveyors and the army for a number of purposes. Rangefinders are mainly of two types’ optical rangefinder and laser rangefinder. Radars are also used as non optical rangefinder. They measure the time lapse of an electromagnetic wave which it emits and translates the time lag its echo into distance.
Military rangefinders are usually long tube with eye pieces at the centre. Lenses and prisms are located at each end of the tube.
One prism is fixed and the other can be moved so that it can be adjusted on to the object. The operator looks through the eye pieces and adjusts the movable prism so coincide with each other. The difference in the angles of the two lines of sight from the ends of the tube is called the parallaxes angle. The angle depends upon the distance of the object. The angle is measured on a dial fro which the distance of the tarter can be read directly.
There are two types of optical rangefinders coincidence a stereoscopic.
In the coincidence rangefinder the operator looks through a single eye piece and sees two images of the target. By turning a knob these two images move together target can be read on a dial.
In case of a stereoscopic rangefinder the operator look through a pair of eye pieces. One sees a single image of the target as well as a marker that seems to be floating I space near the target. The operator turns a knob until the marker seems to be at an equal distance as that of the target. Than the distance is read on dial.
The laser rangefinder works on the principles of radar. A laser beams is emitted at the target and the time taken by the laser reflected beam from the rangefinder to the target and by the velocity of laser light gives double the distance to the target. All one has to do now is to divide it by two to find the distance to the object.
These rangefinders today give the range to the object directly. In modern warfare the laser range finder is absolutely necessary to locate targets and enemy positions from a distance. Spy satellites are also used for the same purpose.