Rangefinder is a device for measuring distances. The military uses rangefinders to determine the distance to a target.
The basis of a military rangefinder is a long tube with eyepieces at the center and an arrangement of lenses and prisms at each end. By adjusting the prisms, the operator can sight the target simultaneously from both ends of the tube. The difference in direction of the two lines of sight is called the parallactic angle. This angle will be large at short distances, and small at long distances. The parallactic angle is measured on a dial from which the range in yards can be read directly.
There are two principal types of rangefinders. The operator of a coincidence instrument looks through a single eyepiece and sees two distinct images of the target. By turning a knob, the operator can make these two images merge. When this happens, the distance can be read on the range dial. The operator of a stereoscopic instrument looks through a pair of eyepieces like binoculars, and sees a single image of the target. The operator also sees a marker that appears to be floating in space near the target. The operator moves a knob until the marker and the target appear to be at the same distance. Then the distance is read on the range dial.
Since World War I (1914-1918), rangefinders have been used in naval gunnery as a part of director systems which aim the guns automatically. During World War II (1939-1945), the Army adopted director systems for antiaircraft fire. But radar, which can measure ranges more accurately, largely replaced the rangefinder in World War II. Since the early 1970’s, the Army has equipped its tanks and other large weapons with laser rangefinders. These instruments measure the time needed for a light pulse to travel to and from a target. Laser rangefinders work during the day and at night. Some can measure the distance to a target up to 12 miles (20 kilometers) away.
