A telemeter is an instrument for measuring the distance to a remote target. Basically, the three main techniques to perform an optical measurement of distance are the following:
- Triangulation. The target is aimed at from two points separated by a known base D, placed perpendicular to the line of sight. By measuring the angle α formed by the two line of sights, the distance is found as L=D/α.
- Time of flight. A light beam from a high-radiance source is propagated to the target and back, and the time delay T=2L/c is measured (c is the speed of light). The distance follows as L=cT/2.
- Interferometry. A coherent beam is used in the propagation to the target. The returned field is detected coherently by beating with ...