Triangulation
Definition: a method for distance measurements
German: Triangulation
Categories: optical metrology, methods
Author: Dr. Rüdiger Paschotta
Triangulation is a method for distance measurements, often using a laser. It exploits the ability of a laser beam to propagate in a well-collimated form (i.e. with small beam divergence) over large distances. In a typical case, the laser beam illuminates a point the distance of which from the laser device is going to be measured; the laser is essentially used as a pointer. Diffuse or specular reflections from that point are monitored with a position-sensitive detector which is mounted in some distance from the laser beam, such that the laser source, the object and the detector form a triangle. Distance information is obtained from the recorded beam position on the detector.
A high detection speed enabled by some detectors (e.g. lateral effect photodiodes) makes it possible to monitor the position of a moving or vibrating part e.g. of some machinery. The accuracy obtained may typically be one-thousandth of the measured distance.
For diffuse reflections, the distance may be limited by the requirement to receive a sufficient amount of reflected optical power; with specular reflections, much larger distances can be measured, but some kind of angular alignment is required.
The principle of triangulation can also be realized with a laser beam the direction of which is rapidly scanned in two dimensions. In that way, one can acquire three-dimensional (3D) images of a scene.
Requirements on the Laser Source
Ideally, a laser source for triangulation has a high beam quality in order to illuminate a small spot over a large distance. A certain optical power level is also required, particular for targets with diffuse reflection. An eye-safe laser wavelength (e.g. in the 1.5-μm region) may be beneficial with respect to laser safety, although a visible laser pilot beam helps to ensure that the correct point is targeted. Red laser diodes with a power of a few milliwatts are frequently used in combination with some beam shaper. A helium–neon laser with diffraction-limited beam quality can also be used, but is more bulky and expensive.
Bibliography
| [1] | R. G. Dorsch et al., “Laser triangulation: fundamental uncertainty in distance measurement”, Appl. Opt. 33 (7), 1306 (1994), DOI:10.1364/AO.33.001306 |
See also: distance measurements with lasers, laser pointers, position-sensitive detectors
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