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 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 detector which is mounted in some distance from the laser beam, such that the laser source, the object and the detector form a triangle. At the detector, a lens focuses the reflected light onto a CCD chip, and the position of the bright spot on the chip reveals the direction of the incoming light, i.e., the angle between the laser beam and the returning light, from which the distance can be calculated.
The high detection speed 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 can 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.
|||R. G. Dorsch et al., “Laser triangulation: fundamental uncertainty in distance measurement”, Appl. Opt. 33 (7), 1306 (1994)|
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