Distance Measurements with Lasers
Lasers can be used in various ways to measure distances or displacements without physical contact. In fact they allow for the most sensitive and precise length measurements, for extremely fast recordings (sometimes with a bandwidth of many megahertz), and for the largest measurement ranges, even though these qualities are usually not combined by a single technique. Depending on the specific demands, very different technical approaches can be appropriate. They find a wide range of applications, for example in architecture, inspection of fabrication halls, criminal scene investigation (CSI), and in the military.
Techniques for Distance Measurements
Some of the most important techniques used for laser distance meters are as follows:
- Triangulation is a geometric method, useful for distances in the range of ≈ 1 mm to many kilometers.
- Time-of-flight measurements (or pulse measurements) are based on measuring the time of flight of a laser pulse from the measurement device to some target and back again. That method is implemented in various kinds of laser rangefinders. Typical accuracies of simple devices for short distances are a few millimeters or centimeters.
- The phase shift method uses an intensity-modulated laser beam.
This principle is also used by some laser rangefinders.
One measures the phase shift of an intensity modulation which is related to the time of flight.
Compared with interferometric techniques, its accuracy is lower, but it allows unambiguous measurements over larger distances and is more suitable for targets with diffuse reflection.
Note that the phase shift technique is sometimes also called a time-of-flight technique, as the phase shift is proportional to the time of flight, but the term is more suitable for methods as described above where the time of flight of a light pulse is measured.
- For small distances, one sometimes uses ultrasonic time-of-flight methods, and the device may contain a laser pointer just for getting the right direction, but not for the distance measurement itself.
- Frequency modulation methods involve frequency-modulated laser beams, for example with a repetitive linear frequency ramp. The distance to be measured can be translated into a frequency offset, which may be measured via a beat note of the sent-out and received beam.
- Interferometers allow for distance measurements with an accuracy which is far better than the wavelength of the light used.
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The RP Photonics Buyer's Guide contains 12 suppliers for distance measurements with lasers. Among them:
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|||T. R. Schibli et al., “Displacement metrology with sub-pm resolution in air based on a fs-comb wavelength synthesizer”, Opt. Express 14 (13), 5984 (2006), doi:10.1364/OE.14.005984|
|||K. Joo et al., “Distance measurements by combined method based on a femtosecond pulse laser”, Opt. Express 16 (24), 19799 (2008), doi:10.1364/OE.16.019799|
|||I. Coddington et al., “Rapid and precise absolute distance measurements at long range”, Nature Photon. 3, 351 (2009), doi:10.1038/nphoton.2009.94|
|||G. Berkovic and E. Shafir, “Optical methods for distance and displacement measurements”, Adv. in Opt. and Photon. 4 (4), 441 (2012), doi:10.1364/AOP.4.000441|
See also: laser rangefinders, LIDAR, triangulation, time-of-flight measurements, interferometers, phase shift method for distance measurements, laser safety, laser applications
and other articles in the categories optical metrology, methods