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Encyclopedia of Laser Physics and Technology

© Dr. Rüdiger Paschotta

Last update: 2008-05-08

Lasers can be used in various ways to measure distances or displacements. 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 concrete demands, very different approaches can be appropriate.

Techniques for Distance Measurements

Some of the most important techniques used by 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. Such methods are typically used for large distances such as hundreds of meters or many kilometers. Using advanced techniques, it is possible to measure the distance between Earth and the Moon with an accuracy of a few centimeters. Typical accuracies of simple devices for short distances are a few millimeters or centimeters.
• 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.
• Interferometers allow for distance measurements with an accuracy which is far better than the wavelength of the light used.
• The phase shift method uses an intensity-modulated laser beam. 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.

Laser Radar

A laser radar is a device which uses one of the distance measurement techniques as described above, and scans the direction of the distance measurement in two dimensions. This allows the acquisition of an image, or more precisely a depth profile of some object, as required e.g. in robotics. For acquiring such depth profiles at a higher rate, there are sensor chips similar to CCDs (charge-coupled devices) with internal electronics to detect phase shifts, so that the distance for each pixel can be measured simultaneously. This allows for rapid three-dimensional imaging with very compact devices.

Compared with ultrasonic or radio and microwave frequency devices (radar), the main advantage of laser distance measurement techniques is that laser light has a much smaller wavelength, allowing for a higher spatial resolution.

Various Issues

As essentially all other measurement techniques using lasers, laser distance measurements can be affected by laser noise.

Note that range finding with lasers can raise serious laser safety issues, particularly when Q-switched pulses are used. The related hazards can be strongly reduced by applying eye-safe lasers.

Bibliography

See also: triangulation, time-of-flight measurements, interferometers, phase shift method for distance measurements, laser noise, laser safety, laser applications

Category: metrology