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. Such laser radar methods (see below) 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.
- The phase shift method uses an intensity-modulated laser beam.
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.
A laser radar (more precisely: LIDAR = light detection and ranging) is a device which uses one of the distance measurement techniques as described above – usually the time-of-flight method or the phase-shift method –, and scans the direction of the distance measurement in one or typically two dimensions. That allows the acquisition of an image, or more precisely a depth profile of some object, as required e.g. in robotics or for self-driving cars. 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. Similarly, there are sensor chips containing an array of avalanche photodiodes for the application of the time-of-flight method with single-photon detection. Such compact devices can greatly facilitate the development of laser radars with rapid three-dimensional imaging.
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 one to send out a much more concentrated probe beam and thus to achieve a higher transverse spatial resolution. Another advantage that an optical bandpass filter makes it possible to very effectively remove noise influences at other optical frequencies.
Laser Safety Aspects
Range finding with lasers can raise serious laser safety issues, particularly when intense pulses from Q-switched lasers are used; that is often required for large detection distances in order not only to obtain a detectable amount of returned light, but also to avoid a dominating influence of ambient light. Then, however, inconvenient additional measures may have to be taken to ensure safety, particularly for human eyes.
Frequently, one tries to design devices to operate a laser safety class I, so that special additional laser safety measures are not required. That, however, can severely limit the optical power which can be sent to the target, and therefore the detection capabilities.
Such trade-offs can be mitigated by applying eye-safe lasers, e.g. in the 1.5-μm spectral region, where far more optical power can be safely used than e.g. in the 1-μm region. However, both the choice of lasers and photodetectors (and their performance) are then substantially limited, and the system cost may be significantly higher.
There are also other ways to improve the trade-off between hazards and detection capabilities, e.g. using improved methods to suppress influences of disturbing ambient light. For example, a photodetector may be used in conjunction with a bandpass filter having a small transmission bandwidth around the laser wavelength, so that most ambient light is strongly suppressed. Further, one can optimize the temporal structure of the used light as well as the sensitivity of the detection apparatus.
As essentially all other measurement techniques using lasers, laser distance measurements can be affected by laser noise – although detection noise is usually the dominant issue. Other noise-related issues can arise from stray light and speckle effects.
The targets can have very different reflection and scattering properties. Problems can arise for very low reflection or for specular reflections. In the latter case, a lot of the incident light may be reflected in directions which are not useful for the detection.
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See also: triangulation, time-of-flight measurements, interferometers, phase shift method for distance measurements, laser safety, laser applications
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