LIDAR was originally understood as laser radar, but is nowadays taken as an acronym meaning light detection and ranging. In any case, its meaning is essentially the creation of three-dimensional images based on distance measurements with the time-of-flight method.
The capitalization of the term is not consistent in the literature. Many authors simply write lidar (just like radar), although for acronyms full capitalization is more common.
Basic Operation Principles of LIDAR
The most fundamental operation principle of LIDAR is based on the following two ideas:
- If laser pulses are sent in a certain direction in the form of a laser beam, and these pulses eventually hit some target which reflects or scatters light back towards the source, the detected time delay of that light can be used to determine the distance of the object (time-of-flight method).
- By doing many such measurements while scanning the direction of the beam, one can acquire information for 3D images (3D laser scanning). The image data can be collected with some kind of computer (e.g. realized with a microprocessor), which can then display them on a screen.
In addition, the following techniques may be applied:
- Light may be scattered not only by solid objects, but also for example by tiny particles in the atmosphere (Rayleigh or Mie scattering), or even in clean air at density fluctuations (Rayleigh scattering). In some cases, Raman scattering is used.
- The laser light may not only be scattered, but may induce fluorescence which can then be detected.
- In atmospheric LIDAR, one may utilize ground reflections for measuring light absorption in the atmosphere (see also the article on laser absorption spectroscopy). Differential absorption LIDAR (DIAL) utilizes differences in absorption at different wavelengths.
- It is possible to measure the Doppler shift of returned light with coherent LIDAR based on optical heterodyne detection (Doppler LIDAR). That way, one can acquire information on the longitudinal velocity of the object. Besides, coherent LIDAR is substantially more sensitive, or can be operated with lower powers.
- It is possible to measure concentrations of substances and temperatures via Raman spectroscopy (Raman LIDAR).
In some cases, data acquired with LIDAR are complemented with data from other sources such as color cameras and radar.
LIDAR can be seen as a whole class of methods for remote sensing; see below for concrete applications.
- Laser safety: the risk of affecting eyes heavily depends on the wavelength. There are devices based on eye-safe lasers, e.g. in the 1.5-μm spectral region, but it is more challenging to achieve a high laser performance in this region, and the choice of photodetectors is also more limited.
- Short wavelengths may be required for obtaining laser-induced fluorescence in certain substances or to cause Raman scattering. Also, in principle they allow for highest transverse spatial resolution.
- Long-distance transmission through the atmosphere works best in certain infrared spectral regions.
Some LIDAR apparatuses use laser light at different wavelengths – for example, 1064 nm, 532 nm and 355 nm, as obtained from a Q-switched YAG laser with frequency doubling and frequency tripling. They may then apply different kinds of detection:
- elastic backscattering at all three wavelengths
- Raman-shifted light from nitrogen (the most abundant substance in air) at 387 nm and 607 nm and water vapor (407 nm)
- parallel- and cross-polarized light components for measuring depolarization ratios
Because so different operation principles can be used, and this under very different circumstances (see below for typical applications), the technical details of LIDAR apparatuses vary substantially.
Laser and Photonics Technology for LIDAR
Different kinds of laser sources are used for LIDAR, because not only basic properties like pulse energy, pulse duration and optical wavelength may be relevant. For example, for coherent LIDAR one usually requires pulsed single-frequency lasers, and for some applications one requires a tunable laser; one such constraint alone can easily force one to use a different kind of laser technology.
For directing the laser light and also for collecting the returned light, one generally requires an optical telescope, or possibly two separate telescopes.
Typically, one also needs photodetectors which are highly sensitive (for a sufficiently high signal-to-noise ratio) and at the same time rather fast. In some cases, single photon counting with photomultipliers or avalanche photodiodes is needed, while in other cases sensitive photodiodes are sufficient. For optical heterodyne detection, the returning light needs to be superimposed with a local oscillator signal from the laser.
Applications of LIDAR
- LIDAR instruments can be used on airplanes for detecting turbulences in the atmosphere over substantial distances, so that the flight path may be adapted or passengers can be warned.
- Similarly, wind energy turbines can use LIDAR for optimizing their operation and/or for switching off to avoid damage.
- Atmospheric LIDAR is used for environmental monitoring and research. One may e.g. detect concentrations of gases (including some trace gases and pollutants), small particles, aerosols, temperatures, wind and turbulences.
- Airborne LIDAR can also be used for security surveillance.
- In autonomous (self-driving) cars, LIDAR may be used for monitoring the surroundings, e.g. other vehicles, pedestrians, lane markings, road features, etc. Especially for long distances, LIDAR has a substantial advantages over radar and ultrasound devices, for example. It can provide relatively high spatial resolution in all directions and can be made relatively immune to disturbing influences. Even before totally autonomous vehicles are realized, LIDAR may be used for adaptive cruise control, detecting hazards and emergency breaking. However, it is challenging to realize this at a reasonable cost.
- Robotics can use LIDAR for various purposes, such as detecting obstacles.
- Compact LIDAR speed guns can be used by the police to monitor velocities of vehicles.
- Geodesy can be done with LIDAR instruments on satellites surrounding Earth or on airplanes. Similar surveying and topographic mapping applications are in geography, forestry, agriculture and archeology. For example, one may detect certain properties of vegetation for optimizing agricultural measures. Laser altimetry has even been applied on spacecrafts to planets and moons.
- LIDAR is also used for military purposes such as reconnaissance and missile navigation.
Laser safety issues can be severely limiting factors for certain applications, such as autonomous cars. Cost constraints may inhibit the use of eye-safe lasers and corresponding photodetectors. The resulting severe limitations of applicable laser intensities can then seriously limit the possible performance.
On airplanes and satellites, laser safety is much less of an issue. One may, for example, automatically deactivate the LIDAR apparatus on an airplane for low flight altitudes, thus ensuring that no person can be close enough to be endangered.
See also: laser rangefinders, time-of-flight measurements, Raman spectroscopy, distance measurements with lasers, laser safety
and other articles in the categories vision, displays and imaging, optical metrology, methods
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