LIDAR
It is a surveying method that measures distance to a target by illuminating the target with pulsed laser light and measuring the reflected pulses with a sensor. Differences in laser return times and wavelengths can then be used to make digital 3-D representations of the target.
It stands for light detection and ranging.
Lidar sometimes is called 3D laser scanning, a special combination of a 3D scanning and laser scanning. It has terrestrial, airborne, and mobile applications.
Working principle of LIDAR
Lidar uses ultraviolet, visible, or near infrared light to image objects. It can target a wide range of materials, including non-metallic objects, rocks, rain, chemical compounds, aerosols, clouds and even single molecules.
A narrow laser beam can map physical features with very high resolutions; for example, an aircraft can map terrain at 30-centimetre (12 in) resolution or better.
Wavelengths vary to suit the target: from about 10 micrometers to the UV (approximately 250 nm). Typically light is reflected via backscattering, as opposed to pure reflection one might find with a mirror. Different types of scattering are used for different lidar applications: most commonly Rayleigh scattering, Mie scattering, Raman scattering, and fluorescence.
Suitable combinations of wavelengths can allow for remote mapping of atmospheric contents by identifying wavelength-dependent changes in the intensity of the returned signal.
Components of LIDAR
Laser
In applications that operate around people Maximum power is limited by the need to make them eye-safe so that at high power wavelength is not strongly absorbed by the eye
for non-scientific applications - 600–1000 nm
longer ranges with lower accuracies - 1550 nm
for military applications with invisibility to in night vision goggles – 1550nm
Airborne topographic mapping lidars- 1064 nm diode pumped YAG lasers
underwater depth research systems - 532 nm frequency doubled diode pumped YAG to penetrate water with much less attenuation.
Laser settings include the laser repetition rate (which controls the data collection speed). Pulse length is generally an attribute of the laser cavity length, the number of passes required through the gain material (YAG, YLF, etc.), and Q-switch (pulsing) speed. Better target resolution is achieved with shorter pulses, provided the lidar receiver detectors and electronics have sufficient bandwidth.
Scanner and optics
mage development speed is affected by the speed at which they are scanned. Options to scan the azimuth and elevation include dual oscillating plane mirrors, a combination with a polygon mirror and a dual axis scanner. Optic choices affect the angular resolution and range that can be detected. A hole mirror or a beam splitter are options to collect a return signal.
Photodetector and receiver electronics
Two main photodetector technologies are used in lidar: solid state photodetectors, such as silicon avalanche photodiodes, or photomultipliers. The sensitivity of the receiver is another parameter that has to be balanced in a lidar design.
Position and navigation systems
Lidar sensors mounted on mobile platforms such as airplanes or satellites require instrumentation to determine the absolute position and orientation of the sensor. Such devices generally include a Global Positioning System receiver and an Inertial Measurement Unit (IMU).
Sensor
Lidar uses active sensors that supply their own illumination source. The energy source hits objects and the reflected energy is detected and measured by sensors. Distance to the object is determined by recording the time between transmitted and backscattered pulses and by using the speed of light to calculate the distance traveled.
3-D imaging can be achieved using both scanning and non-scanning systems. "3-D gated viewing laser radar" is a non-scanning laser ranging system that applies a pulsed laser and a fast gated camera.
Imaging lidar can also be performed using arrays of high speed detectors and modulation sensitive detector arrays typically built on single chips using complementary metal–oxide–semiconductor (CMOS) and hybrid CMOS/Charge-coupled device (CCD) fabrication techniques. In these devices each pixel performs some local processing such as demodulation or gating at high speed, downconverting the signals to video rate so that the array can be read like a camera. Using this technique many thousands of pixels / channels may be acquired simultaneously.
High resolution 3-D lidar cameras use homodyne detection with an electronic CCD or CMOS shutter.
A coherent imaging lidar uses synthetic array heterodyne detection to enable a staring single element receiver to act as though it were an imaging array.
Types of LIDAR
Based on orientation
Lidar can be oriented to nadir, zenith, or laterally. For example, lidar altimeters look down, an atmospheric lidar looks up, and lidar-based collision avoidance systems are side-looking.
Based on platform
Lidar applications can be divided into
Airborne
Airborne lidar (also airborne laser scanning) is when a laser scanner, while attached to an aircraft during flight, creates a 3-D point cloud model of the landscape. This is currently the most detailed and accurate method of creating digital elevation models, replacing photogrammetry. One major advantage in comparison with photogrammetry is the ability to filter out reflections from vegetation from the point cloud model to create a digital surface model which represents ground surfaces such as rivers, paths, cultural heritage sites, etc., which are concealed by trees. Within the category of airborne lidar, there is sometimes a distinction made between high-altitude and low-altitude applications, but the main difference is a reduction in both accuracy and point density of data acquired at higher altitudes.
Terrestrial types.
Terrestrial applications of lidar (also terrestrial laser scanning) happen on the Earth’s surface and can be either stationary or mobile. Stationary terrestrial scanning is most common as a survey method, for example in conventional topography, monitoring, cultural heritage documentation and forensics.
The two types require scanners with varying specifications based on the data’s purpose, the size of the area to be captured, the range of measurement desired, the cost of equipment, and more. Spaceborne platforms are also possible.
Application of LIDAR
| Field | Usage |
|---|---|
| Agriculture | 1)Lidar can help determine where to apply costly fertilizer. It can create a topographical map of the fields and reveal slopes and sun exposure of the farm land. 2) crop mapping in orchards and vineyards, to detect foliage growth 3) Plant species classification |
| Archaeology | 1) planning of field campaigns, mapping features under forest canopy, and overview of broad, continuous features indistinguishable from the ground. 2) reveal micro-topography |
| Autonomous vehicles | obstacle detection |
| Biology and conservation | Faster surveying |
| Atmosphere | range of measurements that include profiling clouds, measuring winds, studying aerosols, and quantifying various atmospheric components. |
| Law enforcement | for speed limit enforcement purposes. |
| Military | Early warning and detection systems |
| Mining | calculation of ore volumes |
| Physics and astronomy | 1)Detect atmosphere, 2) Distance calculation |
| Robotics | perception of the environment |
| Spaceflight | rangefinding and orbital element calculation |
| Solar photovoltaic deployment optimization | determining appropriate roof tops and for determining shading losses |