Editing Lidar (section)

== Theory ==
{{multiple image
| align       = right
| direction   = horizontal
| total_width = 700
| image1      = 20200501 Time of flight.svg
| caption1    = Basic time-of-flight principles applied to laser range-finding
| image2      = Amazon Canopy Comes to Life through Laser Data.webm
| caption2    = Flying over the Brazilian Amazon with a lidar instrument
| image3      = Collecting LIDAR data over the Ganges and Brahmaputra River Basin.ogg
| caption3    = Animation of a satellite collecting digital elevation map data over the Ganges and Brahmaputra River basin using lidar
}}

Lidar uses [[ultraviolet]], [[Interferometric visibility|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 [[molecule]]s.<ref name="cracknell" /> A narrow laser beam can map physical features with very high [[Optical resolution|resolutions]]; for example, an aircraft can map terrain at {{convert|30|cm|adj=on}} resolution or better.<ref>{{cite web |author1=Carter, Jamie |author2=Keil Schmid |author3=Kirk Waters |author4=Lindy Betzhold |author5=Brian Hadley |author6=Rebecca Mataosky |author7=Jennifer Halleran |title=Lidar 101: An Introduction to Lidar Technology, Data, and Applications |date=2012 |page=14 |url=https://coast.noaa.gov/data/digitalcoast/pdf/lidar-101.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://coast.noaa.gov/data/digitalcoast/pdf/lidar-101.pdf |archive-date=2022-10-09 |url-status=live |website=NOAA Coastal Services Center |access-date=2017-02-11}}</ref>

Wavelengths vary to suit the target: from about 10&nbsp;[[micrometer (unit)|micrometers]] ([[infrared]]) to approximately 250&nbsp;[[nanometer]]s ([[ultraviolet]]). Typically, light is reflected via [[backscatter]]ing, 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]].<ref name="cracknell" /> Suitable combinations of wavelengths can allow remote mapping of atmospheric contents by identifying wavelength-dependent changes in the intensity of the returned signal.<ref>{{Cite book |url=https://books.google.com/books?id=zGlQDwAAQBAJ&pg=PA678 |title=Handbook of Optoelectronics: Concepts, Devices, and Techniques (Volume One) |last1=P. Dakin |first1=John |last2=Brown |first2=Robert |publisher=CRC Press |year=2017 |isbn=978-1-4822-4179-2|page=678}}</ref>
The name "photonic radar" is sometimes used to mean visible-spectrum range finding like lidar,<ref name="auto2"/><ref name="auto1"/> although [[photonic radar]] more strictly refers to radio-frequency range finding using [[photonics]] components.

A lidar determines the distance of an object or a surface with the [[formula]]:<ref>{{Cite web |last=Podest |first=Erika |date=2021-03-16 |title=The Fundamentals of LiDAR |url=https://appliedsciences.nasa.gov/sites/default/files/2021-03/SIF_LIDAR_Podest_Final.pdf |access-date=2024-09-07 |website=NASA}}</ref>

:<math>d=\frac{c\cdot t}{2}</math>

where ''c'' is the [[speed of light]], ''d'' is the distance between the detector and the object or surface being detected, and ''t'' is the time spent for the laser light to travel to the object or surface being detected, then travel back to the detector.

The two kinds of lidar detection schemes are "incoherent" or direct energy detection (which principally measures amplitude changes of the reflected light) and [[Coherence (physics)|coherent]] detection (best for measuring [[Doppler effect|Doppler]] shifts, or changes in the phase of the reflected light). Coherent systems generally use [[optical heterodyne detection]].<ref>{{cite book|title=Laser – Surface Interactions|last=Rashid A. Ganeev|publisher=Springer Science & Business Media|url= https://books.google.com/books?id=H8DEBAAAQBAJ&pg=PR2|page=32|isbn=978-94-007-7341-7|year=2013}}</ref> This is more sensitive than direct detection and allows them to operate at much lower power, but requires more complex transceivers.

Both types employ pulse models: either ''micropulse'' or ''high energy''. Micropulse systems utilize intermittent bursts of energy. They developed as a result of ever-increasing computer power, combined with advances in laser technology. They use considerably less energy in the laser, typically on the order of one [[Joule#Microjoule|microjoule]], and are often "eye-safe", meaning they can be used without safety precautions. High-power systems are common in atmospheric research, where they are widely used for measuring atmospheric parameters: the height, layering and densities of clouds, cloud particle properties ([[refractive index#Dispersion and absorption|extinction coefficient]], backscatter coefficient, [[depolarization]]), temperature, pressure, wind, humidity, and trace gas concentration (ozone, methane, [[nitrous oxide]], etc.).<ref name="cracknell" />