LIDAR (Light Detection And Ranging) is an optical remote sensing technology that measures properties of scattered light to find range and/or other information of a distant target.

The prevalent method to determine distance to an object or surface is to use laser pulses. Like the similar radar technology, which uses radio waves, the range to an object is determined by measuring the time delay between transmission of a pulse and detection of the reflected signal. LIDAR technology has application in Geomatics, archaeology, geography, geology, geomorphology, seismology, forestry, remote sensing and atmospheric physics.

Applications of LIDAR include ALSM (Airborne Laser Swath Mapping), laser altimetry or LIDAR Contour Mapping. The acronym LADAR (Laser Detection and Ranging) is often used in military contexts. The term "laser radar" is also in use even though LIDAR does not employ microwaves or radio waves, which is definitional to radar.

Military and law enforcement

Police officer using a hand-held LIDAR speed gunOne situation where LIDAR has notable non-scientific application is in traffic speed enforcement, for vehicle speed measurement, as a technology alternative to radar guns. The technology for this application is small enough to be mounted in a hand held camera "gun" and permits a particular vehicle's speed to be determined from a stream of traffic. Unlike RADAR which relies on doppler shifts to directly measure speed, police lidar relies on the principle of time-of-flight to calculate speed. The equivalent radar based systems are often not able to isolate particular vehicles from the traffic stream. LIDAR has the distinct advantage of being able to pick out one vehicle in a cluttered traffic situation as long as the operator is aware of the limitations imposed by the range and beam divergence.

LIDAR does not suffer from “sweep” error when the operator uses the equipment correctly and when the LIDAR unit is equipped with algorithms that are able to detect when this has occurred. A combination of signal strength monitoring, receive gate timing, target position prediction and pre-filtering of the received signal wavelength prevents this from occurring. Should the beam illuminate sections of the vehicle with different reflectivity or the aspect of the vehicle changes during measurement that causes the received signal strength to be changed then the LIDAR unit will reject the measurement thereby producing speed readings of high integrity.

For LIDAR units to be used in law enforcement applications a rigorous approval procedure is usually completed before deployment. The use of many reflections and an averaging technique in the speed measurement process increase the integrity of the speed reading. Vehicles are usually equipped with a horizontally oriented registration plate that, when illuminated, causes a high integrity reflection to be returned to the LIDAR - despite the shape of the vehicle. In locations that do not require that a front or rear registration plate is fitted, headlamps and rear-reflectors provide almost ideal retro-reflective surfaces overcoming the reflections from uneven or non-compliant reflective surfaces thereby eliminating “sweep” error. It is these mechanisms which cause concern that LIDAR is somehow unreliable.

Most traffic LIDAR systems send out a stream of approximately 100 pulses over the span of three-tenths of a second. A "black box" proprietary statistical algorithm picks and chooses which progressively shorter reflections to retain from the pulses over the short fraction of a second.

Military applications are not yet known to be in place and are possibly classified, but a considerable amount of research is underway in their use for imaging. Higher resolution systems collect enough detail to identify targets, such as tanks. Here the name LADAR is more common.

Utilizing LIDAR and THz interferometry wide area raman spectroscopy, it is possible to detect chemical, nuclear, or biological threats at a great distance. Further investigations regarding long distance and wide area spectroscopy are currently conducted by Sandia National Laboratories.

Five LIDAR units produced by the German company Sick AG were used for short range detection on Stanley, the autonomous car that won the 2005 DARPA Grand Challenge.

• Wikipedia