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A laser range-finder, or LIDAR (LIght Detection And Ranging), is a device which uses a Laser beam in order to determine the distance to an opaque object. It works by sending a laser pulse in a narrow beam towards the object and measuring how long it takes for the pulse to bounce off the target and return to the sender. The pulse may be coded in order to reduce the chance that the range-finder can be Jammed and it is possible to use Doppler Effect techniques to judge whether the object is moving towards or away from the range-finder, and if so how fast. The accuracy of a LIDAR instrument is determined by the brevity of the laser pulse and the speed of the receiver. A LIDAR that uses very short (sharp) laser pulses and has a very fast detector can range on object to within a few centimeters. In order to make laser-range finders and Laser-guided weapons less useful against military targets, various military arms may have developed laser-absorbing paint for their vehicles. Regardless, some objects don't reflect laser light very well and using a laser range-finder on them is difficult. Despite the beam being narrow, it eventually spreads over long distances due to the divergence of the laser beam, as well as to scintillation and beam wander effects, caused by the presence of air bubbles in the air acting as lenses ranging in size from microscopic to roughly half the height of the laser beam's path above the earth. These atmospheric distortions coupled with the divergence of the laser itself and with transverse winds that serve to push the atmospheric heat bubbles laterally may combine to make it difficult to get an accurate reading of the distance of an object, say, beneath some trees or behind bushes, or even over long distances of more than 1 km in open and unobscured desert terrain. Some of the laser light might reflect off leaves or branches which are closer than the object, giving an early return and a reading which is too low. Alternatively, over distances longer than 1200 ft (365 m), the target, if in close proximity to the earth, may simply vanish into a mirage, caused by temperature gradients in the air in close proximity to the heated desert bending the laser light. All these effects have to be taken into account. Some LIDAR instruments are able to determine multiple returns (as above). These instruments use waveform-resolving detectors, which means they detect the amount of light returned over a certain (very short) time. The waveform from a laser pulse that hit a tree and then the ground would have two peaks. The first peak would be the distance to the tree, and the second would be the distance to the ground. The ability for aircraft-mounted LIDAR instruments to see "through" dense canopies and other semi-reflective surface (such as the ocean) provide many applications for airborne LIDAR instruments such as:
LASER RANGEFINDERS AND 3-D COMPUTER MODELLING Laser rangefinders are used extensively in 3-D object recognition, 3-D object modelling, and a wide variety of computer vision related fields. This technology constitues the heart of the so-called ''time-of-flight'' 3D Scanner s. In contrast to the military "LIDAR" described above, laser rangefinders offer high-precision scanning abilities, with either single-face or 360-degree scanning modes. A number of algorithms have been developed to merge the range data retrieved from multiple angles of a single object in order to produce complete 3-D models with as little error as possible. One of the advantages that laser rangefinders offer over other methods of computer vision is that the computer does not need to correlate features from two images (as in stereoscopic methods) to determine depth information. The laser rangefinders used in computer vision applications often have depth resolutions of tenths of millimeters or less. This can be achieved by using triangulation or refraction measurement techniques (as opposed to the time of flight techniques used in LIDAR). SEE ALSO EXTERNAL LINKS |
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