Range Finders

A laser range-finder is a device which uses a laser beam in order to determine the distance to a reflective object. more...

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The most common form of laser range-finder operates on the time of flight principle by sending a laser pulse in a narrow beam towards the object and measuring the time taken by the pulse to be reflected off the target and returned to the sender. Due to the high speed of light, this technique is not appropriate for high precision sub-millimeter measurements, where triangulation and other techniques are often used.

Operation

Pulse

The pulse may be coded in order to reduce the chance that the range-finder can be jammed. 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 the instrument is determined by the brevity of the laser pulse and the speed of the receiver. One that uses very short, sharp laser pulses and has a very fast detector can range on object to within a few centimeters.

Range

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 proximity to the earth, may simply vanish into a mirage, caused by temperature gradients in the air in proximity to the heated desert bending the laser light. All these effects have to be taken into account.

Discrimination

Some 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 time, usually very short. 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.

Read more at Wikipedia.org