Laser range-finder
From Wikipedia, the free encyclopedia
A laser range-finder is a device which uses a laser beam in order to determine the distance to a reflective object. 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.
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[edit] Operation
[edit] 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.
[edit] 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.
[edit] 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.
The ability for aircraft-mounted instruments to see "through" dense canopies and other semi-reflective surface such as the ocean provide many applications for airborne instruments such as:
- Creating "bare earth" topographic maps - removing all trees
- Creating vegetation thickness maps
- Measuring topography under the ocean
- Forest fire hazard
- Overwash threat in barrier islands
[edit] Technologies
Time of flight - this measures the time taken for a light pulse to travel to the target and back. With the speed of light known, and an accurate measurement of the time taken, the distance can be calculated. Many pulses are fired sequentially and the average response is most commonly used. This technique requires very accurate sub-nanosecond timing circuitry.
Multiple frequency phase-shift - this measures the phase shift of multiple frequencies on reflection then solves some simultaneous equations to give a final measure.
Interferometry - the most accurate and most useful technique for measuring changes in distance rather than absolute distances.
[edit] Applications
[edit] Military
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.
[edit] 3-D 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 constitutes the heart of the so-called time-of-flight 3D scanners. In contrast to the military instruments 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 to determine depth information as in stereoscopic methods.
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.
[edit] Handheld distance meters
Leica geosystems introduced the first handheld laser distance meter in 1993,[1] creating its Disto Range. It was followed by Bosch with its DLE range finders. Both are mainly used in the construction field.
Until recently, those instruments were reserved to professional users giving their prices and rich functions like bluetooth measurements transmission, but cheaper models around 100$/€ are emerging from Bosch with the DLE 50 or Stanley Works with the TLM 100.
