- Laser rangefinder
A laser rangefinder is a device which uses a
laserbeam in order to determine the distanceto a reflective object. The most common form of laser rangefinder operates on the time of flight principle by sending a laser pulse in a narrow beam towards the object and measuring the timetaken 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.
The pulse may be coded in order to reduce the chance that the rangefinder can be jammed. It is possible to use
Doppler effecttechniques to judge whether the object is moving towards or away from the rangefinder, and if so how fast.
The accuracy of the instrument is determined by the brevity of the laser
pulseand the speed of the receiver. One that uses very short, sharp laser pulses and has a very fast detector can range an object to within a few centimeters.
beambeing 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.
distortionscoupled with the divergence of the laser itself and with transverse winds that serve to push the atmosphericheat 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 gradientsin the air in proximity to the heated desert bending the laser light. All these effects have to be taken into account.
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 hits 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
canopiesand other semi-reflective surface such as the ocean provide many applications for airborne instruments such as:
* Creating "bare earth"
topographic maps - removing all trees
Bathymetry(measuring topography under the ocean)
Overwashthreat in barrier islands
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-
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.
In order to make laser-range finders and
laser-guidedweapons 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 rangefinder on them is difficult.
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
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
Handheld distance meters
Opti-Logic Corporationintroduced the first consumer level time of flight handheld laser distance meter in 1987. The original handheld consumer priced laser rangefinders were used for golf. Since that time numerous applications have developed. The most popular use is for hunting.
Until 1993, phase shift instruments were reserved to professional users giving their prices and rich functions like
bluetoothmeasurements transmission. Less expensive models around 150$/€ are emerging from Bosch with the DLE 50 or Stanley Workswith the TLM 100.
Industry production processes
An important application is the use of laser Range finder technology during the automation of stock management systems and production processes in steel industry.
* Laser applications
Satellite laser ranging
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