Laser range-finder
Encyclopedia
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
beam to determine the distance
Distance
Distance is a numerical description of how far apart objects are. In physics or everyday discussion, distance may refer to a physical length, or an estimation based on other criteria . In mathematics, a distance function or metric is a generalization of the concept of physical distance...
to an 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 time
Time
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....
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
Triangulation
In trigonometry and geometry, triangulation is the process of determining the location of a point by measuring angles to it from known points at either end of a fixed baseline, rather than measuring distances to the point directly...
and other techniques are often used.
Pulse
The pulse may be coded to reduce the chance that the rangefinder can be jammedElectronic countermeasures
An electronic countermeasure is an electrical or electronic device designed to trick or deceive radar, sonar or other detection systems, like infrared or lasers. It may be used both offensively and defensively to deny targeting information to an enemy...
. It is possible to use Doppler effect
Doppler effect
The Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from...
techniques to judge whether the object is moving towards or away from the rangefinder, and if so how fast.
Precision
The precision of the instrument is determined by the rise or fall timeRise time
In electronics, when describing a voltage or current step function, rise time refers to the time required for a signal to change from a specified low value to a specified high value...
of the laser pulse
Pulse
In medicine, one's pulse represents the tactile arterial palpation of the heartbeat by trained fingertips. The pulse may be palpated in any place that allows an artery to be compressed against a bone, such as at the neck , at the wrist , behind the knee , on the inside of the elbow , and near the...
and the speed of the receiver. One that uses very sharp laser pulses and has a very fast detector can range an object to within a few millimeters.
Range
Despite the beamLight beam
A light beam or beam of light is a narrow projection of light energy radiating from a source into a beam. Sunlight is a natural example of a light beam when filtered through various mediums...
being narrow, it will eventually spread over long distances due to the divergence
Beam divergence
The beam divergence of an electromagnetic beam is an angular measure of the increase in beam diameter or radius with distance from the optical aperture or antenna aperture from which the electromagnetic beam emerges. The term is relevant only in the "far field", away from any focus of the beam...
of the laser beam, as well as due 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
Mirage
A mirage is a naturally occurring optical phenomenon in which light rays are bent to produce a displaced image of distant objects or the sky. The word comes to English via the French mirage, from the Latin mirare, meaning "to look at, to wonder at"...
, caused by temperature gradients in the air in proximity to the heated surface bending the laser light. All these effects have to be taken into account.
Calculation
The distance between point A and B is given by
where c is the speed of light in the atmosphere and t is the amount of time for the round-trip between A and B.
where
is the delay which made by the light traveling and 
is the angular frequency of optical modulation.Then substitute the values in the equation D=ct/2,D=1/2 ct=1/2 c·φ/ω=c/(4πf) (Nπ+Δφ)=c/4f (N+ΔN)=U(N+)
in this equation, U stands for the unit length.
Δφ stands for the delay part which does not fulfill π.
ΔN stands the decimal value.
Discrimination
Some instruments are able to determine multiple returns, as above. These instruments use waveformWaveform
Waveform means the shape and form of a signal such as a wave moving in a physical medium or an abstract representation.In many cases the medium in which the wave is being propagated does not permit a direct visual image of the form. In these cases, the term 'waveform' refers to the shape of a graph...
-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.
Using wavefront sensing, it is possible to determine both the closest and the farthest object at a given point. This makes it possible for aircraft-mounted instruments to see "through" dense canopies and other semi-reflective surface such as the ocean, leading to many applications for airborne instruments such as:
- Creating "bare earth" topographic mapTopographic mapA topographic map is a type of map characterized by large-scale detail and quantitative representation of relief, usually using contour lines in modern mapping, but historically using a variety of methods. Traditional definitions require a topographic map to show both natural and man-made features...
s - removing all trees - Creating vegetationVegetationVegetation is a general term for the plant life of a region; it refers to the ground cover provided by plants. It is a general term, without specific reference to particular taxa, life forms, structure, spatial extent, or any other specific botanical or geographic characteristics. It is broader...
thickness maps - BathymetryBathymetryBathymetry is the study of underwater depth of lake or ocean floors. In other words, bathymetry is the underwater equivalent to hypsometry. The name comes from Greek βαθύς , "deep", and μέτρον , "measure"...
(measuring topography under the oceanOceanAn ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...
) - Forest fire hazard
- OverwashOverwashOverwash is the flow of water and sediment over the crest of the beach that does not directly return to the water body where it originated after water level fluctuations return to normal. There are two kinds of overwash: overwash by runup and overwash by inundation...
threat in barrier islands
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-nanosecondNanosecond
A nanosecond is one billionth of a second . One nanosecond is to one second as one second is to 31.7 years.The word nanosecond is formed by the prefix nano and the unit second. Its symbol is ns....
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
Interferometry
Interferometry refers to a family of techniques in which electromagnetic waves are superimposed in order to extract information about the waves. An instrument used to interfere waves is called an interferometer. Interferometry is an important investigative technique in the fields of astronomy,...
- the most accurate and most useful technique for measuring changes in distance rather than absolute distances.
Military
Point-blank range
In external ballistics, point-blank range is the distance between a firearm and a target of a given size such that the bullet in flight is expected to strike the target without adjusting the elevation of the firearm. The point-blank range will vary with the firearm and its particular ballistic...
shooting to snipers and artillery. They can also be used for military reconciliation and engineering.
Handheld military rangefinders operate at ranges of 2 km up to 25 km and are combined with binoculars
Binoculars
Binoculars, field glasses or binocular telescopes are a pair of identical or mirror-symmetrical telescopes mounted side-by-side and aligned to point accurately in the same direction, allowing the viewer to use both eyes when viewing distant objects...
or monoculars. When the rangefinder is equipped with a digital magnetic compass (DMC) and inclinometer it is capable of providing magnetic azimuth, inclination, and height (length) of targets. Some rangefinders can also measure a target's speed in relation to the observer. Some rangefinders have cable or wireless interfaces to enable them to transfer their measurement(s) data to other equipment like fire control computers. Some models also offer the possibility to use add-on night vision
Night vision
Night vision is the ability to see in low light conditions. Whether by biological or technological means, night vision is made possible by a combination of two approaches: sufficient spectral range, and sufficient intensity range...
modules. Most handheld rangefinders use standard or rechargeable batteries.
The more powerful models of rangefinders measure distance up to 25 km and are normally installed either on a tripod or directly on a vehicle or gun platform. In the latter case the rangefinder module is integrated with on-board thermal, night vision and daytime observation equipment. The most advanced military rangefinders can be integrated with computers.
To make laser rangefinders 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 rangefinder on them is difficult.
3-D Modelling
Computer vision
Computer vision is a field that includes methods for acquiring, processing, analysing, and understanding images and, in general, high-dimensional data from the real world in order to produce numerical or symbolic information, e.g., in the forms of decisions...
-related fields. This technology constitutes the heart of the so-called time-of-flight 3D scanner
3D scanner
A 3D scanner is a device that analyzes a real-world object or environment to collect data on its shape and possibly its appearance . The collected data can then be used to construct digital, three dimensional models....
s. 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 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.
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
LIDAR
LIDAR is an optical remote sensing technology that can measure the distance to, or other properties of a target by illuminating the target with light, often using pulses from a laser...
.
Forestry
Special laser rangefinders are used in forestryForestry
Forestry is the interdisciplinary profession embracing the science, art, and craft of creating, managing, using, and conserving forests and associated resources in a sustainable manner to meet desired goals, needs, and values for human benefit. Forestry is practiced in plantations and natural stands...
. These devices have anti-leaf filters and work with reflector
Retroreflector
A retroreflector is a device or surface that reflects light back to its source with a minimum scattering of light. An electromagnetic wave front is reflected back along a vector that is parallel to but opposite in direction from the wave's source. The device or surface's angle of incidence is...
s. Laser beam reflects only from this reflector and so exact distance measurement is guaranteed. Laser rangefinders with anti-leaf filter are used for example for forest inventories.
Sports
Laser rangefinders may be effectively used in various sports that require precision distance measurement, such as golfGolf
Golf is a precision club and ball sport, in which competing players use many types of clubs to hit balls into a series of holes on a golf course using the fewest number of strokes....
, hunting
Hunting
Hunting is the practice of pursuing any living thing, usually wildlife, for food, recreation, or trade. In present-day use, the term refers to lawful hunting, as distinguished from poaching, which is the killing, trapping or capture of the hunted species contrary to applicable law...
, and archery
Archery
Archery is the art, practice, or skill of propelling arrows with the use of a bow, from Latin arcus. Archery has historically been used for hunting and combat; in modern times, however, its main use is that of a recreational activity...
. Some of the more popular manufacturers are: Opti-logic Corporation, Bushnell, LaserTechnology, Leica, Newcon Optik, Nikon
Nikon
, also known as just Nikon, is a multinational corporation headquartered in Tokyo, Japan, specializing in optics and imaging. Its products include cameras, binoculars, microscopes, measurement instruments, and the steppers used in the photolithography steps of semiconductor fabrication, of which...
, and Swarovski Optik.
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 measuring tools
Laser rangefinders are also used in several industries like construction, renovation and real estate as an alternative to a tape measureTape measure
A tape measure or measuring tape is a flexible form of ruler. It consists of a ribbon of cloth, plastic, fiber glass, or metal strip with linear-measurement markings. It is a common measuring tool. Its flexibility allows for a measure of great length to be easily carried in pocket or toolkit and...
. To measure a large object like a room with a tape measure, one would need two people and a straight run with no obstructions to stretch the tape. With a laser measuring tool, the job can be completed by one operator with just a line of sight. Laser measuring tools typically include the ability to produce some simple calculations, such as the area
Area
Area is a quantity that expresses the extent of a two-dimensional surface or shape in the plane. Area can be understood as the amount of material with a given thickness that would be necessary to fashion a model of the shape, or the amount of paint necessary to cover the surface with a single coat...
or volume
Volume
Volume is the quantity of three-dimensional space enclosed by some closed boundary, for example, the space that a substance or shape occupies or contains....
of a room, as well as switch between imperial and metric units.
Safety
Laser rangefinders for consumers are laser class 1 devices and therefore are considered eyesafe. Some laser rangefinders for military use exceed the laser class 1 energy levels.See also
- Laser applications
- RangefinderRangefinderA rangefinder is a device that measures distance from the observer to a target, for the purposes of surveying, determining focus in photography, or accurately aiming a weapon. Some devices use active methods to measure ; others measure distance using trigonometry...
- LIDARLIDARLIDAR is an optical remote sensing technology that can measure the distance to, or other properties of a target by illuminating the target with light, often using pulses from a laser...
- 3D scanner3D scannerA 3D scanner is a device that analyzes a real-world object or environment to collect data on its shape and possibly its appearance . The collected data can then be used to construct digital, three dimensional models....
- Satellite laser rangingSatellite laser rangingIn satellite laser ranging a global network of observation stations measure the round trip time of flight of ultrashort pulses of light to satellites equipped with retroreflectors...