Laser scanning
Encyclopedia
In modern engineering, the term `laser scanning' is used to described two related, but separate meanings. The first, more general, meaning is the controlled deflection of laser beams, visible or invisible. Scanned laser beams are used in stereolithography
machines, in rapid prototyping
, in machines for material processing, in laser engraving
machines, in ophtalmological laser systems for the treatment of presbyopia
, in confocal microscopy
, in laser printer
s, in laser shows, in Laser TV
, in LIDAR
, and in barcode scanners.
The second, more specific, meaning is the controlled steering of laser beams followed by a distance measurement at every pointing direction. This method, often called 3D object scanning
or 3D laser scanning
, is used to rapidly capture shapes of objects, buildings and landscapes.
This article focuses on the general meaning, i.e., on the methods and applications of scanned laser beams.
Additionally, the mirrors can lead to a periodic motion - like the rotating mirror polygons in a barcode scanner or so-called resonant galvanometer scanners - or to an freely addressable motion, as in servo-controlled galvanometer
scanners. One also uses the terms raster scanning
and vector scanning to distingusih the two situations.
To control the scanning motion, scanners need a rotary encoder
and control electronics that provides, for a desired angle or phase, the suitable electrical current to the motor or galvanometer. A software systems usually controls de scanning motion and, if 3D scanning is implemented, also the collection of the measured data.
In order to position a laser beam in two dimensions, it is possible either to rotate one mirror along two axes - used mainly for slow scanning systems - or to reflect the laser beam onto two closely spaced mirrors that are mounted on orthogonal axes. Each of the two flat or polygonal mirrors is then driven by a galvanometer or by an electric motor. Two-dimensional systems are essential for most applications in material processing, confocal microscopy, and medical science.
Some applications require to position the focus of a laser beam in three dimensions. This is achieved by a servo-controlled lens system, usually called a 'focus shifter' or 'z-shifter'.
Many laser scanners further allow changing the laser intensity.
In laser projectors for laser TV or laser displays, the three fundamental colors red blue and green are combined in a single beam and then reflected together over the two mirrors.
The most common way to move mirrors is, as mentioned, the use of an electric motor
or of a galvanometer
. However piezoelectric actuator
s or magnetostrictive actuator
s are alternative options. They offer higher achievable angular speeds, but often at the expense of smaller achivable maximum angles.
When two optical lenses are moved or rotated against each other, a laser beam can be scanned in a way similar to mirror scanners. (See www.haaslti.com)
or electrooptic deflectors
. These mechanisms allow the highest scanning frequencies possible so far. They are used, for example, in laser TV
systems. On the other hand, these systems are also much more expensive than mirror scanning systems.
s. This method is used to scan RADAR
beams without moving parts. With the use of Vertical-cavity surface-emitting laser (VCSELs), it might be possible to realize fast laser scanners in the forseeable future.
allows enhancing the design process, speeds up and reduces data collection
errors, saves time and money, and thus makes it an attractive alternative to traditional data collection techniques. 3D scanning is also used for mobile mapping
, surveying
, scanning of buildings and building interiors, and in archaeology
.
and laser ablation
, where material is partially removed by the laser. With higher powers the material becomes fluid and laser welding can be realized, or if the power is high enough to remove the material completely, then laser cutting
can be performed. Modern lasers can cut steel blocks with a thickness of 10 cm and more or ablate a layer of the cornea that is only a few micrometers thick.
The ability of lasers to harden liquid polymers, together with laser scanners, is used in rapid prototyping
, the ability to melt polymers and metals is, with laser scanners, to produce parts by laser sintering
.
The principle that is used for all these applications is the same: software that runs on a PC
or an embedded system
and that controls the complete process is connected with a scanner card. That card converts the received vector data to movement information which is sent to the scanhead. This scanhead consists of two mirrors that are able to deflect the laser beam in one level (X- and Y-coordinate). The third dimension is - if necessary - realized by a specific optic that is able to move the laser's focal point in the depth-direction (Z-axis).
Scanning the laser focus in the third spatial dimension is needed for some special applications like the laser scribing of cruved surfaces or for in-glass-marking where the laser has to influence the material at specific positions within it. For these cases it is important that the laser has as small a focal point as possible.
For enhanced laser scanning applications and/or high material throughput during production, scanning systems with more than one scanhead are used. Here the software has to control what is done exactly within such a multihead application: it is possible that all available heads have to mark the same to finish processing faster or that the heads mark one single job in parallel where every scanhead performs a part of the job in case of large working areas.
, especially those with the ability to read bar codes at a distance of a few meters, use scanned laser beams. In these devices, a semiconductor laser beam is usually scanned with the help of a resonant mirror scanner. The mirror is driven electromagnetically and is made of a metal-coated polymer.
Stereolithography
Stereolithography is an additive manufacturing technology for producing models, prototypes, patterns, and in some cases, production parts.-Technology description:...
machines, in rapid prototyping
Rapid prototyping
Rapid prototyping is the automatic construction of physical objects using additive manufacturing technology. The first techniques for rapid prototyping became available in the late 1980s and were used to produce models and prototype parts. Today, they are used for a much wider range of applications...
, in machines for material processing, in laser engraving
Laser engraving
Laser engraving, or laser marking, is the practice of using lasers to engrave or mark an object. The technique does not involve the use of inks, nor does it involve tool bits which contact the engraving surface and wear out...
machines, in ophtalmological laser systems for the treatment of presbyopia
Presbyopia
Presbyopia is a condition where the eye exhibits a progressively diminished ability to focus on near objects with age. Presbyopia’s exact mechanisms are not known with certainty; the research evidence most strongly supports a loss of elasticity of the crystalline lens, although changes in the...
, in confocal microscopy
Confocal microscopy
Confocal microscopy is an optical imaging technique used to increase optical resolution and contrast of a micrograph by using point illumination and a spatial pinhole to eliminate out-of-focus light in specimens that are thicker than the focal plane. It enables the reconstruction of...
, in laser printer
Laser printer
A laser printer is a common type of computer printer that rapidly produces high quality text and graphics on plain paper. As with digital photocopiers and multifunction printers , laser printers employ a xerographic printing process, but differ from analog photocopiers in that the image is produced...
s, in laser shows, in Laser TV
Laser TV
Laser color television , or Laser color video display utilizes two or more individually modulated optical rays of different colors to produce a combined spot that is scanned and projected across the image plane by a polygon-mirror system or less effectively by optoelectronic means to produce a...
, 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...
, and in barcode scanners.
The second, more specific, meaning is the controlled steering of laser beams followed by a distance measurement at every pointing direction. This method, often called 3D object scanning
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....
or 3D laser scanning
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....
, is used to rapidly capture shapes of objects, buildings and landscapes.
This article focuses on the general meaning, i.e., on the methods and applications of scanned laser beams.
Scanning mirrors
Most laser scanners use moveable mirrors to steer the laser beam. The steering of the beam can be one-dimensional, as inside a laser printer, or two-dimensional, as in a laser show system.Additionally, the mirrors can lead to a periodic motion - like the rotating mirror polygons in a barcode scanner or so-called resonant galvanometer scanners - or to an freely addressable motion, as in servo-controlled galvanometer
Galvanometer
A galvanometer is a type of ammeter: an instrument for detecting and measuring electric current. It is an analog electromechanical transducer that produces a rotary deflection of some type of pointer in response to electric current flowing through its coil in a magnetic field. .Galvanometers were...
scanners. One also uses the terms raster scanning
Raster scan
A raster scan, or raster scanning, is the rectangular pattern of image capture and reconstruction in television. By analogy, the term is used for raster graphics, the pattern of image storage and transmission used in most computer bitmap image systems...
and vector scanning to distingusih the two situations.
To control the scanning motion, scanners need a rotary encoder
Rotary encoder
A rotary encoder, also called a shaft encoder, is an electro-mechanical device that converts the angular position or motion of a shaft or axle to an analog or digital code. The output of incremental encoders provides information about the motion of the shaft which is typically further processed...
and control electronics that provides, for a desired angle or phase, the suitable electrical current to the motor or galvanometer. A software systems usually controls de scanning motion and, if 3D scanning is implemented, also the collection of the measured data.
In order to position a laser beam in two dimensions, it is possible either to rotate one mirror along two axes - used mainly for slow scanning systems - or to reflect the laser beam onto two closely spaced mirrors that are mounted on orthogonal axes. Each of the two flat or polygonal mirrors is then driven by a galvanometer or by an electric motor. Two-dimensional systems are essential for most applications in material processing, confocal microscopy, and medical science.
Some applications require to position the focus of a laser beam in three dimensions. This is achieved by a servo-controlled lens system, usually called a 'focus shifter' or 'z-shifter'.
Many laser scanners further allow changing the laser intensity.
In laser projectors for laser TV or laser displays, the three fundamental colors red blue and green are combined in a single beam and then reflected together over the two mirrors.
The most common way to move mirrors is, as mentioned, the use of an electric motor
Electric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...
or of a galvanometer
Galvanometer
A galvanometer is a type of ammeter: an instrument for detecting and measuring electric current. It is an analog electromechanical transducer that produces a rotary deflection of some type of pointer in response to electric current flowing through its coil in a magnetic field. .Galvanometers were...
. However piezoelectric actuator
Piezoelectricity
Piezoelectricity is the charge which accumulates in certain solid materials in response to applied mechanical stress. The word piezoelectricity means electricity resulting from pressure...
s or magnetostrictive actuator
Magnetostriction
Magnetostriction is a property of ferromagnetic materials that causes them to change their shape or dimensions during the process of magnetization. The variation of material's magnetization due to the applied magnetic field changes the magnetostrictive strain until reaching its saturation value, λ...
s are alternative options. They offer higher achievable angular speeds, but often at the expense of smaller achivable maximum angles.
Scanning refractive optics
When two Risley prisms are rotated against each other, a beam of light can be scanned at will inside a cone. Such scanners are used for tracking missiles. (See www.optra.com)When two optical lenses are moved or rotated against each other, a laser beam can be scanned in a way similar to mirror scanners. (See www.haaslti.com)
Material effects
Some special laser scanners use, instead of moving mirrors, acousto-optic deflectorsAcousto-optic modulator
An acousto-optic modulator , also called a Bragg cell, uses the acousto-optic effect to diffract and shift the frequency of light using sound waves . They are used in lasers for Q-switching, telecommunications for signal modulation, and in spectroscopy for frequency control. A piezoelectric...
or electrooptic deflectors
Electro-optics
Electro-optics is a branch of technology involving components, devices and systems which operate by modification of the optical properties of a material by an electric field...
. These mechanisms allow the highest scanning frequencies possible so far. They are used, for example, in laser TV
Laser TV
Laser color television , or Laser color video display utilizes two or more individually modulated optical rays of different colors to produce a combined spot that is scanned and projected across the image plane by a polygon-mirror system or less effectively by optoelectronic means to produce a...
systems. On the other hand, these systems are also much more expensive than mirror scanning systems.
Phased array scanning
Research is going on to achive scanning of laser beams through phased arrayPhased array
In wave theory, a phased array is an array of antennas in which the relative phases of the respective signals feeding the antennas are varied in such a way that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in undesired directions.An antenna array...
s. This method is used to scan RADAR
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
beams without moving parts. With the use of Vertical-cavity surface-emitting laser (VCSELs), it might be possible to realize fast laser scanners in the forseeable future.
3D object scanning
3D object scanning3D 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....
allows enhancing the design process, speeds up and reduces data collection
Data collection
Data collection is a term used to describe a process of preparing and collecting data, for example, as part of a process improvement or similar project. The purpose of data collection is to obtain information to keep on record, to make decisions about important issues, to pass information on to...
errors, saves time and money, and thus makes it an attractive alternative to traditional data collection techniques. 3D scanning is also used for mobile mapping
Mobile Mapping
Mobile mapping is the process of collecting geospatial data from a mobile vehicle, typically fitted with a range of photographic, radar, laser, LiDAR or any number of remote sensing systems. Such systems are composed of an integrated array of time synchronised navigation sensors and imaging sensors...
, surveying
Surveying
See Also: Public Land Survey SystemSurveying or land surveying is the technique, profession, and science of accurately determining the terrestrial or three-dimensional position of points and the distances and angles between them...
, scanning of buildings and building interiors, and in archaeology
Archaeology
Archaeology, or archeology , is the study of human society, primarily through the recovery and analysis of the material culture and environmental data that they have left behind, which includes artifacts, architecture, biofacts and cultural landscapes...
.
Material processing
Depending on the power of the laser, its influence on a working piece differs: lower power values are used for laser engravingLaser engraving
Laser engraving, or laser marking, is the practice of using lasers to engrave or mark an object. The technique does not involve the use of inks, nor does it involve tool bits which contact the engraving surface and wear out...
and laser ablation
Laser ablation
Laser ablation is the process of removing material from a solid surface by irradiating it with a laser beam. At low laser flux, the material is heated by the absorbed laser energy and evaporates or sublimates. At high laser flux, the material is typically converted to a plasma...
, where material is partially removed by the laser. With higher powers the material becomes fluid and laser welding can be realized, or if the power is high enough to remove the material completely, then laser cutting
Laser cutting
Laser cutting is a technology that uses a laser to cut materials, and is typically used for industrial manufacturing applications, but is also starting to be used by schools, small businesses and hobbyists. Laser cutting works by directing the output of a high-power laser, by computer, at the...
can be performed. Modern lasers can cut steel blocks with a thickness of 10 cm and more or ablate a layer of the cornea that is only a few micrometers thick.
The ability of lasers to harden liquid polymers, together with laser scanners, is used in rapid prototyping
Rapid prototyping
Rapid prototyping is the automatic construction of physical objects using additive manufacturing technology. The first techniques for rapid prototyping became available in the late 1980s and were used to produce models and prototype parts. Today, they are used for a much wider range of applications...
, the ability to melt polymers and metals is, with laser scanners, to produce parts by laser sintering
Selective laser sintering
Selective laser sintering is an additive manufacturing technique that uses a high power laser to fuse small particles of plastic, metal , ceramic, or glass powders into a mass that has a desired 3-dimensional shape...
.
The principle that is used for all these applications is the same: software that runs on a PC
Personal computer
A personal computer is any general-purpose computer whose size, capabilities, and original sales price make it useful for individuals, and which is intended to be operated directly by an end-user with no intervening computer operator...
or an embedded system
Embedded system
An embedded system is a computer system designed for specific control functions within a larger system. often with real-time computing constraints. It is embedded as part of a complete device often including hardware and mechanical parts. By contrast, a general-purpose computer, such as a personal...
and that controls the complete process is connected with a scanner card. That card converts the received vector data to movement information which is sent to the scanhead. This scanhead consists of two mirrors that are able to deflect the laser beam in one level (X- and Y-coordinate). The third dimension is - if necessary - realized by a specific optic that is able to move the laser's focal point in the depth-direction (Z-axis).
Scanning the laser focus in the third spatial dimension is needed for some special applications like the laser scribing of cruved surfaces or for in-glass-marking where the laser has to influence the material at specific positions within it. For these cases it is important that the laser has as small a focal point as possible.
For enhanced laser scanning applications and/or high material throughput during production, scanning systems with more than one scanhead are used. Here the software has to control what is done exactly within such a multihead application: it is possible that all available heads have to mark the same to finish processing faster or that the heads mark one single job in parallel where every scanhead performs a part of the job in case of large working areas.
Barcode readers
Many barcode readersBarcode reader
A barcode reader is an electronic device for reading printed barcodes. Like a flatbed scanner, it consists of a light source, a lens and a light sensor translating optical impulses into electrical ones...
, especially those with the ability to read bar codes at a distance of a few meters, use scanned laser beams. In these devices, a semiconductor laser beam is usually scanned with the help of a resonant mirror scanner. The mirror is driven electromagnetically and is made of a metal-coated polymer.