Laser scanning
Laser scanning (also known as laser scanning ) refers to the line-like or raster-like sweeping of surfaces or bodies with a laser beam in order to measure them, process them or to generate an image. Sensors that deflect the laser beam accordingly are called laser scanners . A laser scanner which, in addition to the object geometry, also detects the intensity of the reflected signal, is called an imaging laser scanner. The intensity values of the laser light reflected from the recorded surfaces are recorded in today's laser measurement systems in 16-bit gray levels. The result is an image of the surfaces similar to that of a black and white photo.
The laser scanner
A laser scanner consists of a scan head and driver and control electronics . The electronics consists of a power electronic part that supplies the currents for the drives, and a z. B. on a PC or embedded system running scanner software that addresses the driver electronics.
In measurement applications, the result of the scanning process is received by sensors via the same or a separate optical path and is usually recorded by the same scanner software that also addresses and controls the other components.
Scan head
The laser beam is deflected in the scan head, its deflection angle is measured and (mostly) electronically regulated.
Scanning mechanisms
Scanning a 3D surface with a laser beam requires different scanning mechanisms to move the laser over the surface. The following basic scan modes are distinguished:
- The scanning process is carried out by two orthogonally attached mirrors, which is typical for terrestrial scanners with a limited field of view.
- The laser beam scans in one direction with the scanning mirror, which rotates with the help of a mechanical device. This is typical for terrestrial scanners with a panoramic or hemispherical field of view.
- The laser beam scans in one direction and is mounted in an aircraft. In this case, a second scanning direction is given by the movement of the aircraft, and a system combining GPS and IMS is used to measure the position and orientation.
- In the case of a triangulation scanner, a line is projected instead of a single point. In this case the scan is limited to one direction. Projections of several lines or stripe patterns make it possible to record entire fields.
- Classic scanning can be avoided by using flash lidar technology. More precisely, through advances in CMOS technology. These systems are based on a floodlight device that projects light onto a surface to be examined and enables two-dimensional recording of the TOF data.
Mirror scanner
The simplest method to create a scanning motion is to change the orientation of a mirror on which the laser beam is reflected. In a spatial dimension, this can be done by a galvanometer drive (short: Galvo ), by a continuously rotating mirror or by a continuously rotating mirror prism (polygon mirror), depending on whether a freely programmable movement (vector control) or a periodic movement (line , Picture) is desired. A distinction is therefore usually made between vector scanners and raster scanners .
For two-dimensional deflection, either a mirror has to be deflected in two directions - as is mainly used in slow systems - or two orthogonally rotatable standing mirrors are set up close to one another, through which the laser beam is reflected. The two plane or polygon mirrors are then each driven by a galvanometer drive or an electric motor. Two-dimensional scan heads for high-performance lasers play an essential role in material processing. Two-dimensional scan heads for low power lasers are essential components of confocal microscopes .
For simple show purposes, plane mirrors are often tilted slightly and mounted on a motor shaft so that Lissajous figures and cardioids can be created with the light point .
There are also laser scanners in which an additional spring and corresponding control ensure a resonant torsional vibration. Such resonant laser scanners can be found in both one-dimensional and two-dimensional designs, in barcode scanners and in special applications in printing technology or space travel.
In addition, there are scan heads for three-dimensional laser marking which, in addition to the two mirrors for the X and Y axes, also have adjustable optics for the depth, i.e. the Z axis. This makes it possible to control the laser in the third dimension as well. The laser focus can then be freely positioned in all three spatial dimensions.
Many laser scanners also allow the laser intensity to be changed.
In laser projectors , three laser beams with the three basic colors red, green and blue are guided in a common beam path over the two deflecting mirrors.
Micromirror fields are used in modern video projectors and work in principle like many individual small laser scanners.
Types of mirror scanners
Oscillating mirror
Many commercial airborne laser scanning systems use the technique of the oscillating or pivoting mirror, in which the laser beam is controlled by this pivoting mirror. Point data is generated in both directions of the scan. This results in a zigzag scan pattern on the ground. The point spacing of the laser points within the scan line varies because the mirror is constantly accelerated and decelerated. The largest point distances are found in the middle of the scan line and the smallest point distances at the end, since the mirror direction changes again there. One of the advantages of these scanners is that the scanning angle is variable (between 0 ° and 75 °) and that the scanning speed is also variable (from 0 to 100 Hz). The scanning angle, scanning speed, flight altitude and laser pulse frequency determine the maximum point spacing. This allows the scanner to be controlled so that it can maintain a preferred spacing of the laser points on the ground. In general, such systems operate from 100 m to 6000 m above ground. Due to their flexibility, airborne laser scanners with an oscillating mirror can be configured well for different application requirements.
Rotating polygon mirror
In rotating mirror systems, a rotating polygon mirror is used to deflect rays. The data points are only generated in one scan direction. The scan lines run parallel and, compared to the oscillating mirror system, there is an evenly distributed point pattern on the ground. The systems with a rotating polygon mirror have a scan angle between approximately 30 ° and 60 °.
Palmer scanner
The mirror device that deflects the laser beam is constructed in such a way that the mirror surface and the axis of rotation form an angle that is not equal to 90 °. These scanning systems are mainly used in terrestrial laser scanners. In the case of airborne systems, this results in an elliptical scan pattern on the ground.
Fiber optic scanner
In the case of the fiber optic scanner, individual laser impulses are directed from the mirror into adjacent glass fibers. One advantage of this scanning mechanism is that it is extremely stable because the glass fibers are glued together during manufacture. The scanning angle is also finally determined during manufacture. Such a typical construction has 128 fibers and an angle of 14 °. This configuration creates a different point spacing on the surface.
Prism scanner
Laser beams can also be deflected two-dimensionally by means of two axially rotatable prisms, so-called Risley prisms. Prism scanners are currently only used for a few special applications in the military sector.
Other technologies
There are a few other effects that enable the controlled deflection of a laser beam. Important examples are acousto-optic deflectors and electro-optic deflectors . These scanning methods currently achieve the highest deflection speeds, but are also significantly more expensive than mirror or prism scanners and only allow much smaller scanning angle ranges.
Flash LiDAR
is an extension to sequential scanning with either a collection of LEDs or laser diodes. The demodulation of the returning light is controlled with an external electro-optical device. This can be done using controlled microchannel plates or specially designed CMOS chips. In contrast to other scanners, a complete 3D image is generated here immediately. The disadvantage of Flash LiDAR systems compared to other scanning systems is that Flash LiDAR systems only have a very limited spatial resolution and range, which is related to the spread of the pulse energy over a larger field of view.
Components / requirements for laser scanning
Light sources
Two types of light sources are used in 3D imaging: incoherent light sources (incandescent, luminescent, or sunlight) and lasers (gas, solid, or semiconductor), with the light produced by the laser being more monochromatic, more directional, brighter, and spatial is more coherent than that of other light sources. This spatial coherence allows the laser to stay focused when projected onto a surface. Unfortunately, this high spatial coherence has the consequence that so-called speckles are generated when a rough surface is illuminated with this laser source. Compact lasers for 3D measuring systems are currently available for a large number of different wavelengths.
Laser beam propagation
To understand many optical systems and their limitations, it is important to understand the manipulation and use of laser beams by means of Gaussian beams . These are a solution to Maxwell's equations . Because of diffraction, even in the best laser emitting conditions, collimation cannot be sustained with distance. Geometric optics can be used to analyze laser scanners, but the Gaussian beam must also be included. The spatial resolution depends on the radiation quality and its properties. Even with a high sampling rate, which is given by optical scanning mechanisms, the resolution is limited by the diffraction.
Photodetectors
Each implementation of a laser scanner requires a specific sensor to collect the reflected laser light. Traditional photo sensors for TOF systems are pin photodiodes, avalanche photodiodes and photomultipliers. The first two sensors are photovoltaic detectors. The avalanche photodiodes and the pin photodiodes use internal photoelectric effects, while the photomultipliers use external photoelectric effects. Advances in Complementary Metal Oxide Semiconductor (CMOS ) architecture and device technology have spawned new sensors that represent arrays of photodiodes to process the signal on a per-pixel basis. These are the basic sensors for Flash LiDAR, where the images are recorded simultaneously in one area.
Medium of propagation and surface effects
Laser light has to travel to an object and back again through a transmission medium (air, water, vacuum, etc.). In the case of the TOF systems, a correction in the index of reference in relation to a vacuum is required. This is in the order of 300 ppm. Such corrections can already be made in the calibration process. At high altitudes you may want to consider the temperature gradients between the altitude and the ground; for the accuracy requirements in airborne laser scanning, however, the average air and ground temperature and the pressure measurement are usually sufficient. The strength of an impulse echo characterizes the reflectivity of the illuminated spot on the floor. Advanced systems capture the amplitude (often referred to as "intensity") of each of the incoming echoes as 8- or 16-bit values. The intensity images look like black and white photographs, although they only show the relative reflectivity of objects at a specific wavelength of the laser. An amplitude image as supplementary information to the pure 3D data could, for example, help to identify individual objects that would otherwise only be difficult to recognize using the height data. Depending on the surface there is either a directional reflection, a diffuse reflection or a mixture of both. An echo from a less reflective surface has a lower amplitude than a reflection from a highly reflective surface.
In addition to the reflexivity, the shape of a surface also has an effect on the returning echo. The reflectivity of the illuminated spot on the floor not only determines the precision and reliability of the distance measurements, but also the maximum working range. As a rule, the manufacturers of laser scanners indicate for which specific targets (reflectivity, diffuse or directional reflection) the specified maximum range is valid. Artificial and natural light sources also have an influence on the scanner.
The underlying hypothesis of the active optical geometric measurement is that the measured surface is opaque, diffusely reflective and uniform, and that the environment next to the surface that is being measured does not generate any interference signals. This means that not all materials can be measured properly. Marble, for example, has translucency and inhomogeneity, which leads to a distortion in the distance measurement. The quality of 3D data can also deteriorate when the laser beam crosses objects with severe discontinuities , such as edges or holes.
Distinctions and areas of application
Airborne laser scanning
Airborne laser scanning , ALS for short, or laser altimetry is a geodesy method in which a topography is recorded using point-by-point distance measurements. This method is generally used to record terrain heights and objects on the site and is increasingly replacing classic photogrammetry . The sensors operate from airplanes or helicopters.
Components of an airborne laser scanner
The components that make up an airborne laser scanner are the scanner, airborne GPS antenna, control and data storage unit, user laptop and the flight management system. The scanner is located in the fuselage and sends out laser pulses during the flight. Depending on the airspeed and the altitude, measurement densities of 0.2 to 50 points / m² can be achieved. Modern laser scanners are equipped with roller compensation. As a result, the flight strips overlap, making it possible to record the area flown over without gaps. The GPS antenna is located on the top of the aircraft. This enables undisturbed reception of the GPS satellite signals. The control and data storage unit is responsible for the time synchronization and control of the entire system. The distance and position data recorded by the scanner, the INS and the GPS are saved here. With modern scanners that send out 300,000 laser pulses per second, more than 20 gigabytes of data can be generated in one hour. Only 0.1 gigabytes per hour are created by the GPS or the INS. The user laptop communicates with the control and data storage unit. He is responsible for the correct application of the order parameters and for the correct system settings during the recording. The flight management system is used for orientation for the pilot. Help is offered here to ensure compliance with the predefined flight lanes.
For a correct georeferencing of the distance measurement , it is necessary that the position and orientation of the sensor in space are known at the time of the measurement. In the case of air-assisted laser scanning, a combination of at least one GPS receiver and an inertial navigation system (INS) is used. It is important here that the different measured values of the different sensors can be determined synchronously or at least can be synchronized using suitable methods. With air-assisted laser scanning, according to manufacturer information or service companies, under favorable conditions (areas without vegetation, weak to medium slope), accuracies of 5–15 cm in height and 30–50 cm can be achieved. Using suitable methods, three-dimensional Cartesian coordinates of the measured points can then be derived from the data from the distance measurement and the GPS / INS component.
Measurements using an airborne laser scanner
Different measuring principles can be used to determine the distance to the object to be detected. Processes that utilize the transit time of light use individual, short laser pulses ; the radiation reflected from the object is registered by a sensor . The time between transmitted and received pulses is a measure of the distance between the transmitting and receiving unit. Alternatively, a pulse train with a fixed frequency can be transmitted and its reflection on the object to be measured can be detected. The phase difference between the transmitted and received pulse train is also a measure of the distance.
Airborne laser scanners for land photography work with wavelengths between 800 and 1550 nanometers ( infrared ), the spectral width of which is 0.1 to 0.5 nanometers. The way an object reflects the laser beams depends on the wavelength and the laser system. For example, when using a laser whose wavelengths are close to the visible part of the spectrum, the absorption of water is high. This makes it difficult to evaluate water surfaces in such recordings. Ice and snow absorb strongly from a wavelength of more than 1550 nanometers and are therefore difficult to recognize in the recordings, which is why, depending on the question, one has to pay attention to the wavelength with which the laser is used. In addition, when using a laser scanner, care must be taken to ensure that the laser is not harmful to the human eye, since during aerial flights it cannot be ruled out that there are people on the area that is being scanned by the laser.
The surface scanned by the laser and also its shape are responsible for how strongly the emitted laser signal is reflected. If the laser hits a smooth surface, an echo is reflected. The shape of the waveform is similar to that of the transmitted signal. For example, if the emitted laser signal strikes the roof of a house and the floor next to it, two echoes are generated that are reflected, one from the floor and one from the house. Since the two echoes have different waveforms, they are completely stored in a wave receiving unit. By storing the information of a returned echo, information about the objective of the examination can be obtained.
Airborne laser scanning using drones
Another form of laser scanning is laser scanning using drones or UAS (Unmanned Aerial Vehicle Systems). Since drones have a much lower flight altitude than aircraft-based laser scanners, very detailed recordings with a resolution in the centimeter range are possible. This type of laser scanning is also particularly cost-effective, as the effort is considerably less. Another advantage of this type of laser scanning is that the drone can save its flight route in advance, which then automatically completes the recording flight.
Up to 150 hectares can be measured automatically using drones. The objects of the survey can be fields and landscapes, construction and planning areas, opencast mines, high-voltage lines, forest edges and green strips, road crossings and road courses as well as river and stream courses including the river bed.
A drone is equipped with a GNSS (Global Navigation Satellite System) unit for navigation. In addition to the GNSS unit, the drone also has a LiDAR sensor, the actual laser; However, this must be adapted to the load capacity of the drone. The drone also has a control unit and high-resolution cameras.
Terrestrial laser scanning
In terrestrial laser scanning , or TLS for short, the surface geometry of objects is digitally recorded by means of pulse transit time, phase difference compared to a reference or by triangulation of laser beams. This creates a discrete set of sampling points, which is referred to as a point cloud . The coordinates of the measured points are determined from the angles and the distance in relation to the origin (device location).
In contrast to the air-supported application, static recording situations can be assumed with TLS. With advances in technology, TLS systems are also increasingly being installed on mobile platforms (cars, ships, trains) in order to capture large-scale linear structures, such as clearance profiles of a railway line. In this case one speaks increasingly of kinematic terrestrial laser scanning (k-TLS) up to mobile mapping systems, such as those used for data acquisition at Google Street View .
A further subdivision enables the dimensional approach for 2D and 3D applications.
With 2D laser scanning , the contour of objects is digitally recorded on one level. In safety systems, 2D laser scanning is used as a non-contact protective device to detect whether people or objects are crossing defined (danger) areas in order to be able to initiate appropriate measures (e.g. switching off machines). The main advantages over the light curtain are the ability to program the protective field to be protected and the protection of a large area from a single, relatively small device. The disadvantage is the currently lower computational resolution compared to light curtains, which requires a greater safety distance to the danger point. 2D laser scanners are also used to automatically detect objects, for example on the truck toll control bridges on German motorways . Other areas of application are the creation of maps in robotics and the detection of obstacles in autonomous mobile robots .
The 3D laser scanning delivers three-dimensional point clouds and thus a complete image of the measurement scene. Using the point cloud, either individual dimensions such as B. lengths and angles are determined, or a closed surface is constructed from triangles ( meshing ) and z. B. used in 3D computer graphics for visualization.
The use of terrestrial 3D laser scanning covers numerous areas of inventory and begins with architectural surveying with a focus on building research and monument preservation. Deformed and damaged buildings with spatially complex structures can be roughly identified quickly. The more complex the building structure, however, the more shading the individual scans show and are therefore incomplete, which can only be remedied by further measuring points. In the case of furnished buildings (this is the standard case in monument conservation), the scan results can only be evaluated to a limited extent. Further areas of application are, for example, pipeline and plant construction, archeology, monument protection, reverse engineering and quality assurance as well as tunnel construction, forensics and accident research.
Modern laser measuring systems achieve a point accuracy of up to one millimeter. A laser scanner, whose distance determination works according to the pulse or phase measurement method, also stores the degree of reflection of the laser light. In combination with a (possibly external) digital camera, the point clouds can also be provided with photo-realistic textures. In the field of laser scanners that work according to the phase difference process (phase measurement process), enormous progress has been made in recent years, particularly with regard to the sampling rate. Current devices achieve measurement speeds of over 1 million 3D measurement points per second (1 MHz). In contrast to the pulse transit time, a continuous laser beam is emitted. The amplitude of the emitted laser beam is modulated with several sinusoidal waves of different wavelengths. The resulting time interval between the received signal and the transmitted signal is a result of the distance to the object. When the phase position of the transmitted and received signal is considered at the same time, a phase difference results which allows the object distance to be determined.
Hand-held 3D laser scanner
Handheld 3D laser scanners are also becoming increasingly popular. They allow a very flexible use, but have a shorter range than terrestrial laser scanners because they are more sensitive to disturbing environmental influences such as bright light.
application areas
Hand-held 3D laser scanners are no longer only used in retail stores to read barcodes, but are also used in other areas of application such as mechanical engineering, medical or biomechanical research, the reconstruction of car accidents, measurements in engineering or construction Refurbishment or restoration projects application. The scanner is used together with a tablet and offers real-time visualization of the point cloud data during the scanning process. It is possible for the scanner to record up to 88,000 points / second with an accuracy of less than 1.5 mm from a distance of up to three meters. An optical measuring system with self-compensation enables immediate scanning without a warm-up phase.
Application examples
Architecture and interior design
- Measurement of complex structures and objects
- Project supervision
- Monitoring of deviations
- quality control
- Supplement to Focus3D scans for larger projects
Restoration and 3D modeling
- Monitoring the construction progress
- Recording of the building stock
- Inspection of freeform components
- Deformation control
- reconstruction
- Restoration and conservation
Construction and property management
- As-built documentation
- Planning of structural changes
- New planning of technical modifications
forensic science
- Crime scene investigation and analysis
- digital evidence collection
- Process integration
- Availability
- Determination of the cause of the fire
Accident reconstruction
- Determination and analysis of the cause of traffic accidents
- passive vehicle safety
- Reconstruction of collisions
- digital evidence collection
- digital availability
functionality
At a distance of 0.5 to 3 meters, hand-held scanners can record objects and their surroundings from different angles and colors. Already during the scan it is possible to follow the recorded areas on a connected tablet. This way, no information can be lost when collecting data. The captured 3D data ( point clouds ) are saved on an SD card, which enables data to be transferred to a PC for further processing. The point clouds that have just been recorded can then be edited with various programs. In addition, the data can be exported for use in a CAD system, the point clouds obtained can be merged with other point clouds or shared online.
Workflow
First, the desired area is captured by starting the scanner and pointing it at the object to be scanned. At the push of a button, the scanner records everything that is in its field of vision and saves the point cloud on a storage medium so that data can be transferred to a PC for further processing. The point cloud can then be edited in various programs (e.g. RiSCAN, Faro Scene, PointCap ...) and combined with other point clouds. In order to share scan results that have already been processed, they can be shared using WebCloud, for example.
Confocal laser scanning
Confocal laser scanning is a special three-dimensional laser scanning method that is used in microscopy (see laser scanning microscope and ophthalmoscopy ). In 1957, Marvin Minsky registered a patent in which the basic principle of confocal microscopy is described for the first time. But it took another 30 years and the development of the laser as a light source before confocal microscopy became a standard microscopy technique.
Usual microscopes allow a detailed observation of an object through a two-stage, magnifying image. In this image, the microscope optics have a finite depth of field. This means that the image of the object is an overlay of a sharp image of the points in the focal plane and a fuzzy image of points outside the focal plane, which are still recognized as "sharp" by the detector (eye, camera line). This depth of field prevents object details from being dissolved in the axial direction. The confocal imaging reduces this depth of field extremely and also enables virtual optical cuts through the object with corresponding detailed information in the axial direction.
The principle of confocal laser scanning is based on point-to-point imaging, whereby a focused laser beam scans a sample sequentially point by point and line by line (in microscopy the object itself is sometimes moved instead) and the reflected light behind a small one Point aperture is detected to create an image. The pixel information is combined to form an image. Optical sections of the sample are imaged with high contrast and high resolution in the x, y and z directions. The arrangement of the diaphragm means that only light from the focal plane is detected and a sectional image is only obtained from this plane. How thick this layer is depends on the depth of field of the microscope used. If you change the focus between individual recordings, you can record a whole stack of images and thus obtain a 3D data set (see also confocal microscope ). There are two basic types of confocal microscopes, which differ in the type of scanning in the xy plane: confocal laser scanning microscopes and confocal microscopes with a rotating disk.
Material processing and manufacturing
If the laser power is sufficiently high, the scanned surface can be processed. Laser scanners are used in particular for engraving, welding and hardening. Laser scanning processes can also be used in rapid prototyping , for example to build up a prototype with the so-called laser sintering process from powder layers sintered locally by laser heating. Stereolithography machines use laser scanners to selectively harden liquid plastic and thereby build up three-dimensional plastic parts. Also, laser marker and machines for processing of ophthalmic lenses - for correction of refractive error - put a laser scanner. Another area of application is inside glass engraving .
Barcode reader
Many barcode readers use laser scanners. More on this under barcode reader .
See also
- tomography
- TOF camera
- Strip light scanning
- Lidar
- Survey aircraft
- List of programs for point cloud processing
Web links
- Imaging laser altimetry
- Laser scanning - a new method of city surveying. Laser scanning flight with a two-seater light aircraft from Diamond Airborne Sensing GmbH for the MA41 city survey over Vienna. Flight recordings Zeggl GmbH 2007 for PID Vienna. Available: Video 2:33. - An apparatus pod under each wing.
literature
- E. Heritage: 3D laser scanning for heritage. Advice and guidance to users on laser scanning in archeology and architecture. 2011. (historicengland.org.uk)
- G. Heritage, A. Large (Ed.): Laser scanning for the environmental sciences. John Wiley & Sons, 2009, ISBN 978-1-4051-5717-9 .
- M. Maltamo, E. Næsset, J. Vauhkonen: Forestry Applications of Airborne Laser Scanning: Concepts and Case Studies. (= Managing forest ecosystems. Vol. 27). Springer Science & Business Media, 2014, ISBN 978-94-017-8662-1 .
- J. Shan, CK Toth (Ed.): Topographic laser ranging and scanning: principles and processing. CRC press, 2008, ISBN 978-1-4200-5142-1 .
- G. Vosselman, HG Maas (Ed.): Airborne and terrestrial laser scanning. Whittles Publishing, 2010, ISBN 978-1-4398-2798-7 .
Individual evidence
- ↑ a b c d e f g h i j k l George Vosselman, Hans-Gerd Maas: Airborne and terrestrial laser scanning . Whittles Publishing, 2012, ISBN 978-1-904445-87-6 .
- ↑ Evaluating point clouds from drones (UAV - Unmanned Aerial Vehicle). Retrieved February 16, 2016 .
- ↑ RIEGL RiCOPTER with VUX-SYS. (No longer available online.) Archived from the original on January 31, 2016 ; accessed on February 16, 2016 .
- ^ C. Teutsch: Model-based Analysis and Evaluation of Point Sets from Optical 3D Laser Scanners. (= Magdeburg writings for visualization. Volume 1). Shaker Verlag , 2007, ISBN 978-3-8322-6775-9 .
- ↑ Confocal Scanning Microscopes. Retrieved February 16, 2016 .
- ↑ Confocal microscopy: Measure surfaces non-destructively and with high resolution. (No longer available online.) Archived from the original on February 2, 2016 ; accessed on February 16, 2016 .