Laser scanning

Last updated April 07, 2024

Laser scanning is the controlled deflection of laser beams, visible or invisible.^[1] Scanned laser beams are used in some 3-D printers, in rapid prototyping, in machines for material processing, in laser engraving machines, in ophthalmological laser systems for the treatment of presbyopia, in confocal microscopy, in laser printers, in laser shows, in Laser TV, and in barcode scanners. Applications specific to mapping and 3D object reconstruction are known as 3D laser scanner .

Technology

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 polygon mirror in a barcode scanner or so-called resonant galvanometer scanners - or to a freely addressable motion, as in servo-controlled galvanometer scanners. One also uses the terms raster scanning and vector scanning to distinguish the two situations. To control the scanning motion, scanners need a rotary encoder and control electronics that provide, for a desired angle or phase, the suitable electric current to the motor (for a polygon mirror) or galvanometer (also called galvos). A software system usually controls the 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 polygon (polygonal) mirrors is then driven by a galvanometer or by an electric motor respectively. Two-dimensional systems are essential for most applications in material processing, confocal microscopy, and medical science. Some applications require positioning 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 with two mirrors.

The most common way to move mirrors is, as mentioned, the use of an electric motor or of a galvanometer. However, piezoelectric actuators or magnetostrictive actuators are alternative options. They offer higher achievable angular speeds, but often at the expense of smaller achievable maximum angles. There are also microscanners, which are MEMS devices containing a small (millimeter) mirror that has controllable tilt in one or two dimensions; these are used in pico projectors.

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.

When two optical lenses are moved or rotated against each other, a laser beam can be scanned in a way similar to mirror scanners.

Material effects

Some special laser scanners use, instead of moving mirrors, acousto-optic deflectors or electro-optic 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.

Phased array scanning

Research is going on to achieve scanning of laser beams through phased arrays. 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 foreseeable future.

Applications

3D object scanning

Within the field of 3D object scanning, laser scanning (also known as lidar) combines controlled steering of laser beams with a laser rangefinder. By taking a distance measurement at every direction the scanner rapidly captures the surface shape of objects, buildings and landscapes. Construction of a full 3D model involves combining multiple surface models obtained from different viewing angles, or the admixing of other known constraints. Small objects can be placed on a revolving pedestal, in a technique akin to photogrammetry.^[2]

3D object scanning 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,^[3] and in archaeology.

Material processing

Depending on the power of the laser, its influence on a working piece differs: lower power values are used for laser engraving 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 or laser melting.

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 curved 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 readers, 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.^{[ citation needed ]}

Space flight

When a space transporter has to dock to the space station, it must carefully maneuver to the correct position. In order to determine its relative position to the space station, laser scanners built into the front of the space transporter scan the shape of the space station and then determine, through a computer, the maneuvering commands. Resonant galvanometer scanners are used for this application.

Laser shows

Laser light shows typically uses two galvanometer scanners on an X-Y configuration to draw patterns or images on walls, ceilings or other surfaces including theatrical smoke and fog for entertainment or promotional purposes.^{[ citation needed ]}

Related Research Articles

Scanner may refer to:

A galvanometer is an electromechanical measuring instrument for electric current. Early galvanometers were uncalibrated, but improved versions, called ammeters, were calibrated and could measure the flow of current more precisely. Galvanometers work by deflecting a pointer in response to an electric current flowing through a coil in a constant magnetic field. The mechanism is also used as an actuator in applications such as hard disks.

Lidar is a method for determining ranges by targeting an object or a surface with a laser and measuring the time for the reflected light to return to the receiver. Lidar may operate in a fixed direction or it may scan multiple directions, in which case it is known as lidar scanning or 3D laser scanning, a special combination of 3-D scanning and laser scanning. Lidar has terrestrial, airborne, and mobile applications.

<span class="mw-page-title-main">Barcode</span> Optical machine-readable representation of data

A barcode or bar code is a method of representing data in a visual, machine-readable form. Initially, barcodes represented data by varying the widths, spacings and sizes of parallel lines. These barcodes, now commonly referred to as linear or one-dimensional (1D), can be scanned by special optical scanners, called barcode readers, of which there are several types.

A mirror galvanometer is an ammeter that indicates it has sensed an electric current by deflecting a light beam with a mirror. The beam of light projected on a scale acts as a long massless pointer. In 1826, Johann Christian Poggendorff developed the mirror galvanometer for detecting electric currents. The apparatus is also known as a spot galvanometer after the spot of light produced in some models.

A laser lighting display or laser light show involves the use of laser light to entertain an audience. A laser light show may consist only of projected laser beams set to music, or may accompany another form of entertainment, typically musical performances.

A barcode reader or barcode scanner is an optical scanner that can read printed barcodes, decode the data contained in the barcode on a computer. Like a flatbed scanner, it consists of a light source, a lens, and a light sensor for translating optical impulses into electrical signals. Additionally, nearly all barcode readers contain decoder circuitry that can analyse the barcode's image data provided by the sensor and send the barcode's content to the scanner's output port.

An image scanner—often abbreviated to just scanner—is a device that optically scans images, printed text, handwriting or an object and converts it to a digital image. Commonly used in offices are variations of the desktop flatbed scanner where the document is placed on a glass window for scanning. Hand-held scanners, where the device is moved by hand, have evolved from text scanning "wands" to 3D scanners used for industrial design, reverse engineering, test and measurement, orthotics, gaming and other applications. Mechanically driven scanners that move the document are typically used for large-format documents, where a flatbed design would be impractical.

A coordinate-measuring machine (CMM) is a device that measures the geometry of physical objects by sensing discrete points on the surface of the object with a probe. Various types of probes are used in CMMs, the most common being mechanical and laser sensors, though optical and white light sensors do exist. Depending on the machine, the probe position may be manually controlled by an operator, or it may be computer controlled. CMMs typically specify a probe's position in terms of its displacement from a reference position in a three-dimensional Cartesian coordinate system. In addition to moving the probe along the X, Y, and Z axes, many machines also allow the probe angle to be controlled to allow measurement of surfaces that would otherwise be unreachable.

3D scanning is the process of analyzing a real-world object or environment to collect three dimensional data of its shape and possibly its appearance. The collected data can then be used to construct digital 3D models.

The following are common definitions related to the machine vision field.

A structured-light 3D scanner is a 3D scanning device for measuring the three-dimensional shape of an object using projected light patterns and a camera system.

A laser projector is a device that projects changing laser beams on a screen to create a moving image for entertainment or professional use. It consists of a housing that contains lasers, mirrors, galvanometer scanners, and other optical components. A laser projector may contain one laser light source for single-color projection or three sources for RGB full color projection.

Laser Design Inc. is a company headquartered in Minneapolis, MN that designs, manufactures, and sells 3D laser scanners, used to digitally capture the shape of physical objects such as free-form surfaces and geometries.

<span class="mw-page-title-main">Digital micromirror device</span>

The digital micromirror device, or DMD, is the microoptoelectromechanical system (MOEMS) that is the core of the trademarked Digital Light Processing (DLP) projection technology from Texas Instruments (TI). Texas Instrument's DMD was created by solid-state physicist and TI Fellow Emeritus Dr. Larry Hornbeck in 1987. However, the technology goes back to 1973 with Harvey C. Nathanson's use of millions of microscopically small moving mirrors to create a video display of the type now found in digital projectors.

<span class="mw-page-title-main">Microscanner</span>

A microscanner, or micro scanning mirror, is a microoptoelectromechanical system (MOEMS) in the category of micromirror actuators for dynamic light modulation. Depending upon the type of microscanner, the modulatory movement of a single mirror can be either translatory or rotational, on one or two axes. In the first case, a phase shifting effect takes place. In the second case, the incident light wave is deflected.

Endomicroscopy is a technique for obtaining histology-like images from inside the human body in real-time, a process known as ‘optical biopsy’. It generally refers to fluorescence confocal microscopy, although multi-photon microscopy and optical coherence tomography have also been adapted for endoscopic use. Commercially available clinical and pre-clinical endomicroscopes can achieve a resolution on the order of a micrometre, have a field-of-view of several hundred µm, and are compatible with fluorophores which are excitable using 488 nm laser light. The main clinical applications are currently in imaging of the tumour margins of the brain and gastro-intestinal tract, particularly for the diagnosis and characterisation of Barrett’s Esophagus, pancreatic cysts and colorectal lesions. A number of pre-clinical and transnational applications have been developed for endomicroscopy as it enables researchers to perform live animal imaging. Major pre-clinical applications are in gastro-intestinal tract, toumour margin detection, uterine complications, ischaemia, live imaging of cartilage and tendon and organoid imaging.

Three-dimensional (3D) microfabrication refers to manufacturing techniques that involve the layering of materials to produce a three-dimensional structure at a microscopic scale. These structures are usually on the scale of micrometers and are popular in microelectronics and microelectromechanical systems.

Laser welding of polymers is a set of methods used to join polymeric components through the use of a laser. It can be performed using CO₂ lasers, Nd:YAG lasers, Diode lasers and Fiber lasers.

References

↑ Gerald F. Marshall Handbook of Optical and Laser Scanning, Marcel Dekker, Inc., 2004, ISBN 0-8247-5569-3
↑ Dassot, M., Constant, T., & Fournier, M. (2011). The use of terrestrial LiDAR technology in forest science: application fields, benefits and challenges. Annals of forest science, 68(5), 959-974.
↑ "3D models help preserve landmarks like Notre Dame". PBS .

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[1] Gerald F. Marshall Handbook of Optical and Laser Scanning, Marcel Dekker, Inc., 2004, ISBN 0-8247-5569-3

[2] Dassot, M., Constant, T., & Fournier, M. (2011). The use of terrestrial LiDAR technology in forest science: application fields, benefits and challenges. Annals of forest science, 68(5), 959-974.

[3] "3D models help preserve landmarks like Notre Dame". PBS .

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