Laser scanning is governed by the LiDAR technology and is defined as the controlled deflection of laser beams, visible or invisible. 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. 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, and in archaeology.
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