Many different technologies can be used to build these 3D-scanning devices; each technology comes with its own limitations, advantages and costs. Many limitations in the kind of objects that can be digitised are still present, for example, optical technologies encounter many difficulties with shiny, mirroring or transparent objects. For example, industrial computed tomography scanning can be used to construct digital 3D models, applying non-destructive testing.
Collected 3D data is useful for a wide variety of applications. These devices are used extensively by the entertainment industry in the production of movies and video games. Other common applications of this technology include industrial design, orthotics and prosthetics, reverse engineering and prototyping, quality control/inspection and documentation of cultural artifacts.
4.5.5 "Plastico di Roma antica"
The purpose of a 3D scanner is usually to create a point cloud of geometric samples on the surface of the subject. These points can then be used to extrapolate the shape of the subject (a process called reconstruction). If colour information is collected at each point, then the colours on the surface of the subject can also be determined.
3D scanners share several traits with cameras. Like cameras, they have a cone-like field of view, and like cameras, they can only collect information about surfaces that are not obscured. While a camera collects colour information about surfaces within its field of view, a 3D scanner collects distance information about surfaces within its field of view. The "picture" produced by a 3D scanner describes the distance to a surface at each point in the picture. This allows the three dimensional position of each point in the picture to be identified.
For most situations, a single scan will not produce a complete model of the subject. Multiple scans, even hundreds, from many different directions are usually required to obtain information about all sides of the subject. These scans have to be brought into a common reference system, a process that is usually called alignment or registration, and then merged to create a complete model. This whole process, going from the single range map to the whole model, is usually known as the 3D scanning pipeline.
The time-of-flight 3D laser scanner is an active scanner that uses laser light to probe the subject. At the heart of this type of scanner is a time-of-flight laser range finder. The laser range finder finds the distance of a surface by timing the round-trip time of a pulse of light. A laser is used to emit a pulse of light and the amount of time before the reflected light is seen by a detector is measured. Since the speed of light <math>c</math> is known, the round-trip time determines the travel distance of the light, which is twice the distance between the scanner and the surface. If <math>t</math> is the round-trip time, then distance is equal to <math> \textstyle c \! \cdot \! t / 2</math>. The accuracy of a time-of-flight 3D laser scanner depends on how precisely we can measure the <math>t</math> time: 3.3 picoseconds (approx.) is the time taken for light to travel 1 millimetre.
Recently, there has been research on compensating for distortion from small amounts of vibration  and distortions due to motion and/or rotation.
Conoscopic holography measures distances by using the polarization properties of a converging light cone that reflect from an object. At the core of the technology stands an anisotropic crystal: a ray that traverses it splits into two components that share the same path but have orthogonal polarizations. The crystal’s anisotropic structure forces each of the polarized light rays to propagate at a different velocity, thus creating a phase difference between them. This phase difference enables the formation of an interference pattern that varies with the distance from the object under measurement.
In classical holography, a hologram is created by recording an interference pattern formed between an object beam and a reference beam using a coherent light source. The two beams propagate at the same velocity (same refractive index), but follow different geometric paths. This means that when overlapped, the phase difference between the two beams depends only on the geometric path difference. This phase difference is responsible for the creation of a measurable interference pattern that can later be used to reconstruct the original light field.
In conoscopic holography, however, a light beam that traverses an optically anisotropic crystal is split into two beams that share the same geometric path but have orthogonal polarization modes. The refractive indices of these two beams generally differ from each other. Therefore, after the two beams exit the crystal an interference pattern is generated. The features of this pattern depend on the distance from the light's source.
Since both beams propagate through the same geometric path, conoscopic holography is highly stable in comparison to interferometry-based measurement techniques. Moreover, it is also possible to perform measurements using incoherent light. Conoscopic Holography was first introduced by Optimet (Optical Metrology Ltd) and the company has several technological patents in this field. It was founded in 1995 by Ophir Optronics Solutions a part of Newport Corporation. The sensor sometimes called Laser displacement sensor, or distance sensor, eventually provide a cloud of points which a 3D model can be extract of it.
The sensor emits an eye-safe laser beam, which is focused by an objective lens, and hits the specimen being measured. Part of the scattered light travels back from the specimen into the sensor, and enters the conoscopic unit that contains the optically anisotropic crystal. The resulting interference pattern is detected, and signal processing algorithms are then used to retrieve the distance information from the measured data.
Conoscopic Holography sensor allows both diffusive and reflective material measurement, and its unique advantage is wide angle coverage of up to 170 deg, high accuracy of down to 1 micron, and the ability to go into small holes to measure bottom and side walls.
Using a periscope allows to into small diameter holes and measure bottom and side walls.
Hand-held laser scanners create a 3D image through the triangulation mechanism described above: a laser dot or line is projected onto an object from a hand-held device and a sensor (typically a charge-coupled device or position sensitive device) measures the distance to the surface. Data is collected in relation to an internal coordinate system and therefore to collect data where the scanner is in motion the position of the scanner must be determined. The position can be determined by the scanner using reference features on the surface being scanned (typically adhesive reflective tabs, but natural features have been also used in research work ) or by using an external tracking method. External tracking often takes the form of a laser tracker (to provide the sensor position) with integrated camera (to determine the orientation of the scanner) or a photogrammetric solution using 3 or more cameras providing the complete Six degrees of freedom of the scanner. Both techniques tend to use infra red Light-emitting diodes attached to the scanner which are seen by the camera(s) through filters providing resilience to ambient lighting.
Data is collected by a computer and recorded as data points within Three-dimensional space, with processing this can be converted into a triangulated mesh and then a Computer-aided design model, often as Non uniform rational B-spline surfaces. Hand-held laser scanners can combine this data with passive, visible-light sensors — which capture surface textures and colours — to build (or "reverse engineer") a full 3D model.
The advantage of structured-light 3D scanners is speed and precision. Instead of scanning one point at a time, structured light scanners scan multiple points or the entire field of view at once. Scanning an entire field of view in a fraction of a second generates profiles that are exponentially more precise than laser triangulation. This reduces or eliminates the problem of distortion from motion. Some existing systems are capable of scanning moving objects in real-time. VisionMaster creates a 3D scanning system with a 5-megapixel camera – 5 million data points are acquired in every frame.
A real-time scanner using digital fringe projection and phase-shifting technique (certain kinds of structured light methods) was developed, to capture, reconstruct, and render high-density details of dynamically deformable objects (such as facial expressions) at 40 frames per second. Recently, another scanner has been developed. Different patterns can be applied to this system, and the frame rate for capturing and data processing achieves 120 frames per second. It can also scan isolated surfaces, for example two moving hands. By utilising the binary defocusing technique, speed breakthroughs have been made that could reach hundreds of  to thousands of frames per second.
Surface models: The next level of sophistication in modeling involves using a quilt of curved surface patches to model our shape. These might be NURBS, TSplines or other curved representations of curved topology. Using NURBS, our sphere is a true mathematical sphere. Some applications offer patch layout by hand but the best in class offer both automated patch layout and manual layout. These patches have the advantage of being lighter and more manipulable when exported to CAD. Surface models are somewhat editable, but only in a sculptural sense of pushing and pulling to deform the surface. This representation lends itself well to modelling organic and artistic shapes. Providers of surface modellers include Rapidform, Geomagic, Rhino 3D, Maya, T Splines etc.
Solid CAD models: From an engineering/manufacturing perspective, the ultimate representation of a digitised shape is the editable, parametric CAD model. After all, CAD is the common "language" of industry to describe, edit and maintain the shape of the enterprise's assets. In CAD, our sphere is described by parametric features which are easily edited by changing a value (e.g., centre point and radius).
3D reconstruction of the brain and eyeballs from CT scanned DICOM images. In this image, areas with the density of bone or air were made transparent, and the slices stacked up in an approximate free-space alignment. The outer ring of material around the brain are the soft tissues of skin and muscle on the outside of the skull. A black box encloses the slices to provide the black background. Since these are simply 2D images stacked up, when viewed on edge the slices disappear since they have effectively zero thickness. Each DICOM scan represents about 5mm of material averaged into a thin slice.
The laser power used for laser scanning equipment in industrial applications is typically less than 1W. The power level is usually on the order of 200 mW or less.
In 2003, Subodh Kumar, et al. undertook the 3D scanning of ancient cuneiform tablets. Again, a laser triangulation scanner was used. The tablets were scanned on a regular grid pattern at a resolution of Script error: No such module "convert"..
The process of comparing 3D data against a CAD model is referred to as CAD-Compare, and can be a useful technique for applications such as determining wear patterns on moulds and tooling, determining accuracy of final build, analysing gap and flush, or analysing highly complex sculpted surfaces. At present, laser triangulation scanners, structured light and contact scanning are the predominant technologies employed for industrial purposes, with contact scanning remaining the slowest, but overall most accurate option.
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40x40px Wikimedia Commons has media related to 3D scanning.
What’s the Right 3D Scanner for You?.
"Evaluation of Conoscopic Holography for Estimating Tumor Resection Cavities in Model-Based Image-Guided Neurosurgery". IEEE Transactions on Biomedical Engineering 61: 1833–1843. doi:10.1109/TBME.2014.2308299.
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