3D scanning interprets real objects or environments

by:Sinowon     2021-01-10

In the digital acquiring of geometric shapes, there are two types of classifications: contact and non-contact scanners. Non-contact scanners can further be categorized into active and passive.

Contact scanners

Contact 3D scanners use physical contact to get through the subject, and are mostly used to manufacture items. One of the most widely-used examples of a contact 3D scanner is a coordinate measuring machine (CMM). The disadvantage of this technology, however, depends on the contact with the object, as a simple act of scanning an object might damage it. This unfortunate circumstance is especially critical when scanning historical artifacts. These CMM's are relatively slow compared to laser scanners, as the latter can operate from 10 to 500 kHz, compared to contact 3D scanners' a few hundred.

Non-contact active scanners: Two of the most widely used examples:

Time-of-flight

This type of 3D scanning uses laser light to interpret the subject. A time-of-flight rangefinder usually calculates for the distance of a surface by measuring the time it takes for a pulse of light to return. This pulse of light comes from a laser and the amount of time it takes for the reflected light to be seen is noted down as well. Given that the rangefinder of these scanners only determines the distance of one point in its direction of view, the scanner will tend to scan the field one point at a time. The scan will then change the direction of the view to a new and different point through rotating the rangefinder by itself, or through mirrors. The advantage of this method lies in the capability to operate over long distances, making it ideal for buildings or landscapes. The main disadvantage, however, is accuracy. Given the speed of light, timing the complete trip is difficult and the distance measurement may not be accurate once the laser hits the edge of an object. Information will then be sent back from two different locations for only one pulse.

Triangulation

These scanners, similar to the previous example, also use lasers to gather data from the environment. This gets the camera to look for the location of the point. Depending on the distance from the laser to the surface, the point appears at different areas in the camera's field of view. In this process, both the angle of the laser emitter corner and the angle of the distance between the camera and the emitter are known. The angle of the camera corner can be determined by looking at where the laser point is in the camera's field of view. These three pieces of information determine the size and shape of the triangle and provides the exact location of the laser point of the system.

Non-contact passive

These scanners do not emit radiation themselves, but rely on detecting ambient radiation that is readily available, including infrared. This method can be cheap, given that in most cases only a simple digital camera is used. Examples of this classification include stereophonic systems (2 cameras, 1 scene), photometric systems (1 camera, various scenes in different lighting), and silhouette techniques (outlines created from a sequence of photographs around a three-dimensional object).

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