How Does 3D Scanning Work?
One of the central concepts in laser 3D scanning is time-of-flight. This refers to the use of a laser range-finder, in order to time the round trip of a light pulse travelling from the scanner to the object and back again. Since the speed of light is a known constant, the distance between the scanner and the object can be accurately calculated by measuring the time it takes for a pulse of light to return to the scanner. Hence, the formula for calculating distance based on time-of-flight is:
Therefore, the accuracy of a 3D scanners time-of-flight sensor depends on the accuracy of the scanner’s internal chronometer. A key advantage of laser scanners is their ability is accurately measure longer distances. This is why they tend to be used for large-scale surveying applications. However, accurately measuring a pulse of light traveling at 186,000 miles per second is still very challenging.
3D Scanning – How Triangulation Works
Most 3D handheld scanners use triangulation to off-set the diminished accuracy that comes with time-of-flight measurements.
Some handheld laser scanners incorporate a camera that tracks the laser light projected onto scanned surface. This enables the 3D scanner to triangulate the distance of the objects much more accurately than with time-of-flight measurements alone. However, although triangulation enables higher accuracy, it also limits the effective range of the scanner, which is another key consideration.
Limitations Of 3D Scanning Accuracy & Realistic Expectations
The latest high-resolution 3D scanning technologies provide a highly efficient means of digitally acquiring objects in three dimensions, at relatively high detail. However, it is important to remember that such 3D scanning systems have a maximum resolution accuracy, which is determined by optical configuration, hardware processing and software design.
High-end 3D scanning systems incorporate measurement calibration into their software, in order to ensure a consistent measurement accuracy within a given tolerance. For example, a 3D scanning system with a resolution accuracy of up to + or – 0.5mm, means that the system is capable of wrapping polygons around the chosen object, which can be as small as + or – 0.5mm.
Although 0.5mm is generally considered to be a high level of accuracy for any mobile 3D scanning device, one needs to be realistic about the limitations of capture, especially when trying to 3D scan sharp edges on an object. In this situation, the software looks to wrap polygons around any sharp edge, which tends to have a softening effect due to the minimum size of polygons it can wrap.
For this reason, precision engineering applications tend to import 3D scans (usually in STL file format) into professional CAD software. These STL files requires need to be converted from a 3D polygon mesh file into a file format that can be read by the CAD software. Hence we tend to convert STL to STEP or perhaps IGES file format. This then enables a CAD technician to import the 3D file into CAD engineering software in order to tighten everything up. Once everything has been mastered in CAD, it can be output as a physical model using either CNC milling or 3D printing.
Hence, 3D scanning should not be thought of as a way of capturing an exact copy of an object, but more a means of capturing something very close to the original object, which can then be further developed and perfected in a number of different stages.