3D scanning technologies and the 3D scanning process
3D scanning is a technique to capture the shape of an object using a 3D scanner. The result is a 3D file of the object on a computer, which can be saved, edited, and even 3D printed. Many different technologies can be used to 3D scan objects, environments and people; each technology comes with its own limitations, advantages and costs.
The basic principle is to use a 3D scanner to collect data about a subject.
The subject can be:
- an object
- an environment (such as a room)
- a person
Some 3D scanner can collect simultaneously shape and color data. A 3D scanned color surface is called a texture.
The 3D scans are compatible with Computer Aided Design (CAD) software and also 3D printing, after a little preparation on the computer. A 3D scan can give a lot of information about the design of an object, in a process called reverse engineering.
3D scanners are powerful tools for professionals in several industries, such as automotive, aeronautics, dental, jewelry, video games, special effects and animation movies.
The 3D scanning technologies rely on different physical principles and can be classified in categories:
- Laser triangulation 3D scanning technology, as illustrated on the image, projects a laser beam on a surface and measures the deformation of the laser ray.
- Structured light 3D scanning technology measures the deformation of a light pattern on a surface to 3D scan the shape of the surface.
- Photogrammetry, also called 3D scan from photographies, reconstructs in 3D a subject from 2D captures with computer vision and computational geometry algorithms.
- Contact based 3D scanning technology relies on the sampling of several points on a surface, measured by the deformation of a probe.
A 3D scanner in action (source: Artec).
- Laser pulse (also called time of flight) 3D scanning technology is based on the time of flight of a laser beam. The laser beam is projected on a surface and collected on a sensor. The time of travel of the laser between its emission and reception gives the surface’s geometrical information.
3D scanning technologies
Laser triangulation based 3D scanners use either a laser line or a single laser point to scan across an object.
The laser is first casted by the 3D scanner. As the laser light reflects off the scanned object, its initial trajectory is modified and picked up by a sensor.
From the modification of the laser trajectory and trigonometric triangulation, the system can discern a specific deviation angle. The calculated angle is directly linked to the distance from the object to the scanner. When the 3D scanner collects enough distances, it is capable of mapping the surface’s object and to create a 3D scan.
The main advantages 0f the laser triangulation technology for 3D scanning are its resolution and accuracy.
One downside of the laser triangulation technology is its sensibility to the properties of the surface to 3D scan. Very shiny or transparent surfaces are particularly problematic.
Structured light 3D scanners use trigonometric triangulation but do not rely on a laser. Instead, the structured light 3D scanning technology works with the projection of a series of linear patterns onto an object.
The system is then capable to examine the edges of each line in the pattern and to calculate the distance from the scanner to the object’s surface.
The structured light used for 3D scanning can be white or blue and generated by numerous types of projectors, such as Digital Light Processing (DLP) technology. The projected pattern is usually a series of light rays but can also be a randomized dot matrix.
The main advantages of the structured light technology for 3D scanning are its speed, resolution and ability to 3D scan people.
One downside of the structured light technology is its sensibility to lighting conditions and issues to work outside.
Photography – Photogrammetry
Photogrammetry is the science of making measurements from photographs, especially for recovering the exact positions of surface points. Photogrammetry is based on a mix of computer vision and powerful computational geometry algorithms.
The principle of photogrammetry is to analyze several photographs of a static subject, taken from different viewpoints, and to automatically detect pixels corresponding to a same physical point.
The data input required from the user are the parameters of the camera such as focal length and lens distortion.
The main challenge for this 3D scanning technology is to analyse many photos and thousands of points with high accuracy. A very powerful computer is required to run photogrammetry algorithms.
The main advantages of the photogrammetry technology for 3D scanning are its precision and acquisition speed. The photogrammetric technology is also capable to reconstruct subjects of various scales, photographed from the ground or from the air.
One downside of the photogrammetry technology is its sensibility to the resolution of the input photographs and the time it takes to run the algorithms.
Contact based 3D scanning
Contact based 3D scanning is also known as digitizing. The contact technology for 3D scanning implies a contact based form of 3D data collection.
Contact 3D scanners probe the subject through physical touch, while the object is firmly hold in place. A touching probe is moved on the surface to various points of the object to record 3D information.
The probe is sometimes attached to a articulated arm capable of collecting all its respective configurations and angles for more precision. Some specific configurations of contact based 3D scanners are called Coordinated Measuring Machines (CMM).
Contact 3D scanning is widely used for performing quality control of parts after fabrication or during maintenance operations.
The main advantages 0f the contact technology for 3D scanning are its precision and ability to scan transparent or reflective surfaces.
The downsides of the contact 3D scanning technology is its speed and inadequacy to work with organic freeform shapes.
Laser pulse-based 3D scanners
The Laser pulse-based 3D scanners, also known as time-of-flight scanners or Lidar, measure how long a casted laser takes to hit an object and come back.
Because the speed of light is exactly known, the time it takes for the laser to do the way back trip gives the exact distance between the 3D scanner and the object.
In order to measure precisely the distance, the 3D scanner requires to compute millions of laser’s pulses with a picosecond (0.000000001 second!) accuracy.
Because each measure only collects one point, the 3D scanner needs to cast its laser 360 degrees around it. To perform this feature, the 3D scanner is usually fitted with a mirror which changes the orientation of the laser.
Time of flight 3D scanners encompass both laser pulse and phase shift lasers. Phase shift laser 3D scanners are a sub-category of laser pulse 3D scanners. In addition to pulsing the laser, the phase shift systems also modulate the power of the laser beam. The phase shift lasers offer a better overall performance.
The main advantage of the laser pulse 3D scanners is their ability to 3D scan very big objects and environments. They are however quite slow.