Laser Scanning involves 3D point capture. The points are oriented in a point cloud and are a representation of 3D space. Multiple scans are unorganized until they are registered (stitched together).
Scanning is a non-invasive technology that can assist in the documentation and recording of existing buildings, features, and objects. A combination of scanning formats to achieve highly accurate results efficiently within millimeter or sub-millimeter accuracy. Point cloud data is registered to create a 3D model. Once registered, it can be exported to multiple formats to create 2D drawings or polygonal 3D CAD models, which can assist in the research/interpretation of historic structures.
Laser scanning is used in:
- Surveys (infrastructure)
- Cultural Heritage
There are a wide range of laser scanners. Time of Flight (ToF) operates by emitting a light pulse. Range coordinates are determined by calculating the difference between when the laser sends out the signal and when it returns. It can measure distances from a few feet to thousands of feet. Accuracy of the laser is greater over longer ranges in comparison to the phase-based scanner. Time of Flight scanners are more appropriate for scans of large buildings and landscapes or objects without high levels of details. Leica C10 is a great example of a Time of Flight scanner.
Phase-based Scanners (what we are using in class) emit a continuous laser scanner. Range coordinates are determined by calculating the difference between the wave-length when it was sent and the wave-length when it returns, rather than time. It can acquire points much faster than ToF. Objects must be in a certain distance for it to capture details.
Optical Triangulation Scanners use a laser and camera to work in unison to calculate the distance. Range coordinates are determined by calculating the distance between the project laser dot, the camera, and the laser. They have a limited range but are highly accurate and are ideal for scanning ornaments or detailed objects.
Structured Light Scanners use a combination of a projected light source and camera to determine 3D point values. A series of light patterns are projected onto an object and the 3D points are calculated by analyzing distortions in the pattern. These scanners have a limited range but are extremely fast and work best in low light conditions.
Considerations for choosing the right scanner:
- Scope of project (more than one scanner?)
- Size of the project
- Amount of detail to be captured
Limitations for scanners are numerous. Shiny or reflective surfaces will not reflect light accurately back to the scanner. Dark surfaces will absorb the light and not reflect it back to the scanner. Transparent objects allow light to pass through, making it difficult to capture shapes.
- Data Storage
- Power Source (internal and external)
The data capture process includes collecting point cloud data of the object or structure being scanned. Making use of multiple scan locations with 30% overlap. Processing and registering the collected information follows. The point cloud data is converted to a 3D mesh. Scan registration can either be manual or automatic. Manual involves manually identifying points between scans. Automatic registration registers the scans using software.Output can include 2D drawings in AutoCAD using the point cloud or as a basis for 3D models.
The scanner we will be using in class is the NextEngine Scanner, an optical triangulation scanner.
Image Sources: http://aqua.epfl.ch/page-96308-en.html http://www.tankonyvtar.hu/en/tartalom/tamop425/0032_precizios_mezogazdasag/ch02s04.html http://3dscanningservices.net/blog/need-know-3d-scanning/ http://www.3ders.org/articles/20150409-teacher-builds-diy-structured-light-3d-scanner-using-a-video-projector-and-webcams.html