Optical Measuring (white-light scanning)
Nowadays, valuable cultural treasures are increasingly being measured by means of white-light scanning in order to produce digital “back-up copies”. These copies are more accurate in detail than was possible in the photogrammetric measuring used previously. Optical measuring takes over the role of traditional plaster replicas.
Beams of light are projected onto the object (image 4). A stereo-camera records the lines of projection on both sides and derives measuring points from them with great precision. In this way, millions of points in space (point clouds) are generated. Between them, a triangulated mesh is generated with which it is possible to depict the surface of the object accurately. By means of registration marks (image 2), individual measurements from various spatial angles can be put together. As a result, objects of up to many meters in size can be measured with pinpoint accuracy.
The data set is saved as an STL file* and can be processed further using rapid prototyping** or with machining tools (5-axis-milling). The Atos system from the company Gom has particularly proven itself for the measuring of complex objects.
* STL = Stereo-lithography
** "Rapid prototyping", in the Kunstgiesserei this includes: laser sintering, stereo-lithography or 3D printing
Measuring with computer tomography
Using computer tomography, we position 900 to 1,200 x-ray images on a rotary disk across a central axis. The individual cross-sections are assembled into a three-dimensional data set from with the object can be reconstructed with a level of detail of up to 2 microns (two-thousandths of a millimeter). The point clouds obtained in this manner are here also supplemented to become a spatial mesh.
Since its invention in 1973, computer tomography has been important particularly in the field of medicine. Its advantage in comparison with optical measuring is that it makes it possible to obtain information about the interior of an object. Hollow cavities that are not recognizable from the outside, as well as indentations and complex surfaces can, therefore, also be depicted.
The Kunstgiesserei regularly works with 3D scanning in cooperation with regional and international partners.
Images 1–3: Urs Fischer "Thank You Fuck You"
Images 4–6: Urs Fischer "Fuck You Thank You"