Computed Tomography and Metrology
Fraunhofer Institute for Integrated Circuits
Data Generation
Testing time, image quality and precision are the criteria on which the method of data generation is chosen.
Measurement Using 2D CT / 3D CT / Helical CT
CT systems
being used in industrial metrology can be divided into two classes, according to their measurement principle: Fan beam (2D CT) and cone beam (3D CT) computed tomography. The decisive advantage of 3D CT over 2D CT is its much shorter measurement time. However, a stronger influence of scattered radiation is to be expected, thus reducing image quality.
One further development is the Helical CT
. Using this method image acquisition is enhanced by reducing (Feldkamp) artifacts. Due to continuous object movement on a helical trajectory a complete data set throughout the entire volume is obtained. This way , in contrast to the conventional method, even fine inner structures and geometries are measurable.
Surface Extraction (STL Data Generation)
The generation of STL data from volume data is a standard method to represent objects and to inspect them metrologically. These geometrical values are required for further data preparation (e.g. registration, actual/nominal comparison). The number of triangles has an important influence on the accuracy of object representation, especially when edges are concerned.
Correction techniques for the Improvement of Precision
- This figure shows a slice of the reconstruction volume of an aluminium casting before (top) ans after (bottom) an Interative Artefact Reduction (IAR) was applied.
Apart from device specific perturbations various physical effects are also reducing the image quality of CT reconstructions. The main effects here are scattered radiation and beam hardening. The reason for that is the polychromatic nature of the X-rays used. Artifacts represent themselves by blurring, loss of contrast and geometry deviations.
For artifact reduction the in-house developed software tool IAR is available. This tool has the following properties:
- Correction of beam hardening for single- or multi-material specimens
- Synchronous artifact reduction
- Correction of scattered radiation
Set Bookmark