The automated calibration of test specimens for X-ray microcomputed tomography is achieved by means of non-contact optical measurement with recourse to computer vision methods.
Microcomputed tomography is a 3D X-ray imaging technique. The method corresponds to the CT scans used in clinics, although the comparatively smaller systems of microcomputed tomography have a greatly increased resolution. They display the internal structure of objects non-destructively as a three-dimensional image with very fine resolution. Test specimens, so-called phantoms, are used for the geometric calibration of these tomographs. The phantoms consist of a plastic carrier with reflective metallic spheres attached to it. Relevant are the spatial distances between the sphere centres with an accuracy in the micrometre range and the traceability to an internationally standardised length standard. The non-contact determination of the distances between the sphere centres is achieved by means of industrial theodolites according to the principle of forward cutting. The calibrations of the phantoms, which were previously carried out manually, are now automated by modular digital camera theodolites based on the Leica TM5100 motorised theodolites. With the help of digital image processing, targeted reflections or their virtual image are determined with high precision on the reflecting, metallic spheres, supported by the geometric optics.
The modular digital camera theodolites are enhanced by LED lights mounted eccentrically on the telescope. This targeted illumination of the spheres enables automatic detection and centre determination. Manual aiming of the sphere centres is not necessary. In order to be able to verify the automatic calibration, a laboratory setup was designed so that a second independent measurement procedure could be applied. In addition, to observe the repeatability, a test specimen was made that allows the position of the phantom to be changed without changing the distance between the balls.
At the end of 2018, the automated calibration of test specimens for microcomputer tomographs successfully replaced the previous, manual measurement procedure. The reflections of the eccentric LEDs are used to take automatic theodolite targets to determine the sphere distances. In 2016, it had already become apparent in the theoretical and practical observations that the modelling effectively eliminates all relevant systematic deviations due to the eccentric attachment of the LEDs with regard to the precise angle determination to the sphere centres: The result of the verification showed a distance difference of 7μm. The repeated measurement with position change of the test specimen showed an achieved standard deviation of the mean value of ±2μm.