X-ray computed tomography

The ability of X-rays to penetrate objects is utilised for X-ray tomography. On its way through an object, some of the incident radiation is absorbed. The longer the radiographic length in the object, the less radiation escapes behind the object. However, absorption also depends on the material. An X-rays detector captures the incident X-rays as a two-dimensional radiographic image. With detector side lengths of approx. 50 mm to 400 mm, a large proportion of the measuring objects can be captured in one image.

Similar to measurement with image processing, it is possible to change the magnification in tomography scans in order to capture small parts with high magnification or larger parts completely with lower magnification (Fig. 37b). To do this, either the measuring object within the beam path or the X-ray components (X-ray source and detector) are shifted relative to the measuring object in an axial orientation. In some cases, however, the size of the sensors or the number of pixels available is not sufficient to scan tomographically large parts or small features with sufficient resolution. In such cases, the rotary table with the measuring object and the X-ray components are moved relative to each other. The images or volume sections recorded in this way are then precisely stitched together (Raster Tomography, Fig. 37c).

The application possibilities of X-ray tomography are practically only limited by the radiolucency of the objects to be measured and the accuracy requirements. This technique is therefore most widely used in plastic injection moulding. In addition to the rapid first article inspection of many features, the deviations from the CAD model caused by the manufacturing process can be captured.

The model for the tool is subsequently modified automatically or manually in a specific way using appropriate software functions. This allows corrected tools to be produced and process influences to be compensated for. Practical applications include the measurement of car headlights, connector modules, cutting edges of shaving heads for electric razors and the overall geometry of diesel injection nozzles. The method can also be used to check the size of components of mounted assemblies and internal geometries as well as to analyse materials (inclusions).

X-ray tomography has developed into an independent specialist field within coordinate metrology. The physical background, other measuring methods, areas of application and the topic of "accuracy" are described in more detail in volume 331 of the series Die Bibliothek der Technik[8].