Metrological investigation of radiation filtration in x-ray computed tomography

X-ray Computed Tomography (CT) is a non-destructive evaluation method that gains recognition in dimensional metrology applications. During a CT scan, 2D radiographs are collected that are then reconstructed to a 3D model which allows non-destructive evaluation of the specimen and assists with the characterisations of defects. This technology has the potential to provide information unobtainable by other non-destructive, non-contact techniques. There are numerous factors that can affect the CT measurement results with varying measurement uncertainty such as shifts and changes of x-ray focal spot, geometrical alignment issues and environmental issues. This study investigates the effect of physical pre and post filtration on the resultant dimensional measurements. Here, a calibrated specimen is used based on one of the considered designs for the proposed ISO 10360-11 CT for dimensional measurements. The resultant in local variations of grey values in the volume is measured for different thicknesses of copper filtering placed before and after the scanned specimen, and the resultant threshold dependent and independent measurements are evaluated. The results provide information that assists the selection of filtration in order to reduce measurement error and improve dimensional measurements, and also demonstrate that post filtration should be considered when scanning specimens that generate large amounts of scatter.