Beam hardening correction and its influence on the measurement accuracy and repeatability for CT dimensional metrology applications

The beam hardening effect is one of the major artifacts of X-ray computed tomography. It not only complicates medical inspection and material analysis, but also influences the accuracy and repeatability of dimensional measurements. Therefore, many efforts have been devoted to develop beam hardening correction methods since the early 1970s. In practice, the combination of hardware filters (pre-filtration) and linearization algorithms is frequently used. In general, this combination can largely eliminate the cupping effect, resulting in more homogeneous gray values throughout the same material and improving the surface appearance. However, while correcting for the beam hardening effect, other problems are generated, including noise magnification and contrast reduction. Moreover, experiments reveal that in some cases, beam hardening correction methods induce edge offsets of internal structures that are largely dependent on the amount of surrounding material. This worsens the measurement uncertainty. Therefore, this paper investigates the influence of beam hardening correction on the measurement accuracy and uncertainty for CT metrology applications.