Calibration of X-ray computed tomography for surface topography measurement using metrological characteristics

X-ray computed tomography (XCT) has been shown to be capable of capturing surface topography information comparable to that acquired using established surface measurement systems. However, calibration of XCT for surface measurement has not yet been achieved. The factors that influence XCT measurement are numerous, and the ways in which these factors affect measurements are not always clear. However, the problem of understanding complex, numerous influence factors exists for other measurement technologies as well, with factors being difficult or impossible to quantify in many cases, often because they cannot be measured in isolation from other influence factors. In measurement of areal surface topography, a calibration framework based on understanding the metrological characteristics (MCs) of an instrument has been developed to address the uncertainty problem; recently attaining international acceptance with the publication of ISO 25178-600. This framework was developed to allow calibration without characterising all the influence factors individually. Instead of examining all possible factors within a measurement system, the framework examines a few standardised characteristics of the system (i.e. MCs), as an approximate means to understanding measurement uncertainty. While some individual influence factors are inevitably double-counted (in that some factors contribute to more than one MC), the MC framework can be used to provide calibration for complex systems, with the caveat that double counting leads to slight increases in the determined measurement uncertainties. In this work, we apply the ISO 25178-600 MC framework to an XCT measurement system, demonstrating a method of calibrating such a system for surface measurement. We present a material measure to be used for this purpose. Given the novelty of applying the MC framework to XCT measurement and the associated complexities, we also provide an assessment of the MC framework with respect to its suitability for calibrating XCT systems.