X-ray Computed Tomography for Dimensional Measurements

Industrial X-ray computed tomography (CT) systems have the ability to map internal and external structures simultaneously in a non-destructive way with high imaging resolution. Recently, there has been an increase of surveys in the field of dimensional metrology referring to CT as a tool for nondestructive dimensional quality control (i.e., traceable measurement and geometrical tolerance verification of industrial components). This increase in surveys runs parallel to the growth of commercial markets for industrial X-ray CT technologies and research institutes as well as metrology-governing bodies’ growing interest in creating standarization. Currently, there is a lack of international standards that provide comprehensive procedures and guidelines for dealing with the verification of CT systems’ dimensional metrology performance and developing task-specific measurement uncertainty budgets in compliance with the Guide to the Expression of Uncertainty in Measurement (GUM). To overcome this, some CT manufactures have opted to design their own calibration methods so that they can provide an estimate of maximum permissible error (MPE) for the measurements obtained with systems dedicated to metrology tasks. Essentially, the traceability of the instrument to the meter is provided with an expanded uncertainty upper-bounded by the MPE. In an effort to clarify some of these concepts, this paper gives a brief review of the use of X-ray CT for dimensional metrology with an update on the international attempt to create standards for metrological testing and uncertainty assessment with this technique. An example of in-house calibration is presented, which found deviations in the range -4.4 μm to 3.5 μm between CT measurements and calibrated references obtained at the National Institute of Standards and Technology (NIST), and this is contrasted to the MPE limits pre-established for CT measurement. A particular emphasis is made in the understanding of the terms “trueness,” “precision,” “accuracy,” and “uncertainty,” so the main metrology-related terminology is revisited with reference to international standards and other guidelines. It is concluded that while in-house calibrations might suffice, international standards are still needed, not only to reach homogeneity in the commercial market but also to avoid misinterpretations. In addition, users and manufacturers from the industry of measuring equipment need to better understand the terms “accuracy” and “uncertainty,” which are often misused and interchanged.

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