One might think a measurement using the same calibrated tool would have the same accuracy regardless of the object being measured.  But metrology is more than that, as this whitepaper will show. “Uncertainty of measurement” can be influenced by the object being measured, as well as the specific measurement task. 

The gold standard for 3D measurement equipment is the DIN EN ISO 10360 Geometrical Product Specification (GPS). It is designed to help compare different equipment and create a clear understanding of when a system is within the specification and when it is not.

In a CT system, X-ray photons emitted from an X-ray tube will travel through the object and hit a detector. The detector senses the dose at each pixel of the detector area. This creates digital images while the object is rotated. The reconstruction software calculates virtual slices based on the dose at each location from different angles. The result is called a tomogram. Each pixel in the tomogram represents a density of material, a voxel.

There is a range of terminology for CT system accuracy and uncertainty that should be understood when evaluating 3D scanning systems, this blog post illustrates some of those terms, such as Precision and Accuracy, Tolerance, Uncertainty of Measurement, Resolution, Error, and Suitability.

Computed tomography (CT) is the most effective technology for non-destructive testing (NDT).The dataset obtained from a CT scan allows users to examine material samples, observe defects, perform measurements and identify materials with different densities. Engineers, builders, researchers and scientists can therefore discover features that could not previously be examined without destroying the test object.

In this video Dirk Steiner compares two examples of scans at 550mm vs 1100mm detector distance using a micro-focus tube to illustrate how scan speed can be increased, and he shows the impact on image quality and dimensional deviation. 
 

Thanks to most recent developments, computed tomography (CT) is now an affordable and efficient
technology for non-destructive testing of workpieces made of a vast variety of materials. While digital radiography provides two-dimensional fluoroscopic images, computed tomography typically rotates the object between the X-ray tube and the flat-panel detector 360° and captures several hundred to well over a thousand X-ray images from various angles.