On the Relationship of Edge Responses to Length Measurements in Industrial X-ray Computed Tomography

Matern, Dierck; YXLON International GmbH; Germany

Matern, D.; YXLON International GmbH; Germany
Herold, F.; YXLON International GmbH; Germany

ID: ECNDT-0038-2018
Download: PDF
Session: CT-Applications 1
Room: G3
Date: 2018-06-13
Time: 09:00 - 09:20

The determination of lengths is one of the tasks in the dimensional measurements. Using tactile coordinate measurement machines (CMMs), we can find the length using two states: there is either contact with the surface or no contact, and the length is the difference between two contact points. In contrast, using X-ray computed tomography (CT), the information is included in volume element (voxel) positions, sizes and grey values. Latter usually are estimated absorption coeffcients at the respective coordinates, averaged over the specifc element. Hence, the information where specifcally in this element the absorption occurs is unknown. Furthermore, there are many influencing factors which also alter the estimation of absorption, like scattering and beam hardening, the used spectrum of the X-ray source, and the absorption spectrum of the detector or the number of used projections. Those properties can be shortly described as -image quality- if we understand the tomogram as a three dimensional image, resulting from the CT. Due to all of those factors, the surface is not as well defined as with CMMs. As a lot of those factors are usually properties of the individual scan and arise from the object itself, it is problematic to transfer results of the study of test artefacts onto the -real- objects. Therefore it sometimes can be more useful to analyse the local grey value distribution to determine an uncertainty of the length measurement, rather than transferring results from the tactile standards to dimensional measurement in X-ray CT. Especially, if we are interested in the lengths, it can be important to study the local edge responses (which is the influence of an edge of the scanned object onto the tomogram), as the length can be described as the distance between two edges. In this paper, we discuss this relationship with exemplary tomograms.