EN 16016-3:2011

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.Zerstörungsfreie Prüfung - Durchstrahlungsverfahren - Computertomographie - Teil 3: Durchführung und AuswertungEssais non destructifs - Moyens utilisant les rayonnements - Tomographie informatisée - Partie 3: Fonctionnement et interprétationNon destructive testing - Radiation Methods - Computed Tomography - Part 3: Operation and interpretation19.100Neporušitveno preskušanjeNon-destructive testingICS:Ta slovenski standard je istoveten z:EN 16016-3:2011SIST EN 16016-3:2012en,fr,de01-februar-2012SIST EN 16016-3:2012SLOVENSKI
STANDARD
EUROPEAN STANDARD NORME EUROPÉENNE EUROPÄISCHE NORM
EN 16016-3
August 2011 ICS 19.100 English Version
Non destructive testing - Radiation methods - Computed Tomography - Part 3: Operation and interpretation
Essais non destructifs - Méthodes par rayonnements - Tomographie numérisée - Partie 3: Fonctionnement et interprétation
Zerstörungsfreie Prüfung - Durchstrahlungsverfahren - Computertomographie - Teil 3: Durchführung und Auswertung This European Standard was approved by CEN on 29 July 2011.
CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN member.
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Spatial resolution measurement using line pair gauges . 23A.1Line pair gauges . 23A.2Principle of measurement . 24Bibliography . 27 SIST EN 16016-3:2012

4.2 CT system set-up 4.2.1 General The CT system set-up is oriented towards the requirements for the given task. The required spatial resolution (taking into account the tube focal spot size), contrast resolution, voxel size and the CT image quality can be derived from these requirements. The quality of the CT image is determined by different parameters, which under certain circumstances counteract each other. In the following, system parameters are described and information is provided on setting up a CT system for inspection. Due to the interactions of the different system parameters, it may be necessary to run through the set-up steps several times in order to acquire optimal data. The optimal energy is that which gives the best signal-to-noise ratio and not necessarily that which gives the clearest radiograph (the dependency of the detector efficiency on the energy is to be taken into account). However, in order to differentiate between materials of different chemical composition it may be necessary to adjust the accelerating voltage to maximise the difference in their linear attenuation coefficients. 4.2.2 Geometry The source-detector and source-object distances and thus also the beam angle used should be specified. In order to achieve high resolutions, the projection can be magnified onto the detector. The magnification is equal to the ratio of the source-detector distance to the source-object distance. Increasing source-detector SIST EN 16016-3:2012

EN 16016-2:2011, 8.2. For the best measurement results, an attenuation ratio of approx. 1:10 should be used. That is the grey level through the sample should be about 10% of the white level (both measured with respect to the dark level). The optimal range can be achieved through the use of prefilters. It should be noted that every prefilter reduces the intensity. Prefilters have the additional advantage of reducing beam hardening, though further improvements can be made with software correction. 4.2.4 Detector The following detector settings need to be set appropriately for the sample being scanned:  Exposure time (Frame rate);  Number of integrations per projection;  Digitisation gain and offset;  Binning. If necessary, corrections for offset, gain and bad pixels (which may depend on X-ray settings) should be applied. The individual CT projection is determined by the detector properties: its geometric resolution, its sensitivity, dynamics and noise. The gain and exposure time can be adjusted together with the radiation intensity of the source so that the maximum digitised intensity does not exceed 90 % of the saturation level. SIST EN 16016-3:2012

The measurement of the geometric properties of an object using CT is an indirect procedure, in which the dimensional measurement takes place in or is derived from CT images. For this reason, in order to facilitate precise measurements, an accurate knowledge of two important variables is necessary:  the precise image scale or voxel size and  the boundary surface of two materials, for example the component surface (material-to-air transition), which can be determined via a CT grey value threshold in the CT image. 4.5.3.2 Determination of precise image scale The precise image scale or voxel size must be determined through the measurement of a suitable calibration standard (together with the measurement object and directly before/after the object inspection) or using a reference geometry at the object. For this, the voxel size or magnification M specified by the CT system is compared with the actual available and precisely determined (using the reference body/geometry) voxel size or magnification M*. Thus, for example, the exact voxel size can be determined with high precision via measurements without the disturbing influence of other variables (for example, the precise position of the component surface (grey value threshold) in the CT image) for the centre distances of a test piece (e.g. dumbbell, see Figure 1). In this procedure, it must be taken into account that the CT grey values of the test item can, under certain circumstances, be influenced by the accompanying reference bodies (for example, through changes to the contrast ratios, interferences and artefacts). Using the actual voxel sizes determined in this way, the visualisation software can be correspondingly scaled/corrected as regards the voxel size specified by the system.
Figure 1  Reference objects (dumbbell) 4.5.3.3 Threshold value determination In order to be able to carry out dimensional measurements, the component surface or material contact surface must be determined in the CT image. The component surface is generally derived from the transition from SIST EN 16016-3:2012

4.5.3.4 Adjustment of geometrically primitive bodies In addition to simple point-to-point operations (see 4.5.3), methods from coordinate measurement technology, such as reference geometry adjustment may be employed. In this connection, so-called geometric primitive bodies or reference elements (for example planes, cylinders, spheres or similar) are fitted, using software, to object contours of interest in the correspondingly calibrated data. At the reference elements, geometric features (for example, diameter, distances, angles, etc) are determined directly or by combining reference elements. By fitting to the typically several thousand measurement points of the corresponding data, there is thus, due to the statistic averaging and reduction of the user influence, an often much higher precision than via the manual distance measurement of two points. 4.5.3.5 Generation of geometric data So-called triangular models can be extracted from the voxels and calibrated grey value threshold. These models represent the calibrated threshold value-Iso-surface, i.e. the material surface in the form of linked triangles. The triangular model contains – as part of the extraction process precision (see below) – the geometry information on the object surface. It consists of only two types of information: the so-called vertices and the information as to which vertices belong to a triangle. The vertices are 3D points, which lie on the threshold value-Iso-surface. The quantity of all vertices is also designated a point cloud. It is initially the linking information, i.e. the information as to which three vertices in each case form a surface triangle, which defines the course of the object surface. A standard format for data exchange is the so-called STL file format (ASCII or binary and dimension-less). Alternatively, the point cloud (vertices without triangle information) can be exported, whereby in general important information on adjacent vertices is lost and if necessary must subsequently be reproduced.
The geometric quality of the generated point cloud or triangular model depends entirely on the number and position of the vertices. Since only triangles are assumed between the vertices in the triangular model, detailed surface structures, contained in the voxels,
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