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WinWerth® artifact corrections

WinWerth® Artifact Corrections – Novelties at a glance

When measuring with computed tomography, systematic measurement errors, known as artifacts, occur in the volume. Correction methods integrated into the WinWerth® measurement software can greatly reduce artifacts and increase measurement accuracy accordingly.

Empirical Artifact Correction (EAC) is a proven method for reducing artifacts caused by beam hardening and scattered radiation. For measuring objects of one material and one density, the relationship between the attenuation of the X-rays by the measuring object and the irradiated length can be described by a characteristic line. This relationship is determined experimentally on a calibrated material sample or on the workpiece itself. The characteristic line can be used in a subsequent measurement to greatly reduce beam hardening and some scattered beam artefacts. This method has proven itself in use over many years, but has its limits for some tasks. The newer methods described below enable the specific reduction of artifacts caused by various physical effects.

 

WinWerth® Artifact Corrections – Novelties at a glance

Empirical artifact correction: volume without (left) and with correction (right)

Large and dense workpieces

Scattered radiation generated in the measuring object causes falsified intensity values to be measured, which lead to artifacts in the volume. These scattered radiation artifacts are simulated using a volume from a master part measurement and then used to correct the volume.

The scattered radiation is caused by scattering of X-ray photons in the workpiece due to the Compton effect. This occurs in particular during tomography scans of relatively large objects made of dense materials that are difficult to penetrate. At low magnifications or a short distance between the measuring object and the detector, a larger proportion of the scattered radiation is captured. The main area of application of scattered radiation artifact correction is therefore the reduction of systematic measurement errors when measuring large objects made of highly attenuating materials, e.g. large turbine blades, engine blocks and gear housings.

 

WinWerth® Artifact Corrections – Novelties at a glance

Scattered radiation artifact correction: volume without (left) and with correction (right)

Large cone beam angles

Tomography scans with a cone-shaped X-rays enable short measurement times by capturing large areas of the workpiece simultaneously. However, as the cone beam angle increases, the workpiece is scanned more poorly. The resulting cone beam artifacts can be simulated on the target geometry using a patented process and used to correct the measuring volume.

The correction can be calculated by simulation on the CAD model or on a point cloud of a master part measurement of the workpiece. Once calculated, the correction can be applied to point clouds of the same workpiece or other workpieces of the same type, e.g. for a series measurement. Volumes can also be corrected. The correction is calculated on the basis of a simulation on a point cloud of the measured workpiece.

By measuring with a larger cone beam angle, it is possible to reduce the measuring time with the same repeatability or improve the repeatability with the same measuring time. This is achieved by using a larger proportion of the available radiation energy, for example by reducing the focus-detector distance (FDD) at the same imaging scale. The resulting cone beam artifacts and the resulting systematic measurement errors can be greatly reduced by using cone beam artifact correction.

 

WinWerth® Artifact Corrections – Novelties at a glance

Cone beam artifact correction: volume without (left) and with correction (right)

Ring artifacts in the measurement of low attenuation materials

The sensitivity determines the conversion of X-rays into a measured intensity. Due to differences in sensitivity that are not fully corrected, the same radiation intensity leads to darker or lighter gray values for two adjacent pixels. The back projection of these differences in all rotational positions results in ring-shaped artifacts in the reconstructed volume.

With the new WinWerth® ring artifact correction, the information contained in the intensity images about the sensitivity differences between the pixels is determined from the current measurement and used for fine correction of the images. The ring artifacts are greatly reduced in the reconstructed volume. These are more significant with weakly absorbing workpieces. This correction method is therefore particularly useful for measurements of workpieces with a low attenuation capacity, such as plastic micro-gears, foam material samples or lens packages for smartphones.

 

WinWerth® Artifact Corrections – Novelties at a glance

Ring artifact correction: volume without (left) and with correction (right)

Further information:

Computed tomography

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