OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 24 No. 4 2022 The advantages of the CL scheme are as follows: - full 360° rotation is available even for large sample; - constant tilt of the sample during analysis provides a close average value of the intensity of the transmitted radiation; - the rotational symmetry of the accessible Fourier region is a factor contributing to the further reconstruction of the image. These advantages have been demonstrated by Feng Xu et al. [3]. In those cases when implementing the CT method a limiting angle occurs after which it is not possible to obtain an image of the sample the CL method can be used. It is noted that artifacts in the image of the sample surface in the direction of the normal to the movement of X-rays in the implementation of the CT method limit the achievable resolution to a greater extent than in the CL method. In addition, when implementing computed tomography, nonisotropic artifacts in these planes create more noticeable distortions in the images compared to isotropic artifacts from CT. One of the advantages of the method of synchrotron computed tomography over synchrotron computed laminography is the ability to select the optimal signal/noise ratio – the number of artifacts after scanning the object. If it is possible to scan the sample with a rotation of 360° using the CT method so in this case it is possible to select such an amount of experimental data that will allow one to reconstruct the image with a higher signal-to-noise ratio and fewer artifacts. The advantages of the CL method make it suitable in cases where the range of the missing angle limited by the geometry of the sample or the design features of the equipment is large for the CT method. Zuber et al. developed the augmented laminography method [16], which uses X-ray tubes as radiation sources. Its merits were demonstrated in the study of fossils. This method is a combination of both types of scanning: CT and CL. Its idea is to supplement the Fourier space of laminography with information obtained using computed tomography with a lower resolution (Fig. 7). However, when examining elongated samples, some regions of the Fourier space are still missing due to the larger field of view and, as a result, the low resolution typical for computed tomography of such objects. The implementation of the augmented laminography method implies the need to increase the field of view when scanning a sample with zero tilt. This is due to the condition of the CT scan. To demonstrate the quality of images obtained using various research methods, Fig. 8 shows the results of the analysis of test samples which consisted of several layers, different in shape and materials [16]. The image of the x–y plane obtained by the CL method is characterized by good resolution and quality. The resolution of the analyzed plane, reconstructed by the CT method, is noticeably worse. The augmented laminography method demonstrates the most qualitative result. When considering the x–z plane using the computed laminography method, artifacts that distort the structural features of the analyzed object significantly are noted. The image obtained by computed tomography doesn’t have this drawback. At the same time, as well as the image in the x–y plane, it is characterized by low spatial resolution. The reconstruction obtained using the augmented laminography method (Fig. 8) is characterized by the absence of significant artifacts and high resolution. Table 1 presents the main characteristics of these three methods. Fig. 7. Sampled areas in the Fourier space of the reconstructed volume. The green volume outside the two inner 2θ cones refers to CT laminography and the red volume refers to low-resolution CT. The blue volume corresponds to the area, where missing information in the laminography data can be reconstructed using CT [16]
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