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Fig. 3. In the first row, a gray-scale value image of the material density (left) and the corresponding black (particles) and white (pores) image is shown. The figures show a sector of the sand model with a side length of 50 voxels and voxel size of 60 µm. This voxel size equals the resolution of the measurement with micro-X-ray tomography. Not all voxels determined as pores are entirely within the pore space (gray value 0). The fraction of these gray voxels is given in the figure in the second row. This fraction decreases with decreasing voxel size. The large symbols denote the resolution of the tomography with X-rays from tubes (black) or thermal neutrons (gray), respectively. The positions of the arrows correspond to the resolution with 50% of voxels denoted as pores with a gray value of 0. For the tomography of the glass bead column, this requirement was fulfilled.
