The thickness of the layers was estimated from the XRD tomograms by extracting one-dimensional cross sections perpendicular to the layers and calculating the distance between the two inflection points of the layer edges as shown in Fig. 5. This is achieved by fitting two erf step functions to the cross-sectional profile and calculating the difference in their positions. The width parameter σ of the derivatives of both step functions is taken as a measure of the effective spatial resolution in the XRD tomograms. We consider that the actual interface between the subsequent layers in the multilayer sample is much narrower than the dimensions of the microbeam (15 μm) employed for this investigation; this is also confirmed by the absorption tomogram (Fig. 1). For each of the eight layers, one representative crystalline constituent was selected and ten cross-sectional profiles were extracted from the XRD tomogram at different positions. The average thickness and average interface width are given in Table 3 for XRD tomograms reconstructed with the FBP and MLEM method. The calculated average thickness is the same for both methods within the observed standard deviation. Comparison with values obtained from the absorption tomogram in Fig. 1, representing a cross section of the paint fragment at a different height than the XRD tomograms, reveals differences in average thickness that are larger than the standard deviation. This indicates an inhomogeneous layer deposition, especially for layers 3 to 5, which contain
talc and aluminum flakes.