This work investigates the combined approach of sheet splitting by heat-seal pouch lamination and fibre segment orientation analysis by steerable filtering to characterize both in-plane and z-directional variations in local fibre orientation. The use of steerable filters allows the orientation of individual fibre segments to be estimated from a series of rotated versions of a two-dimensional fibre template. The performance of steerable filtering is compared to the widely spread method based on local gradients in the image. The results are based on a 60 g/m2 oriented handsheet formed with a dynamic sheet former from decrilled softwood kraft pulp. The sheet was split in 10 z-directional layers and the fibre orientation was estimated on a 10×10 mm2 sub-area. Despite an observed visual difference in the 2D filter response between the two techniques, the result showed only small differences in the estimated fibre orientation angle and anisotropy. The effect of a decreased signal-to-noise ratio, due to e.g. fines and filler aggregates, was simulated by adding a statistical small-scale noise. The fibre orientation distribution estimated by steerable filtering was more or less unaffected by the introduced noise, whereas the estimation based on gradient filtering showed a decreased anisotropy by 35%. The results indicate that, for routine measurements of local fibre orientation, the more selective properties of steerable filtering do not motivate the more complex computations involved. For most cases, gradient filtering should give similar results. The use of steerable filtering is however recommended as an alternative for non-standard applications, e.g. in the presence of a high "noise" level or to minimize problems related to scanning imperfections. The theoretical possibility to use steerable filtering to determine fibre width distributions is also discussed.
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