Analysis of web defects by correlating 1-D Morlet and 2-D Mexican hat wavelet transforms

The manufacturing processes for paper and similar non-woven fiber webs can affect end-use properties. In this paper, we document a new wavelet-based method of product diagnostic. The methods combines 1-dimensional Morlet and Isotropic 2-dimensional Mexican hat wavelets, with wavelet-based filtering and denoising techniques. Two samples produced in pilot machines by different forming methods are examined for variations in their mass per unit area - the grammage. The grammage maps are decomposed into three layers: one associated with the nearly-periodic grammage streaks at large scale, one associated with flocs or related medium-size structures, and the background which combines pixel-size fluctuations and large-scale stochastic modulations. By correlating the structure and background fluctuations with the streaks local phase, we show that one sample exhibits formation streaks (statistical variations in properties other than mean grammage), which are not found in the other.