The authors have proposed a non-destructive method for discriminating between different types of paper using image analysis, Fourier transformation, and cross-correlation matching; i.e., fast Fourier transform (FFT) is used to extract the periodicity in the structure of paper that results from the manufacturing processes. Cross-correlation matching quantifies the similarity between power spectra (PS). In applications of this method to photocopier paper and newsprints, however, some of numerical similarity calculations were not successful. To clarify the difficulty, this paper describes the changes of wire-marks; changes of wire-marks in paper with unused wire and with long-term-used wire and also cross direction wire-mark profile as well were examined. Position changes in the coordinates of typical power peaks in PS were carefully examined; a top most peak (or bottom most) can be considered to stand for typical horizontal periodicity, and a right most (or left most as well) one for typical vertical periodicity as well. Results showed; (1) no plastic elongation was observed after the wire had been used for a long period, (2) wire-marks in paper was elongated in machine direction and shrunk in cross direction comparing to the periodicity in the original wire itself. And (3) wire-mark profile showed some distortion of power spectrum in cross direction. (3) can be considered as a cause of difficulty of the discrimination of paper.
View full abstract