NC State
Y.J. Sung, C.H. Ham, O. Kwon, H.L. Lee and D.S. Keller. Applications of thickness and apparent density mapping by laser profilometry. In Advances in Paper Science and Technology, Trans. of the XIIIth Fund. Res. Symp. Cambridge, 2005, (S.J. I’Anson, ed.), pp 961–1007, FRC, Manchester, 2018.


This paper describes the development of a method for mapping the apparent density of paper, nonwovens and other fibrous webs based on non-contact laser profilometry and β-transmission radiographic imaging. The method is applied in three complimentary studies that examine the in-plane non-uniformity of thickness, grammage and apparent density in printing papers. The first section focuses on the development and verification of the analytical method for mapping thickness. Through simultaneous scanning of the topography of both sides of the specimen by opposing range sensors, the surface contour of each side, the local thickness, and the out of plane deformation were measured. It was demonstrated that the laser based method provides results that closely approximate the intrinsic thickness that is independent of paper formation. The method was then used to examine the structural differences in laboratory sheets pressed by soft and hard nip calenders. The well known difference in web densification mechanisms of the two was reaffirmed by mapping discrete changes in thickness and statistically comparing these with corresponding points on grammage maps. Densification was shown to be dependent on grammage for the hard calender, and independent of grammage for the soft calender. The final study used the thickness mapping method to monitor the hygroexpansivity of representative printing papers as equilibrium humidity was varied between 9% and 80%. Thickness maps were obtained for newsprint, SC-A (calendered and uncalendered), bulk offset and office copy. Differential thickness maps were used to compare the in-plane non-uniformity of hygroexpansion. The inplane hygroexpansion was characterized and corrected for using a recently developed algorithm known as Enhanced Digital Image Correlation (EDIC). Thickness change did not appear to correspond to grammage maps. The results suggest that a significant irreversible increase in thickness occurs for papers that are heavily calendered.

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