Multiparameter sensing of paper sheets using terahertz time-domain spectroscopy: Caliper, fiber orientation, moisture, and the role of spatial inhomogeneity

Abstract

Terahertz radiation is used to sense multiple parameters in paper and cardboard sheets. The work advances from previous results by (i) using a stratified dispersive model approach to (ii) accurately determine caliper (sheet thickness), fiber orientation, and moisture while (iii) evaluating the impact of the spatial inhomogeneity of the sample sheets. A multitude of paper samples with varying thickness was analyzed using spatially resolved terahertz measurements in combination with a highly accurate material characterization approach. The spatial inhomogeneity of the caliper across the paper samples was found to play a significant role, which cannot be resolved by mechanical reference methods. The feasibility to detect low concentrations of moisture, e.g., through absorption of atmospheric humidity, is also demonstrated.