Tuukka PRYKÄRI^*, Jakub CZAJKOWSKI, Erkki ALAROUSU, and Risto MYLLYLÄ

Optoelectronics and Measurement Techniques Laboratory, University of Oulu, P. O. Box 4500, 90014 University of Oulu, Finland

(Received September 18, 2009; Accepted December 30, 2009)

Optical coherence tomography (OCT), a technique for the noninvasive imaging of turbid media, based on low-coherence interferometry, was originally developed for the imaging of biological tissues. Since the development of the technique, most of its applications have been related to the area of biomedicine. However, from early stages, the vertical resolution of the technique has already been improved to a submicron scale. This enables new possibilities and applications. This article presents the possible applications of OCT in paper industry, where submicron or at least a resolution close to one micron is required. This requirement comes from the layered structure of paper products, where layer thickness may vary from single microns to tens of micrometers. This is especially similar to the case with high-quality paper products, where several different coating layers are used to obtain a smooth surface structure and a high gloss. In this study, we demonstrate that optical coherence tomography can be used to measure and evaluate the quality of the coating layer of a premium glossy photopaper. In addition, we show that for some paper products, it is possible to measure across the entire thickness range of a paper sheet. Furthermore, we suggest that in addition to topography and tomography images of objects, it is possible to obtain information similar to gloss by tracking the magnitude of individual interference signals in optical coherence tomography.

Key words: optical measurement, noninvasive, surface quality, roughness, gloss

^*E-mail address: tuukka.prykari@ee.oulu.fi