Study on the use of optical coherence tomography in measurements of paper properties

This study proposes a new method for the characterization of paper surface and bulk structure: optical coherence tomography (OCT). Although having been used in medicine for years, this technique is still relatively unknown in the paper-making industry. First, we provide a brief description of a conventional PC-controlled measurement system for the OCT imaging of paper. In this set-up, a powerful superluminescent diode is used to illuminate a Michelson interferometer with a free-space configuration, and a piezo-transducer is placed in the optical delay line to modulate the measurement signal for optical heterodyne detection. The set-up is then applied to demonstrate the ability of the technique to visualize the surface structures of widely disparate paper samples using a single device. Next, the paper provides 3D images of a fibre network and of typical copy paper. The results prove that OCT is applicable not only to the 3D imaging of simple wood fibre networks, but also to the imaging of complex commercial paper, provided that an appropriate clearing agent is used. Finally, the effect of filler on the OCT signal slope calculated by averaging several A-scans from the sample is demonstrated, showing that increasing filler content produces a corresponding decrease of decay of the A-scan in depth.