The OptiTopo technique for fast assessment of paper topography - Limitations, applications and improvements

摘要

The printability of paper is extremely dependent on the topographical properties of the substrate. Imaging instruments make it possible to obtain detailed 3D scans of paper surfaces that can be further used to calculate valuable quality predictors. A new imaging instrument, OptiTopo, based on the photometric stereo principle was developed at the Swedish Pulp and Paper Research Institute (STFI) with the advantages of an extreme acquisition speed and the possibility of simultaneously acquiring topographic and reflectance information. The topographical imaging of paper surfaces using this technique has now been investigated and improved. Eleven paper samples covering a wide range of different grades have been analyzed by OptiTopo and their scans compared to those obtained using a reference imaging technique. By applying a suitable signal treatment it is possible to improve the instrument's performance in terms of detail rendering capability. The positive results have been confirmed using visual assessment, classical statistical indicators and frequency analysis. The present limitations of the technique in relation to the physical properties of the substrate are discussed and absolute boundaries for the instrument are proposed.