Simultaneous characterization of coated paper topography and optical contrast by near-field scanning optical microscopy (NSOM)

Paper coatings are composite materials with their dielectric functions varying randomly with position. Their optical properties are determined by both refractive index fluctuations and topography. We demonstrate that near-field scanning optical microscopy (NSOM) provides a unique capability of measuring simultaneously the information about both local topography and optical contrast at resolutions better than conventional far-field techniques. We present results of studying fine kaolin blends with broad and narrow particle size distribution ground calcium carbonates in top coat of double-coated paper from a high speed pilot coating. We found that the optical contrast in the proximity of the coating layer is a direct indication of samples diffusiveness. Also, our results demonstrate that the addition of fine platy kaolin increased the topographic correlation lengths resulting in a smoother varying surface. The NSOM measurements suggest that surface roughness and correlation length of the topography affect surface anisotropy and optical properties. The complex characterization approach introduced here may have potential applications in studies of optical loss, adhesion, and friction.