Mid Sweden University

This paper presents a method to describe, with good accuracy, the relation between variations in paper properties and variations of the spectral optical response of an uncoated paper. The dependence between density, filler content, grammage, and the spectral optical response is characterized by a multivariate model. The model is based on large-scale measurements data on a set of paper samples that have been produced with different values of grammage, density and filler content, representing the variations within a normal 80 g/m² uncoated paper. From the optical measurements the light scattering (s) and light absorption (k) coefficients have been estimated according to the Kubelka-Munk theory.The results from this study will give valuable input to optical modeling activities, where the optical variations are predicted from measured small-scale variations in underlying paper properties.The variations in the paper properties can be used to model the light scattering coefficient, s, but there were too small variations in the light absorption coefficient, k, to find any significant dependence to the paper properties for the samples studied in this work. Furthermore, linear models give sufficient accuracy in the intervals studied. Additional findings from this study are the different effects of wet-pressing and calendering on the light scattering coefficient. 

In this work, a non-destructive method is presented that enables the measurement of filler content in paper with high spatial resolution. The method uses an X-ray fluorescence setup that enables high resolution measurements of calcium content in the paper, together with a beta radiography measurement method, to assess the local filler content in the paper. An image registration method is used to combine the two measurement maps, and a calibration polynomial is applied to the point-wise values in order to calculate the local filler content. The measurement methods show good accuracy. The grammage and the calcium content can be measured at a spatial resolution of 0.1 mm × 0.1 mm but the resolution for the filler content map was in this work chosen to 1 mm × 1 mm in order to minimize image registration errors. The method is illustrated using two paper samples, a laboratory paper and a commercial 80 g/m2 copy paper. From the methods used in this work, a difference is shown between the two paper samples in how the filler content distribution is related to the paper formation. With the help of image registration, point-wise measurements of filler content from both sides of the samples can be compared. The method can be used together with other high resolution measurements in order to analyze the simultaneous interrelation between different paper properties. The high resolution measurements of filler content will be particularly valuable for the analysis of the underlying causes to optical variations in paper and print.

A method to predict optical variations from high resolution measurementsof paper properties is evaluated in this work. The method combines thepoint-wise values of high resolution maps of filler content and grammagewith an empirical model derived in an earlier study to predict thespatial optical variations in paper.The method has been applied on two paper samples, a laboratory paper anda commercial 80 g/m(2) copy paper. The optical variations have beenpredicted at a scale of 1 mm(2). Validation has been made by using ahigh resolution spectrophotometric setup to measure the spatialreflectance variations in the paper. The results show that for thesamples used, the influence of filler content variations and densityvariations on the optical variations is small compared to influence ofthe grammage variations.