Angular variations of reflectance and fluorescence from paper - the influence of fluorescent whitening agents and fillers

摘要

It has earlier been shown that light reflected from the body of paper exhibit anisotropic behavior. On the other hand, fluorescence emission is often assumed to be distributed in a Lambertian manner. The angular behavior of light reflected and fluoresced from paper is examined using measurements from a spectral goniophotometer. The angular dependency of the radiance factors was measured for a range of excitation wavelengths. Moreover, the influence of fillers and fluorescent whitening agents (FWA) on the anisotropy was studied. The measurements show that the anisotropy of the total radiance factor of paper decreases when an increasing amount of FWA is added to the paper. The same effect was also observed when an increased amount of filler was added to the paper. In addition, it was shown that the presence of fillers reduce the effect of the FWA. The results show that in comparison to the anisotropy of the total radiance factor from the paper samples, the anisotropy of the fluorescence alone is negligible. Hence, for paper samples containing FWA evenly distributed in the bulk, the fluorescence alone should not induce significant differences between color measuring instruments of different measurement geometries.