Limitations in the efficiency of fluorescent whitening agents in uncoated paper

The fluorescence efficiency of one fluorescent whitening agent (FWA) in uncoated and unfilled paper is characterised at different FWA concentrations. An extended Kubelka-Munk model proposed earlier by several authors is applied to quantify the effect on CIE whiteness of light absorption by the FWA in the visible spectrum. At high FWA concentration, chemical interactions slightly modify the quantum efficiency, which then depends on the FWA concentration. This effect has however a negligible effect on the CIE whiteness for FWA concentrations used in practice. Light absorption by the FWA in the visible spectrum is shown to be the main cause of greening (a shift of the chromaticity towards green) and saturation of the fluorescence effect. With increasing FWA concentration, the positive effect of fluorescence is neutralised by the reduction of the reflected radiance factor in the violet-blue region of the spectrum induced by a significant absorption by the FWA in that region. The effect is well predicted with a simple extension of the Kubelka-Munk model for light scattering in paper.