Limitations in the efficiency of fluorescent whitening agents in uncoated paper

The fluorescence efficiency of one fluorescent whitening agent (FWA) inuncoated and unfilled paper was characterised at different FWA concentrations. An extendedKubelka-Munk model proposed earlier by several authors was applied to quantify the effect onCIE whiteness of absorption of the FWA in the visible spectrum . At high FWA concentration,chemical interactions slightly modify the emission spectrum of the FWA, and the quantumefficiency depends on the FWA concentration. This effect has however a negligible effect onthe CIE whiteness for FWA concentrations used in practice. The overlap of the absorption andemission bands of the FWA is shown to be the main cause of greening (a shift of thechromaticity towards green) and saturation of the fluorescence effect. With increasing FWAconcentration, the positive effect of fluorescence is neutralised by the reduction of thereflectance factor in the violet-blue region of the spectrum induced by a significant absorptionof the FWA in that region.