EFFECT OF STRONG ABSORPTION ON THE KUBELKA-MUNK SCATTERING COEFFICIENT

The influence of chromophore absorption on the Kubelka-Munk specific s cattering and absorption coefficients (s and k) is examined to determi ne the origin of the anomalous behavior of the scattering coefficient in regions of strong absorption. Specifically, when chromophores in th e cell wall of a paper fibre absorb light strongly, the Kubelka-Munk s cattering coefficient decreases. Our studies on the optical properties of dyed cellophane and dyed paper show that changes in s are the comb ination of two effects: the influence of the absorption on the surface reflectance or reflectivity of the cell wall, and the absorption of l ight internal to the cell wall. The reflectivity or the cell wall chan ges most when the rate of change in the absorption coefficient (dk/d l ambda) is high and is caused by a known interdependence of the optical constants called ''anomalous dispersion''. Near an absorption peak, t he line shape of s(lambda) depends upon bath the dk/d lambda from disp ersion and the k(lambda) from the internal absorption. The surface ref lectance and the internal absorption are integrated into a net ''slab' ' reflectance used with the Scallan-Borch layer theory. Care must be t aken to distinguish between changes in s that arise from fibre bonding and those that are a result of the interdependence of s and k.