The variation of reflectance (in the range of 503–508 nm) for thefirst three is related to the interaction of chitosan with the papercomponents (mainly cellulose fibers), which induced a modifica-tion of the environment of the chitosan derivative and a shift inthe absorption wavelength. For the fourth and fifth layers, thewavelength maximum stayed at 508 nm, given the fact that thecoverage of the paper surface reached completion.The fact that the fluorescent chitosan derivative was also de-tected on the uncoated side of the sheets confirmed that it pene-trated progressively throughout the paper thickness all the wayto the other side