Improvement of ultraviolet-visible diffuse reflectance spectroscopy for chromophores research

Paper represents a striking example of optically inhomogeneous material characterized by the presence of light scattering which affects both absorption and reflectance measurements. In the visible (Vis) and ultraviolet (UV) spectral regions scattered light usually exceeds absorbed light so largely that it is no longer possible to obtain a useful absorption spectrum of paper without taking account of scattering even for thinned samples. On the other hand absorption spectra measured over the UV-Vis region give direct information on the nature and chemistry of the chromophores that regulate the optical quality of paper. Kubelka-Munk (K-M) theory, recently extended by Yang and Miklavcic, allows an estimation of paper absorption spectra from measurements of diffuse reflectance spectra. This is true for low-basis weight sheets, i.e. optically thin samples. For optically thick sheets, singularities in the K-M equation for the scattering coefficient (S) prevents for the recovering of the optical absorption. In this presentation a new approach to estimate S and, consequently, the optical absorption from reflectance measurements of highly absorbing paper sheets will be shown. This approach is based on the estimation of S in the wavelength region where samples are optically thick, obtained by using the measured diffuse reflectance spectra. This allows the optical properties to be recovered up to the UV region where paper sheets can show strong chromophores' absorption. Results have been compared to theoretical ab-initio quantum-mechanical computational simulations carried out within time-dependent density functional theory. In this way quantitative chemical information on the chromophores causing paper yellowing can be obtained in non-destructive way. Applications to modern paper samples aged in different environmental conditions and highly absorbing in the ultraviolet region, as well as ancient paper used by Leonardo da Vinci for its drawings, will be shown.