Optical response of strongly absorbing inhomogeneous materials: Application to paper degradation

Abstract

In this paper, we present a new noninvasive and nondestructive approach to recover scattering and absorption coefficients from reflectance measurements of highly absorbing and optically inhomogeneous media. Our approach is based on the Yang and Miklavcic theoretical model of light propagation through turbid media, which is a generalization of the Kubelka-Munk theory, extended to accommodate optically thick samples. We show its applications to paper, a material primarily composed of a web of fibers of cellulose, whose optical properties are strongly governed by light scattering effects. Samples studied were ancient and industrial paper sheets, aged in different conditions and highly absorbing in the ultraviolet region. The recovered experimental absorptions of cellulose fibers have been compared to theoretical ab initio quantum-mechanical computational simulations carried out within time-dependent density functional theory. In this way, for each sample, we evaluate the absolute concentration of different kinds of oxidized groups formed upon aging and acting as chromophores causing paper discoloration. We found that the relative concentration of different chromophores in cellulose fibers depends on the aging temperature endured by samples. This clearly indicates that the oxidation of cellulose follows temperature-dependent reaction pathways. Our approach has a wide range of applications for cellulose-based materials, like paper, textiles, and other manufactured products of great industrial and cultural interest, and can potentially be extended to other strongly absorbing inhomogeneous materials

Terahertz Absorption by Cellulose: Application to Ancient Paper Artifacts

“…In order to assess the volume fraction v of cellulose fibers in the samples and recover their absolute THz absorption coefficient, the density (ρ paper ) of each paper sample was measured. Then v = V c /V paper = ρ paper /ρ c , where V c and V paper are the volume occupied by cellulose fibers and the volume of the paper sheets used in this study, respectively, while ρ c = 1.5g/cm 3 is the average density of cellulose fibers [17] (see Supplemental Materials for details). The frequency dependent behavior of the complex refraction indexn(ω) = n(ω)+iα(ω) c/2ω (c is the speed of light and ω/2π is the frequency) can be extracted from the THz-TDS transmission mode signals by using the experimental complex transfer functionT exp (ω j ) (j is a data array index):…”

Visual degradation in Leonardo da Vinci's iconic self-portrait: A nanoscale study

“…[11][12][13] For this reason, in order to interpret the results (and following on from our previous work [11][12][13] ), we modelled oxidized cellulose using eight different configurations of ketone, diketone, and aldehyde groups within the b-D-glucopyranose units (see Figs. 4-6 of Ref.…”

“…7 The white appearance of paper is due to its light scattering effect. 11,12 The yellowing seen in ancient papers is mainly due to the fact that chromophores in cellulose 13 absorb in the higher energy range of visible light (corresponding to violet and blue) and largely scatter the yellow and red portion, thereby producing the characteristic yellow-brown hue.…”

NiO Pseudocapacitance and Optical Properties: Does The Shape Win?

“…An evaluation was made by recording diffuse reflectance spectra R, which provided information if the particle size was comparable to or smaller than the incident wavelength of light and much smaller than the total thickness of the sample. In this case, reflectance spectra can be converted into absorption ones by using the Kubelka-Munk function [39,40]:…”