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      Thickness Dependence of Optical Transmittance of Transparent Wood: Chemical Modification Effects

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          Abstract

          Transparent wood (TW) is an emerging optical material combining high optical transmittance and haze for structural applications. Unlike nonscattering absorbing media, the thickness dependence of light transmittance for TW is complicated because optical losses are also related to increased photon path length from multiple scattering. In the present study, starting from photon diffusion equation, it is found that the angle-integrated total light transmittance of TW has an exponentially decaying dependence on sample thickness. The expression reveals an attenuation coefficient which depends not only on the absorption coefficient but also on the diffusion coefficient. The total transmittance and thickness were measured for a range of TW samples, from both acetylated and nonacetylated balsa wood templates, and were fitted according to the derived relationship. The fitting gives a lower attenuation coefficient for the acetylated TW compared to the nonacetylated one. The lower attenuation coefficient for the acetylated TW is attributed to its lower scattering coefficient or correspondingly lower haze. The attenuation constant resulted from our model hence can serve as a singular material parameter that facilitates cross-comparison of different sample types, at even different thicknesses, when total optical transmittance is concerned. The model was verified with two other TWs (ash and birch) and is in general applicable to other scattering media.

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          Most cited references31

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          Highly Anisotropic, Highly Transparent Wood Composites

          For the first time, two types of highly anisotropic, highly transparent wood composites are demonstrated by taking advantage of the macro-structures in original wood. These wood composites are highly transparent with a total transmittance up to 90% but exhibit dramatically different optical and mechanical properties.
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            The modified Beer-Lambert law revisited.

            The modified Beer-Lambert law (MBLL) is the basis of continuous-wave near-infrared tissue spectroscopy (cwNIRS). The differential form of MBLL (dMBLL) states that the change in light attenuation is proportional to the changes in the concentrations of tissue chromophores, mainly oxy- and deoxyhaemoglobin. If attenuation changes are measured at two or more wavelengths, concentration changes can be calculated. The dMBLL is based on two assumptions: (1) the absorption of the tissue changes homogeneously, and (2) the scattering loss is constant. It is known that absorption changes are usually inhomogeneous, and therefore dMBLL underestimates the changes in concentrations (partial volume effect) and every calculated value is influenced by the change in the concentration of other chromophores (cross-talk between chromophores). However, the error introduced by the second assumption (cross-talk of scattering changes) has not been assessed previously. An analytically treatable special case (semi-infinite, homogeneous medium, with optical properties of the cerebral cortex) is utilized here to estimate its order of magnitude. We show that the per cent change of the transport scattering coefficient and that of the absorption coefficient have an approximately equal effect on the changes of attenuation, and a 1% increase in scattering increases the estimated concentration changes by about 0.5 microM.
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              Optically Transparent Wood from a Nanoporous Cellulosic Template: Combining Functional and Structural Performance.

              Optically transparent wood (TW) with transmittance as high as 85% and haze of 71% was obtained using a delignified nanoporous wood template. The template was prepared by removing the light-absorbing lignin component, creating nanoporosity in the wood cell wall. Transparent wood was prepared by successful impregnation of lumen and the nanoscale cellulose fiber network in the cell wall with refractive-index-matched prepolymerized methyl methacrylate (MMA). During the process, the hierarchical wood structure was preserved. Optical properties of TW are tunable by changing the cellulose volume fraction. The synergy between wood and PMMA was observed for mechanical properties. Lightweight and strong transparent wood is a potential candidate for lightweight low-cost, light-transmitting buildings and transparent solar cell windows.
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                Author and article information

                Journal
                ACS Appl Mater Interfaces
                ACS Appl Mater Interfaces
                am
                aamick
                ACS Applied Materials & Interfaces
                American Chemical Society
                1944-8244
                1944-8252
                04 September 2019
                25 September 2019
                : 11
                : 38
                : 35451-35457
                Affiliations
                []Wallenberg Wood Science Center, Department of Fiber and Polymer Technology, KTH Royal Institute of Technology , Teknikringen 56, Stockholm 100 44, Sweden
                []Department of Applied Physics, School of Engineering Sciences, KTH Royal Institute of Technology , Isafjordsgatan 22, Kista 164 40, Sweden
                Author notes
                [* ]Email: miya@ 123456kth.se (M.Y.).
                [* ] ilyas@ 123456kth.se (I.S.).
                Article
                10.1021/acsami.9b11816
                6776381
                31483595
                ad7db9ea-9270-49ba-8d78-ebebdf186177
                Copyright © 2019 American Chemical Society

                This is an open access article published under a Creative Commons Non-Commercial No Derivative Works (CC-BY-NC-ND) Attribution License, which permits copying and redistribution of the article, and creation of adaptations, all for non-commercial purposes.

                History
                : 06 July 2019
                : 04 September 2019
                Categories
                Research Article
                Custom metadata
                am9b11816
                am-2019-118167

                Materials technology
                transparent wood,transmittance,photon diffusion equation,attenuation coefficient,anisotropic scattering

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