Biological materials—wood is a typical example—are widely used in a state where not only the cellular structure but also its bulky shape is retained. NIRS (near-infrared spectroscopy) may be called for as a promising technique to analyze the physical state of such materials as well as the chemical composition. In this report, the effects of physical conditions found in wood on the absorption of NIR radiation are examined. In the experiments, conifers (Sitka spruce) that had various degrees of surface roughness and different orientations of fibers to the direction of incident light were used. Results of these measurements showed that the orientation of fibers and the surface roughness of wood were directly related to the absorbance. In addition, it became clear that the behavior of diffusely reflected light in wood could be expressed by Kubelka-Munk theory and fell into three categories according to the characteristic of the absorption and scattering coefficient. On the basis of these results, a new concept for the behavior of NIR light passing through material made of hollow fibers has been proposed.

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