Analytical Characterization of Reflected and Transmitted Light from Cellular Structural Material for the Parallel Beam of NIR Incident Light

The results of several studies show clearly that the behavior of light transmitted in wood samples is considerably different from that of an ideal scattering medium. These observations suggest that the Kubelka–Munk theory (the K-M theory) cannot be applied directly to biological material with a cellular structure. We have developed new concepts to understand the optical characteristics of a sample having cellular structure for the illumination condition of the available spectrometers. Through a series of examinations, it became clear that the behavior of diffusely reflected light in wood was accurately described by the K-M theory. However, the behavior of its transmitted light was considerably different from that of an ideal scattering medium under the conditions of Lambert's cosine law. A two-component model of the diffusion process with respect to a parallel beam component of incident light was considered. Furthermore, the nth power cosine model of radiant intensity was introduced to be fitted for the actual state of the radiation in wood. On the basis of these optical models, the mean optical pathlength through the sample was calculated, which allowed the concept of the equivalent sample thickness to be derived. Over and above these considerations, generalized input/output equations for radiation were constructed by introducing the equivalent sample thickness into K-M equations. The absorbance calculated by such concepts agreed well with the experimental values.