Application of Time-of-Flight Near-Infrared Spectroscopy to Wood with Anisotropic Cellular Structure

The newly constructed optical measurement system, which was mainly composed of a parametric tunable laser and a near-infrared photoelectric multiplier, was introduced to clarify the optical characteristics of wood with anisotropic cellular structure on the basis of the time-of-flight near-infrared spectroscopy (TOF-NIRS). The combined effects of the cellular structure of the wood sample, the wavelength of the laser beam, and the detection position of transmitted light on the time-resolved profiles were investigated in detail. The substantial optical pathlength calculated from the time-resolved profile was outstandingly larger than the sample thickness. The substance of the wood was directly related to the time delay of light propagation. In applying TOF-NIRS to the cellular structural materials, it is very important to give attention to the difference in the light scattering within the cell wall and that caused by the multiple specular-like reflections between cell walls. The newly proposed empirical equation, considering the effects of optical parameters, could well express the wavelength and sample thickness dependency of the time delay of the peak maxima. The time-resolved profile was intensively governed by the combination of light-absorbing and -scattering conditions and the degree of anisotropy. These basic data will be essential for this system to put an in-process measurement system in the wood industry to practical use.