Simultaneous detection of density, moisture content and fiber direction of wood by THz time-domain spectroscopy
2020
Kashima, Moe | Tsuchikawa, Satoru | Inagaki, Tetsuya
In the wood product-manufacturing process, the physical properties of wood must be monitored. The terahertz (THz) radiation has a high potential for sensing and imaging wood with a good spatial resolution. This study demonstrates the simultaneous prediction of the density, moisture content, and fiber direction of wood, which are important in determining the wood quality, using THz time-domain spectroscopy. We measured the spectra of 12 kinds of wood at various moisture contents while rotating the sample against the THz wave polarization. The fiber direction was predicted by observing the birefringence, which is the optical property of a material, in which the refractive index depends on the polarization direction. The density and the moisture content were predicted by multiple linear regression using the refractive indices and absorption coefficients obtained from the THz measurement. In a series of THz measurements, we successfully identified the wood fiber direction with an R² of 0.90 and a root mean square error (RMSE) of 16.51° (total measured range = 0–170°), the density with an R² of 0.97 and an RMSE of 0.022 (total measured range = 0.25–0.67 g/cm³), and the moisture content with an R² of 0.78 and an RMSE of 4.06 (total measured range = 0.00–30.98%).
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