Simultaneous detection of density, moisture content and fiber direction of wood by THz time-domain spectroscopy

• Published in: Journal of wood science Vol. 66; no. 1; pp. 1 - 8
• Main Authors: Kashima, Moe, Tsuchikawa, Satoru, Inagaki, Tetsuya
• Format: Journal Article
• Language: English
• Published: Singapore Springer Singapore 01.12.2020; Springer Nature B.V; SpringerOpen
• Subjects: Absorptivity; Biomedical and Life Sciences; Birefringence; Characterization and Evaluation of Materials; Density; Error analysis; Fiber direction; Life Sciences; Materials Science; Moisture content; Optical properties; Original Article; Physical properties; Polarization; Refractivity; Root-mean-square errors; Spatial resolution; Spectrum analysis; Terahertz frequencies; THz time-domain spectroscopy; Time domain analysis; Wood fibers; Wood Science & Technology; Fiber direction; Birefringence; THz time-domain spectroscopy; Moisture content; Density
• ISSN: 1435-0211; 1611-4663
• DOI: 10.1186/s10086-020-01874-3

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 R 2 of 0.90 and a root mean square error (RMSE) of 16.51° (total measured range = 0–170°), the density with an R 2 of 0.97 and an RMSE of 0.022 (total measured range = 0.25–0.67 g/cm 3 ), and the moisture content with an R 2 of 0.78 and an RMSE of 4.06 (total measured range = 0.00–30.98%).