FMCW THZ radar and X-ray analysis of wood properties: A comparative study

Wood is a material valued for its mechanical properties and sustainability. It exhibits substantial variability in density due to its growth being influenced by the external environment. The measurement of its local properties is therefore crucial for various applications such as in the construction and transport industries. X-ray attenuation densitometry measurement is a well-established method, but it uses ionizing radiation which can pose hazards to human health. Its cost is significant in terms of investment and consumables. Terahertz (THz) technology, being non-ionizing and promising, emerges as an alternative for density imaging. Therefore, this study employs THz frequency modulated continuous wave (FMCW) radar, a novel approach, to assess its ability to predict local density in a pool of 110 samples from diverse wood species, with different thicknesses, and a wide density range (from 111 kg.m -3 to 1,086 kg.m -3 ) representative of the natural variability of wood density — both at the local scale of growth rings and at a global scale. The beating signal of the FMCW radar was modeled by considering the crossed medium as uniform, to extract both the optical index and the absorption coefficient. Additionally, the local density was measured using an X-ray industrial timber scanner as a reference for the actual local density. Results reveal strong correlations between density and THz parameters. However, the study highlights limitations in the THz modeling, such as wood vessel scattering, thickness influence, potential polarization effect, or non-uniformity of the medium between earlywood and latewood. • The optical parameters of wood in the terahertz range are correlated with its density • THz wave-matter interactions other than reflection and attenuation are not negligible • Density variation between early and late wood gives rise to edge effect • Wood vessels, when voluminous enough, may scatter THz rays