Relation between anisotropic relative permittivity and density of wood evaluated using THz time domain transmission spectroscopy

To confirm the feasibility of applying THz time domain spectroscopy (TDS) technique to nondestructive evaluation (NDE) of wood, the relation of relative permittivity to density for specimens of 12 wood species was investigated using THz-TDS transmission measurement system. The dielectric anisotropy, the difference in the relative permittivity in the fiber and transverse direction of wood, was also examined. The relative permittivity increased with wood density. The relation between the anisotropy and density was concave downward. These findings indicate that THz-TDS technique can be applied to NDE of wood density and grain direction.