Abstract
A practical method to detect wood annual ring structure has been limited to the ionizing radiation such as X-ray though the structure is important in determining the dimensional stability and strength of wood. A millimeter wave technique has attracted much attention as a new technique that can detect the annual ring structure. The relation of annual ring structure to the millimeter wave image, however, has been obscure because the wave is diffracted in wood. In this paper, the effect of diffraction on the millimeter wave image was investigated. The transmission images of the MMW transmitted through 2 mm thick quarter-sawn specimens of six wood species were obtained as the distributions of phase shift and attenuation, and they were compared with the density distributions obtained by the X-ray radiography. The phase shift and attenuation distributions did not always match the density distribution. The electromagnetic wave transmission model considering diffraction well explained this mismatch. This indicates that the diffraction affects the millimeter wave transmission image of wood.
