Nondestructive detection of termites using a millimeter-wave imaging technique

Termites inhabiting an air-dried wood block of western hemlock were detected by a nondestructive imaging technique using millimeter waves at a frequency of 100 GHz with 10-mW output power. The millimeter waves were radiated onto the wood through an antenna and lens system, and the reflected wave was received with the same antenna, discriminated, and filtered with a Gaussian high-pass filter that had a 40 seconds cut-off cycle. The amplitude of the intermittent change that appeared in the detected wave was analyzed. A significant intermittent change was confirmed in the detected wave from the wood inhabited by only one termite, whereas no change was found in the wave from the wood without termites. The amplitude of the detected wave increased with an increase in the number of termites in the wood. The intermittent change was less easily detected as the thickness of plywood attached on the wood surface as a mask increased, and as the distance increased between the wood surface and the hole inhabited by the termites. Those observations suggested that the millimeter waves are reflected from the electromagnetic boundary between termites and wood, which is recognized as a spot area of higher MC and a background of lower MC, respectively.