Transmission terahertz (THz) spectroscopy on heterogeneous materials often shows structured spectra due to interference of waves passing through the object along different optical paths. Of special interest is wood with its strong birefringence and pronounced orientation- ependent structure. In this paper, we show that at specific polarization angles, the THz transmission spectra show gaps at distinct frequencies. The frequency changes of these gaps can be used for a fast and accurate determination of the dependence of birefringence on humidity. We present results for spruce and beech with different sample thicknesses and orientations, and study and model their optical properties as a function of water content.

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