Fibre content of solid wood plays an important role in the wood products industry in terms of value. Additionally,
fibre structure in composite wood products such as Oriented Strand Board (OSB) and paper products plays an
important role in terms of strength properties. The effect of moisture content on wood properties is important
in the manufacturing process and final product performance, and therefore its effect on the birefringence is
of considerable interest. Since solid wood exhibits strong birefringence at terahertz frequencies, there may be
potential applications of terahertz spectroscopy to fibre content and structure sensing. There are two potential
sources for this strong birefringence: (i) form birefringence resulting from the porous structure of solid wood
and (ii) intrinsic birefringence resulting from the dielectric properties of the material itself. In this report, the
variability of birefringence within and between species, the dependence of the birefringence on moisture content
and the relative contributions from form and intrinsic birefringence are examined. In order to clarify the role
of these contributions to the measured birefringence, polarized terahertz reflection spectroscopy is examined
and compared to the results obtained in a transmission geometry. Comparison of the birefringence measured in
transmission and reflection geometries suggests that form birefringence may dominate.
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