In the wood products industry, terahertz (THz) radiation is showing promise for new types of sensing and imaging applications, which depend on the ability of THz radiation to probe the gross fiber structure of wood. The sensitivity to this gross fiber structure is a result of the strong birefringence of wood at THz frequencies. Wood is a complex structure, and the large birefringence observed may be due to either intrinsic or form birefringence. In this paper, the origin of birefringence in wood at THz frequencies is examined in detail. Although the source of birefringence varies according to species, the trend shows contributions from both types of birefringence. This has implications for application development in the wood products industry, where the possibility of probing both the gross physical structure of wood and wood products, and the intrinsic properties of wood such as crystallinity and microfibril angle, simultaneously may allow for nondestructive noncontact strength testing of wood and composite wood products.
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