Prediction of oven-dry density of wood by time-domain terahertz spectroscopy

Abstract

Wood is relatively transparent to terahertz (THz) radiation with wavelengths in the submillimeter range. This radiation has a high potential for sensing and imaging wood with a good spatial resolution. THz is especially sensitive to moisture content, fiber alignment, and density – all of which are critical in the manufacturing of wood products. In this work, a systematic study was undertaken on 46 very different wood species by means of THz time-domain spectroscopy with density determination in focus. The dielectric response of wood was modeled based on the Maxwell-Garnett effective medium theory. The dielectric function of the cell wall material was found to be extremely consistent over this large number of species with very different properties. This renders possible to determine wood density by THz time-domain spectroscopy. A strong correlation between the measured and predicted densities has been observed for all the samples investigated.

Single-frequency terahertz Michelson interferometer for applications in wood products

“…Wood is an extremely complex material and variances in its structure (e.g., hardwoods vs softwoods) will result in different densities between species and within the same species. The densities of 46 different species of wood have been determined by Inagaki et al using a MaxwellGarnet effective medium theory based on cylindrical geometry [59]. They found that this model agreed well with the measured densities obtained by using the oven-dry weight and volume measurements.…”

“…The index of refraction can be used to compute the dielectric constant of the wood , which is correlated with the density of the wood sample [59] . Wood is an extremely complex material and variances in its structure (e.g., hardwoods vs softwoods) will result in different densities between species and within the same species.…”

Determination of density and moisture content of wood using Terahertz time domain spectroscopy.

“…According to Duvillaret et al, the measured signal is very small compared to the noise level at low frequencies [38], which results in a phase that may not go to zero at zero frequency as must physically happen. The distorted phase at the low frequency points is linearly extrapolated [39] to account for this, and implemented in the analysis conducted in the experiments that follow.…”

“…There have been several reports on the interaction of THz radiation with wood [5,14,38,[48][49][50][51], which have highlighted applications of THz radiation to the wood products industry, including density mapping [5], defect detection [14], properties of paper [52] and wood-plastic composites [53], dendrochronology [6], and sensitivity to internal moisture suitable for imaging of MC [51].…”

“…To date, the prediction accuracy of various species of wood by THz spectroscopy has been demonstrated on oven-dried wood [38], and requires quantification over a range of moisture contents in order to further our understanding of THz-wood interactions so that new applications of this emerging technology related to wood science and the wood industry can be explored further. We showed an excellent correlation between the predicted and measured volume fractions of cell wall material (density) over a large range of volume fractions (f= 0.2-0.8), by applying a Maxwell Garnett (MG) effective medium theory (EMT) approach [38], Here, we show the prediction accuracy is retained over a range of moisture contents below the fiber saturation point, and also demonstrate the possibility for simultaneous extraction of MC and density.…”