## 5 Applications
### Wood  
An interesting field for diffuse optics with potential industrial applications is the non-destructive spectroscopy of wood. This field has been explored at large by different NIR techniques directly focused on non-destructive retrieval of wood composition or conservation status [144; 145; 146; 147; 148]. The key advantage of broadband TD-DOS in this field is the intrinsic capability of disentangling absorption from scattering contributions, resulting in a more accurate estimate of the optical properties. An example of an application is the monitoring of archeological wood and in general of age-related degradation of wood artifacts, as shown in Figure 18 [149]. A clear reduction in the scattering coefficient is observed upon wetting--as for waterlogged wood--and even further by artificially procured degradation, due to the decrease of refractive index mismatch among cells and disruption of the cellular structure. The absorption spectra clearly display the water contribution in wet samples (around 980 nm) as well as the increased absorption of oxidative products in the region <800 nm. The strong optical anisotropy of wood materials is evident from the large difference in the scattering coefficient using a source-detector pair parallel (up) or perpendicular (down) with respect to the wood fibers.  
The capability to detect water content within the diffusive medium can be exploited for the monitoring of wood moisture--a key parameter with relevance to timber industrial applications [150]. Figure 19 shows a sample specimen kept in a chamber under controlled humidity and continuously monitored using broadband TD-DOS. Upon increasing the humidity level--and therefore the moisture content in wood--the water absorption peak around 980 nm increases correspondingly, demonstrating a sensitivity of the technique down to a few percents of moisture content.  
Figure 17: Absorption spectra measured on the abdomen of 4 healthy volunteers with source-detector distance: (**a**) \(\rho\) = 3 cm, and (**b**) \(\rho\) = 1 cm. The legend shows the thickness of the adipose tissue layer in (**a**) and the subject age in (**b**) [143].  
Figure 19: Spectral changes in the absorption spectrum of wood samples under increasing moisture content [150].  
Figure 18: Monitoring the effect of degradation on wood samples measured along the fibers (up), perpendicular to them (down). The scattering spectrum (right) is progressively decreased moving from the dry sample (open diamonds), to wet sample (gray diamonds), and finally to degraded sample (black diamonds). Likewise, the absorption spectrum (left) is increased for the wet sample in the water-absorbing region (around 980 nm) and for the degraded sample in the red region (\(<\)800 nm) due to oxidation products. The strong optical anisotropy of wood materials is evident from the large difference in the scattering coefficient in the configuration parallel (up) or perpendicular (down) to the wood fibers [149].  
A further attractive industrial application was pursued in the quantification of monomer uptake in wood polymer composites [151]. The key interest derives from recent attempts to generate structurally strong wood materials starting from low-quality softwood structures soaked in a (possibly biodegradable) monomer undergoing polymerization. Figure 20a shows the alteration in the absorption spectrum of wood caused by the uptake of a mix of monomers. The wood specimen was immersed in a solution containing the monomer mix (45% glycidyl methacrylate, 45% methylmethacrylate, and 10% ethylene glycol dimethacrylate) and uptake was forced applying mild vacuum to the chamber, while the sample was continuously monitored by broadband TD-DOS. The difference between the treated and untreated wood corresponds to the absorption spectrum of the monomer mix, thus permitting absolute quantitation. Figure 20b demonstrates capabilities to monitor the monomer uptake non-destructively across a 1 cm thick wood sample at different positions on the surface along the specimen with a precision better than 1% [152].