11 Time-of-Flight Spectroscopy
271
are set perpendicular or parallel to wood grain orientation) and obtained many
interesting results. They reported that the μ
s ( 10–200 cm
−1 ) was much larger than the
μ a ( 0.05–1.00 cm
−1 ) for all wood samples. μ
s spectra were almost constant over the
measured wavelength ranges. It was also found that μ
s highly depends on the wood
species (μ
s value differs between silver fir and sweet chestnut wood greatly). μ
s
of wet wood was significantly small compared to dried wood because the refractive
index mismatch between the wood cell wall substance and water in the pores is much
smaller than that between wood cell and air. D’Andrea et al. also evaluated the moisture content of wood using the μ a and found a high relationship between moisture
content and the μ a at a specific wavelength [11]. Kienle et al. investigated the origin
of scattering in wood by comparing the light propagation in the microstructure of
silver fir measured experimentally to simulation modeled by MC method [12]. They
determined μ
s (wet wood: 1.79 mm
−1 , dried wood: 6.68 mm
−1 ) due to tracheids by
solving Maxwell’s equation. They also determined μ
s−iso , which is the scattering
coefficient due to all other scattering media (rough border between the lumen and
wood cell substance, pits, ray cells), calculated by fitting measured light propagation
to simulated data. The light scattering in wood is significantly complex as wood is a
hygroscopic, heterogeneous, cellular, and anisotropic material. Although the wood
samples were regarded as a homogenous material when the μ
s values were estimated
using TR spectroscopic method, in fact, the scattering properties highly depended
on the wood species and fiber direction because the cellular structure, which caused
multiple light reflections at the boundary between cell wall and air (water), significantly differs between wood species (i.e., hardwood has various cell arrangements
like ring-porous, diffuse-porous, radial-porous, and figured-porous). Kitamura et al.
tried to determine true μ a and μ
s values of wood cell wall substance itself in order
to construct the robust calibrations wood properties by NIR spectroscopy [13]. They
expected that the μ a and μ
s values of the cell wall substance are identical or similar
between species because the density of wood cell wall itself is about 1.4–1.5 g cm
−3
regardless of species ( wood density depends on the ratio of pore and cell wall volume
in wood). As the density of cell wall is identical between species, it is thought that
the factor affecting the optical properties might be the concentration ratio of the three
main polymers in the cell wall (cellulose, hemicellulose, and lignin). As there was no
specific absorption band at the wavelength used in their study (846 nm) as shown by
Hans et al. [14], it implied that the concentration ratio of the cellulose, hemicellulose,
and lignin does not strongly affect μ a . In order to decide the true optical parameters
of wood cell wall, Douglas fir wood samples were immersed in hexane, toluene,
or quinolone and saturated with them to minimize the multiple light reflections at
the boundary between pore cell wall substance in wood. TR transmittance result
of organic liquid saturated wood samples was fitted to the diffusion approximation
equation to decide μ a and μ
s . μ
s showed the minimum value when the wood was
saturated with toluene because the refractive index of toluene is close to wood cell
wall substance. In the toluene saturated wood sample, Fresnel reflection (1) at small
particle, (2) between lumen and wood cell wall substance for all cell types (tracheid,
ray cells, vessel), and (3) at the rough border are minimized because refractive index
mismatch between toluene and cell wall was very small. The optical parameters of
271
are set perpendicular or parallel to wood grain orientation) and obtained many
interesting results. They reported that the μ
s ( 10–200 cm
−1 ) was much larger than the
μ a ( 0.05–1.00 cm
−1 ) for all wood samples. μ
s spectra were almost constant over the
measured wavelength ranges. It was also found that μ
s highly depends on the wood
species (μ
s value differs between silver fir and sweet chestnut wood greatly). μ
s
of wet wood was significantly small compared to dried wood because the refractive
index mismatch between the wood cell wall substance and water in the pores is much
smaller than that between wood cell and air. D’Andrea et al. also evaluated the moisture content of wood using the μ a and found a high relationship between moisture
content and the μ a at a specific wavelength [11]. Kienle et al. investigated the origin
of scattering in wood by comparing the light propagation in the microstructure of
silver fir measured experimentally to simulation modeled by MC method [12]. They
determined μ
s (wet wood: 1.79 mm
−1 , dried wood: 6.68 mm
−1 ) due to tracheids by
solving Maxwell’s equation. They also determined μ
s−iso , which is the scattering
coefficient due to all other scattering media (rough border between the lumen and
wood cell substance, pits, ray cells), calculated by fitting measured light propagation
to simulated data. The light scattering in wood is significantly complex as wood is a
hygroscopic, heterogeneous, cellular, and anisotropic material. Although the wood
samples were regarded as a homogenous material when the μ
s values were estimated
using TR spectroscopic method, in fact, the scattering properties highly depended
on the wood species and fiber direction because the cellular structure, which caused
multiple light reflections at the boundary between cell wall and air (water), significantly differs between wood species (i.e., hardwood has various cell arrangements
like ring-porous, diffuse-porous, radial-porous, and figured-porous). Kitamura et al.
tried to determine true μ a and μ
s values of wood cell wall substance itself in order
to construct the robust calibrations wood properties by NIR spectroscopy [13]. They
expected that the μ a and μ
s values of the cell wall substance are identical or similar
between species because the density of wood cell wall itself is about 1.4–1.5 g cm
−3
regardless of species ( wood density depends on the ratio of pore and cell wall volume
in wood). As the density of cell wall is identical between species, it is thought that
the factor affecting the optical properties might be the concentration ratio of the three
main polymers in the cell wall (cellulose, hemicellulose, and lignin). As there was no
specific absorption band at the wavelength used in their study (846 nm) as shown by
Hans et al. [14], it implied that the concentration ratio of the cellulose, hemicellulose,
and lignin does not strongly affect μ a . In order to decide the true optical parameters
of wood cell wall, Douglas fir wood samples were immersed in hexane, toluene,
or quinolone and saturated with them to minimize the multiple light reflections at
the boundary between pore cell wall substance in wood. TR transmittance result
of organic liquid saturated wood samples was fitted to the diffusion approximation
equation to decide μ a and μ
s . μ
s showed the minimum value when the wood was
saturated with toluene because the refractive index of toluene is close to wood cell
wall substance. In the toluene saturated wood sample, Fresnel reflection (1) at small
particle, (2) between lumen and wood cell wall substance for all cell types (tracheid,
ray cells, vessel), and (3) at the rough border are minimized because refractive index
mismatch between toluene and cell wall was very small. The optical parameters of