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T. Inagaki and S. Tsuchikawa
wood cell wall substance calculated taking into account the volume fraction of wood
cell wall substance were μ a = 0.030 mm
−1 and μ
s = 18.4 mm
−1 . Konagaya et al.
[15] fully investigated the effect of boundary between cell wall substance and air
or water (refractive index mismatch) on the μ
s . They investigated optical properties
of drying wood with the moisture contents ranging from 10 to 200% by TR spectroscopy. They divided the source of light scattering into two factors, 1. scattering
from large scatters (i.e., scattering diameter is much larger than the wavelength λ of
the light), which can be described by geometric optics, expressing light propagation
in terms of rays, and 2. scattering from small scatters (i.e., when the scattering diameter is the same as or less than λ), where Mie theory (≈λ) or Rayleigh theory (<λ) is
applicable. They revealed the contribution of scattering source at each stage of wood
drying (constant rate, initial part of the first decreasing rate, later part, and second
decreasing rate period). Scattering from dry pores dominated during the constant
drying rate period, and the drying process of smaller pores dominated during the
period of decreasing drying rate. The surface layer and interior of the wood exhibit
different moisture states, which affect the scattering properties of the wood. The
light propagation in wood complex cell structure is simulated by MC method taking
into account the light reflection and transmission at the boundary between wood
cell wall substance and pore by Ban et al. [16]. They investigated the relation of
wood texture parameters calculated from cross-sectional microscopic images of the
13 species of wood samples and μ
s at 846 nm. They found that μ
s has linear relation
to the air-dry density (R
2
= 0.56), quadratical relation to the cell–wall area ratio
(R
2
= 0.76), and exponentially relation to the median pore area (R
2
= 0.54). 85
percent of the variation in μ
s between many wood species can be explained by these
three parameters. They simulated the light propagation in wood using the measured
cross-sectional microscopic image of wood. After they performed segmentation of
the microscopic image and recognized the pixel corresponding to cell wall substance
and air area, the simulations were performed in the MC code, MCVM. The refractive index mismatch at the boundaries is also considered to improve the precision of
simulations in MCVM code. Figure 11.2 shows simulated photon propagation in (a)
agathis, (b) yellow poplar, and (c) rubber wood. It is observed that photon spreads
farther in wood cell wall woods through continuously connected cell walls. The high
correlation of cell–wall area ratio and median pore area on μ
s can be attributed to
the thicker, more connected cell walls associated with large cell–wall area ratio and
small median pore area. Accordingly, increasing the area ratio of the cell wall and
decreasing the pore area increased the μ
s .
11.7 Brief Explanation for SR Spectroscopy
Not only the TR spectroscopy, some techniques are used to determine the optical
properties. SR technique was developed to understand light propagation in turbid
media. Compared to TR spectroscopy, SR technique is well suitable for use in postharvest applications thanks to its low instrumentation cost, easy implementation.
T. Inagaki and S. Tsuchikawa
wood cell wall substance calculated taking into account the volume fraction of wood
cell wall substance were μ a = 0.030 mm
−1 and μ
s = 18.4 mm
−1 . Konagaya et al.
[15] fully investigated the effect of boundary between cell wall substance and air
or water (refractive index mismatch) on the μ
s . They investigated optical properties
of drying wood with the moisture contents ranging from 10 to 200% by TR spectroscopy. They divided the source of light scattering into two factors, 1. scattering
from large scatters (i.e., scattering diameter is much larger than the wavelength λ of
the light), which can be described by geometric optics, expressing light propagation
in terms of rays, and 2. scattering from small scatters (i.e., when the scattering diameter is the same as or less than λ), where Mie theory (≈λ) or Rayleigh theory (<λ) is
applicable. They revealed the contribution of scattering source at each stage of wood
drying (constant rate, initial part of the first decreasing rate, later part, and second
decreasing rate period). Scattering from dry pores dominated during the constant
drying rate period, and the drying process of smaller pores dominated during the
period of decreasing drying rate. The surface layer and interior of the wood exhibit
different moisture states, which affect the scattering properties of the wood. The
light propagation in wood complex cell structure is simulated by MC method taking
into account the light reflection and transmission at the boundary between wood
cell wall substance and pore by Ban et al. [16]. They investigated the relation of
wood texture parameters calculated from cross-sectional microscopic images of the
13 species of wood samples and μ
s at 846 nm. They found that μ
s has linear relation
to the air-dry density (R
2
= 0.56), quadratical relation to the cell–wall area ratio
(R
2
= 0.76), and exponentially relation to the median pore area (R
2
= 0.54). 85
percent of the variation in μ
s between many wood species can be explained by these
three parameters. They simulated the light propagation in wood using the measured
cross-sectional microscopic image of wood. After they performed segmentation of
the microscopic image and recognized the pixel corresponding to cell wall substance
and air area, the simulations were performed in the MC code, MCVM. The refractive index mismatch at the boundaries is also considered to improve the precision of
simulations in MCVM code. Figure 11.2 shows simulated photon propagation in (a)
agathis, (b) yellow poplar, and (c) rubber wood. It is observed that photon spreads
farther in wood cell wall woods through continuously connected cell walls. The high
correlation of cell–wall area ratio and median pore area on μ
s can be attributed to
the thicker, more connected cell walls associated with large cell–wall area ratio and
small median pore area. Accordingly, increasing the area ratio of the cell wall and
decreasing the pore area increased the μ
s .
11.7 Brief Explanation for SR Spectroscopy
Not only the TR spectroscopy, some techniques are used to determine the optical
properties. SR technique was developed to understand light propagation in turbid
media. Compared to TR spectroscopy, SR technique is well suitable for use in postharvest applications thanks to its low instrumentation cost, easy implementation.