266
T. Inagaki and S. Tsuchikawa
Behavior of transmitted or diffuse reflected light from an agricultural, forest
product or human body (i.e., highly scattering media) is strongly affected by
both physical and chemical properties of the tissues, making it complicate to
examine the optical characteristics of the tissue in detail and to evaluate the
sample constituents accurately. Especially in the 500–1100 nm wavelength range
for most biological media, the scattering coefficient is much higher than the absorption coefficient. Although many studies have reported that the chemical, physical, and mechanical properties of biological material can be predicted by NIR
diffuse reflectance/transmittance spectroscopy with the aid of statistical methods
(i.e., chemometrics), such chemometric NIR approaches have some disadvantages.
First, the contribution of the light absorption and scattering phenomena in acquired
spectra cannot be explained independently. Second, the construction of a calibration
model, which is usually not transferable among instruments, requires a considerable
amount of spectral and objective data. Additionally, the light scattering contribution
to NIR spectra is significant when the material has complex cellular structure result in
the high scattering of light. In order to construct robust calibrations for organic materials by NIR spectroscopy, it is of importance to independently evaluate the spectral
contribution from light absorption (absorption resulting from harmonics or overtones
of the fundamental absorptions of molecular vibrations) and light scattering (mainly
due to the cellular structure and refractive index mismatch at the boundary).
In order to understand such a complex phenomena of light propagation in organic
materials, many researchers have given attention to time-of-flight (TOF) or timeresolved (TR) spectroscopy using short pulses of light emission and observe the
reflected or transmitted light as a function of time in nano or pico order. A timeresolved measurement, or time domain system, could provide the TOF information
of the detected light. In the TOF approach, tens of picosecond light pulses are usually
injected into the tissue, usually using a suitable optical fiber.
The intensity of light pulse propagates through the tissue is detected at a certain
distance from the injection point (Fig. 11.1). It is also possible to examine biological
tissue using the transmission approach. In this approach, the source and detector
fibers are placed on opposite sides of the tissue. Time domain intensity of photons
propagated into the tissue, known as the photon distribution of time of flight (DTOF),
results delayed, broadened, and attenuated because of the scattering and absorption of
light inside the diffusive medium. Although it is possible to estimate the optical properties of materials by spatially resolved technique (SR) or spatial frequency domain
technique, it is considered that TR technique is more accurate in the measuring of
optical properties.
Patterson et al. [1] proposed the usefulness of the time-resolved reflectance and
transmittance spectroscopy for the noninvasive measurement of tissue optical properties theoretically. They developed a model based on the diffusion approximation of
radiative transfer, which yielded an analytical expression for pulse shape in terms of
the interaction with a homogeneous slab, for the determination of optical properties
[i.e., absorption coefficient (μ a ) and reduced scattering coefficient (μ
s )] in tissue. μ
s
is defined as
T. Inagaki and S. Tsuchikawa
Behavior of transmitted or diffuse reflected light from an agricultural, forest
product or human body (i.e., highly scattering media) is strongly affected by
both physical and chemical properties of the tissues, making it complicate to
examine the optical characteristics of the tissue in detail and to evaluate the
sample constituents accurately. Especially in the 500–1100 nm wavelength range
for most biological media, the scattering coefficient is much higher than the absorption coefficient. Although many studies have reported that the chemical, physical, and mechanical properties of biological material can be predicted by NIR
diffuse reflectance/transmittance spectroscopy with the aid of statistical methods
(i.e., chemometrics), such chemometric NIR approaches have some disadvantages.
First, the contribution of the light absorption and scattering phenomena in acquired
spectra cannot be explained independently. Second, the construction of a calibration
model, which is usually not transferable among instruments, requires a considerable
amount of spectral and objective data. Additionally, the light scattering contribution
to NIR spectra is significant when the material has complex cellular structure result in
the high scattering of light. In order to construct robust calibrations for organic materials by NIR spectroscopy, it is of importance to independently evaluate the spectral
contribution from light absorption (absorption resulting from harmonics or overtones
of the fundamental absorptions of molecular vibrations) and light scattering (mainly
due to the cellular structure and refractive index mismatch at the boundary).
In order to understand such a complex phenomena of light propagation in organic
materials, many researchers have given attention to time-of-flight (TOF) or timeresolved (TR) spectroscopy using short pulses of light emission and observe the
reflected or transmitted light as a function of time in nano or pico order. A timeresolved measurement, or time domain system, could provide the TOF information
of the detected light. In the TOF approach, tens of picosecond light pulses are usually
injected into the tissue, usually using a suitable optical fiber.
The intensity of light pulse propagates through the tissue is detected at a certain
distance from the injection point (Fig. 11.1). It is also possible to examine biological
tissue using the transmission approach. In this approach, the source and detector
fibers are placed on opposite sides of the tissue. Time domain intensity of photons
propagated into the tissue, known as the photon distribution of time of flight (DTOF),
results delayed, broadened, and attenuated because of the scattering and absorption of
light inside the diffusive medium. Although it is possible to estimate the optical properties of materials by spatially resolved technique (SR) or spatial frequency domain
technique, it is considered that TR technique is more accurate in the measuring of
optical properties.
Patterson et al. [1] proposed the usefulness of the time-resolved reflectance and
transmittance spectroscopy for the noninvasive measurement of tissue optical properties theoretically. They developed a model based on the diffusion approximation of
radiative transfer, which yielded an analytical expression for pulse shape in terms of
the interaction with a homogeneous slab, for the determination of optical properties
[i.e., absorption coefficient (μ a ) and reduced scattering coefficient (μ
s )] in tissue. μ
s
is defined as