11 Time-of-Flight Spectroscopy
267
Fig. 11.1 Summary of TR NIR spectroscopy. The injected pulse (IRF: instruments response function) propagate into scattering media with absorption coefficient and reduced scattering coefficient.
The detected light intensity at certain distance from injected point (ρ) with time domain broadens
due to light scattering (DTOF: distribution of time-of-flight)
μ
s = (1 − g)μ s
where μ s is the linear scattering coefficient, and g is the mean cosine of the scattering
angle. A value of g = 1 represents forward scattering, while g = 0 represents isotropic
scattering. Under the assumption that μ a μ
s (i.e., high scattering media), the
diffusion of a photon can be considered to be in a random walk of step size 1/μ
s ,
where each step involves isotropic scattering. Patterson et al. solve the diffusion
equation using Green’s function with two assumptions; 1. All the incident photon
are initially scattered at the depth of z 0 = 1/μ
s and 2. diffuse photon rate at the
physical boundary between tissue and non-scattering medium would be 0. They
successfully express the reflectance and transmittance ratio with the function of
distance from light source and time. The usefulness of the function they reported
has been proven by many researches. However, Leonardi and Burns [2] investigated
quantitative measurements in scattering media on the basis of TOF spectroscopy with
analytical descriptors. They found that experimental analysis from time-resolved
profiles is efficient in estimating absorption and scattering coefficients. Numerical
methods such as adding-doubling and Monte Carlo (MC) methods simulating light
propagation in biological tissues are also often used.
Many researches revealed that the determination of μ a and μ
s in agricultural and
food products can be used for the evaluation of chemical and physical properties
in samples [3–5]. In the field of medical science, time domain method is expected
to develop optical tomography techniques, which can be used for the noninvasive
detection of cancer [6] or the changes of hemoglobin concentration associated with
neural activation in human brain [7]. In this chapter, we introduce the principle of
TOF-NIR spectroscopy and some applications to agricultural, medical area and forest
products.
267
Fig. 11.1 Summary of TR NIR spectroscopy. The injected pulse (IRF: instruments response function) propagate into scattering media with absorption coefficient and reduced scattering coefficient.
The detected light intensity at certain distance from injected point (ρ) with time domain broadens
due to light scattering (DTOF: distribution of time-of-flight)
μ
s = (1 − g)μ s
where μ s is the linear scattering coefficient, and g is the mean cosine of the scattering
angle. A value of g = 1 represents forward scattering, while g = 0 represents isotropic
scattering. Under the assumption that μ a μ
s (i.e., high scattering media), the
diffusion of a photon can be considered to be in a random walk of step size 1/μ
s ,
where each step involves isotropic scattering. Patterson et al. solve the diffusion
equation using Green’s function with two assumptions; 1. All the incident photon
are initially scattered at the depth of z 0 = 1/μ
s and 2. diffuse photon rate at the
physical boundary between tissue and non-scattering medium would be 0. They
successfully express the reflectance and transmittance ratio with the function of
distance from light source and time. The usefulness of the function they reported
has been proven by many researches. However, Leonardi and Burns [2] investigated
quantitative measurements in scattering media on the basis of TOF spectroscopy with
analytical descriptors. They found that experimental analysis from time-resolved
profiles is efficient in estimating absorption and scattering coefficients. Numerical
methods such as adding-doubling and Monte Carlo (MC) methods simulating light
propagation in biological tissues are also often used.
Many researches revealed that the determination of μ a and μ
s in agricultural and
food products can be used for the evaluation of chemical and physical properties
in samples [3–5]. In the field of medical science, time domain method is expected
to develop optical tomography techniques, which can be used for the noninvasive
detection of cancer [6] or the changes of hemoglobin concentration associated with
neural activation in human brain [7]. In this chapter, we introduce the principle of
TOF-NIR spectroscopy and some applications to agricultural, medical area and forest
products.