12 Biomedical Engineering Fundamentals
591
Here, c is the speed of light in vacuum (c = 3 × 10 8 meter/second), μ r is the
relative magnetic permeability (μ r ≈ 1 in biological media), and ε r is the relative
electric permittivity (ε r 1 in biological media).
The light energy (photon energy) can also exist and it can be given as follows
[35]:
W =
h × υ
λ
(12.38)
Here, W is the photon energy and h is Plank’s constant (h = 6.6 × 10 −34 Joule
second).
The induced light is subjected to volume and inhomogeneity effects. In volume
effects, the light absorption takes place which attenuates the propagation of light
beam in homogenous medium. On the other hand, the heterogeneous medium
causes scattering, diffraction, reflection, and refraction effects which attenuate and
redirect the light beam in a particular direction in inhomogeneity effects. These
effects are not fully independent to each other [35]. The basic inhomogeneity
effects are shown in Fig. 12.38 [36]. In Fig. 12.38, the reflection of light occurs
at biological tissue interface and the refraction of light occurs when the light
enters in tissue that has different refractive index. The absorption and scattering
of light also take place in between the tissue structure. The physical parameters
such as refractive index, absorption coefficients, and scattering coefficients related to
these inhomogeneity effects vary continuously at biological tissue boundaries. The
different biological tissues have different strengths of absorption coefficients which
determine penetration power and energy absorption into a specific tissue from a
particular light source. The absorption degree is depending on the type of tissue and
wavelength of light in many cases. The mainly light absorption takes place between
the wavelength range of UV (<400 nm) and IR (>2 μm). Hence, the light cannot
deeply penetrate in this spectral range and attenuation due to scattering is less in this
range. The scattering causes broadening of light beams and the light beams decay
as it travels through the tissue due to this scattering phenomenon. This scattering
phenomenon dominates over absorption in the spectral range of 600–1600 nm and
the forward and backward scattering of incident light within tissue are used in
various optic biomedical applications such as Raman vibrational spectroscopy and
surface-enhanced Raman scattering (SERS) [36].
The reflection and transmitted modes of light can also be used for various optic
biomedical applications such as optical plethysmography. The different arrangements of light source and sink are used for reflection and transmitted modes of
operation. Figures 12.39 and 12.40 show the different arrangements of light source
and sink for reflection and transmitted modes of operation applied on a finger,
respectively. In reflection mode, red and near-IR lights are generally used due to
the fact that these lights can penetrate tissue to relatively large depths as compared
to other lights. The arrangement of light source and sink for red and near-IR lights
are shown in Fig. 12.41. In Fig. 12.41, the light source and sink for red and near-IR
lights are arranged in reflection mode which yields different pathways by photons
591
Here, c is the speed of light in vacuum (c = 3 × 10 8 meter/second), μ r is the
relative magnetic permeability (μ r ≈ 1 in biological media), and ε r is the relative
electric permittivity (ε r 1 in biological media).
The light energy (photon energy) can also exist and it can be given as follows
[35]:
W =
h × υ
λ
(12.38)
Here, W is the photon energy and h is Plank’s constant (h = 6.6 × 10 −34 Joule
second).
The induced light is subjected to volume and inhomogeneity effects. In volume
effects, the light absorption takes place which attenuates the propagation of light
beam in homogenous medium. On the other hand, the heterogeneous medium
causes scattering, diffraction, reflection, and refraction effects which attenuate and
redirect the light beam in a particular direction in inhomogeneity effects. These
effects are not fully independent to each other [35]. The basic inhomogeneity
effects are shown in Fig. 12.38 [36]. In Fig. 12.38, the reflection of light occurs
at biological tissue interface and the refraction of light occurs when the light
enters in tissue that has different refractive index. The absorption and scattering
of light also take place in between the tissue structure. The physical parameters
such as refractive index, absorption coefficients, and scattering coefficients related to
these inhomogeneity effects vary continuously at biological tissue boundaries. The
different biological tissues have different strengths of absorption coefficients which
determine penetration power and energy absorption into a specific tissue from a
particular light source. The absorption degree is depending on the type of tissue and
wavelength of light in many cases. The mainly light absorption takes place between
the wavelength range of UV (<400 nm) and IR (>2 μm). Hence, the light cannot
deeply penetrate in this spectral range and attenuation due to scattering is less in this
range. The scattering causes broadening of light beams and the light beams decay
as it travels through the tissue due to this scattering phenomenon. This scattering
phenomenon dominates over absorption in the spectral range of 600–1600 nm and
the forward and backward scattering of incident light within tissue are used in
various optic biomedical applications such as Raman vibrational spectroscopy and
surface-enhanced Raman scattering (SERS) [36].
The reflection and transmitted modes of light can also be used for various optic
biomedical applications such as optical plethysmography. The different arrangements of light source and sink are used for reflection and transmitted modes of
operation. Figures 12.39 and 12.40 show the different arrangements of light source
and sink for reflection and transmitted modes of operation applied on a finger,
respectively. In reflection mode, red and near-IR lights are generally used due to
the fact that these lights can penetrate tissue to relatively large depths as compared
to other lights. The arrangement of light source and sink for red and near-IR lights
are shown in Fig. 12.41. In Fig. 12.41, the light source and sink for red and near-IR
lights are arranged in reflection mode which yields different pathways by photons