1.2 The Structure and Optical Properties of Biological Tissues
5
oxygen present in the blood. The absorption spectra of the two forms of hemoglobin
are slightly different from each other: oxyhemoglobin has an absorption band near
405 nm (Sore band) and the characteristic double peak absorption in the area of 545–
575 nm; deoksigemoglobin strongly absorbs near 430 nm and a weak close to 550 nm
[13, 14].
In the infrared region of the spectrum all biomolecules have quite intense vibrational absorption bands.
Starting with λ = 1500 nm and above, the absorption spectrum of the skin is
largely determined by the absorption spectrum of water.
The absorption of subcutaneous fatty tissue is defined as absorption bands of
lipids, water, and β- carotene. The main absorption band of fatty tissue lies in the
ultraviolet and infrared regions of the spectrum. Skin tissue is characterized by a
significant light scattering, as it consists of a large number of randomly distributed
scattering centers in volume [15]. Light scattering happens because of fluctuations
in the density of scatterer and refractive index fluctuations in the volume of tissue.
The nature of scattering depends on the correlation of the wavelength of the
scattered radiation and the size of the light scattering particles, and the ratio of the
refractive index of the scattering particle and its environment [16]. Light scattering
in media consisting of a large number of particles is significantly different from the
scattering of light by individual particles.
This is explained by firstly the interference of the waves scattered by the individual
particles with each other and with the incident wave, and second, in many cases,
multiple scattering effects are important (reradiation), when the light is scattered by
a single particle, others are dissipated again and thirdly, the interaction between the
particles does not allow them to consider independent movement.
To account for multiple scattering and absorption of the laser the beam is broadened and attenuated during propagation in the skin. Volume scattering is the cause of a
significant proportion of the radiation propagation in the reverse direction (backscattering). Cell membranes, nucleus and organelles are the main scatterers in many
biological tissues. The absorbed light is converted into heat, is reradiated as fluorescence or phosphorescence, and spent photobiochemistry reaction.
The absorption spectrum is determined by the type of dominant absorption centers
and water content in the tissue. The natural photo of laser radiation of biological tissue
is determined by its composition and the absorption coefficient at the wavelength of
radiation. The ultraviolet and infrared (λ > 2 nm) spectral region dominates the
absorption and scattering, and the contribution is relatively small and shallow. Light
penetrates into the biological tissue, only to one or more cell layers in the short-visible
and spectrum of light penetration depth for a typical tissue that is 0.5–2.5 mm. In
this case the main role is absorption and thus scattering, which predominates in
the reflected radiation from the skin (affects approximately 15–50% of the incident
beam). At wavelengths from 600–1500 nm scattering prevails over absorption and
penetration depth is increased to 8–10 mm.
Depending on the type of tissue the wavelength of the reflection coefficient can
vary widely. Thus, the optical properties of biological tissue are determined by its
structure, physiological condition, the level of hydration, homogeneity, specific vari-
5
oxygen present in the blood. The absorption spectra of the two forms of hemoglobin
are slightly different from each other: oxyhemoglobin has an absorption band near
405 nm (Sore band) and the characteristic double peak absorption in the area of 545–
575 nm; deoksigemoglobin strongly absorbs near 430 nm and a weak close to 550 nm
[13, 14].
In the infrared region of the spectrum all biomolecules have quite intense vibrational absorption bands.
Starting with λ = 1500 nm and above, the absorption spectrum of the skin is
largely determined by the absorption spectrum of water.
The absorption of subcutaneous fatty tissue is defined as absorption bands of
lipids, water, and β- carotene. The main absorption band of fatty tissue lies in the
ultraviolet and infrared regions of the spectrum. Skin tissue is characterized by a
significant light scattering, as it consists of a large number of randomly distributed
scattering centers in volume [15]. Light scattering happens because of fluctuations
in the density of scatterer and refractive index fluctuations in the volume of tissue.
The nature of scattering depends on the correlation of the wavelength of the
scattered radiation and the size of the light scattering particles, and the ratio of the
refractive index of the scattering particle and its environment [16]. Light scattering
in media consisting of a large number of particles is significantly different from the
scattering of light by individual particles.
This is explained by firstly the interference of the waves scattered by the individual
particles with each other and with the incident wave, and second, in many cases,
multiple scattering effects are important (reradiation), when the light is scattered by
a single particle, others are dissipated again and thirdly, the interaction between the
particles does not allow them to consider independent movement.
To account for multiple scattering and absorption of the laser the beam is broadened and attenuated during propagation in the skin. Volume scattering is the cause of a
significant proportion of the radiation propagation in the reverse direction (backscattering). Cell membranes, nucleus and organelles are the main scatterers in many
biological tissues. The absorbed light is converted into heat, is reradiated as fluorescence or phosphorescence, and spent photobiochemistry reaction.
The absorption spectrum is determined by the type of dominant absorption centers
and water content in the tissue. The natural photo of laser radiation of biological tissue
is determined by its composition and the absorption coefficient at the wavelength of
radiation. The ultraviolet and infrared (λ > 2 nm) spectral region dominates the
absorption and scattering, and the contribution is relatively small and shallow. Light
penetrates into the biological tissue, only to one or more cell layers in the short-visible
and spectrum of light penetration depth for a typical tissue that is 0.5–2.5 mm. In
this case the main role is absorption and thus scattering, which predominates in
the reflected radiation from the skin (affects approximately 15–50% of the incident
beam). At wavelengths from 600–1500 nm scattering prevails over absorption and
penetration depth is increased to 8–10 mm.
Depending on the type of tissue the wavelength of the reflection coefficient can
vary widely. Thus, the optical properties of biological tissue are determined by its
structure, physiological condition, the level of hydration, homogeneity, specific vari-