6
1 Methods Describing the Interaction of Laser Radiation with Biological Tissues
ance, the nature of the measurements in-vivo - in-vitro and others. The attenuation
of the laser beam in biological tissue follows the exponential law. The intensity of
the collimated radiation is estimated under Bouguer law. Other important optical
parameters of the tissue is the optical depth penetration.
The significant value of the scattering anisotropy of biological tissues and multiple
scattering gets a deviation from the Bouguer law. In the description of the effects that
occur in the tissues under the influence of radiation, absorption of water is important
because it is the main component of most tissues. The human body contains from
about 55–65% water. An adult with a body weight of 65 kg contains an average of
40 liters of water, of which about 25 L is inside cells, 15 L are in the extracellular
fluids. Water is the primary medium in which many chemical reactions take place
and the physical and chemical processes (assimilation, dissimilation, osmosis, diffusion, transport and others) that important for life. In the ultraviolet, visible and near
infrared wavelengths the absorption coefficient of water is very small. In these areas,
the absorption of tissue determines the absorption spectra of pigments, especially
for the skin - the absorption spectra of melanin and blood count (hemoglobin and
oxyhemoglobin). Melanin absorption is the most important component of the total
absorption of the epidermis and the stratum corneum.
For the calculation of interest the optical density (OD) of the epidermis is needed,
which is the result of the following product:
OD = μ melanin · h,
where μ melanin is the absorption coefficient of melanin, h is the thickness of the
epidermis.
Optical density depends on the amount of melanin in the basal layer, which
depends on many factors, the main one of which is the type of skin. Note that
the dermis is very different from the epidermis in the composition and structure.
The scattering coefficient of the dermis stronger at shorter wavelengths. Scattering
plays a major role in determining the depth of penetration of radiation at different
wavelengths in the dermis. Therefore, longer wavelengths penetrate deeper rather
than shorter. It is explained by the presence of melanin, which absorbs more shorter
wavelengths than long. According to [15] for a sample consisting of the epidermis and
dermis, the depth of which is 0.15–0.2 mm (wavelength 632.8 nm) and 0.21–0.4 nm
(wavelength 675 nm).
1.3 Structure and Optical Properties of Blood
Blood is one of the most important biological fluids. Blood is the liquid part of
the plasma (57% of blood volume) and suspended in its cell (enzymatic) elements
(43%). Plasma consist from 90–91% water; 6.5–8.0% are protein molecules and the
remaining 2% are low molecular substances. In addition, the blood contains platelets
99% of the blood cells are red blood cells, and 1% are white blood cells and platelets.
1 Methods Describing the Interaction of Laser Radiation with Biological Tissues
ance, the nature of the measurements in-vivo - in-vitro and others. The attenuation
of the laser beam in biological tissue follows the exponential law. The intensity of
the collimated radiation is estimated under Bouguer law. Other important optical
parameters of the tissue is the optical depth penetration.
The significant value of the scattering anisotropy of biological tissues and multiple
scattering gets a deviation from the Bouguer law. In the description of the effects that
occur in the tissues under the influence of radiation, absorption of water is important
because it is the main component of most tissues. The human body contains from
about 55–65% water. An adult with a body weight of 65 kg contains an average of
40 liters of water, of which about 25 L is inside cells, 15 L are in the extracellular
fluids. Water is the primary medium in which many chemical reactions take place
and the physical and chemical processes (assimilation, dissimilation, osmosis, diffusion, transport and others) that important for life. In the ultraviolet, visible and near
infrared wavelengths the absorption coefficient of water is very small. In these areas,
the absorption of tissue determines the absorption spectra of pigments, especially
for the skin - the absorption spectra of melanin and blood count (hemoglobin and
oxyhemoglobin). Melanin absorption is the most important component of the total
absorption of the epidermis and the stratum corneum.
For the calculation of interest the optical density (OD) of the epidermis is needed,
which is the result of the following product:
OD = μ melanin · h,
where μ melanin is the absorption coefficient of melanin, h is the thickness of the
epidermis.
Optical density depends on the amount of melanin in the basal layer, which
depends on many factors, the main one of which is the type of skin. Note that
the dermis is very different from the epidermis in the composition and structure.
The scattering coefficient of the dermis stronger at shorter wavelengths. Scattering
plays a major role in determining the depth of penetration of radiation at different
wavelengths in the dermis. Therefore, longer wavelengths penetrate deeper rather
than shorter. It is explained by the presence of melanin, which absorbs more shorter
wavelengths than long. According to [15] for a sample consisting of the epidermis and
dermis, the depth of which is 0.15–0.2 mm (wavelength 632.8 nm) and 0.21–0.4 nm
(wavelength 675 nm).
1.3 Structure and Optical Properties of Blood
Blood is one of the most important biological fluids. Blood is the liquid part of
the plasma (57% of blood volume) and suspended in its cell (enzymatic) elements
(43%). Plasma consist from 90–91% water; 6.5–8.0% are protein molecules and the
remaining 2% are low molecular substances. In addition, the blood contains platelets
99% of the blood cells are red blood cells, and 1% are white blood cells and platelets.