A three-layered skin model (stratum corneum, epidermis, and dermis) and engineering formulas for radiative transfer theory are used to study absorption spectra and light penetration depths of normal and pathologically altered skin. The formulas include small-angle and asymptotic approximations and a layer-addition method. These characteristics are calculated for wavelengths used for low-intensity laser therapy. We examined several pathologies such as vitiligo, edema, erythematosus lupus, and subcutaneous wound, for which the bulk concentrations of melanin and blood vessels or tissue structure (for subcutaneous wound) change compared with normal skin. The penetration depth spectrum is very similar to the inverted blood absorption spectrum. In other words, the depth is minimal at blood absorption maxima. The calculated absorption spectra enable the power and irradiation wavelength providing the required light effect to be selected. Relationships between the penetration depth and the diffuse reflectance coefficient of skin (unambiguously expressed through the absorption coefficient) are analyzed at different wavelengths. This makes it possible to find relationships between the light fields inside and outside the tissue.

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We have used the absorption spectra of whole blood, erythrocytes, and plasma to study photochemical reactions initiated by exposure of blood in vivo to UV radiation (UV irradiation) in the UV-visible and IR regions of the spectrum. We have established that when blood is exposed to therapeutic doses of UV radiation (0.5 J/cm 2 ), the absorption of blood proteins decreases as monitored using the UV absorption and luminescence bands of the proteins; photochemical reactions are initiated in the protein and heme components of the hemoglobin. For the studied doses, the reversible reaction of photodissociation of hemoglobin complexes with oxygen is one of the most likely primary reactions initiated by UV irradiation of blood. We conclude that changes in the position and relative intensities of the IR absorption bands of the peptide groups (stretching and bending vibrations of NH, CN, and CO bonds) may be due to conformational transitions in the blood protein macromolecules, induced with a change in the intermolecular hydrogen bonds on absorption of the UV radiation by the blood. The changes in the absorption spectra of blood initiated by UV irradiation are compared with the results of laboratory blood analyses.Key words: UV irradiation, spectrum of blood, photodissociation of hemoglobin, conformational transitions in proteins. Introduction.Exposure of blood to UV radiation (UV irradiation) is widely used in medical practice for treatment of various diseases as a method based on modification of blood by optical radiation in the UV range followed by returning the blood back into the body. UV irradiation is recognized as an effective phototherapy method that has multiple impacts on the body [1, 2]. However, to date opinions vary concerning the primary mechanisms of action on the blood by UV irradiation. The effect of UV irradiation has been studied in the most detail in cell cultures, for which several important effects have been identified: the high reversibility of the changes induced by UV irradiation; the different sensitivities of both different cells and the same cell, depending on its physiological state; stimulation of protein and nucleic acid synthesis by exposure to small UV doses [3].The effect of UV irradiation at the molecular level has been most studied for exposure of cell cultures and biomolecules to high doses (D UV >> 1 J/cm 2 ). For such doses, the molecular structure of DNA breaks down, intramolecular crosslinking of pyrimidine bases and different types of intermolecular crosslinking appear. The most general primary response of proteins to treatment with high doses of UV irradiation is the process of deamination and decarboxylation [4]. The photodissociation products are NH 3 , CO 2 , and aldehydes. A number of observed effects are associated with denaturing changes in the protein molecules. Among the effects detected at the molecular level, we also note changes in the biological activity of proteins and formation of new compounds having higher biological activity. Some of these compounds, structurally...