Role of epidermis in the optics and thermal physics of human skin

Some engineering approaches to the problems of optics and thermophysics of biological tissues have been developed. The light fields both within and beyond biological medium have been studied. The role of the epidermis in the formation of these fields is revealed. It is shown that in the general case epidermis cannot be considered as a spectral filter that only absorbs and does not scatter light. A convenient approximation of the fluence rate in a two-layer (epidermis and dermis) tissue is proposed, which is the sum of two exponential functions of depth in the epidermis. This approximation made it possible to obtain an analytical solution to the problem of heating biological tissues by an external narrow light beam. It is found that epidermis only slightly affects the temperature fields under blue light irradiation. However, when an external source of red light is used, the epidermis works as a heater and can increase several times the dermis temperature in comparison with a single-layer tissue. The reasons for these heating features are discussed. Examples of corresponding calculations are given.