Gregory B. Altshuler1 and Valery V. Tuchin2,3
1Palomar Medical Technologies, Inc., Burlington, MA, USA 2Institute of Optics and Biophotonics, Saratov State University, Saratov, Russia 3Institute of Precise Mechanics and Control of RAS, Saratov, Russia
Gurpreet S. Ahluwalia (ed.), Cosmetic Applications of Laser and Light-Based Systems, 49-123,
© 2009 William Andrew Inc.
Skin as a biological tissue is an optically inhomogeneous and absorbing medium whose average refractive index is higher than that of air. This is responsible for the partial reflection of radiation at the skin/air interface, while the remaining part penetrates the skin. Multiple scattering and absorption are responsible for laser beam broadening and eventual decay as the radiation travels through the skin, whereas bulk scattering within skin dermis and underlying tissues is a major cause of the dispersion of a large fraction of radiation in the backward direction. Therefore, light propagation within the skin depends on the scattering and absorption properties of its compartments: cells, cell organelles, and various fiber structures [1-11]. The size, shape, and density of these structures, their refractive index, relative to the interstitial ground substance, and the polarization state of the incident light all play important roles in the propagation of light in tissues.
Light interaction with a multilayer and multicomponent skin is a very complicated process [1-11]. The horny skin layer (stratum corneum) reflects about 5-7% of the incident light. A collimated light beam is transformed to a diffuse one by microscopic inhomogeneities at the air/horny layer interface. A major part of reflected light results from backscattering in different skin layers (stratum corneum, epidermis, dermis, blood, and fat). The absorption of diffuse light by the major skin pigments, such as melanin, hemoglobin, and its oxygenated form, is an informative feature for diagnosis and monitoring of skin pathology and aging.
Light-induced thermal effects in skin are important for diagnostics, therapy, and surgery. The optothermal diagnostic methods are based on detection of the time-dependent heat generation, induced in skin by a comparably low-intensive pulsed or modulated optical radiation. They allow one to estimate optical, thermal, and acoustic properties of skin and underlying tissues that depend on peculiarities of tissue structure.
For thermal phototherapy and surgery, much higher light intensities are used. In these cases, controllable temperatures rise, and thermal and/or thermo-mechanical damage (coagulation, vaporization, vacuolization, pyrolysis, ablation) of skin are important.
In this chapter we discuss the basic physics of light and light interaction with skin that defines light propagation in skin and light photothermal action. Light refraction, scattering, absorption, as well as spectral and polarization properties are analyzed. Different light sources and fibers for light delivery are briefly described. Skin’s optical properties, its penetration depth, transmittance, reflectance, and fluorescence spectra formation are also discussed. The prospective use of skin optical clearing technology for more effective applications of various optical methods is also presented. Mechanisms of light tissue interaction of inducing photochemical, photothermal, and photomechanical reactions are discussed in the framework of skin selective photothermolysis and extensions of this technology.