Chapter 3 - Physics Behind Light-Based Systems: Skin and Hair Follicle Interactions with Light

Abstract

This chapter discusses 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 further 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. 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 ski/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