Principles of light-skin interactions

Abstract

Skin is the largest human organ. It covers between 1.5 and 2 m2, comprising about one-sixth of total body weight. Skin performs a complex role in human physiology. It serves as a barrier to the environment and acts as a channel for communication to the outside world. For example, skin protects us from water loss, ultraviolet (UV) rays of the sun, friction, and impact wounds. It also helps in regulating body temperature and metabolism. All photobiological responses are influenced heavily by the optical properties of skin. Therefore, for the successful development of photomedicine, in-depth knowledge and understanding of light-skin interactions, specifically known as skin optics, is required. The transfer of optical radiation into human skin depends on the absorption and scattering properties of three functional skin layers: epidermis, dermis, and hypodermis. The structures and component chromophores of these layers determine the attenuation of radiation in skin. The enhanced penetration of optical radiation as well as selective targeting of pathology can be achieved by studying and analyzing the wavelength-dependent interactions of light with skin. For example, considering that melanin exhibits maximum absorption in the UV and blue spectral ranges, whereas blood preferentially absorbs blue and yellow light, the treatment protocol have been devised that target pigmented and vascular lesions, respectively.1 The chromophores, such as melanin, blood, water, and lipid determine skin absorption. Scattering largely determines the depths to which light penetrates through skin, as it dominates absorption in the visible and near-infrared (NIR) spectral ranges by at least one order of magnitude. It has also been shown that light scattering from dermal collagen significantly modifies skin color.2 Thus, detailed information on scattering is required for the accurate estimation of the light penetration through skin. An optical window between 600 and 1300 nm offers the possibility of treating large tissue volumes3 and using exogenous chromophores/fluorophores for contrast-enhancing.4-6 Optical imaging and spectroscopy allow for noninvasive assessment of skin pathology and treatment efficacy.7-9 In particular, reflectance imaging and spectroscopy provide information on the distribution and quantities of the scatterers and chromophores,10 whereas fluorescence responses determine the biochemical composition of the interrogated biotissue.11,12 The development of lasers and light-based medical devices has been stimulated by the achievements of diagnostic and therapeutic photomedicine.13-15 This chapter provides a brief summary and description of the properties of light, its interaction with human skin, the list of the medical light sources, and the diagnostic and/or therapeutic uses of currently available light-based devices within the visible to NIR spectral range in dermatology. © 2009 Springer-Verlag London.