23.2.1 Chromophores in the Skin
Skin optics has been evaluated by multiple investigators with the intent of improving phototherapy [36-38]. Though the optics of human skin is extraordinarily complex, there are a few key considerations, as it relates to lasers and human safety. When considering transmission of light into tissue, there are three components: (i) reflection, (ii) scattering, and (iii) absorption. Reflectance is of minor importance compared to absorption and scattering, particularly when considering the depth of penetration of light into the skin. Scattering in the dermis is most probably the interaction of light with collagen. This event plays a large role in the depth of penetration of light with greater scattering at shorter wavelengths, in accordance with the Kubelka-Munk theory [38]. The chromophores in the skin which are the targets of lasers used in dermatology and absorb the photon energy from such devices are melanin, oxyhemaglobin/hemoglobin, and water [39]. The absorption profile of melanin and oxyhemaglobin are presented in Fig. 23.2. Depending on the wavelength of light, oxyhemaglobin or melanin can be preferentially targeted. The photon energy absorbed by melanin or oxyhemaglobin/hemoglobin is dissipated as heat.
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Alexandrite Wavelength (nm) Figure 23.2 Absorption profile for melanin and oxyhemaglobin. The shaded region identifies the range of wavelengths for lasers commonly used in nonablative dermatological procedures, e. g., tattoo removal, hair epilation. |