In vivo study of age-related changes in the optical properties of the skin

The optical properties of the skin (absorption coefficient, scattering coefficient, refractive index) may serve to characterize the skin and are important for correct light dosimetry in many optical diagnostic procedures and laser treatments especially photodynamic therapy and laser therapy. We determined in vivo the optical properties of tissues near the wrist, elbow and knee in subjects of different ages using diffuse reflectance spectroscopy, having in view the establishment of laser system types for the laser treatment of posttraumatic lesions in subjects of different ages. Diffuse reflection of light from biological tissue is due to the variation in refractive index of tissular and cellular components and the surrounding medium and depends on the wavelength of the incident optical radiation. The diffuse reflectance spectrum of the tissues tested showed two maxima localized at λM1 ≈ 610 nm and λM2 ≈ 675 nm. Laser systems which emit radiation at these wavelengths are not efficient for the treatment of joints, regardless of the subject’s age. The deep tissues have a strong absorption in the range 630–700 nm, which indicates that for treating posttraumatic lesions we can use laser systems such as the He–Ne laser, the GaAlAs laser, and the InGaAlAs laser. Using Kramers-Kronig analysis of the diffuse reflectance spectra, the optical parameters n(ω) and k(ω) were determined. The age-dependent changes in these optical parameters of tissue must be taken into consideration and the use of laser treatments or optical diagnosis methods must be based on a knowledge of these properties and of the optical radiation parameters.