The Optics of Human Skin

Abstract

An integrated review of the transfer of optical radiation into human skin is presented, aimed at developing useful models for photomedicine. The component chromophores of epidermis and stratum corneum in general determine the attenuation of radiation in these layers, moreso than does optical scattering. Epidermal thickness and melanization are important factors for UV wavelengths less than 300 nm, whereas the attenuation of UVA (320-400 nm) and visible radiation is primarily via melanin. The selective penetration of all optical wavelengths into psoriatic skin can be maximized by application of clear lipophilic liquids, which decrease regular reflectance by a refractive-index matching mechanism. Sensitivity to wavelengths less than 320 nm can be enhanced by prolonged aqueous bathing, which extracts urocanic acid and other diffusible epidermal chromophores. Optical properties of the dermis are modelled using the Kubelka-Munk approach, and calculations of scattering and absorption coefficients are presented. This simple approach allows estimates of the penetration of radiation in vivo using noninvasive measurements of cutaneous spectral remittance (diffuse reflectance). Although the blood chromophores Hb, HbO2, and bilirubin determine dermal absorption of wavelengths longer than 320 nm, scattering by collagen fibers largely determines the depths to which these wavelengths penetrate the dermis, and profoundly modifies skin colors. An optical "window" exists between 600 and 1300 nm, which offers the possibility of treating large tissue volumes with certain long-wavelength photosensitizers. Moreover, whenever photosensitized action spectra extend across the near UV and/or visible spectrum, judicious choice of wavelengths allows some selection of the tissue layers directly affected.

An in-shoe laser Doppler sensor for assessing plantar blood flow in the diabetic foot

“…2.3.3 Optical scattering in skin tissue Anderson and Parrish (1981) found that the scattering processes in the dermis were mainly determined by erythrocytes, blood vessels, and structural fibres such as collagen.…”

“…In cutaneous tissue the erythrocyte concentration typically accounts for 4.5% of the total tissue volume (Anderson and Parrish 1981). The erythrocytes are non-uniformly distributed in tissue due to vessel containment.…”

Iris damage and acute pigment dispersion following photo-epilation

“…1 Confocal microscopy has become a popular technique to observe the structure of cornea in vivo in several disease states afflicting the cornea including diabetes mellitus. 2,3 Among the less common observations in the corneas of diabetic patients are the faint vertical lines at the level of Descemet's membrane and endothelium, initially described by Waite and Beetham 4 and Henkind and Wise. 5 The purpose of this paper was to report the characteristics and incidence of Waite-Beetham lines in the corneas of diabetic patients.…”

“…2,3 Hair follicles lie about 3 mm below the skin surface. However, thermal damage from the alexandrite laser can occur at several millimetres depending on user settings and skin type.…”

“…It relies on the principle of selective photothermolysis whereby use of an appropriate wavelength and pulse duration of light causes injury that is confined to the desired target tissue while sparing surrounding structures. 1,2 We report a case of iris damage and acute pigment dispersion after the use of long-pulsed infra-red (LPIR) alexandrite laser for photo-epilation of the eyebrow.…”