Dermoscopy: Looking skin deep

Dermoscopes are specialized microscopes used to examine the skin. Skin has several layers, and viewing one layer is difficult if another lies above it (see Figure 1). The corneal layer (corneum) is made primarily of the protein keratin. The corneum has many randomly oriented surfaces at air-tissue boundaries formed by air pockets between keratin surfaces. When light passes between two materials (e.g. air and keratin) with indices of refraction $n_1$‍ and $n_2$‍ the fraction of light reflected, $R,$‍ is predicted by (Equation 1) and the angle of reflection equals the angle of incidence. Non-polarizing dermoscopes (NPDs; Figure 1A) counteract this reflected “glare” by introducing a refractive index matching immersion fluid to make features in the pigmented epidermis visible. Indices of refraction for various substances are included in Table 1.

Equation 1. Fraction Reflected from Index Mismatch

Table 1. Indices of Refraction

Polarizing dermoscopes (PDs; Figure 1B) shine linearly polarized light to display structures that lie deeper than those usually visible with NPDs. Most of the photons that enter tissue will scatter at surfaces between materials with different refractive indices. Each scattering interaction carries a probability of changing the polarization of the photon. Photons returning to the light detector with randomized polarization likely went through 10 or more scattering interactions and penetrated 50-100μm into the skin. Light returning to the detector passes through a second linear polarizing filter that is orthogonal to the incident beam’s polarization, largely absorbing photons that have not penetrated deeply into the tissue.

Figure 1. Photon Paths in Non-Polarizing and Polarizing Dermoscopes

A. Non-polarizing dermoscope: immersion fluid permeates corneum

B. Polarizing dermoscope: heavily scattered light predominates in detector