1. Skin reflex

Ultraviolet radiation is directly reflected from the interface due to the change of interlayer refractive index (which is the largest at the air/cuticle interface).

Because of the different refractive index between air and cuticle, ultraviolet rays are reflected back from the skin surface. The reflection is related to the projection angle, which is large. The reflectivity is also high.

2. Scattering in skin

Due to the presence of molecules, particles, fibers, organelles and cells in the skin layer, ultraviolet rays scatter through the skin. The degree of scattering depends on the nature of the matter through which the radiation passes. When the particle size and wavelength of the passing material are 1/100, the scattering amount is inversely proportional to the fourth power of the wavelength (Rayleigh or molecular scattering law); when the particle size and wavelength are the same order of magnitude, the scattering is not inversely proportional to the wavelength, and the scattering is the largest. For large particles, the scattering is no longer dependent on wavelength, and belongs to Mie scattering. The strong inverse relationship between molecules and small particles and wavelength leads to a mechanism that shorter wavelength radiation penetrates shallower skin in scattering media. In the tissue, the increase of scattering also prolongs the effective path length of radiation. The absorption of photons in the medium is proportional to the length of the photon path, so in a layer of tissue, the absorption of scattered tissue will exceed that of non-scattered tissue, and this effect is more obvious for the radiation with shorter wavelength.

3. The absorption of ultraviolet rays by biological substances in the skin

The absorption of ultraviolet radiation by biological substances (nucleic acid, protein, etc.) in skin is detailed in Chapter 3. In addition, pigments in the skin also absorb ultraviolet light of different wavelengths. Studies of cut skin layers have also shown that ultraviolet radiation is most absorbed in areas containing pigmentation in specimens.

When ultraviolet rays are absorbed, photochemical changes occur, such as heating, fluorescence or phosphorescence, which results in energy loss.

4. To penetrate deeper skin tissue

After these three basic optical processes, some of the ultraviolet energy is consumed, while the remaining energy continues to penetrate deeper skin tissue.