When light "strikes" the human skin, part of the light is reflected back from the surface of the skin, and only the remaining part enters the deep layer of the skin. The amount of reflected light depends on factors such as projection angle, skin condition and wavelength. True absorption of ultraviolet rays in all layers of skin is high. Although many methods have been used, only the amount of ultraviolet radiation penetrating the whole or a certain number of layers of the skin can be measured, but the true absorption of the whole or all layers of the skin can not be measured. Because of the decrease of penetration, besides true absorption, it is more due to reflection and scattering in different tissue layers of the skin.

The perforation of various skin layers is summarized as follows:

I. cuticle and epidermis

Everett et al. (1966) showed that about 50% of the UV-A radiation projected to the skin can pass through the cuticle of white people and reach the living cells of the epidermis. 35-50% of the projected UV-A can penetrate white skin specimens. Therefore, it is reasonable to believe that the epidermal tissues (keratinocytes, melanocytes, basal cells) and dermal tissues (blood, lymphocytes, nerves and fibroblasts in capillaries) exposed to UV-A can directly produce photobiological effects. The transmittance and reflectivity of UV-A decreased with the increase of melanin content. Therefore, about 50% of the projected UV-A radiation from white people can reach the basal cell layer, while only 5-15% of the projected UV-A radiation from black people can reach the basal cell layer.

The radiation of about 280 nm can be absorbed by aromatic amino acid residues with less content in keratin, so it can penetrate the cuticle less. This is the reason why the radiation of about 280 nm plays less role in skin erythema caused by ultraviolet radiation.

Evertt et al. used in vitro skin to study ultraviolet absorption. Its precise calculation method is that the spectral absorption of any skin layer cut is equal to the total incident amount minus the total transmittance and reflectance. The spectral absorption curves of cuticle and epidermis of the cut white skin are in good agreement with those measured by Lucas (1930) a long time ago. However, this value is different from that in vivo, because there is light interaction between skin layers in vivo.

2. Dermis

There are few studies on ultraviolet optical properties of dermis. Short-wave ultraviolet rays can not penetrate human dermis 1-4 nm thick, but a small amount of UV-A can penetrate subcutaneous tissue.

Although it can be proved by experiments that the absorption of ultraviolet rays by the isolated layers of the skin is completely different. In fact, it is more important to further understand the ability of ultraviolet rays of different wavelengths to penetrate the whole skin.

The in vitro experimental data of Caucasians also confirm that the penetration depth increases with the increase of wavelength from 250 to 1230 nm, and the amount of penetration into the deep layer increases gradually. In the ultraviolet band, the sites of direct histological changes induced by UV-A in the skin are deeper than those induced by UV-B and UV-C.