The skin is pigmented by melanin, and an essential component of the skin tissue is water. Regarding the skin, the penetration depth of the optical radiation depends on the wavelength as well. Accordingly, the various layers of skin are affected differently. A large part of the optical radiation in the UV spectral range is absorbed by the epidermis. The radiation in the long-wavelength UV-A spectral range penetrates significantly more deeply. Long-wavelength IR radiation is also absorbed in the epidermis.
Chronic UV exposure can lead to the premature ageing of the skin, which is characterised by a wrinkled "leather type" skin. This is largely attributable to radiation in the UV-A spectral range. Exposure in the UV-A spectral range can furthermore trigger an immediate, albeit temporary change in the skin pigmentation (immediate pigmentation). The principal effect of optical radiation in the UV-B spectral range is the formation of erythema. As a result of erythema, a delayed pigmentation of the skin occurs. UV-C radiation is absorbed by all the biological tissues so strongly, however, that the radiation is only able to penetrate a thin upper layer. UV-B radiation can also induce mutations of the tumour suppressor gen p53, which can lead to skin cancer. Skin cancer is the most severe long-term consequence of strong exposure to UV.
Intensive radiation in the visible spectral range can cause the skin to become hot and induce photosensitive reactions. The exposure of the skin to intensive IR radiation can cause burning to the skin. During the long-term irradiation of the skin, both the conduction and dissipation of heat through the blood can be seen to play a role. Due to the circulation of blood and the associated dissipation of heat, the increase in temperature of the tissue is limited.