Présentation:

Ultraviolet (UV) radiation is electromagnetic radiation of a wavelength intermediate between visible light and X-rays that, the name means "beyond violet", violet being the color of length of the shortest wavelength of visible light. About 5% of the sun's energy is emitted in the form of ultraviolet radiation, these rays are classified in three categories according to their wavelength: UV-A, UV-B and UV-C. However, due to absorption by the ozone layer of the atmosphere, 99% of the ultraviolet light that reaches the surface of the earth is the range of UV-A. These three types of radiation are not the same biological activity or the same power of penetration of the skin. More UV radiation has a wavelength, the less harmful it is (it is close to visible light) but it has the power to penetrate the skin more important. When the wavelength decreases, it has more energy and is closer to X-rays and therefore is more destructive (UV-C). These rays are reflected from the ground depending on the nature of the latter, this reflection is measured by a fraction called albedo which is between 0 and 1 (the reflection is particularly strong in the snow). Here are the UV-A and UV-B play a role in our skin and therefore those who are most interested.

1) Les UVA (400-315 nm):

UV-A, the wavelength is relatively long, nearly 95% of UV radiation that reaches the Earth's surface. They can penetrate the deeper layers of the skin. These are the most penetrating Ultraviolet even if they are the least energy. They manage to cross a glass and penetrate the skin to the dermis.
They are responsible for the immediate tanning effect. In addition, they also promote skin aging and wrinkling, disrupting the balance of protein synthesis (especially collagen degradation) and the production of free radicals in cells is also due to the range UV.
For a long time it was thought that UV-A could be causing lasting damage. Recent studies strongly suggest that they could also foster the development of skin cancer (they affect the DNA of the cell).

2) Les UVB (315-280 nm):

UV-B wavelength average, have significant biological activity, but do not penetrate beyond the superficial layers of the skin, they are absorbed by the relatively stratum corneum of the epidermis (melanin). Part of solar UV-B are filtered by the atmosphere.
They are responsible for tanning and burning delayed. They are capable of producing high amounts of oxygen free radicals in skin cells are responsible for short-term sunburn and inflammation. In addition to these short-term effects, they promote the aging of the skin (collagen fibers destroying them) and the onset of skin cancer.
However, they can be beneficial for certain types of skin conditions such as psoriasis or synthesis of vitamin D. Some studies also suggest that the benefits of short-term exposure to UV-B (10 minutes a few times per week) would be greater than the risks (see tan).

3) Les UVC (280-100 nm):

UV-C, short-wavelength UV are the most harmful, but they are completely filtered by the ozone layer of the atmosphere and thus do not reach the theoretical surface of the Earth. However, UV-C lamps are used in biology lab for germicidal effects to sterilize parts or equipment (laminar flow hood, for example).