In the range 300-200 nm it is known as the far ultraviolet . Below 200 nm it is known as the extreme ultraviolet or the vacuum ultraviolet , as absorption by the oxygen in the air makes the use of evacuated apparatus essential. The sun is a strong emitter of UV radiation but only the near UV reaches the surface of the earth as the ozone layer of the atmosphere absorbs all wavelengths below 290 nm. Ultraviolet radiation is classified in three ranges according to its effect on the skin. It is also used to induce vitamin D formation in patients that are allergic to vitamin D preparations.
Excessive exposure can cause severe blistering.
UV-C, with the shortest wavelengths, is particularly damaging. It is thought that short-wavelength ultraviolet radiation causes skin cancer and that the risk of contracting this has been increased by the depletion of the ozone layer. Most UV radiation for practical use is produced by various types of mercury-vapour lamps . Ordinary glass absorbs UV radiation and therefore lenses and prisms for use in the UV are made from quartz. A Dictionary of Science, Oxford University Press 2.