Solar and Ultraviolet Radiation

Terrestrial life is dependent on radiant energy from the sun. Solar radiation is largely optical radiation [radiant energy within a broad region of the electromagnetic spectrum that includes ultraviolet (UV), visible (light) and infrared radiation], although both shorter wavelength (ionizing) and longer wavelength (microwaves and radiofrequency) radiation is present. The wavelength of UV radiation (UVR) lies in the range of 100–400 nm, and is further subdivided into UVA (315–400 nm), UVB (280–315 nm), and UVC (100–280 nm). The UV component of terrestrial radiation from the midday sun comprises about 95% UVA and 5% UVB; UVC and most of UVB are removed from extraterrestrial radiation by stratospheric ozone. Approximately 5% of solar terrestrial radiation is UVR, and solar radiation is the major source of human exposure to UVR. Before the beginning of last century, the sun was essentially the only source of UVR, but with the advent of artificial sources the opportunity for additional exposure has increased. 1.1 Nomenclature and units