The solar spectrum contains a strong ultraviolet component ranging from 200 nm to 380 nm, the lower wavelength limit of the human visible spectrum. At sea level, this ultraviolet spectrum is greatly reduced by absorption in the atmospheric layer, chiefly by ozone. It was divided, essentially on physiological criteria, into three areas: UVC, from 200 to 280 nm, nearly nonexistent at sea level; UVB, from 280 to 315 nm; and UVA, from 315 to 380 nm. Without a real physical basis, these limits can fluctuate among the authors by a few nm (Fig. 20.1).