The global solar UV index (UVI) was developed as an easy-to-understand measure of biologically effective UV radiation with a view to promote public awareness of the risks of UV radiation exposure and sun protection, using the CIE erythemal action spectrum as basis.
Link to - Eyrthema amd skin exposure to UV
In the initial work done on UVI in Canada, a typical midday summer erythemal irradiance of 250 mW.m-2 was found, leading to the definition of one UV index equal to an erythemally weighted irradiance of 25 mW.m-2. This gives a typical range from UVI 0 at night to UVI in excess of 11 in sunnier climes.
UVI can be evaluated directly through a measurement of global solar irradiance in a location with an un-obscured hemispherical view. The spectral range can be reduced to 290-400nm given that the shorter wavelengths aren't transmitted to the earths surface thanks to the ozone layer.
- "Global Solar UV Index - A Practical Guide." A joint recommendation of: World Health Organization, World Meteorological Organization, United Nations Environment Programme and International Commission on Non-Ionizing Radiation Protection