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The Definition of the International Global Ultraviolet Index

(source: http://www.soleil.info/defscient.php)

The recommended standard definition for the Global UV Index follows as :

A quantity developed for the public domain, which expresses the ultraviolet radiation exposure to the skin. The International Global UV Index (Iuv) is a unitless quantity defined by the formula :

where El is the solar spectral irradiance expressed in W.m^-2.nm^-1 at wavelength l and dl is the wavelength interval used in the summation. S_er(l) is the erythema action spectrum developed by CIE. and ker is a constant equal to 40.

Example : an effective irradiation of 0.2 W.m^-2 corresponds to a Iuv of 8.0.

If daily maxima are reported or forecast a30 minute time averaged values of the effective irradiance should be used when calculating the UV Index.

If real time observations of the UV Index are presented, it is recomended to 5-10 minutes averages.

Other usefull definitions

CIE Erythema action spectrum [S_er(l)]	Spectral dependence of the ability of UV radiation to produce erythema in human skin. Note: It is usual to tabulate and plot the S_er(l) normalized to its maximum.
Erythemal effective irradiance (E_er)	Quantity defined by the equation : where El is the spectral irradiance in W.m^-2.nm^-1 and S_er(l) is the erythema action spectrum normalized to its maximum.
Erythemal effective radiant exposure (H_er), also called the effective dose or erythemal dose	The time integral of erythemal effective irradiance defined by the equation : where El is the spectral irradiance in W.m^-2.nm^-1 and S_er(l) is the erythema action spectrum.
Standard erythema dose (SED)	Standardised measure of erythemogenic UV radiation, 1 SED is equivalent to an erythemal effective radiant exposure of 100 J.m^-2.
Ultraviolet radiation (UVR)	Optical radiation in the 100 nm to 400 nm wavelength range.