How is the UV-Index Calculated?
 

UV is radiation in the Ultra-Violet spectrum with wavelengths varying from 280 to 400 nanometers. (by comparision, the visible light spectrum represents radiation with wavelengths from 400 to 700 nm)

Because not all wavelengths have the same damaging effects on biological organisms, scientists have developed a means to measure separately the radiation reaching the earth for different wavelenghts and calculate the potential for sunburn based on the incidence of the most damaging radiation.

The radiation at different wavelengths can be quite different, for example on a very clear summer day, the spectrum could have radiation levels (mW m -2 nm -1 ) of

Wavelength

Incidence

295.0

0.6

305.0

74.0

325.0

478.0

This information is then weighted according to how human skin responds to each wavelength; it is more important to protect people from wavelengths that harm skin than from wavelengths that do not damage people's skin. The weighting function is called the McKinlay-Diffey Erythema action spectrum. According to this function, to obtain a similar effect as 1 unit of 295 nm radiation you need to be exposed to 4.5 units of 305 nm radiation and 34.5 units of 325 nm radiation. To account for this, 305 and 325 nm radiation are multiplied by smaller numbers than 295 nm, to get the weighted irradiance at each wavelength:

 

Wavelength

Incidence

Weight

Result

295.0

0.6

1.00

0.6

305.0

74.0

0.22

16.3

325.0

478.0

0.03

13.8

Note that the highest weighted irradiance is at the wavelength with the intermediate energy (305 nm) because it has still much damaging potential. These weighted irradiances are totalled, or integrated, over the 290 to 330 nm range resulting in a value representing the total effect a given minute's UV radiation will have on skin. This calculation uses the full UV spectrum not just 3 wavelengths, but the principle is the same. When all wavelengths are summed in our example spectrum, the total is 264.

This value is then scaled (divided) by a conversion factor of 25 and rounded to the nearest whole number. This results in a number that usually ranges from 0 (when there is no sun light) to the mid teens. The average value for the one hour period around noon (1 PM Daylight time) is the noon UV Index. Thus, the UV Index for the example would be:

264 / 25 = 10.6