What is the UV Index?
Measurements of total UV radiation are made over the range 290 - 400 nm, either by broadband or spectral instruments.
The measured intensities are converted to irradiance values in W/m2 and then into a UV Index value for public information reporting. The UV index is a warning of the likely risk of sunburn and hence long-term skin damage.
For spectral data the CIE Action Spectrum has to be applied over the wavelength range in order to convert measured irradiance to a measure of health-risk. The CIE spectrum is one of a number of models for the susceptibility of the caucasian skin to sunburn (erythema). The erythemally weighted data are then summed across the wavelengths to provide the Total UV index.
The graph below shows how the most significant effects are at the lower wavelength range once the intensities have been erithemally weighted. Without erithemal weighting irradiance basically increases as you scan across the wavelengths.
Graph of Erithemally weighted UV irradiance data
Page last modified: 04 November 2008