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Deriving the UV Index from Field Measurement

UV Radiation
The ultraviolet (UV) part of the solar spectrum has several beneficial effects but it may also be very harmful if UV radiation exceeds “safe” limits. An indicator of UV exposures, the UV Index, warns for UV radiation and its possible detrimental effects. The calculation method of the UV Index is given on the right hand side. Because skin types differ considerably in their sensitivity to UV doses, different groups were established according to the skin's ability to tan. This classification is shown in Table 1.


Table 1. Classification of skin types (adopted from TB Fitzpatrick and JL Bolognia, 1995)
Source: Global Solar UV Index, A Practical Guide, World Health Organisation 2002

The Global Solar UV Index (UVI) described in Table 1. was developed in an international effort by WHO in collaboration with WMO, UNEP and ICNIRP, and is a simple measure of the UV radiation level at the Earth's surface. It serves as an important vehicle to raise public awareness and to alert people about the need to adopt protective measures when exposed to UV radiation.

UV Radiometers
For the measurement of the Erithemally weighted UV radiation, Kipp & Zonen offers UV-S-E-T precision radiometers. Equipped with quartz domes and diffusers the radiometers provide a uniform cosine response function for accurate radiative energy measurements. The thermostat control of the detection system guarantees the highest accuracy. Rugged construction allows the radiometers to be used under all weather conditions as well as in very hot and cold regions.

The UV-S-E-T radiometer has an output of 0 - 3 VDC which corresponds to 0 - 0.6 W/m² of Erythemal radiation. The UV Index can be calculated according to the method illustrated below.

UV index
From UV-E radiation measurements, the UV-Index is calculated as follow:

Take the output from the UV-E radiometer according to ISO 17166:1999/CIE S007/E-1998. Transform the output voltage to W/m² with the instruments sensitivity.



Erythemal Action Spectrum
As sunburn is a consequence of excessive UV radiation exposure the UV radiometer should mimic the human skin. Therefore a special Erythemal action spectrum was defined which corresponds to the sensitivity of the human skin on UV radiation. The UV-S-E-T radiometers are equipped with special filters which match the Erythemal action spectrum.