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The erythemal UV irradiance plotted on the Y axis is the biologically weighted radiation being responsible for the sunburn (erythema). It results from an integration of the physical UV irradiance (in the unit W/m²) at ground level weighted by the wavelength-dependent biological sensitivity of the skin to get sunburnt (Erythemal action spectrum). The dimension of sunburn effective UV radiation is also power per area. As the occurring values are considerably smaller than 1 W/m² generally the unit mW/m² is used (1 mW/m² = 0.001 W/m²).
The UV Index is an internationally standardized measure of this sunburning effect of UV radiation. It is derived as an integer value without any unit from the measured (or calculated) erythemal effective UV irradiance on a horizontal surface. The current UV index (in the figure top right in the blue box) is calculated as a moving average over the last 10 minutes and varies throughout the day. The daily high of the UV index (UVImax) is the highest UVI of a day averaged over a period of 30 minutes. If the current UV index is known, suitable protective measures may be taken.
For detailed information on the UV index and its application, see e.g. at the World Health Organization WHO.
The sensor used here is a broadband radiometer of the type UV-Mess-Sonde UV-Cosine_UVI of the company sglux, Berlin, DE.