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How do I calculate UV-index from GUV data?

There are two methods to calculate the UV Index from GUV measurements, a simple and a more advanced method. The UV Index discussed here refers to the action for erythema (sunburn) as defined in the CIE norm by McKinlay and Diffey .

1. Simple method

The simple method is described in the BSI Application Note "GUV 'Diffey Dose' Calculations," available at GUV Diffey Dose Calculations.pdf. In brief, the UV Index is calculated from a linear combination of measurements of the GUV channels at 305, 320, and 340 nm:

UV Index = a1 E305 + a2 E320 + a3 E340

where a1 = 0.8911, a2 = 0.0818, and a3 = 0.007751. These coefficients refer to the "2B. Coefficients for 96 UV INDEX" listed in Table 1 of the aforementioned application note. E305, E320, and E340 are the spectral irradiances at 305, 320, and 340 nm, which are calculated by applying offsets and scale factors to the GUV's measurements as described in the instrument's calibration certificate.

If a GUV radiometer is equipped with a channel at 313 nm, the UV Index can also be calculated from a linear combination of measurements of four GUV channels, namely 305, 313, 320, and 340 nm:

UV Index = a1 E305 + a2 E313 + a3 E320 + a4 E340

where a1 = 0.8058, a2 = 0.0887, a3 = 0.0324, and a4= 0.0131. Using measurements of four, rather than three, channels leads to slightly more accurate results, in particular for low-Sun conditions.