A UV Index is calculated from a skin-damaging UV radiation metric called the Erythemal action spectrum.
Implemented by the International Commission on Illumination (CIE), the Erythemal curve represents the average skin response over the solar UV spectrum. Weighing factors are provided for the UV wavelength spectrum (280-400nm) –the more damaging the wavelength, the larger the factor. The World Health Organization’s recommendation for calculating UVI multiplies the intensity at each wavelength with the Erythemal action function and the resulting sum is the total Erythemal-weighted UV intensity in milliwatts/meter2.
This sum can be multiplied by 0.04 to obtain the standardized UVI. True UVI calculation requires an expensive and large spectrophotometer to scan and measure the intensities of each UV wavelength.
ROHM’s broad spectrum UV sensor, capable of measuring 280-400nm wavelengths while having a similar Erythemal response, can be used to approximate the UV Index within ±1 UVI error..