The UV Index is a number linearly related to the intensity of sunburn-producing UV radiation at a given point on the earth’s surface. It cannot be simply related to the irradiance (measured in W/m2) because the UV of greatest concern occupies a spectrum of wavelength from 295 to 325 nm, and shorter wavelengths have already been absorbed a great deal when they arrive at the earth’s surface. Skin damage from sunburn, however, is related to wavelength, the shorter wavelengths being much more damaging. The UV power spectrum (expressed as Watts per square metre per nanometre of wavelength) is therefore multiplied by a weighting curve known as the erythemal action spectrum, and the result integrated over the whole spectrum. This gave Canadian scientists a weighted figure (sometimes called Diffey-weighted UV irradiance, or DUV, or erythemal dose rate) typically around 250 mW/m2 in midday summer sunlight. So, they arbitrarily divided by 25 mW/m2 to generate a convenient index value, essentially a scale of 0 to 11+ (though ozone depletion is now resulting in higher values, as mentioned above). (Source Wiki, where else)