The Vernier UVA and UVB Sensors respond to specific regions of the electromagnetic spectrum. The wavelength region from 320 to 400 nm is commonly called UVA radiation, and 280 to 320 nm is called UVB radiation. Wavelengths shorter than 280 nm fall into the UVC spectrum. Neither Vernier sensor is sensitive to UVC radiation.
Plants and animals respond differently to the three types of UV radiation. Although very harmful to plants and animals, UVC radiation is nearly completely absorbed by the ozone in the Earth’s atmosphere. Some UVB radiation makes it through the atmosphere, although the degree of absorption depends critically on the angle of the sun and the amount of ozone along the light path. UVB radiation is thought to be responsible for reddening of the skin (erythema), cataracts, and skin cancers. UVA can also cause these effects on human skin, but to a lesser extent. It is generally agreed that UVB radiation is the primary danger to humans, but increasingly UVA is being shown to cause delayed, but significant, damage to skin and eyes.
The standard erythemal (or sunburning) action spectrum (McKinlay and Diffey, 1987) represents a combined estimate of the relative sensitivity of skin as a function of wavelength. Since knowledge of how UV light affects skin improves with time, the erythemal spectrum may not represent the latest thinking of UV danger.
There are several ways of measuring UV light intensity and exposure. The usual irradiance unit for measurement is mW/cm2, but a simplified UV Index system is also in use. The UV Index is actually a forecast, not a measurement. For comparison with the forecast, some UV sensors can be calibrated in terms of UV Index. Since the UV Index includes a wavelength weighting corresponding to the erythemal action spectrum, only sensors matching the erythemal spectrum can logically be calibrated in terms of UV Index. An erythemally-weighted irradiance measurement of 0.25 mW/cm2 corresponds to a UV Index of 10.
Since the Vernier UV sensors allow the separate measurement of UVA and UVB irradiance (instead of an erythemally weighted average) the individual readings of the Vernier sensors cannot strictly be converted to UV Index units. The erythemal action spectrum is predominately UVB, however, so an estimate of the UV index can be calculated by multiplying the UVB sensor reading by a factor of 40 index-cm2/mW—but this is only an estimate. The UVA sensor reading cannot be used to estimate UV index.
Here are some useful links regarding the effects of UV:
http://earthobservatory.nasa.gov/Library/UVB/
http://www.cpc.ncep.noaa.gov/products/stratosphere/uv_index/
The July 2001 issue of Scientific American had a good article on sunburn vs. tan (page 90)