The Vernier UVA Sensor is built around a broadband UV sensitive silicon photodiode. The diode produces a current proportional to the UV intensity. A wavelength selective filter limits light striking the diode to only the UVA region. The signal from the diode is amplified and sent to the output.
The Vernier UVA and UVB (order code UVB-BTA) 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 of 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. However, 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. The UVA sensor therefore cannot report UV index.