How to measure UV radiation
In general, monitoring Ultraviolet radiation has not been the historical interest or responsibility of meteorological organisations. As it is largely a public health issue it normally falls within the parameters of environmental protection or pollution monitoring authorities.
Three main types of radiometers have been used for the measurement of UV radiation:
These filter out only a certain very narrow part of the spectrum, normally at one UVA, or one UVB, wavelength as defined by the World Meteorological Organisation. This type of instrument has lost popularity because the measurements are not good representations of the UVA or UVB irradiance under a range of sky and atmosphere conditions. They have been replaced by broadband radiometers.
Typically these receive radiation from the whole sky. Some types have a response that covers most of the UVA and UVB solar radiation spectrum and these are often called ‘Total UV’ radiometers. An example is the Kipp & Zonen model CUV 5. More complex broadband radiometers are designed to monitor specifically UVA, UVB or UVE (Erythemal) radiation and these are becoming the most widely used type of UV sensor, such as the Kipp & Zonen UVS range of instruments.
Broadband radiometers use a variety of spectral selection and detection techniques, but they cannot accurately match the ideal ‘rectangular and flat’ spectral response for UVA and UVB, or the tri-slope logarithmic erythemal action spectrum response for measuring UVE. This can lead to significant errors that can only be partially corrected by determination of the individual instrument characteristics and the use of algorithms such as those used in the Kipp & Zonen ‘UVIATOR’ software package that is used to correct measurements by the UVS range of radiometers.
Spectrophotometers measure the intensity of the radiation over a spectral range at a large number of discrete wavelengths. The Brewer MkIII can perform spectral scans of ultraviolet radiation, either in the direct solar beam or from the whole sky, in the wavelength range from 286.5 nm to 363 nm with a resolution of 0.6 nm. This enables the Brewer to make integrated measurements of UVA, UVB and UVE (and hence, the UV Index) with high accuracy.
Brewer spectrophotometers are used for scientific issues such as the world-wide determination of UV and ozone values within the scope of the WMO Global Ozone Observing System (GO3OS) programme and Global Atmosphere Watch (GAW). These WMO programmes were launched specifically to monitor changes in the earth’s atmosphere over several decades with a high level of accuracy.
The low intensity of solar UVB radiation compared to that of the visible radiation makes it critical that the instrument filters out the visible radiation very effectively. Spectrophotometers best achieve this by using two spectrometers in series, as in the Brewer MkIII, and a high sensitivity UV-enhanced photomultiplier tube photon detector.
Less sophisticated ‘solid state’ instruments without moving parts use a CCD or photodiode array as the detector, which measures the complete spectral range simultaneously. However, these instruments are usually intended for measurement in laboratories with artificial light sources. They do not normally have the stability, sensitivity or stray light rejection for measuring UVB and UVE outdoors from sunlight.
The main limitation on the UV measurement performance of the Brewer is the uncertainties in the calibration of the instrument response to radiation at each wavelength.