These typically make use of metallic interference filters that transmit radiation in wavelength intervals several nanometers wide. The multi-filter approach allows simultaneous measurement at several wavelengths, which eliminates complications found in scanning instruments that can result from changes in cloud cover during the time period of a spectral scan. The long-term degradation of the optical filters in these instruments must be considered, but the recent development of ion deposition technology has significantly improved filter stability. Scanning spectroradiometers make continuous spectrally-resolved measurements across the entire UV spectrum, as is required to answer questions about the chemical composition of the atmosphere, or to study effects on receptors with different action spectra. Several different experimental configurations are used for these measurements.
Most contemporary instruments employ photomultiplier detectors with single or double monochromators.
Furthermore, the preservation of accurate long-term calibration of such complex instruments is a challenge. Currently operating sites are shown in the first figure above. The design concept behind the network is that a broad base of relatively simple instruments would be operated over a wide range of geographic locations, with a smaller number of more sophisticated instruments at a subset of locations where more detailed data are desired. The Table Mountain facility is the home of the Central Calibration Facility set up to serve all US UV monitoring activities as a result of earlier interagency agreement. The Central UV Calibration Facility.