Multifilter Rotating Shadowband Radiometers (MFRSR)
The multi filter rotating shadowband radiometer (MFRSR) is a Sun photometer that collects sky radiation through a horizontal diffuser, and uses a rotating shadow band to separately measure the global and diffuse components of the radiative field; the direct component is derived as the difference between the two measurements [Harrison, L., J. Michalsky, and J. Berndt, Automated multifilter rotating shadow-band radiometer: An instrument for optical depth and radiation measurements, Appl. Opt., 33, 5118–5125, 1994]. Two version of he instrument are operational at Lampedusa. A visible MFRSR, model MFR-7, has a channel for the total shortwave radiation, and six channels centered respectively at 416, 496, 615, 673, 869, and 940 nm. The bandwidth of the last six channels is around 10 nm.
The second MFRSR, model UV-MFR-4, has four channels centered at 299.8, 305.4, 311.3, and 317.6 nm, respectively, with bandwidths of about 2 nm FWHM.
The aerosol optical depth at the visible MFRSR bands is calculated by calibrating the photometer with the Langley method, selecting for the calibration as many as possible half-days characterized by clear and stable atmospheric conditions. The calibration of the MFRSR is obtained with the Langley plot method, after correcting the signals for changes of the Sun-Earth distance. The aerosol optical depth is derived by applying the Beer-Lambert law.
The band at 940 nm corresponds to a strong water vapour absorption band; consequently, column water vapour amount can be derived from the MFRSR observations.