The Multi-Filter Rotating Shadowband Radiometer (MFRSR) measures both global and diffuse radiation in six narrow bands, approximately 10 nm wide, centered on 415, 500, 615, 673, 870, and 940 nm. The first four channels are in the visible and last two are in the near infrared part of the solar spectrum. These particular bands were selected to allow for the computation of optical depths for aerosols, water vapor and ozone. The MFRSR also has one silicon broadband detector for measuring total solar irradiance. The receiving surface is a small unprotected horizontal diffuser disk covering the aperture atop of a cylindrical, temperature-controlled enclosure. Within this enclosure selective waveband sampling is accomplished by interference filters, and photodetectors beneath the filters measure the signal strengths. These signals can be converted to an equivalent radiant energy flux by applying calibration factors or optical depth calculations can be made using the Langley slope method [1]; which circumvents the need for absolute calibrations. The ability to obtain both global and diffuse measurements is made possible by the rotating curved metal strip (the shadowband). While the band is at rest, below the receiving enclosure, the instrument measures downwelling global radiation. Periodically (four times per minute at most), the curved band swings over the top and shades the aperture, restricting the measured solar radiation to the diffuse component. Measurements are made every 15 seconds and one-minute averages are recorded. The MFRSR is heated to keep its components at a constant temperature, and to keep it free of snow and ice.
[1] In applying the Langley slope method to compute optical depth, the log of the output signals (voltages) of several measurements of monochromatic radiation taken throughout the day are plotted against the secant of the solar zenith angle (or the airmass). The slope of the resultant line is the mean optical depth.