Out-of-band rejection studies of the UV multi-filter rotating shadow-band radiometers

The Central UV Calibration Facility (CUCF) annually calibrates and characterizes 47 Ultraviolet Multi-Filter Rotating Shadow-band Radiometers (UV-MFRSR) for the USDA UV Monitoring and Research Program (UVMRP). The UV-MFRSR instrument has seven 2-nm wide channels with nominal centroids at 300, 305, 311, 317, 325, 332, and 368 nm. The first two channels 300 and 305 nm use silicon-carbide (SiC) photodiodes, and in the original design the remaining five channels used gallium-phosphide (GaP) photodiodes. Because of the high rate of failure in the channels with GaP photodiodes, channels 3 through 7 were replaced with silicon (Si) photodiodes starting in June 2000 by the manufacturer Yankee Environmental Systems, Inc. The newer design radiometers were tested for out-of-band rejection with two sources, in the laboratory using a 1000W FEL quartz tungsten halogen lamp and in the field using the sun. Out-of-band light measurements were completed in the field on all 47 radiometers and show there is no appreciable signal from out-of-band light contributing to the total solar horizontal irradiance in each of the seven wavelength bands. However, in the calibration procedure, using a 1000W FEL quartz- tungsten-halogen lamp there is significant out-of-band signal contributing to the measured signal. The out-of-band signal is measured at the time of the calibration and corrections are applied to the calibration factors of the radiometer in each channel. At the Table Mountain Test Facility, solar irradiance from a calibrated filter radiometer with and without the out-of-band correction factors are compared to filter weighted solar irradiance from the U111 reference spectroradiometer.