A modified Langley technique is used in a preliminary evaluation of the stability of ion-assisted deposition interference filters. The technique uses lamp-calibrated direct-normal irradiance measurements to return estimates of the extraterrestrial solar irradiance over the passband of the filter. Results from prototype and production UV-multifilter rotating shadow-band radiometers deployed in the U.S. Department of Agriculture UV Radiation Monitoring program indicate a 1%/yr drift in filter measurements. Measurements are reported to be within 8% of the SUSIM Atlas-3 extraterrestrial solar irradiance.
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