AbstractMultifilter instruments such as the Norwegian Institute for Air Research (NILU)-UV Irradiance Meter (NILU-UV) are suitable for long-term monitoring of UV radiation, as long as their irradiance scale is maintained. The sensitivity of these instruments varies with time; thus, periodic standard absolute calibrations are required. However, standard calibrations may be insufficient to characterize the short-term variations of channels sensitivity. Therefore, results from standard calibrations have to be combined with the information derived from frequent relative calibrations, leading to a stable irradiance scale. This paper focuses on the long-term application of three calibration methods, hereinafter referred to as CC, CC2, and CC2(ϕ), as applied to the NILU-UV 119 multifilter radiometer. Here a generalization is proposed, consisting of using empirical functions in order to guarantee the accurate calibration of multifilter instruments over long time periods. These function-based methods need to be updated regularly to account for changes on instrument sensitivity. The changes in sensitivity of the NILU-UV 119 were found to be channel dependent. The detected drifts range from 23% to 42% for a 4-yr period of study. Time series of calibrated irradiance are obtained by applying the proposed methods. Results suggest that calibration methods using empirical functions perform better than classic calibration approaches that use constant calibration factors. When methods CC2 and CC2(ϕ) are applied, calibrated irradiances agree with the measurements of reference within ±5% for channel 305 and within ±3% for channels 312, 320, and 340.