We have developed a new method for characterizing the irradiance responsivity of filter radiometers. The method is based on a spatially uniform, known irradiance, generated by combining several identical laser beams. The measurement setup and the experimental demonstration at one wavelength are presented. The diffraction correction related to the generated irradiance is studied experimentally. The uncertainty analysis of the method indicates a relative standard uncertainty of 1 × 10−3. The results with the new method are compared with the characterization measurements based on our present spectral-irradiance scale. The results have a relative deviation of 1 × 10−3, which is well within the combined standard uncertainty of the comparison.