Laboratory Calibration Comparison of Hyperspectral Ocean Color Radiometers in the Frame of the FRM4SOC Phase 2 Project

Variability across different calibration laboratories can impact the consistency of ocean color data; this study addresses that challenge through a coordinated comparison of spectral irradiance and radiance calibrations. As part of the Fiducial Reference Measurements for Satellite Ocean Color (FRM4SOC) Phase 2 project, the metrological consistency across six international laboratories was tested in the years 2022–2023. Each participant determined the responsivity for four transfer radiometers using their own SI-traceable radiometric standards and calibration procedures. This was among the first laboratory comparisons for Ocean Color Radiometry (OCR) using hyperspectral radiometers. The main objective was to verify that the instrument manufacturers and research laboratories can fulfill the updated International Ocean Color Coordination Group (IOCCG) protocols to perform SI traceable calibrations with an uncertainty of 1% (k = 1) for irradiance and slightly more for radiance. The comparison revealed biases among participants and provided an overview of the calibration capabilities of OCRs. The differences between the participants varied from ±1 … 2% up to ±5%. Biases due to different measurement conditions were corrected by the Pilot. Furthermore, biases due to traceability and different conditions revealed several data handling errors. However, after uniform data processing, the metrological compatibility between the participants was reached within ±3%.