Radiometric Characterization and Absolute Calibration of the Marine Optical System (MOS) Bench Unit

The Marine Optical System (MOS) is a dual charge-coupled device (CCD)-based spectrograph system developed for in-water measurements of downwelling solar irradiance E-d and upwelling radiance L-u. These measurements are currently used in the calibration and validation of satellite ocean color measurement instruments such as the moderate resolution imaging spectroradiometer (MODIS) and the Sea-viewing Wide Field-of-view Sensor (SeaWiFS). MOS was designed to be deployed from a ship for single measurements and also integrated into the Marine Optical Buoy (MOBY) for longer time series datasets. Measurements with the two spectrographs in the MOS systems can be compared in the spectral interval from about 580 to 630 nm. In this spectral range, they give different values for Lu or Ed at a common wavelength. To better understand the origin of this observation and the sources of uncertainty in the calibration of MOBY, an MOS bench unit was developed for detailed radiometric characterization and calibration measurements in a laboratory setting. In the work reported here, a novel calibration approach is described that uses a tunable-laser-based, monochromatic, spatially uniform, Lambertian, large area integrating sphere source (ISS). Results are compared with those obtained by a conventional approach using a lamp-illuminated ISS. Differences in the MOS bench unit responsivity between the two calibration approaches were observed and attributed to stray light. A simple correction algorithm was developed for the lamp-illuminated ISS that greatly improves the agreement between the two techniques. Implications for water-leaving radiance measurements by MOS are discussed.