The Marine Optical Spectrograph (MOS) is a dual CCD-based spectrograph system developed for in-water measurements of down-welling solar irradiance, Εd, and up-welling radiance, Lu. These measurements are currently used in the calibration and validation of satellite ocean color instruments such as MODIS and 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 data sets. Measurements with the two spectrographs in the MOS systems can be compared in the spectral interval from about 580 nm to 630 nm. In this spectral range, they give different values for Lu or Εd at a common wavelength. To better understand the origin of this observation and the sources of uncertainty in the calibration of MOBY, a MOS bench unit was developed for detailed radiometric characterization and calibration measurements in a laboratory setting. In the work reported here, we describe a novel calibration approach using a tunable-laser-based, monochromatic, uniform, Lambertian, large area integrating sphere sources (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 broadband source that greatly improves the agreement between the two techniques. Implications for water-leaving radiance measurements by MOS are discussed.
Citation: Journal of Atmospheric and Oceanic Technology
Issue: no. 3
Pub Type: Journals
CCD, ocean color, radiance responsivity, spectrograph, stray light