Radiometric Characterization and Calibration of the Marine Optical System (MOS) for the Marine Optical Buoy (MOBY) Project

Published

October 1, 2000

Author(s)

C Habauzit, Steven W. Brown, Keith R. Lykke, B. Carol Johnson, M Yarbrough, M Feinholz, D K. Clark

Abstract

The scientific objective of the Marine Optical Buoy (MOBY) project is to measure the downwelling spectral irradiance, E_d, and the upwelling spectral radiance, L_u, at various depths in the ocean. From these measurements, other quantities can be derived, such as the abundance of microscopic marine plants. MOBY is also used as a calibration reference station for ocean color satellite instruments such as the Sea-viewing Wide Field-of-view Sensor (SeaWiFS), the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Ocean Color Temperature Scanner (OCTS), thereby assisting in assessing the accuracy of those instruments.The MOBY system uses fiber optics to couple the radiant flux to the Marine Optical System (MOS), which is a CCD-based dual spectrograph. A dichroic beamsplitter that transmits with high efficiency from 700 nm to beyond 1000 nm and reflects with high efficiency from 400 nm to 650 nm directs the reflected flux to one spectrograph, and the transmitted flux to the other. The measurements can be compared in the spectral interval from about 600 nm to 650 nm. However, in this spectral range they give different values for E_d or L_u at a common wavelength. To address and reconcile the discrepancy, tests were performed in the NIST Laboratories with both spectrographs. The results of the radiometric characterization of to the calibration sources, the initial tests focused on the instrument parameters that are varied to span the required dynamic range, such as integration time and bin factor. The stray-light rejection in the single grating spectrographs was also quantified. The majority of the tests were performed using a new NIST facility, the Spectral Irradiance and Radiance Responsivity Calibrations with Uniform Sources (SIRCUS). The SIRCUS source, which is the exit aperture of a laser-illuminated integrating sphere, is monochromatic (and broadly tunable), uniform, lambertian, and large area. The radiance is determined with a relative standard uncertainty of about 0.2% using a calibrated detector standard. The SIRCUS results for the MOS absolute radiance response, slit profile, stray light, wavelength accuracy, and pixel-to-pixel uniformity are reported. These characterization functions were applied to the calibration algorithms of MOS to evaluate the impact on the in-water measurements and determine the uncertainty budget.

Proceedings Title

Oceans from Space 2000

Volume

October

Issue

2000

Conference Dates

October 13-14, 2000

Conference Location

Venice, 1, IT

Conference Title

NASA Web Site

Pub Type

Conferences

Keywords

CCD, exposure time, irradiance, linearity, pixel responsivity, radiance, spectrograph, stray-light

Citation

Habauzit, C. , Brown, S. , Lykke, K. , Johnson, B. , Yarbrough, M. , Feinholz, M. and Clark, D. (2000), Radiometric Characterization and Calibration of the Marine Optical System (MOS) for the Marine Optical Buoy (MOBY) Project, Oceans from Space 2000, Venice, 1, IT (Accessed April 24, 2024)