CALCON September B. Carol Johnson, Steven W. Brown and Howard W. Yoon National Institute of Standards and Technology Gaithersburg, MD Radiometric Calibration History of MOBY/NIST Single Channel Dual Mode Radiometers

CALCON September Ocean Color Radiometry and Bio-Optics MODIS & SeaWiFS: Global Exoatmospheric Radiances Corrections for Atmospheric and Reflected Components Water-leaving Radiance, L w ( , , ) Remote Sensing Reflectance  Phytoplankton  Carbon Ground Truth using Buoy and Ship Deployments

CALCON September Marine Optical Buoy (MOBY) & Marine Optical Characterization Experiments NOAA/NESDIS Program (D. K. Clark, PI) Methods –MOBY deployed off coast of Lanai, Hawaii: 1994—present –MOCE cruises Goals –daily values for L w in SeaWiFS and MODIS bands—need 5% –bio-optical algorithm development

CALCON September Field Radiometers in the NOAA Program Moss Landing Marine Laboratories—partner in instrument design, deployment, and calibration MOBY—dual spectrograph; 350nm to 950nm; rebuilt, recalibrated, and redeployed every three months at field laboratory (Honolulu, Hawaii) MOS—Marine Optical Spectrograph; a profiling version of the MOBY system Various other commercial radiometers for in-water and at surface measurements (filter radiometers and grating instruments)

CALCON September Calibration of MOBY and MOS Spectral Radiance Responsivity –Integrating sphere source(s) –Calibrated by commercial standards laboratory (1%-2%) –Re-lamped every 50 h –Calibrated twice—at the beginning and end of the 50 h –Lamp current and voltage always recorded –Used before and after field deployments Sphere Source with External 45 W Lamp Schematic of Radiance Input on buoy arm (window, lens, and fiber optic) L cal ( )

CALCON September Calibration of MOBY and MOS Spectral Irradiance Responsivity –1000 W lamp standards of spectral irradiance (m/n FEL) –Calibrated by NIST (0.4%) –Recalibrated every 50 h –Lamp current and voltage recorded during radiometer calibrations –Used before and after field deployments Lamp Source with Reference Plate Schematic of Irradiance Input on buoy arm (diffuser and fiber optic) FEL at 50 cm E cal ( )

CALCON September Verification of Standard Sources NOAA required direct verification of L cal ( ) and E cal ( ) NIST response—transfer radiometers –Standard Lamp Monitors (SLMs) Dual Mode (Irradiance and Radiance) Interchange Fore Optics for E and L modes Single Channel using Interference Filters Temperature-controlled filter and detector 7 decade custom preamp Rugged design Recalibrated at NIST to verify stability of SLMs Used in Comparisons at Snug Harbor with SXR and VXR (six channel filter radiometers from NIST for SeaWiFS and EOS)

CALCON September SLM Specifications Wavelength –412 nm and 870 nm –10 nm bandpass Field-of-View –5  (radiance) –hemisphere (irradiance) Temperature-stabilized –28  C Accuracy & Stability –> VXR Sphere Source SLMs Calibration Hut, Honolulu

CALCON September Characterization and Calibrations at NIST Characterizations Performed: –Electronics Gain Range Factors –Spatial Cosine response (E mode) Point Spread (L mode) –Stability Interchange Fore Optics Repeat Absolute Calibrations –Spectral Relative (using monochromator & lamp) Absolute (using lasers) Calibrations Performed: –Radiance Mode NIST-calibrated sphere sources (in 1996, 1998, and 2000) SIRCUS Facility (2000) –Irradiance Mode NIST FELs (in 1996, 1998, and 2000) SIRCUS Facility (2000)

CALCON September Calibration Method 1—Broadband Source STEP 1 –Determine the Relative Spectral Responsivity of the SLMs in radiance and irradiance mode Visible Spectral Comparator Facility was used in 1996 –could not fill SLM radiance field of view –finite bandwidth, ~ 1 nm –low flux Sources Alignment Laser Wavelength Drive Light Tight Box Monitor Detector Shutter Baffle X/Y scanning detector carriage

CALCON September Calibration Method 1, continued STEP 2 –Calibrate the NIST Sources –FASCAL Spheres calibrated in radiance mode (shown) Irradiance scale is based on the radiance scale FASCAL supplies NIST- calibrated FEL Lamps (irradiance mode)

CALCON September Calibration Method 1, continued STEP 3 –Calibrate the SLM with the broadband source –r( ) = relative spectral responsivity –S = net signal –L NIST ( ) = source radiance (or irradiance) SLM Calibration SLM NIST Sphere or FEL Lamp Channel Calibration Coefficient

CALCON September Calibration Method 2—Tunable Lasers ONLY STEP –SIRCUS (Spectral Irradiance and Radiance Responsivity using Uniform Sources) –Determine the Absolute Spectral Radiance (Irradiance) Responsivity, R( ) –Reduced Uncertainties Channel Calibration Coefficient

CALCON September Spectral Response—Radiance Mode FAKE 870 Sircus data

CALCON September Spectral Response: Irradiance Mode

CALCON September History of SLM 412 Radiance Irradiance

CALCON September History of SLM 870 Radiance Irradiance

CALCON September Conclusions—SLMs for MOBY/MOS Traceability to NIST and EOS Calibration Program Continuous Monitoring of Standard Sources Verification of FEL-derived radiance scales from commercial standards laboratories Uncertainties of between 1.2% and 0.5% using broadband source calibration method Uncertainties of about 0.25% using tunable laser source