Radiometric Calibration History of MOBY/NIST Single Channel Dual Mode Radiometers

1. 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 2000 19-21 September

2. Ocean Color Radiometry and Bio-Optics MODIS & SeaWiFS: Global Exoatmospheric Radiances Corrections for Atmospheric and Reflected Components Water-leaving Radiance, Lw(,,) Remote Sensing Reflectance  Phytoplankton  Carbon Ground Truth using Buoy and Ship Deployments CALCON 2000 19-21 September

3. 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 Lw in SeaWiFS and MODIS bands—need 5% • bio-optical algorithm development CALCON 2000 19-21 September

4. 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 2000 19-21 September

5. Sphere Source with External 45 W Lamp Schematic of Radiance Input on buoy arm (window, lens, and fiber optic) 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 Lcal() CALCON 2000 19-21 September

6. 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 Ecal() FEL at 50 cm Schematic of Irradiance Input on buoy arm (diffuser and fiber optic) CALCON 2000 19-21 September

7. Verification of Standard Sources • NOAA required direct verification of Lcal() and Ecal() • 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 2000 19-21 September

8. Wavelength 412 nm and 870 nm 10 nm bandpass Field-of-View 5 (radiance) hemisphere (irradiance) Temperature-stabilized 28 C Accuracy & Stability > 0.985 Calibration Hut, Honolulu Sphere Source VXR SLMs SLM Specifications CALCON 2000 19-21 September

9. 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) Characterization and Calibrations at NIST CALCON 2000 19-21 September

10. Light Tight Box Sources Alignment Laser Wavelength Drive Monitor Detector X/Y scanning detector carriage Baffle Shutter 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 CALCON 2000 19-21 September

11. 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 2000 19-21 September

12. SLM Calibration NIST Sphere or FEL Lamp SLM Calibration Method 1, continued • STEP 3 • Calibrate the SLM with the broadband source • r() = relative spectral responsivity • S = net signal • LNIST() = source radiance (or irradiance) Channel Calibration Coefficient CALCON 2000 19-21 September

13. 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 2000 19-21 September

14. Spectral Response—Radiance Mode FAKE 870 Sircus data CALCON 2000 19-21 September

15. Spectral Response: Irradiance Mode CALCON 2000 19-21 September

16. History of SLM 412 Radiance Irradiance CALCON 2000 19-21 September

17. History of SLM 870 Radiance Irradiance CALCON 2000 19-21 September

18. 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 CALCON 2000 19-21 September