Multi-channel, Hyperspectral Spectrograph System using a Volume Phase Holographic Transmission Grating for In-Water Radi

1. Simultaneous Acquisition The new system is similar to MOBY regarding spectral parameters, but now multiple fiber optics directly illuminate the entrance slit, coupling flux directly for Es and Lu(z) determinations. Testing to date with two commercial spectrographs has proven the concept and shown that the multi-channel system response, dynamic range, and stray light performance are very favorable compared to MOBY. In these new systems, The result is reduced measurement uncertainty and increased versatility for experiment design and sampling protocols. Sequential Acquisition MOBY Is a high resolution (< 1 nm), hyperspectral (340 nm to 960 nm), fiber-coupled, dual CCD spectrograph-based sensor system that is a primary source of vicarious calibration for ocean color sensors. In MOBY, As a result, variability is introduced due to the changing solar zenith angle and bio-optical, sea state, and atmospheric conditions, with the solar component requiring normalization procedures using the bracketing Es scans. • acquiring multiple scans can be exploited to reduce the measurement uncertainty in Lw() from random variability – the number of scans can be optimized for the measurement conditions; • highly-correlated fluctuations such as wave focusing do not impact the measurement uncertainty because the measurements are synchronized and simultaneous; and • shorter integration times allow for a more effective sampling protocol in order to reduce environmentally-induced noise e.g., sea state. • the scans are discrete and sequential (Es, Lu,MID, Es, Lu,TOP, Es, Lu,BOT, Es), and include dark scans; • integration times are ~ 30 sec; • acquisition of these scans requires ~ 20 min; and an • optical multiplexer switches between fiber-optic inputs. Marine Optical BuoY (MOBY) Lanai, Hawaii, July 1997 - present Multi-channel, Hyperspectral Spectrograph System using a Volume Phase Holographic Transmission Grating for In-Water Radiometry B. Carol Johnson,1 Steven W. Brown,1 Dennis K. Clark,2 Michael E. Feinholz,3 Stephanie Flora,3 Mark Yarbrough,3 Michael Kehoe,4 and Casey Dodge4 Abstract Determination of the water-leaving spectral radiance using in-water instrumentation requires measurements of the up-welling spectral radiance, Lu(), at multiple depths. If these measurements are separated in time, changes in the measurement conditions result in increased variance in the results. In this project, we will finalize the optical design, incorporate the optics and hardware, and characterize, a multi-channel, hyperspectral spectrograph system that uses a volume phase holographic transmission grating to disperse the light. The system’s optical path will be in a linear configuration to accommodate the optimal housing profile for incorporation into radiometric oceanic autonomous buoys. We plan to build two spectrographs, one optimized for the blue spectral region and the other for the red. Both will have high spectral resolution (an instrument bandpass of 1 nm to 2 nm with pixel-to-pixel resolution of less than 0.5 nm). The system design is for fiber optical coupling of the radiant flux from six separate input channels; the measurements are synchronized and simultaneous in time because the individual fiber optics illuminate separate sub-areas of the entrance slit (in the vertical dimension), resulting in six spectrally dispersed tracks on the CCD array. Measurement Uncertainty This project is part of an effort to reduce the measurement uncertainty for in situ, in-water radiometry in support of the vicarious calibration of ocean color satellites (SeaWiFS, MODIS Aqua/Terra). Simultaneous measurements of the downwelling surface irradiance Es(, 0+) andLu(, z) at multiple depths, when compared to presently implemented techniques, result in reduced measurement uncertainties. New sensor design is the first step in the development of a vicarious calibration observatory designed to replace MOBY. Previous testing using commercial spectrographs One system tested was a Horiba Jobin Yvon CP140 spectrograph, which utilized a concave high efficiency holographic grating, in combination with an Andor CCD camera and four 1mm diameter optical fibers. The other system was a Kaiser HoloSpec f/1.8i spectrograph, which utilized a volume-phase holographic transmission grating in a folded configuration. An Apogee camera and six 0.8mm fibers completed this system. Both systems have been operated from small boat in the waters off Oahu, Hawaii. Two general configurations were used: with a small buoy and as a shadowing experiment. The effect of correlations in the light field was investigated by deriving Lw() using simultaneous Lu()s (the new approach), and by randomly sampling the Lu() scans in time to simulate the current MOBY sampling statistics. The measurement uncertainty was reduced by 20% to 60% for the ocean color bands. As mentioned above, increasing the number of scans becomes a viable method to reduce measurement uncertainty in Lw() when the simultaneous approach is utilized. In these Oahu tests with the small buoy, the integration times were from 4 – 6 s; averaging 100 scans took about 15 min and the measured standard deviation of the mean was <0.2% for the HoloSpec system at the primary ocean color bands. This work We learned in the preparatory work that the concept is sound and that the superior stray-light performance, decreased polarization sensitivity, and imaging characteristics of the spectrograph based on the volume-phase holographic transmission grating suit our requirements. We are pursuing the concept in a custom in-line design, which allows for compact cylindrical packaging and enhanced versatility in camera selection. Blue The preliminary designs are shown, one primarily for the blue (370 nm to 720 nm), and the other primarily for the red (500 nm to 900 nm). Both are high spectral resolution, prism-grating-prism, f/2.2, systems with 13 mm square focal planes. Costs are manageable by use of identical optical elements where possible. The spectral stray light characteristics of the JY and the HoloSpec spectorgraphs were determined using tunable lasers. Both were favorable compared to the MOBY MOS. This will also result in reduced measurement uncertainty. • Tasks • Implement into hardware the optical design; • Acquire associated components (e.g., two CCD cameras); • Integrate the systems; • Completely characterize at NIST laboratories; • Develop and implement correction algorithms (spatial and spectral stray light, temperature, etc.). Reference M. Yarbrough, S. J. Flora, M. E. Feinholz, T. Houlihan, Y. S. Kim, S. W. Brown, B. C. Johnson, and D. K. Clark, "Simultaneous measurement of up-welling spectral radiance using a fiber-coupled CCD spectrograph," Proc. SPIE 6680, 6680-6618 (2007). Red The authors acknowledge the current support of NASA’s Ocean Biology and Biogeochemistry Program (NNH08AH65I) as well as previous support: NASA (NNG04HK33I) and NOAA/NESDIS/STAR. Dennis Clark (Marine Optical Consulting, Arnold, MD) is affiliated with Space Dynamics Laboratory as part of the Joint NIST/Utah State University Program in Optical Sensor Calibration. Certain commercial equipment, instruments, or materials are identified in this poster to foster understanding. Such identification does not imply recommendation or endorsement by NIST, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose. 1National Institute of Standards and Technology, Gaithersburg, MD; 2Marine Optical Consulting, Arnold, MD; 3Moss Landing Marine Laboratories, Moss Landing, CA; 4Resonon, Inc., Bozeman, MT