Imaging in scattering media with the purpose of object identification has always been a challenging task. In the ocean, and especially in coastal areas, the situation is one of the worst: absorption and scattering by suspended and dissolved particles take away most of the information and blur the image of the target to be identified. In addition, one has also to take into account the variability of the bottom which, being close to the surface, plays an important role in the resulting integrated light field. Our goal in this study is to gain insight into the effects of the variable environments on the complex polarized underwater realm. We analyze the polarized tridimensional underwater environment. The instruments deployed were an underwater hyperspectral and multi-angular polarimeter, whose accuracy and exactness of results have been previously validated by the means of different radiative transfer calculations; and a green band full-Stokes polarimetric video camera, enclosed in a custom made underwater housing. The results presented here were collected during the first field deployment of the imaging camera. An in-situ validation of the camera with the polarimeter has been obtained and the results have been used to validate the values of the Stokes elements in the images, both for the water column itself and for the underlying bottom.
Optical remote sensing of coastal waters from space is a basic requirement for monitoring global water quality and assessing anthropogenic impacts. However, this task remains highly challenging due to the optical complexity of the atmosphere-water system in coastal areas. In order to support present and future multi- and hyper-spectral calibration/validation activities for the Ocean Color Radiometry (OCR) satellites, as well as the development of new measurements and retrieval techniques for coastal waters, City College of New York along with the Naval Research Laboratory (Stennis) has established a scientifically comprehensive observation platform, the Long Island Sound Coastal Observatory (LISCO). As an integral part of the NASA AERONET - Ocean Color Network, LISCO is equipped with a multispectral SeaPRISM system. In addition, LISCO expands its observational capabilities through hyperspectral measurements with a HyperSAS system. The related multi- and hyperspectral data processing and data quality analysis are described. The three main OCR satellites, MERIS, MODIS and SeaWiFS, have been evaluated against the LISCO dataset of quality-checked measurements of SeaPRISM and HyperSAS. Adjacency effects impacting satellite data have been analyzed and found negligible. The remote sensing reflectances retrieved from satellite and in situ data are also compared. These comparisons show satisfactory correlations (R2 > 0.91 at 547nm) and consistencies (median value of the absolute percentage difference ~ 7.4%). It is also found that merging of the SeaPRISM and HyperSAS data at LISCO site significantly improve the overall data quality which makes this dataset highly suitable for satellite data validation purposes or for potential vicarious calibration activities.
Polarization characteristics of coastal waters were recently measured during a cruise on the R/V "Connecticut" in the areas of New York Harbor - Sandy Hook, NJ region using a new Stokes vector instrument developed by the Optical Remote Sensing Laboratory at CCNY. The instrument has three hyperspectral Satlantic radiance sensors each with a polarizer positioned in front of it, with polarization axes aligned at 0, 90 and 45°. The measured degrees of polarization (DOPs) and normalized radiances as a function of angle and wavelength match very well with simulated ones obtained with a Monte Carlo radiative transfer code for the atmosphere-ocean system. In order to numerically reproduce the polarized images for underwater horizontal imaging system the measured typical underwater polarized radiance was used to estimate the polarized components of the background veiling light and the blurring effects were modeled by point spread functions obtained from the measured volume scattering functions from this cruise and other typical oceanic environments. It is shown that the visibility can be improved for unpolarized target by placing a polarizer oriented orthogonally to the partially polarized direction of the veiling light before camera. The blurring effects strongly depend on the small angle scattering in the forward directions. For polarized targets the Monte Carlo simulation of slab geometry for polarized pencil light shows that the scattering medium with high g value has a very strong ability to retain the polarization status of the incident light, which can be utilized to improve the image contrasts for targets with very different polarized reflection properties.
The attenuation coefficient of the water body is not directly retrievable from measurements of unpolarized water-leaving radiance.Based on extensive radiative transfer simulations using the vector radiative transfer code RayXP, it is demonstrated that the underwater degree of linear polarization (DoLP) is closely related to the attenuation-to-absorption ratio (c/a) of the water body, a finding that enables retrieval of the attenuation coefficient from measurements of the Stokes components of the upwelling underwater polarized light field.The relationship between DoLP and the c/a ratio is investigated for the upwelling polarized light field for a complete set of viewing geometries, at several wavelengths in the visible part of the spectrum; for varying compositions of the aquatic environment, whose constituents include phytoplankton, non-algal particles, and color dissolved organic matter (CDOM); and for varying microphysical properties such as the refractive index and the slope of the Junge-type particle size distribution (PSD).Consequently, this study reveals the possibility for retrieval of additional inherent optical properties (IOPs) from air-or space-borne DoLP measurements of the water-leaving radiation.
synthetic bio-optical dataset of inherent optical properties (IOPs) was created based on Chlorophyll concentrations ranging between 0.01 and 30 mg m-3. Dissolved and particulate fractions of absorption were varied to account for the natural ranges in values. The IOPs will then be used as inputs to a time-resolved Monte-Carlo radiative transfer model to generate accurate lidar backscatter time history wave forms. Test experiments were performed to validate the model, where the primary lidar geometry in the model matched an existing system developed at HBOI under NOAA-OAR funding. The system uses blue and green pulsed laser sources (473 and 532 nm, respectively) and has two telescopes arranged at a 10° offset (on and off axis) from one another. The field of view of the telescopes is set at 1°. Approaches are being investigated to invert simulated and measured lidar results to derive input water column IOP properties. Results are tested through application to lidar measurements collected in an experimental tank with known suspended particle type and concentration.
Algorithms for retrieving inherent optical properties (IOPs) in coastal waters from remote sensing of water leaving reflectance spectra, are increasingly focused on red and near infrared (NIR) spectral bands, since the simple blue - green ratio approaches, valid in open oceans, fail when in coastal waters with strongly scattering inorganic particles and colored dissolved organic matter (CDOM). NIR spectra can however be significantly impacted by overlapping chlorophyll a fluorescence, and considerable progress has been made to quantify its contribution, and hence achieve more accurate [Chl] retrievals. Recently we have been studying multiangular hyperspectral polarization characteristics of underwater scattered light, using our recently developed Stokes vector polarimeter to fully measure Stokes parameters. From these studies, information on IOPs, in particular the characteristics of non - algal particles (NAP), which are the primary source of underwater polarized elastic scattering, can be obtained. Multiangular hyperspectral polarization measurements, combined with those of IOPs collected in eutrophic waters of Chesapeake/Virginia and New York Harbor/Hudson River areas, showed that chlorophyll a fluorescence markedly impacts (reduces) the underwater degree of polarization (DOP) in the 650 - 700 nm spectral region. By noting the unpolarized nature of algal fluorescence and the partially polarized properties of elastic scattering, we are able to separate the chlorophyll a fluorescence signal from the total reflectance. The analysis is based on comparisons of experimental measurements with vector/scalar radiative transfer computations using measured IOPs as inputs. Relationships between change in observed DOP and fluorescence contributions are examined, and the possibility of using DOP measurements for underwater fluorescence retrieval is evaluated for different scattering geometries.
We report large values of the two- and three-photon absorption coefficients at 532nm (252 cm/GW) and 1064nm (160 cm 3 /GW 2 ) in surfactant-capped CdS quantum dots. The cross-sections of absorption were approximately 1e-44 cm 4 s/photon (532nm) and 8e-73 cm 6 s 2 /photon 2 (1064nm).
Water-leaving radiances, retrieved from in situ or satellite measurements, need to be corrected for the bidirectional properties of the measured light in order to standardize the data and make them comparable with each other. The current operational algorithm for the correction of bidirectional effects from the satellite ocean color data is optimized for typical oceanic waters. However, versions of bidirectional reflectance correction algorithms specifically tuned for typical coastal waters and other case 2 conditions are particularly needed to improve the overall quality of those data. In order to analyze the bidirectional reflectance distribution function (BRDF) of case 2 waters, a dataset of typical remote sensing reflectances was generated through radiative transfer simulations for a large range of viewing and illumination geometries. Based on this simulated dataset, a case 2 water focused remote sensing reflectance model is proposed to correct above-water and satellite water-leaving radiance data for bidirectional effects. The proposed model is first validated with a one year time series of in situ above-water measurements acquired by collocated multispectral and hyperspectral radiometers, which have different viewing geometries installed at the Long Island Sound Coastal Observatory (LISCO). Match-ups and intercomparisons performed on these concurrent measurements show that the proposed algorithm outperforms the algorithm currently in use at all wavelengths, with average improvement of 2.4% over the spectral range. LISCO's time series data have also been used to evaluate improvements in match-up comparisons of Moderate Resolution Imaging Spectroradiometer satellite data when the proposed BRDF correction is used in lieu of the current algorithm. It is shown that the discrepancies between coincident in-situ sea-based and satellite data decreased by 3.15% with the use of the proposed algorithm. This confirms the advantages of the proposed model over the current one, demonstrating the need for a specific case 2 water BRDF correction algorithm as well as the feasibility of enhancing performance of current and future satellite ocean color remote sensing missions for monitoring of typical coastal waters.
