A novel polarization camera (Teledyne DALSA) based on the Sony first polarization imaging sensor provides a high resolution broadband image of the object in 400-900 nm spectral range, where each pixel contains four subpixels bearing built-in linear polarizers oriented at the 0, 45, 90 and -45 degrees. With an additional lens it has a field of view (FOV) of about 40° degrees. The camera was equipped with a filter wheel containing five band-pass filters, thus providing multispectral, multi-angular high quality polarimetric data with high spatial resolution. The camera has undergone radiometric calibration and was used in various illumination conditions and water environments in NYC area. Spatial and temporal distribution of water and sky Stokes vector components were characterized at various wind speeds. Polarization images were also used for the estimation of wave slope statistics from the ocean surface. Results are compared with concurrent measurements of total and polarized radiances by a state-of-the-art snapshot hyperspectral imager, which simultaneously acquires spectra with 4 nm spectral resolution in the wavelength range of 450- 750nm, also with a 40° FOV for 20 - 60° viewing angles. A computer-controlled filter wheel was installed in front of the imager, allowing division-of-time Stokes vector images from the ocean surface. The combination of these two instruments allows to observe spectral and polarization properties of the wind-roughened ocean at high spatial and temporal resolution, with the goal of advancing our understanding of the air-water interface and in-water light propagation.
Results are presented for measurements of the total and polarized radiances from the ocean surface by a state-of-the-art snapshot hyperspectral imager, which simultaneously acquires spectra with 4nm spectral resolution in the wavelength range of 450-950nm within a 40° field-of-view. The imager does not require any along track movement and allows the continuous collection of hyperspectral imagery from stationary structures or slow moving platforms such as ships or helicopters. In addition, a computer controlled filter wheel is installed in front of the imager allowing for division-oftime Stokes vector images from the ocean surface. Experiments are aimed at the application of the multi-angular polarimetric measurements for the retrieval of water parameters in addition to the ones retrieved from traditional unpolarized methods. Several sets of measurements used in the analysis were acquired from ocean platform in the NYC area, Duck, NC and from shipborne observations in the Gulf of Mexico and along the Florida coast. Measurements made by the imager are compared with simulations using a vector radiative transfer code showing good agreement. Analysis of pixel-to-pixel variability of the total and polarized above water radiance for the viewing angles of 20°-60° in different wind conditions enable the estimation of uncertainties in measurements of these radiances in un-polarized and polarized modes for the whole spectral range, thus setting requirements for the quality of polarized measurements. Impacts of aerosols on spectral variations of both the radiance and the polarized terms of the Stokes vector are studied.
Results of measurements by a novel snapshot hyperspectral polarimetric imager are presented using several data sets acquired from ocean platforms. Based on the unique availability of the pixel-to-pixel total, sky and water leaving radiances at multiple wavelengths, variations of these parameters for wind-roughened surface are assessed and possible errors in measurements of these parameters are estimated. Measurements made by the imager are compared with coincident ones from the green-band SALSA Stokes vector imaging camera, a push-broom hyperspectral polarimetric imager operated by Naval Research Laboratory (NRL), and with simulations using a vector radiative transfer code, all demonstrating excellent agreement.
Imaging of underwater targets is challenging because of the significant attenuation of the propagating light field due to the absorption and scattering by water and suspended/dissolved matter. Some living and manmade objects in water have surfaces which partially polarize the light, whose properties can be used to camouflage or, conversely, to detect such objects. The attenuation of light by the intervening water (so-called veiling light) changes both the intensity and polarization characteristics at each pixel of the image, but does not contain any information about the target and contributes to image degradation and blurring. Its properties need to be understood in order to isolate the true optical signature of the target. The main goal of this study is to retrieve the polarization characteristics of the target from the image in different water environmental and illumination conditions by taking into account coincidentally measured inherent water optical properties (IOPs) during recent field campaigns outside the Chesapeake Bay and in New York Bight. Data, in the form of images and videos, were acquired using a green-band full-Stokes polarimetric video camera. Analysis of the acquired images show reasonable agreement in Stokes vector components with the measurements by the underwater polarimeter and modeled polarized signals. In addition, Stokes vector components of the veiling light were also estimated and compared with the models. Finally, retrieval of the attenuation coefficient for the light from the target is attempted from the measurements and compared with the results of the independent measurements of IOPs.
We have examined, in earlier work, the relationship between naturally induced chlorophyll-a fluorescence and the underwater polarized oceanic light field. This shows the un-polarized fluorescence causes a reduction in the degree of polarization over the fluorescence spectral range. Theory shows that the peak of the reduction in polarization occurs at or near the fluorescence peak. Furthermore, it also shows that the magnitude of this reduction in degree of polarization can be related to both the magnitude of the fluorescence as well as the intensity of the underwater light field over the fluorescence spectral range. To examine this relationship in detail, a vector radiative transfer code (VRTE) for the coupled atmosphere-ocean system was employed for a variety of oligotrophic and eutrophic water conditions. The VRTE used measured inherent optical properties (IOPs) for these water conditions as inputs to simulate the complete elastic and inelastic components of the underwater light field, as well as the degree of linear polarization (DoLP) associated with it. These theoretical predictions were then compared with the results of DoLP measurements carried out using by our multiangular hyperspectral polarimeter. A comparison of the measured reduction in degree polarization of the underwater light field over the fluorescence spectral range, and the magnitude of the fluorescence causing it, confirmed the validity of our theoretical relationship, and the feasibility of determining the natural fluorescence existing in an underwater light field from polarization measurements.
Knowledge of the underwater light field is fundamental to determining the health of the world's oceans and coastal regions. For decades, traditional remote sensing retrieval methods that rely solely on the spectral intensity of the water-leaving light have provided indicators of marine ecosystem health. As the demand for retrieval accuracy rises, use of the polarized nature of light as an additional remote sensing tool is becoming necessary. For two weeks in December 2015, the NOAA NPP-VIIRS Calibration/Validation cruise continuously observed the polarized radiance of the ocean and the sky using a HyperSAS-POL system. Additionally, a full Stokes imaging polarimetric camera was used to acquire images and videos of the sea surface and sky during stations at coincident angles with HyperSAS-POL. Polarized remote sensing reflectance is computed for all viewing elevations present in the polarization images, and the results are compared to vector radiative transfer calculations.
During two cruises in 2014, the polarized radiance of the ocean and the sky were continuously acquired using a HyperSAS-POL system. The system consists of seven hyperspectral radiometric sensors, three of which (one unpolarized and two polarized) look at the water and similarly three at the sky. The system autonomously tracks the Sun position and the heading of the research vessel to which it is attached in order to maintain a fixed relative azimuth angle with respect to the Sun (i.e. 90°) and therefore avoid the specular reflection of the sunlight. For the duration of both cruises, (NASA Ship Aircraft Bio-Optical Research (SABOR), and NOAA VIIRS Validation/Calibration), in situ inherent optical properties (IOPs) were continuously acquired using a set of instrument packages modified for underway measurement, and hyperspectral radiometric measurements were taken manually at all stations. During SABOR, an underwater polarimeter was deployed when conditions permitted. All measurements were combined in an effort to first develop a glint (sky + Sun) correction scheme for the upwelling polarized signal from a wind driven ocean surface and compare with one assuming that the ocean surface is flat.
Polarized image of underwater light field contains rich information of and the targets strongly affected by the water inherent optical properties. We present a comprehensive analysis of the polarimetric images of a manmade underwater target with known polarization properties acquired by a full Stokes vector imaging camera in underwater environment. The effects of the camera’s parameters such as numerical aperture and orientation are evaluated. With the knowledge acquired in the analysis of such a forward polarimetric imaging process, a method for retrieval of the inherent optical properties of the water and the target polarization is explored.
The relationship between the degree of linear polarization (DoLP) and attenuation-to-absorption coefficients ratio (c/a) of the water from which the scattering coefficient is readily computed (b = c-a) for two main types of oceanic waters (Case I and II) was investigated using the vector radiative transfer simulation. It is found the for Case I waters that only the green channels of the spectrum can be used to retrieve the scattering coefficient of the water whereas blue and red channels are dominated by the pure water effects of either Rayleigh scattering or high water absorption showing no variability between DoLP and c/a. On the other hand, Case II waters showed a strong relationship between DoLP and c/a for all wavelength of light under study (440, 550, 665 nm). Those relationships have been parameterized for all possible viewing geometries (sensor zenith and azimuth relative to the Sun’s principle plane) and for varying Sun zenith angles. That relationship has been tested and validated against a dataset of in-situ measurements using a custom developed underwater polarimeter that measures the DoLP and an in-water package of instruments (WetLabs ac-s) that measure the absorption and the attenuation coefficients. Another polarimeter fixed on a platform in Long Island Sound at the LISCO station measures the DoLP of the light above water while a moored instrument package (WQM and C-star) that measures in-water optical properties have been used for a time serious validation.
The analysis of images of several underwater targets that exhibits different polarization properties measured using an underwater camera in various water conditions is presented. The measurements are compared with an imaging model which combines vector radiative transfer simulations by the RayXP program for the propagation of light in the atmosphere-interface-ocean system and the Monte Carlo simulations for the near horizontal imaging in the water. Modeling includes analysis of the vector point spread function (PSF) from the target and the contribution of the veiling light between the target and the camera.
Polarized light fields contain more information than simple irradiance and such capabilities provide an advanced tool for
underwater imaging. We used a Monte Carlo technique to simulate the vector point spread function for a broad range of
water parameters from clear to turbid coastal waters. We also analyzed the impact of light scattered by suspended
particles between the target and the camera on the polarized image together with the light from the target. This
knowledge is expected to contribute to solutions of the inverse problem of the restoration of the target polarization
characteristics from its underwater image.
Underwater imaging is challenging because of the significant attenuation of light due to absorption and scattering of light in water. Using polarization properties of light is one of the options for improving image quality. We present results of imaging of a polarized target in open ocean (Curacao) and coastal (NY Bight) waters. The target in the shape of a square is divided into several smaller squares, each of which is covered with a polarizing film with different polarization orientations or transmission coefficients was placed on a mirror and imaged under water by a green-band full-Stokes polarimetric video camera at the full range of azimuth angles against the Sun. The values of the Stokes vector components from the images are compared with the modeled image of the target using radiative transfer code for the atmosphere-ocean system combined with the simple imaging model. It is shown that even in clear water the impact of the water body on the polarized underwater image is very significant and retrieval of target polarization characteristics from the image is extremely challenging.
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.
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