Carlos Carrizo

Open-ocean fish reveal an omnidirectional solution to camouflage in polarized environments.

Disappearing act Unlike coastal regions and reefs, the open ocean is mostly empty. Many fish species, nonetheless, spend most of their lives there. Such emptiness makes camouflage exceedingly difficult, so how does an organism hide in water filled with bouncing and reflected light? Brady et al. show that some families of fish have evolved skin that reflects and polarizes light, allowing them to blend into their mirrorlike conditions more easily. These results help to explain the silvery coloration found in sea-living fish across the worlds oceans. Science, this issue p. 965 Light-reflecting and -polarizing platelets in their skin permit fish to blend into the mirrorlike open ocean. Despite appearing featureless to our eyes, the open ocean is a highly variable environment for polarization-sensitive viewers. Dynamic visual backgrounds coupled with predator encounters from all possible directions make this habitat one of the most challenging for camouflage. We tested open-ocean crypsis in nature by collecting more than 1500 videopolarimetry measurements from live fish from distinct habitats under a variety of viewing conditions. Open-ocean fish species exhibited camouflage that was superior to that of both nearshore fish and mirrorlike surfaces, with significantly higher crypsis at angles associated with predator detection and pursuit. Histological measurements revealed that specific arrangements of reflective guanine platelets in the fish’s skin produce angle-dependent polarization modifications for polarocrypsis in the open ocean, suggesting a mechanism for natural selection to shape reflectance properties in this complex environment.

Polarimetric imaging and retrieval of target polarization characteristics in underwater environment.

Polarized light fields contain more information than simple irradiance and such capabilities provide an advanced tool for underwater imaging. The concept of the beam spread function (BSF) for analysis of scalar underwater imaging was extended to a polarized BSF which considers polarization. The following studies of the polarized BSF in an underwater environment through Monte Carlo simulations and experiments led to a simplified underwater polarimetric imaging model. With the knowledge acquired in the analysis of the polarimetric imaging formation process of a manmade underwater target with known polarization properties, a method to extract the inherent optical properties of the water and to retrieve polarization characteristics of the target was explored. The proposed method for retrieval of underwater target polarization characteristics should contribute to future efforts to reveal the underlying mechanism of polarization camouflage possessed by marine animals and finally to generalize guidelines for creating engineered surfaces capable of similar polarization camouflage abilities in an underwater environment.

Characterization of sun and sky glint from wind ruffled sea surfaces for improved estimation of polarized remote sensing reflectance

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.

Polarimetric imaging of underwater targets

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.

Impact of fluorescence on the underwater polarized light field: comparison of theory and field measurements

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.

Analysis of polarimetric image by full stokes vector imaging camera for retrieval of target polarization in underwater environment

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.

Polarization analysis of target imaging in an underwater environment

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.

Ocean surface characterization using snapshot hyperspectral polarimetric imager

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.

Speckle-based sequential optimization of adaptive receivers in downlink laser communications

Free-space optical communications (FSOC) are rapidly becoming a key technology for terrestrial, aerial, and space communication, mainly because of its very high throughput capacity. To achieve multi-gigabit laser downstream, an efficient single-mode fiber coupling is required. However, atmospheric turbulence remains one of FSOC’s main limitations. The turbulence affects the communications performance by inducing wavefront distortions that develop into coupled power fluctuations. In regimes of very strong turbulence, the use of traditional adaptive optics systems is limited due to strong scintillation and higher number of phase singularities. These limitations could be solved by relying on systems based on the stochastic iterative maximization of the coupled power. The drawback of such systems is that a high number of iterations are required for signal optimization. We address this problem and propose a different iterative method that compensates the distorted pupil phasefront by operating directly on the focal plane. The technique works by iteratively updating the phases of individual speckles to maximize the received power coupled into a single-mode fiber. We show numerically and experimentally that the method can improve the quality of the received signal with reduced bandwidth utilization.

Imaging of polarized target in underwater environment

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.