Polarimetric imaging of underwater targets Alex Gilerson, Carlos Carrizo, Alberto Tonizzo, Amir Ibrahim, Ahmed El-Habashi, Robert Foster, Samir Ahmed Optical Remote Sensing Laboratory, The City College of the City University of New York, USA In collaboration with: Molly Cummings, University of Texas, Austin James Sullivan and Mike Twardowski, WET Labs George Kattawar and Dayou Chen, TAMU SPIE Security and Defense, April 30, 2013

Polarization in the ocean and underwater imaging Solar radiation is initially unpolarized Polarization by atmosphere (molecules Rayleigh scattering, aerosols) Polarization by atmosphere to ocean interface and refraction of light Polarization by water molecules (Rayleigh scattering) and water’s constituents or hydrosols Absorption impact polarization by modulating scattering events Underwater Polarization carries important information on microphysical and bio-optical properties (particle shape, size, refractive index) which can be used for watery environment remote sensing and monitoring

Polarization in the ocean and underwater imaging Underwater imaging is difficult because of the significant attenuation of light by water and suspended/dissolved matter and rapid blurring and degradation of an image Using polarization properties of light is one of the options for improving image quality Some living and manmade objects in water have partially polarized surfaces, whose properties can be advantageous for camouflage or, conversely, for easier detection Imaging with polarimetry is a powerful tool for target detection; it enhances image contrast and gives more information on the target itself Water body between the target and the camera can impact the image and make retrieval of polarization characteristics of the target difficult The goal of this work is the study of the imaging of a polarized target in various water and illumination conditions and evaluation of impact of these conditions on the image of the target

Contents Instrumentation: polarimeter and full Stokes vector imaging camera Radiative transfer and imaging model Imaging results for clear waters – Curacao, 2012 Imaging results for coastal waters – NY Bight, 2012 Conclusions

Stokes vector components Instrumentation Stokes vector components If I90, I0, and I45 are the intensity values recorded by the sensors, then the Stokes components can be calculated as: Stokes vector where El and Er are the components of the electric field parallel and perpendicular to the plane, which contains the vector of light propagation V component characterizes circular polarization, it is usually very small and can be neglected. Degree of linear polarization Angle of linear polarization (characterizes orientation of polarization)

Degree of polarization dependence on the scattering angle Instrumentation Degree of polarization dependence on the scattering angle Water surface 30° 60° 40° Sun angle viewing direction 48° Snell’s window Hyperspectral multi-angular polarimeter 120° With horizontal viewing degree of polarization is lower than DoLPmax and is expected to be 0.4-0.5 in the open ocean and 0.25-0.3 in the coastal waters Optics Express, 2009, Applied Optics, 2011(2)

Full Stokes vector imaging camera Instrumentation Full Stokes vector imaging camera

Polarimeter-camera system with thrusters Instrumentation Polarimeter-camera system with thrusters HYPERSPECTRAL RADIOMETERS (0°, 45°, 90°, LH CP) FULL STOKES POLARIZATION CAMERA THRUSTERS DATA LOG & STEPPER MOTOR 135°

Schematic of the equipment and the target used for the measurements Instrumentation Schematic of the equipment and the target used for the measurements The polarizers and a piece of silver mirror are stuck together and they are placed vertically in water. The camera is about 1m away from the mirror. The camera-target system can be rotated as shown in accompanying image. The frame to which the camera and the mirror were attached is allowed to rotate, both clockwise (as shown) and counterclockwise with computer-controlled thrusters. Measurements were taken on July 10th, 2012 in Curacao (near oil terminal). Video duration was 4min 28sec while sun azimuth angle (clockwise from North) was 81 deg, sun elevation (from horiz): 43 deg, depth (mean, in meters)    2.91  +/- 0.09, and wind speed: ~ 3m/s. Transmission direction of polarizers in front side of the mirror

Radiative Transfer simulations Imaging model Radiative Transfer simulations Simulations using RayXP program by Zege. Optimize computational time by incorporating various techniques of solving the RT equation (very fast). Plane-parallel homogenous layers for AIO system. Assumptions: Rayleigh, non-absorbing atmosphere Wind ruffled surface (speed of 3 m/s) Optically deep waters (no bottom boundary effects) Sensor position Stokes components and DOP calculated for geometries: θsun Atmosphere Interface Ocean Bottom Viewing angle (0° looking straight down, 180° up) Relative azimuth with the sun

Imaging model RT simulations Inputs to the RayXP are based on the field measured data. Atmospheric aerosols (AOT). Oceanic hydrosols (c, a, ω) measured by ac-s. Scattering Matrices are based on either MASCOT measurements or estimated from Mie calculations. Atmospheric parameters (AOT) RayXP Radiative Transfer Simulations ac-s (WET Labs) (c, a, ω) [I,Q,U,V]T MASCOT (WET Labs) (Scattering Matrix)

Imaging model

Imaging model Iw = Is* exp(-c*l) Stokes vector Iw = Is/c for 0 to ∞  where Is is the radiance of light scattered in the forward direction, c is the attenuation coefficient, l is the distance. The Stokes vector of the light which illuminated the target is   Ibw = Isb/c  where Isb is the Stokes vector of light scattered in the backward direction. The Stokes vector of light which entered the target and after transformations returned back from the target for each of target elements i Itar(i) = η2 *t2*Mp(i)*Mmir*Mp(i)*Ibw; The Stokes vector of light from water directly reflected from the mirror outside the target Iwr(i) = Mmir*Ibw The contribution of the veiling light scattered by water between the target and the camera to each of the target element images Iv = Is(1-exp(-cL))/c The Stokes vectors Ic of the images at the camera for each element Ic(i) = Itar(i)exp(-cL) + Iv

Mueller matrices for the target components and the mirror Imaging model Mueller matrices for the target components and the mirror 1 1 0 0 1 1 0 0 0 0 0 0 0 0 0 0 1 −1 0 0 −1 1 0 0 0 0 0 0 0 0 0 0 Mphor = Mpvert = 1 0 1 0 0 0 0 0 1 0 1 0 0 0 0 0 1 0 −1 0 0 0 0 0 −1 0 1 0 0 0 0 0 Mp45pos = Mp45neg = 1 0 0 0 0 1 0 0 0 0 −1 0 0 0 0 −1 Mmir = Mmir33 was replaced by 0.5 based on comparison with the model

Water properties (WET Labs) Imaging results for clear water, Curacao 2012 Water properties (WET Labs) Jul 10, 2012 Sun elevation 43° Depth of the instrument 2.7m Wind 3m/s

Imaging results for clear water Curacao 2012 Comparison of water I component and DoLP simulated by RayXP code and measured by the polarimeter and the camera in the horizontal plane as a function of azimuth angle

Imaging results for clear water, Curacao 2012 I, 0 deg DoLP, 0 deg I, 90 deg DoLP, 90 deg

Imaging results for clear water, Curacao 2012 Simulation and experimental data Solid lines – simulations, dashed lines -measurements

Imaging results for clear water, Curacao 2012 Simulation and experimental data Solid lines – simulations, dashed lines -measurements

Imaging results for clear water, Curacao 2012 Simulations x3 and experimental data Solid lines – simulations, dashed lines -measurements

Imaging results for coastal water, NY Bight 2012 Water properties Aug 23, 2012 Sun elevation 60 deg Depth of the instrument 5.7m No wind

Imaging results for coastal water, NY Bight 2012 Experimental data Comparison of water I component and DoLP simulated by RayXP code and measured by the polarimeter and the camera in the horizontal plane as a function of azimuth angle

Imaging results for coastal water, NY Bight 2012 Experimental data I, 90 deg DoLP, 90 deg Images of I component and DoLP for the target on the mirror and surrounding water area 90deg.

Imaging results for coastal water, NY Bight 2012 Experimental data

Conclusions Imaging of polarized targets was performed in two different water and illumination conditions using a full Stokes vector imaging camera and compared with the imaging model. Depending on the strength of the target polarization signal, water body between the target and the camera can change polarization characteristics even in clear waters and retrieval of the target properties from the camera image can require development of the special algorithms. The contribution of the water body to the image changes with the azimuth angle as target signal and scattering contribution from veiling light in the water change in different proportions. That is expected to create difficulties for polarization camouflage as it should vary as a function of illumination, viewing and azimuth angles.

Acknowledgement This work was supported by grant from ONR MURI program