1. 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

2. 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

3. 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

4. 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

5. 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)

6. 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 in the open ocean and in the coastal waters
Optics Express, 2009, Applied Optics, 2011(2)

7. Full Stokes vector imaging camera
Instrumentation
Full Stokes vector imaging camera

8. 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°

9. 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

10. 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

11. 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)

12. Imaging model

13. 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

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

15. 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

16. 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

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

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

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

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

21. 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

22. 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

23. 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.

24. Imaging results for coastal water, NY Bight 2012
Experimental data

25. 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.

26. Acknowledgement This work was supported by grant from ONR MURI program