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

Polarimetric imaging of underwater targets

SPIE Security and Defense, April 30, 2013

In collaboration with:

Molly Cummings, University of Texas, AustinJames Sullivan and Mike Twardowski, WET LabsGeorge Kattawar and Dayou Chen, TAMU

Polarization in the ocean and underwater imaging

2

• 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

• 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

Polarization in the ocean and underwater imaging

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

4

5

Stokes vector components

Stokes vector }',,,{I VUQI

**

**

**

**

,

,

,

rllr

lrrl

rrll

rrll

EEEEiV

EEEEU

EEEEQ

EEEEI

where El and Er are the components of the electric field parallel and

perpendicular to the plane, which contains the vector of light propagation

IUQDoLP /22 Degree of linear polarization

V component characterizes circular polarization, it is usually very small and can be neglected.

If I90, I0, and I45 are the intensity values recorded by the sensors, then the Stokes components can be calculated as:

Instrumentation

Angle of linear polarization (characterizes orientation of

polarization)

)/arctan(5.0 QUAoLP

6

Degree of polarization dependence on the scattering angle

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

0 20 40 60 80 100 120 140 160 180 2000

0.2

0.4

0.6

0.8

Scattering Angle, sca (°)

DO

P

412nm440nm488nm510nm532nm555nm650nm

0 20 40 60 80 100 120 140 160 180 2000

0.1

0.2

0.3

0.4

Scattering Angle, sca (°)

DO

P

412nm440nm488nm510nm532nm555nm650nm

Water surface

30°

60°

40°Sun angle

viewing direction

48°

Snell’s window

Instrumentation

Optics Express, 2009, Applied Optics, 2011(2)

Hyperspectral multi-angular polarimeter

120°

Full Stokes vector imaging camera

7

Instrumentation

Polarimeter-camera system with thrusters

HYPERSPECTRAL

RADIOMETERS

(0°, 45°, 90°, LH CP)

FULL STOKES

POLARIZATION

CAMERA

THRUSTERS

DATA LOG &

STEPPER

MOTOR

135°

Instrumentation

Transmission direction of polarizers in front side of the mirror

Schematic of the equipment and the target used for the measurements

Instrumentation

10

Atmosphere

Interface

Ocean

Bottom

sun

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

0 :10 : 360sun

180:5:0viewing

20

40

60

80

30

210

60

240

90

270

120

300

150

330

180 0Relative azimuth with the sun

Viewing angle (0° looking straight down, 180° up)

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.

ac-s (WET Labs)

(c, a, ω)

MASCOT(WET Labs)(Scattering

Matrix)

Atmospheric parameters

(AOT)

RayXP Radiative Transfer

Simulations

[I,Q,U,V]T

Imaging model

12

Imaging model

13

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

Imaging model

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Imaging model

[1100110000 0000 00

]Mphor = Mpvert =

Mp45pos = Mp45neg =

Mmir =

[1− 10 0−110 000 0000 00

][10 −1 0

00 00−101 000 00

][101 000 0 0101 000 0 0

]

Mueller matrices for the target components and the mirror

Mmir33 was replaced by 0.5 based on comparison with the model

Water properties (WET Labs)

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Imaging results for clear water, Curacao 2012

450 500 550 600 650 7000

0.1

0.2

0.3

0.4

0.5

Wavelength, (nm)

a (m

-1)

TotalWaterPhytoNAPCDOM

450 500 550 600 650 7000

0.05

0.1

0.15

0.2

Wavelength, (nm)

c (m

-1)

0 20 40 60 80 100 120 140 160 18010

-2

10-1

100

101

102

103

Scattering Angles [ ]

Ph

ase

Fu

nct

ion

, sr-1 Jul 10, 2012

Sun elevation 43° Depth of the instrument 2.7mWind 3m/s

16

0 20 40 60 80 100 120 140 160 1800

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Azimuth angle (deg)

I Nor

mai

lzed

I Normalized to Io at 0 azimuth angle

I (Meas. camera)Error I - CameraI (Meas. polarimeter)I (Simul. RayXp)

0 20 40 60 80 100 120 140 160 1800

10

20

30

40

50

60

70

80

90

100

Azimuth angle (deg)

DO

LP

- P

erce

ntag

e (%

)

DOLP as a function of azimuth angle

DOLP (Meas. camera)Error DOLP - CameraDOLP (Meas. polarimeter)DOLP (Simul. RayXp)

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 2012I, 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

0 50 100 150 2000

50

100

150

200

250

Azimuth angle, deg

I

waterhorvert45p45nwater refl

0 50 100 150 200-1

-0.5

0

0.5

1

Azimuth angle, deg

Q/I

waterhorvert45p45nwater refl

0 50 100 150 200-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

Azimuth angle, deg

U/I

waterhorvert45p45nwater refl

19

Imaging results for clear water, Curacao 2012

Simulation and experimental dataSolid lines – simulations,

dashed lines -measurements

0 50 100 150 2000

0.2

0.4

0.6

0.8

1

Azimuth angle, deg

DO

LP

waterhorvert45p45nwater refl

0 50 100 150 200-80

-60

-40

-20

0

20

40

Azimuth angle, deg

AO

LP,

deg

waterhorvert45pwater refl

20

0 50 100 150 2000

0.2

0.4

0.6

0.8

1

Azimuth angle

DOLP

waterhorvert45p45nw

refl

0 50 100 150 200-100

-80

-60

-40

-20

0

20

40

60

80

100

Azimuth angle

AO

LP

waterhorvert45p45nw

refl

Imaging results for clear water, Curacao 2012

Simulations x3 and experimental dataSolid lines – simulations,

dashed lines -measurements

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Imaging results for coastal water, NY Bight 2012Water properties

400 450 500 550 600 650 7000

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

Wavelength [nm]

[m-1

]

Station 9

cpg

apg

ag

atotal

400 450 500 550 600 650 7001.1

1.15

1.2

1.25

1.3

1.35

1.4

1.45

1.5

Wavelength [nm]

[m-1

]

Station 9: Fitting PSD slope with cp()

p =0.4461

cp

best fit

Aug 23, 2012Sun elevation 60 degDepth of the instrument 5.7mNo wind

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Imaging results for coastal water, NY Bight 2012

0 20 40 60 80 100 120 140 160 1800

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1I (Normalized) as a function of azimuth [0...180]

I (N

orm

aliz

ed)

Azimuth angle (deg)

PolarimeterCameraRayXp simulation

0 20 40 60 80 100 120 140 160 1800

10

20

30

40

50

60

70

80

90

100DOLP as a function of azimuth [0...180]

DO

LP (P

erce

ntag

e)Azimuth angle (deg)

PolarimeterCameraRayXp simulation

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

Experimental data

23

Imaging results for coastal water, NY Bight 2012

Experimental data

Images of I component and DoLP for the target on the mirror and surrounding water area 90deg.

I, 90 deg DoLP, 90 deg

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Imaging results for coastal water, NY Bight 2012

Experimental data

0 50 100 150 2000.1

0.2

0.3

0.4

0.5

0.6

Azimuth angle, deg

DO

LP

waterhorvert45pwater refl

0 50 100 150 200-80

-60

-40

-20

0

20

Azimuth angle, deg

AO

LP,

deg

waterhorvert45pwater refl

0 50 100 150 20020

30

40

50

60

70

80

90

100

Azimuth angle, deg

I

waterhorvert45pwater refl

• 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.

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Conclusions

AcknowledgementThis work was supported by grant from

ONR MURI program

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