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Shape-from-Polarimetry: Recovering Sea Surface Topography. Howard Schultz Department of Computer Science University of Massachusetts 140 governors Dr Amherst, MA 01003 hschultz @cs.umass.edu >. October 2011. Outline. Why recover the spatial -temporal structure of ocean waves?

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slide1

Shape-from-Polarimetry:

Recovering Sea Surface Topography

Howard Schultz

Department of Computer Science

University of Massachusetts

140 governors Dr

Amherst, MA 01003

[email protected]>

October 2011

outline
Outline
  • Why recover the spatial-temporal structure of ocean waves?
  • Requirements
  • What is polarimetry?
  • What is the Shape-from-Polarimetry?
  • Build and Test an Imaging Polarimeter for Ocean Apps.
  • Recent Experiment and Results
  • Optical Flattening
  • Seeing Through Waves
slide3
Why recover the structure of the ocean surface?
    • Characterize small small-scale wave dynamics and microscale breaking
    • Air-sea interactions occur at short wavelengths
    • Non-linear interaction studies require phase-resolved surface topography
    • Enable through-the-wave imaging
    • Detect anomalies in surface slope statistics
  • Why use a passive optical technique
    • Probes disturb the air-sea interaction
    • Radar do not produce phase-resolved surfaces
    • Active techniques are complex and expensive
  • Requirements
    • Spatial resolution (resolve capillary waves) ~ 1mm
    • Temporal resolution ~60Hz sampling rate
    • Shutter speed < 1 msec
what is polarimetry
What is polarimetry?
  • Light has 3 basic qualities
  • Color, intensity and polarization
  • Humans do not see polarization
slide5

Linear Polarization

http://www.enzim.hu/~szia/cddemo/edemo0.htm

what is polarimetry1

Amount of circular polarization

Orientation and degree of linear polarization

Intensity

What is polarimetry?
  • A bundle of light rays is characterized by intensity, a frequency distribution (color), and a polarization distribution
  • Polarization distribution is characterized by Stokes parameters

S = (S0, S1, S2, S3)

  • The change in polarization on scattering is described by Muller Calculus

SOUT = M SIN

  • Where M contains information about the shape and material properties of the scattering media
  • The goal: Measure SOUT and SIN and infer the parameters of M

Incident Light

Muller Matrix

Scattered Light

what is shape from polarimetry sfp
What is Shape-from-Polarimetry (SFP)?
  • Use the change in polarization of reflected skylight to infer the 2D surface slope, , for every pixel in the imaging polarimeter’s field-of-view
what is shape from polarimetry sfp2
What is Shape-from-Polarimetry (SFP)?

SAW = RAWSSKYand SWA = TAWSUP

what is shape from polarimetry sfp3
What is Shape-from-Polarimetry (SFP)?
  • For RaDyO we incorporated 3 simplifying assumptions
    • Skylight is unpolarized SSKY = SSKY(1,0,0,0)

good for overcast days

    • In deep, clear water upwelling light can be neglected SWA = (0,0,0,0).
    • The surface is smooth within the pixel field-of-view
how well does the sfp technique work
How well does the SFP technique work?
  • Conduct a feasibility study
    • Rented a linear imaging polarimeter
    • Laboratory experiment
      • setup a small 1m x 1m wavetank
      • Used unpolarized light
      • Used wire gauge to simultaneously measure wave profile
    • Field experiment
      • Collected data from a boat dock
      • Overcast sky (unpolarized)
      • Used a laser slope gauge
slide15

Looking at 90 to the waves

Looking at 45 to the waves

Looking at 0 to the waves

slide17

X-Component

Y-Component

Slope in Degrees

slide18

X-Component

Y-Component

Slope in Degrees

build and test an imaging polarimeter for oceanographic applications
Build and Test an Imaging Polarimeter for Oceanographic Applications
  • Funded by an ONR DURIP
  • Frame rate 60 Hz
  • Shutter speed as short as 10 μsec
  • Measure all Stokes parameters
  • Rugged and light weight
  • Deploy in the Radiance in a Dynamic Ocean (RaDyO) research initiative

http://www.opl.ucsb.edu/radyo/

slide21

Camera 3

Camera 4

Camera 1

(fixed)

Polarizing

beamsplitter

assembly

Objective

Assembly

Camera 2

Motorized Stage

12mm travel

5mm/sec max speed

slide23

FLIP INSTRUMENTATION SETUP

Scanning Altimeters

Visible Camera

Air-Sea Flux Package

Infrared Camera

Polarimeter

sample results
Sample Results
  • A sample dataset from the Santa Barbara Channel experiment was analyzed
  • Video 1 shows the x- and y-slope arrays for 1100 frames
  • Video 2 shows the recovered surface (made by integrating the slopes) for the first 500 frames
convert slope arrays to a height array
Convert slope arrays to a height array

Use the Fourier derivative theorem

seeing through waves
Seeing Through Waves
  • Sub-surface to surface imaging
  • Surface to sub-surface imaging
optical flattening1
Optical Flattening
  • Remove the optic distortion caused by surface waves to make it appear as if the ocean surface was flat
    • Use the 2D surface slope field to find the refracted direction for each image pixel
    • Refraction provides sufficient information to compensate for surface wave distortion
    • Real-time processing
image formation subsurface to surface
Image FormationSubsurface-to-surface

Observation Rays

Air

Water

Imaging Array

Exposure Center

image formation surface to subsurface
Image Formationsurface-to-subsurface

Exposure Center

Imaging Array

Air

Imaging Array

Water

Exposure Center

seeing through waves1
Seeing Through Waves

0 20 40 60 80

0 10 20 30 40

optical flattening2
Optical Flattening
  • Remove the optic distortion caused by surface waves to make it appear as if the ocean surface was flat
    • Use the 2D surface slope field to find the refracted direction for each image pixel
    • Refraction provides sufficient information to compensate for surface wave distortion
    • Real-time processing
un distortion use the refraction angle to straighten out light rays
Un-distortionUse the refraction angle to “straighten out” light rays

Image array

Air

Water

Distorted Image Point

un distortion use the refraction angle to straighten out light rays1
Un-distortionUse the refraction angle to “straighten out” light rays

Image array

Air

Water

Un-distorted Image Point

real time un distortion
Real-time Un-Distortion
  • The following steps are taken Real-time Capable
    • Collect Polarimetric Images ✔
    • Convert to Stokes Parameters ✔
    • Compute Slopes (Muller Calculus) ✔
    • Refract Rays (Lookup Table) ✔
    • Remap Rays to Correct Pixel ✔
image formation surface to subsurface1
Image Formationsurface-to-subsurface

Exposure Center

Imaging Array

Air

Imaging Array

Water

Exposure Center

detecting submerged objects lucky imaging
Detecting Submerged Objects“Lucky Imaging”
  • Use refraction information to keep track of where each pixel (in each video frame) was looking in the water column
  • Build up a unified view of the underwater environment over several video frames
  • Save rays that refract toward the target area
  • Reject rays that refract away from the target area
slide48

For more information contact

Howard Schultz

University of Massachusetts

Department of Computer Science

140 Governors Drive

Amherst, MA 01003

Phone: 413-545-3482

Email: [email protected]

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