Introduction
Introduction
We are developing a new passive optical technique based on polarimetry. Conventional optical remote sensing techniques rely on light amplitude and frequency to carry information about the scattering surface. The polarimetric method exploits these properties, as well as the polarization properties of light to sense information about the scattering media. When the polarimetric properties of light are included, the increased information about the scattering media is striking. We demonstrated in a recent exploratory experiment that the two-dimensional slope field of short gravity wave can be recovered from a distance without interfering with the fluid dynamics of the air or water.
Nondestructive remote sensing methods for measuring the dynamics of ocean surface waves are critical to many important oceanographic and fluid mechanics research topics. Understanding how energy is transferred from the atmosphere to the ocean, the growth and decay of waves, and gas exchange are a few examples of research topics that depend on a good knowledge of the ocean surface dynamics. Investigators have often built instruments that exploit the scattering properties of light to sense the air-sea interface. Some examples of light sensing devices include stereo photography, sun glint photography, specular surface stereo, laser slope gauges, laser profiling, and color table slope gauges. The problem with these methods has been extracting sufficient information from passive measurements and constructing a nondestructive instrument for active devices.
Proof-of-concept study
A proof-of-concept study "Ocean Surface Wave Optical Roughness: Innovative Polarization Measurement," was funded by a grant from the Office of Naval Research, Steven Ackleson, Program Manager. The goal of this experiment was to test the feasibility of computing the orientation of surface facets from polarimetric images. We contracted with Equinox Corporation to supply a three-component polarimetric camera and provide an initial analysis of the data. The experiment had two parts -- a laboratory wave tank experiment and a field experiment. The laboratory data collection is complete and we have some preliminary results, which are summarized below. The analysis of the field experiment is ongoing.
For the laboratory experiment we set up a 1m × 1m × 0.3m (deep) wave tank, a simple 1-D wave generator, a wave gauge, and a bank of unpolarized lights (see Figure 1). The purpose of the laboratory experiment was to provide ideal conditions for testing the polarimetric slope sensing concept, including:
- Unpolarized source radiance eliminated the need for a Sky Camera.
- Generating 1-D sinusoidal waves simplified the comparison between the wave probe and polarimetric results.
- Generated slow gravity wave (frequency ~5 Hz, phase speed: ~0.3m/sec) to minimize blurring.
Examples
We have several video clips of the experiment. They can be downloaded from
this website.
The video clip MVI_0018.AVI shows the wave tank setup, and the video clips that begin with "exp" show animations of the recovered water surface topographies.
The laboratory study results so far are summarized in Table 1. Sample frames from experiments 1, 5 and 7 are shown in Figure 2. The elevations were found by first computing the instantaneous surface slopes from the polarimetric data and then integrating once to find the elevations. Because the constant of integration cannot be determined without at least one independent range estimate, the elevation at the lower-left corner is set to 0.0.
The slopes computed from the polarimetric data were compared to the wave height measured by the wave probe by assuming a sinusoidal wave profile. For experiment 1, the wavelength error is 3.5% and the waveheight error is 1.2%. For experiment 2 the wavelength error is 6.4% and the waveheight error is 6.9%. These numbers are impressive when you consider that the wave probe is only a point measurement. Also, experiment 2 is not approximated very well by a sine wave.
Summary
 |
Figure 1. The 1m x 1m wave tank used in the feasibility study. The white cylindrical wave generator is seen at the left. There is a blue horse-hair absorbing beach on the right. |
Table 1:
| Exp1Run3a |
Camera perpendicular to wave propagation direction |
0.143117 |
5.0293 |
exp1a_min_seq |
| Exp1Run3b |
Camera perpendicular to wave propagation direction |
0.150291 |
4.9805 |
exp1b_mid_seq |
| Exp1Run3c |
Camera perpendicular to wave propagation direction |
0.163003 |
4.9805 |
|
| Exp2Run3c |
Camera perpendicular to wave propagation direction |
0.367454 |
4.8828 |
exp2a_mid_seq |
| Exp3Run2a |
Camera 45 degrees to wave propagation direction |
0.378367 |
4.8828 |
|
| Exp4Run1a |
Camera 45 degrees to wave propagation direction |
0.218449 |
4.5898 |
|
| Exp5 |
Camera parallel to wave propagation direction |
|
|
exp5a_seq |
| Exp7 |
Wind generated waves |
|
|
exp7a_seq |
| |
Wind generated waves |
|
|
exp7b_seq |
| |
Wind generated waves |
|
|
exp7c_seq |
Figure 2:
