Improving visibility depth in passive underwater imaging by use of polarization

Abstract

Results are presented that demonstrate the effectiveness of using polarization discrimination to improve visibility when imaging in a scattering medium. The study is motivated by the desire to improve visibility depth in turbid environments, such as the sea. Most previous research in this area has concentrated on the active illumination of objects with polarized light. We consider passive or ambient illumination, such as that deriving from sunlight or a cloudy sky. The basis for the improvements in visibility observed is that single scattering by small particles introduces a significant amount of polarization into light at scattering angles near 90 degrees: This light can then be distinguished from light scattered by an object that remains almost completely unpolarized. Results were obtained from a Monte Carlo simulation and from a small-scale experiment in which an object was immersed in a cell filled with polystyrene latex spheres suspended in water. In both cases, the results showed an improvement in contrast and visibility depth for obscuration that was due to Rayleigh particles, but less improvement was obtained for larger scatterers.

Exploring underwater target detection by imaging polarimetry and correlation techniques

“…Passive imaging uses ambient illumination from sunlight (or a cloudy sky) which is generally unpolarized. Light which has not reached the object becomes partially polarized by scattering and can be filtered by an analyzer placed in front of the detector [9,10].…”

“…Polarization filtering is adapted to underwater target detection and identification [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. In this context, combining polarization filtering and a correlation technique is justified because: (i) it is simple to implement, (ii) it is compatible with polarization filtering, and (iii) it allows us for simultaneous detect and identify target objects.…”

Transmission of linearly polarized light in seawater: implications for polarization signaling

“…Polarization imaging, combined with intensity imaging, can increase the detection range of objects in a scattering medium, including those that reflect polarized light (Briggs and Hatcett, 1965;Lythgoe and Hemming, 1967;Lythgoe, 1971;Rowe et al, 1995;Tyo et al, 1996;Chang et al, 2003), transparent objects (Shashar et al, 1995) and nonpolarizing objects (Cariou et al, 2003;Chang et al, 2003;Schechner and Karpel, 2004). Fish and squid exploit this phenomenon, using polarization vision to improve the range of detection of transparent prey (Novales-Flamarique and Browman, 2001;Shashar et al, 1998).…”

Structured Light Methods for Underwater Imaging: Light Stripe Scanning and Photometric Stereo

“…A distant source (direction s) illuminates a surface point P with unit normal n. A camera observing the surface receives irradiance E surf due to the light reflected by the surface (solid red lines) and irradiance E med due to light scattered by the medium (dashed lines) in the viewing direction. The irradiance E surf is the same as for light striping (see equation 3),…”

“…However, an implicit assumption made in most methods is that light is neither scattered nor absorbed by the medium in which the scene and sources are immersed (as in pure air). 1 Work has been done on a related but different problem of analyzing the appearances of scenes in scattering media (underwater or the atmosphere) using passive methods [12], [3], [22], [24], [21], [17] that rely on natural illumination external to the medium Thus, it is critical to take into account the effects of scattering while applying computer vision based structured light methods underwater.…”