Active Polarization Descattering

Abstract

Abstract-Imaging in scattering media such as fog and water is important but challenging. Images suffer from poor visibility due to backscattering and signal attenuation. Most prior methods for scene recovery use active illumination scanners (structured and gated), which can be slow and cumbersome. On the other hand, natural illumination is inapplicable to dark environments. The current paper addresses the need for a non-scanning recovery method, that uses active scene irradiance. We study the formation of images under widefield artificial illumination. Based on the formation model, the paper presents an approach for recovering the object signal. It also yields rough information about the 3D scene structure. The approach can work with compact, simple hardware, having active widefield, polychromatic polarized illumination. The camera is fitted with a polarization analyzer. Two frames of the scene are instantly taken, with different states of the analyzer or light-source polarizer. A recovery algorithm follows the acquisition. It allows both the backscatter and the object reflection to be partially polarized. It thus unifies and generalizes prior polarization-based methods, which had assumed exclusive polarization of either of these components. The approach is limited to an effective range, due to image noise and falloff of widefield illumination. Thus, these limits and the noise sensitivity are analyzed. The approach particularly applies underwater. We therefore use the approach to demonstrate recovery of object signals and significant visibility enhancement in underwater field experiments.

On controlling light transport in poor visibility environments

“…Similarly, if the noise is derived from a Gaussian distribution, z can be the standard deviation. In addition, we should account for the effect of post-processing on image noise [24,17]. …”

“…On the other hand, underwater divers realize that maintaining a good separation between the source and the camera reduces backscatter, and improves visibil- ity [21,9]. Polarization filters have also been used to reduce contrast loss due to haze and murky water [20,24,19,6]. Based on these observations, we attempt to address two key questions.…”

PD Based Determination of Polarized Reflection Regions in Bad Weather

“…Polarization parameter is sensitive to a scattering surface. In such case, polarization imaging techniques often have the ability to produce images with higher inherent visual contrast than conventional image processing of the intensity distribution, especially under adverse weather, such as haze, fog and rain [3][4][5] Unaided human eyes fail to discern the polarization of light. In contrast many animals have visual systems that exploit the polarization of light in many different ways, e.g.…”

Silicon nanofin grating as a miniature chirality-distinguishing beam-splitter

“…The benefits of polarization-resolved imaging are evidenced by the fact that some animal eyes have evolved to achieve this functionality, for example, the mantis shrimp can resolve the full polarization state of light (Stokes vectors) 6 . Indeed, polarization-resolved imaging can enhance contrast 7,8 and enable the classification of chemical isomers 9 . In both fibre-optic 10 and on-chip 11 systems, polarization control is needed to prevent the detrimental effects of polarization-mode dispersion.…”