Experimental Characterization of Polarized Light Backscattering in Fog Environments.

This paper focuses on the experimental characterization of the polarization behavior of light backscattered through fog. A polarimetric orthogonal state contrast imager and an active, purely polarized white illuminator system are used to evaluate both linear and circular polarization signals. The experiments are carried out in a macro-scale fog chamber under controlled artificial fog conditions. We explore the effect of backscattering in each imaging channel, and the persistence of both polarization signals as a function of meteorological visibility. We confirm the presence of the polarization memory effect with circularly polarized light, and, as a consequence, the maintenance of helicity in backscattering. Moreover, the circular cross-polarized channel is found to be the imaging channel less affected by fog backscattering. These results are useful and should be taken into account when considering active polarimetric imaging techniques for outdoor applications under foggy conditions.

Analysis of the performance of a polarized LiDAR imager in fog.

This paper focuses on exploring ways to improve the performance of LiDAR imagers through fog. One of the known weaknesses of LiDAR technology is the lack of tolerance to adverse environmental conditions, such as the presence of fog, which hampers the future development of LiDAR in several markets. Within this paper, a LiDAR unit is designed and constructed to be able to apply temporal and polarimetric discrimination for detecting the number of signal photons received with detailed control of its temporal and spatial distribution under co-polarized and cross-polarized configurations. The system is evaluated using different experiments in a macro-scale fog chamber under controlled fog conditions. Using the complete digitization of the acquired signals, we analyze the natural light media response, to see that due to its characteristics it could be directly filtered out. Moreover, we confirm that there exists a polarization memory effect, which, by using a polarimetric cross-configuration detector, allows improvement of object detection in point clouds. These results are useful for applications related to computer vision, in fields like autonomous vehicles or outdoor surveillance where many variable types of environmental conditions may be present.

Quantitative demonstration of the superiority of circularly polarized light in fog environments.

The polarization behavior of light transmitted through scattering media is studied quantitatively. A division of focal plane (DOFP) imaging polarimeter modified with a wideband quarter-wave plate (QWP) is used to evaluate the linear and circular depolarization signals. This system allows the measurement of the linear and circular co-polarization and cross-polarization channels simultaneously. The experiments are carried out at CEREMA's 30 m fog chamber under controlled fog density conditions. The polarization memory effect with circularly polarized light is demonstrated to be superior in forward transmission compared to the same phenomena with linearly polarized light when imaging inside a scattering medium. This paves the way for its use in imaging through scattering media for hazard detection in different applications.