Increased range and contrast in fog with circularly polarized imaging

Abstract

Fogs, low lying clouds, and other highly scattering environments pose a challenge for many commercial and national security sensing systems. Current autonomous systems rely on optical sensors for navigation whose performance is degraded by highly scattering environments. In our previous simulation work, we have shown that polarized light can penetrate through a scattering environment such as fog. We have demonstrated that circularly polarized light maintains its initial polarization state better than linearly polarized light, even through large numbers of scattering events and thus ranges. This has recently been experimentally verified by other researchers. In this work, we present the design, construction, and testing of active polarization imagers at short-wave infrared and visible wavelengths. We explore multiple polarimetric configurations for the imagers, focusing on linear and circular polarization states. The polarized imagers were tested at the Sandia National Laboratories Fog Chamber under realistic fog conditions. We show that active circular polarization imagers can increase range and contrast in fog better than linear polarization imagers. We show that when imaging typical road sign and safety retro-reflective films, circularly polarized imaging has enhanced contrast throughout most fog densities/ranges compared to linearly polarized imaging and can penetrate over 15 to 25 m into the fog beyond the range limit of linearly polarized imaging, with a strong dependence on the interaction of the polarization state with the target materials.

Performance studies of two active SWIR instrument designs for vision enhancement in indoor applications

“…Furthermore, the performance of using circular polarized light in our field test was found to be better than using unpolarized light, but worse than linear polarization, which appears to be in contrast to studies suggesting circular polarization to be preserved better in fog. [9][10][11] The reason is that the illumination unit produces circular polarization by a combination of linear polarization filter and quarterwaveplate for 1550 nm (i.e. having a retardance ̸ = 0.25 for λ ̸ = 1550 nm), i.e.…”