Resolution enhancement in active underwater polarization imaging with modulation transfer function analysis

Increased range and contrast in fog with circularly polarized imaging

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.

Numerical simulation model of an optical filter using an optical vortex

Vortex beam has the potential to significantly improve the performance of lidar (light detection and ranging) and optical communication applications in which low signal-to-noise ratio (SNR) limits the detection/transmission range. The vortex beam method allows for spatially separating the coherent light (laser signal) from the incoherent light (the background radiation and multiple-scattered light) of the received signal. This paper presents results of a simulation model in which the optical vortex acts as an optical filter. We present instrument parameters that describe the filtering effect, e.g., the form of the vortex phase modulation function, the topological charge of the vortex and the focal length of a virtual Fresnel lens that is used for optical filtering. Preliminary experimental results show that the background radiation within the spectral filter bandwidth can be suppressed by as much as 95%. At the same time, we retain 97% of the coherent laser signal. Our simulation model will be used in future design of lidar instruments and optical communication systems in which the optical vortex method is used for optical filtering of the detected signals.

Time-of-Flight Imaging in Fog Using Polarization Phasor Imaging

Due to the light scattered by atmospheric aerosols, the amplitude image contrast is degraded and the depth measurement is greatly distorted for time-of-flight (ToF) imaging in fog. The problem limits ToF imaging to be applied in outdoor settings, such as autonomous driving. To improve the quality of the images captured by ToF cameras, we propose a polarization phasor imaging method for image recovery in foggy scenes. In this paper, optical polarimetric defogging is introduced into ToF phasor imaging, and the degree of polarization phasor is proposed to estimate the scattering component. A polarization phasor imaging model is established, aiming at separating the target component from the signal received by ToF cameras to recover the amplitude and depth information. The effectiveness of this method is confirmed by several experiments with artificial fog, and the experimental results demonstrate that the proposed method significantly improves the image quality, with robustness in different thicknesses of fog.

Imaging in turbid water based on a Hadamard single-pixel imaging system

Underwater imaging is a challenging task because of the large amounts of noise and the scattering nature of water. Conventional optical methods cannot realize clear imaging in underwater conditions owing to the limitations of low sensitivity, geometrical aberrations, and the narrow spectrum of photoelectric detectors. By contrast, single-pixel imaging (SPI) is a promising tool for imaging in poor-visibility environments. Nevertheless, this challenge is faced even when using traditional SPI methods in highly turbid underwater environments. In this work, we propose a Hadamard single-pixel imaging (HSI) system that outperforms other imaging modes in turbid water imaging. The effects of laser power, projection rate, and water turbidity on the final image quality are systematically investigated. Results reveal that compared with the state-of-the-art SPI techniques, the proposed HSI system is more promising for underwater imaging because of its high resolution and anti-scattering capabilities.

Broadband ellipso-polarimetric camera utilizing tunable liquid crystal achromatic waveplate with improved field of view

An ellipso-polarimetric camera integrated with improved field of view tunable achromatic waveplate (AWP) over wide spectral band based on nematic liquid crystal retarders is presented. The AWP operates as half, quarter and full waveplate over a wide range of 430-780nm and wide field of view. The proposed analysis proved that capturing images at these modes is sufficient to extract the ellipsometric parameters: sin(2ψ), cos(Δ) and the Stokes parameters S₁ and S₃, besides showing the relations in between. Transmission and reflection modes setups are demonstrated in addition to an ellipso-polarimetric smartphone camera. The results show for the first time superiority of cos(Δ) images in which prominent contrast and fine details appear even with scattering objects and higher immunity to device errors. Biometric, remote sensing and archeological improved imaging applications are demonstrated.

Active underwater descattering and image recovery

Underwater imaging is a promising but challenging topic due to the scattering particles in water, which result in serious light attenuation. Therefore, underwater images suffer from low-contrast and low-resolution issues. In this study, in order to recover high-quality underwater images, the point spread functions (PSFs) are estimated by a slant-edge method. The experiment modulates the illumination source to deal with backscattering and the imager to take two images in orthogonally polarized states. This imaging method benefits the satisfactory edge extraction. The PSF estimation is performed based on the extracted slant edge to enable recovery of the image. In addition, the modulation transfer function (MTF) is introduced to evaluate the resolution variation with the spatial frequencies. It manifests considerable resolution enhancement in the recovered images. Moreover, the proposed underwater image recovery method also reduces the effect from the scattering as an effective compensation to the polarization imaging approach.

Polarimetric dehazing method for visibility improvement based on visible and infrared image fusion

Polarimetric dehazing methods have proven effective in enhancing the quality of chromatic hazy images. Considering that the infrared radiance has a better capacity for traveling through the haze, in this paper we propose a polarimetric dehazing method based on visible and infrared image fusion to improve the visibility of hazy images, especially for dense haze conditions. Experimental results demonstrate that the visibility of hazy images can be effectively enhanced, and the color information can be finely maintained. The visibility of dehazed images can be promoted at least 100%. This kind of dehazing method can be used widely in many dehazing applications.

Scattering robust 3D reconstruction via polarized transient imaging

Reconstructing 3D structure of scenes in the scattering medium is a challenging task with great research value. Existing techniques often impose strong assumptions on the scattering behaviors and are of limited performance. Recently, a low-cost transient imaging system has provided a feasible way to resolve the scene depth, by detecting the reflection instant on the time profile of a surface point. However, in cases with scattering medium, the rays are both reflected and scattered during transmission, and the depth calculated from the time profile largely deviates from the true value. To handle this problem, we used the different polarization behaviors of the reflection and scattering components, and introduced active polarization to separate the reflection component to estimate the scattering robust depth. Our experiments have demonstrated that our approach can accurately reconstruct the 3D structure underlying the scattering medium.