Theory of autocalibration feasibility and precision in full Stokes polarization imagers

Vortex retarder-based Stokes polarimeters: optimal data processing and autocalibration capability

We present a full Stokes polarimeter that utilizes a vortex retarder (VR) in conjunction with a polarization camera. We demonstrate its capability to estimate the full Stokes vector in a single shot with optimal precision and to autocalibrate the VR retardance, ensuring precise measurements even in dynamic environments where retardance is variable.

Local optimized Stokes polarimetry for specific polarization states

In this Letter, we propose a locally optimized Stokes polarimetry. Focusing on the effect on polarization measurements by Poisson noise, the studies establish a new, to the best of our knowledge, optimization function combining the equally weighted variance with the condition number. This method considers both the stability and the precision of polarization measurements; by trading an increase in the condition number by 2.48%, we realize a decrease in equal-weighted variance by 19.1% near the north pole. The advantages of this local optimization method are demonstrated based on Monte Carlo (MC) simulations and experiments of continuous polarization state modulation. Finally, an imaging demonstration using a 4 µm pathological section implies the potential of this new local optimization method in improving polarization measurements and applying it to more biomedical research.

Polarized image super-resolution via a deep convolutional neural network

Reduced resolution of polarized images makes it difficult to distinguish detailed polarization information and limits the ability to identify small targets and weak signals. A possible way to handle this problem is the polarization super-resolution (SR), which aims to obtain a high-resolution polarized image from a low-resolution one. However, compared with the traditional intensity-mode image SR, the polarization SR is more challenging because more channels and their nonlinear cross-links need to be considered as well as the polarization and intensity information need to be reconstructed simultaneously. This paper analyzes the polarized image degradation and proposes a deep convolutional neural network for polarization SR reconstruction based on two degradation models. The network structure and the well-designed loss function have been verified to effectively balance the restoration of intensity and polarization information, and can realize the SR with a maximum scaling factor of four. Experimental results show that the proposed method outperforms other SR methods in terms of both quantitative evaluation and visual effect evaluation for two degradation models with different scaling factors.

Self-calibration for Mueller polarimeters based on DoFP polarization imagers

Mueller polarimeters (MPs) based on division of focal plane (DoFP) polarization imagers can achieve fast measurements and significantly improve the effectiveness of Mueller polarimetry. In this Letter, we demonstrate a unique property of the DoFP sensor-based MPs: they can be calibrated without any extra polarizing reference element. We describe a self-calibration method that only requires six image acquisitions; based on our analysis, the calibration accuracy is only limited by the noise.

Definition of an error map for DoFP polarimetric images and its application to retardance calibration

With the recent development of division of focal plane (DoFP) polarization sensors, it is possible to perform polarimetric analysis of a scene with a reduced number of acquisitions. One drawback of these sensors is that polarization estimation can be perturbed by the spatial variations of the scene. We thus propose a method to compute a map that indicates where polarization estimation can be trusted in the image. It is based on two criteria: the consistency between the intensity measurements inside a super-pixel and the detection of spatial intensity variations. We design both criteria so that a constant false alarm rate can be set. We demonstrate the benefit of this method to improve the precision of dynamic retardance calibration of DoFP-based full Stokes imaging systems.

Calibration of a polarization image sensor and investigation of influencing factors

Polarization measurements conducted with a polarization camera using the Sony IMX 250 MZR polarization image sensor are assessed with the super-pixel calibration technique and a simple test setup. We define an error that quantifies the quality of the polarization measurements. Multiple factors influencing the measurement quality of the polarization camera are investigated and discussed. We demonstrate that polarization measurements are generally consistent throughout the sensor if not corrupted by large chief ray angles or large angles of incidence. The central 600×400pixels were analyzed, and it is shown that sufficiently large f-numbers no longer influence measurement quality. We also argue that lens design and focal length have little influence on these central pixels. The findings of this study provide useful guidance for researchers using such a polarization image sensor.

Integration time optimization and starting angle autocalibration of full Stokes imagers based on a rotating retarder

Full Stokes imaging can be performed with a continuously rotating retarder in front of a fixed polarizer and a standard camera (RRFP) or a division of a focal plane polarization camera (RRDOFP). We determine the optimal number and duration of intensity measurements through a cycle of the retarder for these two types of setups as a function of instrument and noise parameters. We show that this number mainly depends on the type of noise that corrupts the measurements. We also show that with these setups, the starting angle of the retarder need not be known precisely and can be autocalibrated, which facilitates synchronization of the rotating retarder with the camera. We investigate the precision and feasibility domain of this autocalibration and show the RRDOFP setup has more attractive properties compared with RRFP setup. These results are important to optimize and facilitate the operation of polarization imagers based on a rotating retarder.