Division Of Focal Plane Polarimeters Research Articles

Calibration method for thermal infrared division-of-focal-plane polarimeters considering polarizer reflection characteristics

Owing to manufacturing defects of micropolarizer arrays and differences in the pixel response of detectors, division-of-focal-plane (DoFP) polarimeters have severe nonuniformity, which affects the measurement accuracy of the polarimeters and the calculation of the polarization information. This study proposes a calibration method for thermal infrared DoFP polarimeters considering polarizer reflection characteristics. The temperature-controlled adjustable infrared polarized radiation source is calibrated by a division-of-time polarimeter and is, in turn, used to calibrate a thermal infrared DoFP polarimeter. Through laboratory blackbody and external scenes, the performance of the proposed method is compared to that of state-of-the-art techniques. The experimental results indicate that the proposed method effectively avoids overcalibration and improves the accuracy of polarization information.

Creep Monitoring of Submersible Observation Windows Using Mueller Matrix Imaging.

Safety of the observation window is one of the core concerns for manned submersibles. When subjected to underwater static pressure, extrusion and creep deformation always occur in the observation window, which can pose a threat to both safety and optical performance. To assess the deformation, real-time and non-contact monitoring methods are necessary. In this study, a conceptual setup based on the waveplate rotation and dual-DoFP (division of focal-plane polarimeter) polarization camera is built for the observation window's creep monitoring by measuring the Mueller matrix images of the samples under different pressures and durations. Then, a series of characteristic parameters, such as t1, R, r, R', are extracted from the Muller matrix images by Mueller matrix transformation (MMT), Mueller matrix polar decomposition (MMPD), correlation analysis and phase unwrapping method. The results demonstrate that these parameters can effectively describe the observation window's creep at different pressure levels which are simulated by finite element analysis. Additionally, more characterization parameters, such as ψ, A and D, are given from the Mueller matrix images and discussed to illustrate the method's potential for further applications and investigations. Ultimately, future devices based on this method could serve as a valuable tool for real-time and non-contact creep monitoring of the submersible observation windows.

Demosaicking DoFP images using edge compensation method based on correlation.

With the development of nanotechnology, the division of focal plane (DoFP) infrared polarization imaging system with real-time imaging has matured. Meanwhile, the demand for real-time acquisition of polarization information is growing, but the super-pixel structure of the DoFP polarimeter will bring instantaneous field of view (IFoV) errors. Existing polarization demosaicking methods cannot satisfy both accuracy and speed in terms of efficiency and performance. According to the characteristics of DoFP, this paper proposes an edge compensation demosaicking method by analyzing the channel correlations of polarized images. The method performs demosaicing in the differential domain, and the proposed method's performance is verified by comparison experiments using synthetic and authentic polarized images in the near-infrared (NIR) band. The proposed method outperforms the state-of-the-art methods in terms of accuracy and efficiency. It achieves an average peak signal-to-noise ratio (PSNR) improvement of 2 db on public datasets compared to current state-of-the-art methods. A typical 768 × 1024 specification short-wave infrared (SWIR) polarized image can be processed in 0.293s on the Intel Core i7-10870 H CPU, and the technique significantly outperforms various existing demosaicking methods.

DoLP and AoP Synthesis from division of focal plane polarimeters using CycleGAN

Division of focal plane (DoFP) polarimeter brings significant advantages for capturing polarization information, and its unique structure enables temporal synchronization of polarization intensity measurements across scenes. While DoFP can obtain polarization information in all four directions simultaneously, it also reduces the spatial resolution of the image to 1/4 of the camera resolution. In order to solve low resolution and the cumulative error caused by the formula calculation. Using CycleGAN to construct a raw mosaic image-to-polarization properties images model, which differs from general interpolation methods and other existing deep learning methods with paired datasets and takes microgrid images as input and outputs the degree of linear polarization (DoLP) and angle of polarization (AoP) directly by learning the mapping function between unpaired the mosaic images and the polarization properties. In addition, Gradient Magnitude Similarity Deviation (GMSD) metric is applied to preserve satisfactory reconstructed image edge contour information and reduce IFOV artifacts and aliasing. The experimental results demonstrate that the quantitative performance evaluation metrics scores of our method are all higher than other methods and the reconstructed DoLP and AoP images are comparable to the ground truth imagery in terms of visual quality. The end-to-end network structure trained on unpaired datasets solves the severe limitations of paired datasets training, is more suitable for DoLP and AoP reconstructing.

Mixed noise removal based on Stokes residual noise removal for division of focal plane polarimetric images

The division of focal plane (DoFP) polarimeter has the advantages of simultaneous imaging and compact optical structure. It is inevitable to introducing the noise in the process of micro-polarizer array integration, polarization image acquisition and transmission. In this paper, we propose a novel mixed noise suppression method based on Stokes residual noise removal (SRNR) for mixed additive white Gaussian noise (AWGN) and impulse noise (IN) removal in DoFP polarimetric images. The Laplacian scale mixture (LSM) model is introduced to estimate the IN and the nonlocal low-rank regularization (NLR) is adopted to enhance the denoising performance. The residual noise of the Stokes parameters is taken into consideration. This crucial processing achieves the acquisition of high-quality polarization data. The experiments demonstrate the superior denoising performance for polarimetric imagery in terms of objective assessment and visual evaluation.

Residual Interpolation Integrated Pixel-by-Pixel Adaptive Iterative Process for Division of Focal Plane Polarimeters.

Residual interpolations are effective methods to reduce the instantaneous field-of-view error of division of focal plane (DoFP) polarimeters. However, their guide-image selection strategies are improper, and do not consider the DoFP polarimeters’ spatial sampling modes. Thus, we propose a residual interpolation method with a new guide-image selection strategy based on the spatial layout of the pixeled polarizer array to improve the sampling rate of the guide image. The interpolation performance is also improved by the proposed pixel-by-pixel, adaptive iterative process and the weighted average fusion of the results of the minimized residual and minimized Laplacian energy guide filters. Visual and objective evaluations demonstrate the proposed method’s superiority to the existing state-of-the-art methods. The proposed method proves that considering the spatial layout of the pixeled polarizer array on the physical level is vital to improving the performance of interpolation methods for DoFP polarimeters.

Temporal and spatial error model for estimating the measurement precision of the division of focal plane polarimeters.

Temporal noise and spatial non-uniformity primarily limit the measurement precision of division of focal plane (DoFP) polarimeters, based on which this study proposes an error model for DoFP polarimeters. The closed-form expressions of the estimation error of the main polarization parameters (Stokes vector, degree of linear polarization, and angle of linear polarization) are derived. Compared with the existing error models for DoFP polarimeters in the presence of temporal noise, the proposed model modifies the normalization condition in traditional calibration methods of DoFP polarimeters and clarifies the selection rule of the coefficient matrix leading to more accurate precision estimation; and experiments using linearly polarized light on a real-world DoFP polarimeter prove its validity.

Conditional generative adversarial network demosaicing strategy for division of focal plane polarimeters.

Division of focal plane (DoFP), or integrated microgrid polarimeters, typically consist of a 2 × 2 mosaic of linear polarization filters overlaid upon a focal plane array sensor and obtain temporally synchronized polarized intensity measurements across a scene, similar in concept to a Bayer color filter array camera. However, the resulting estimated polarimetric images suffer a loss in resolution and can be plagued by aliasing due to the spatially-modulated microgrid measurement strategy. Demosaicing strategies have been proposed that attempt to minimize these effects, but result in some level of residual artifacts. In this work we propose a conditional generative adversarial network (cGAN) approach to the microgrid demosaicing problem. We evaluate the performance of our approach against full-resolution division-of-time polarimeter data as well as compare against both traditional and recent microgrid demosaicing methods. We apply these demosaicing strategies to data from both real and simulated visible microgrid imagery and provide an objective criteria for evaluating their performance. We demonstrate that the proposed cGAN approach results in estimated Stokes imagery that is comparable to full-resolution ground truth imagery from both a quantitative and qualitative perspective.

Implementation of a logarithmic division-of-focal-plane polarimeter to quantify changes in collagen alignment at varying levels of illumination.

We examine the impact of illumination, aperture, and sample thickness on two division-of-focal-plane (DoFP) polarimeters, one created using a standard 3 T pixel and the other with a forward-biased, logarithmic pixel. Across all measured metrics the logarithmic DoFP polarimeter was better able to track real-time changes in collagen alignment than the standard DoFP polarimeter.

Pixel response model for a division of focal plane polarimeter.

A division of focal plane (DoFP) polarimeter includes an array of polarized pixels. The response characteristics of polarized pixels are directly affected by inherent defects of a DoFP polarimeter. Correspondingly, the response characteristics are crucial to correction of the inherent defects. However, research on the response characteristics is rarely reported. Therefore, this paper proposes a pixel response model for a DoFP polarimeter. The response model combines the response characteristics of a traditional photoelectric imager and a micro-polarizer array. The proposed model includes six input parameters. They are the major polarization responsivity, minor polarization responsivity, polarization orientation, exposure time, conversion gain, and gamma correction. An experimental setup is constructed to measure the response of a DoFP polarimeter. The proposed model is evaluated by comparing the calculated results and the measured results. The compared results under different artificial parameters show that the each average root-mean-square error value is less than one gray value, which proves the validity of the proposed model.

Modular division of focal plane polarimeter system

Existing division of focal plane (DoFP) polarimeters generally integrate a micro-polarizer array (MPA) and image sensors, which is useful in many applications. Polarization in different conditions (e.g., with different spectrum and resolution ratio) was studied in some exploratory research. These requirements demand different combinations of the DoFP system. Therefore, a kind of flexible DoFP system should be studied. In this work, we present a novel DoFP polarimeter system which separates the MPA and image sensor using a relay lens. Thus, it can be used to combine different MPAs, image sensors, and lenses for different applications. Segmented local adaptive threshold segmentation and convolution interpolation are implemented to compute the polarization, including the degree of linear polarization (DoLP) and the angle of polarization (AoP). Test results indicate that the mean error deviation of DoLP after calibration is below 1.5% and the mean error deviation of AoP after calibration is below 0.2°. Our camera design has the unique advantage of using different combinations of MPAs and image sensors for measuring multi-dimensional information like intensity and polarization states of an optical field in a single shot, thus allowing it to be adapted to different applications.

Misalignment error analysis in polychromatic division of focal plane Stokes polarimeters

Snapshot imaging polarimeters are used in many different areas. Recently, division of focal plane (DoFP) polarimeters have proved useful as snapshot polarimeters for dynamic applications. For an optimal performance of polarimeters, different works dealing with the error analysis of such devices are proposed in literature. In terms of noise amplification from intensity measurements to the final polarization calculations, well-established quality metrics, as in the condition number or the equally weighted variance criteria, are used. Other studies analyze systematic errors due to deviations in the construction parameters. However, something not considered so far is the effect produced by misalignment between the various pixelated masks over the pixelated structure of the camera sensor, always occurring in experimental implementations of DoFPs. In this work, we study the effect of such misalignments in DoFP polarimeters and demonstrate how they lead to polarimetric systems composed of partially depolarized analyzers. We calculate the combined degree of polarization related to different amounts of misalignment and analyze the corresponding system performance. From this study, we show how an imaging polarimeter based on partially polarized analyzers can still lead to a robust and accurate polarimetric performance, and we also provide the misalignment limits in which an acceptable performance is obtained. We evaluate both the monochromatic and the polychromatic cases with special focus on the latter.

Demosaicking DoFP images using Newton's polynomial interpolation and polarization difference model.

A demand for division of focal plane (DoFP) polarization imaging technology grows rapidly as nanofabrication technologies become mature. For real-time polarization imaging, a DoFP polarimeter often trades off its spatial resolution, which may cause instantaneous field of view (IFoV) errors. To deal with such problems, interpolation methods are often used to fill the missing polarization information. This paper presents an interpolation technique using Newton's polynomial for DoFP polarimeter demosaicking. The interpolation is performed in the polarization difference domain with the interpolation error taken into consideration. The proposed method uses an edge classifier based on polarization difference and a fusion scheme to recover more accurate boundary features. Experiments using both synthetic and real DoFP images in visible and long-wave infrared spectrum demonstrate that the proposed interpolation method outperforms the state-of-the-art techniques quantitatively as well as visually to reduce nonconformities caused by high-frequency energy.

Calibration method for division of focal plane polarimeters.

The division of focal plane polarimeters is fabricated out of nanowire polarization filters attached to the surface of a focal plane array. Due to limitations in the manufacturing process, the transmissivity of each nanowire polarization filter is different, resulting in a combination of inhomogeneities in transmittance and detector response, and which also yields the problem of polarization imaging uniformity, a more complicated method than traditional intensity imaging. This study presents a brand new nonuniformity correction method for solving the correction inaccuracies in the traditional method. The method corrected the polarization filter transmittance with detector gain based on a new method for calculating the degree of linear polarization (DoLP). This paper includes the theoretical derivation, experimental testing, and numerical simulation. In this simulation environment, the DoLP value was accurately calculated.

A 4-Megapixel Cooled CCD Division of Focal Plane Polarimeter for Celestial Imaging

The field of astronomy relies on spectral and polarization imagery recorded across a wide range of spectra to make inferences about imaged objects from nearby and distant galaxies. One of the challenges in recording celestial polarization information is recording multiple images filtered with various polarization optics, such as linear polarization filters or retarders, and with low-noise, low-dark-current sensors. In this paper, we present a division of focal plane polarimeter that can operate at room temperature down to −20 °C. When the imaging sensor operates at −20 °C, the dark currents is reduced by two orders of magnitude, which improves the polarization extinction ratio by ~5-fold. Comprehensive optoelectronic tests are presented with data recorded with the polarimeter.

PCA-based denoising method for division of focal plane polarimeters.

Division of focal plane (DoFP) polarimeters are composed of interlaced linear polarizers overlaid upon a focal plane array sensor. The interpolation is essential to reconstruct polarization information. However, current interpolation methods are based on the unrealistic assumption of noise-free images. Thus, it is advantageous to carry out denoising before interpolation. In this paper, we propose a principle component analysis (PCA) based denoising method, which works directly on DoFP images. Both simulated and real DoFP images are used to evaluate the denoising performance. Experimental results show that the proposed method can effectively suppress noise while preserving edges.

Novel “quasi-Bayer” micro-polarizer patterns for the division-of-focal-plane polarimeters

In this paper, we present two novel “quasi-Bayer” micro-polarizer (MP) patterns for the polarization imaging based on the division-of-focal-plane polarimeters (DoFP). Compared with the traditional equally-weighted MP pattern with four different micro-polarizers, the “quasi-Bayer” pattern requires less photo-lithography-based selective etching steps, leading to a significant reduction of the MP array's fabrication complexity. In addition, for the mainstream bilinear interpolation algorithm, the proposed “quasi-Bayer” pattern with three micro-polarizers exhibits the lowest mean square error (MSE) of 0.43%. Moreover, the “quasi-Bayer” patterns take advantages not only at the fixed illumination level, but also for different illumination levels. Reported experimental results validate the effectiveness of the “quasi-Bayer” patterns by varying the input light intensity from 13lx to 213lx.

Non-uniformity correction for division of focal plane polarimeters with a calibration method.

Division of focal plane polarimeters are composed of nanometer polarization elements overlaid upon a focal plane array (FPA) sensor. The manufacturing flaws of the polarization grating and each detector in the FPA having a different photo response can introduce non-uniformity errors when reconstructing the polarization image without correction. A new calibration method is proposed to mitigate non-uniformity errors in the visible waveband. We correct non-uniformity in the form of a vector. The correction matrix and offset vector are calculated for the following correction. The performance of the proposed method is compared with state-of-the-art techniques by employing simulated data and real scenes. The experimental results showed that the proposed method can effectively mitigate non-uniformity errors and achieve better visual results.

Image interpolation for division of focal plane polarimeters with intensity correlation

Division of focal plane (DoFP) polarimeters operate by integrating micro-polarizer elements with a focal plane. These polarization imaging sensors reduce spatial resolution output and each pixel has a varying instantaneous field of view (IFoV). These drawbacks can be mitigated by applying proper interpolation methods. In this paper, we present a new interpolation method for DoFP polarimeters by using intensity correlation. We employ the correlation of intensity measurements in different orientations to detect edges and then implement interpolation along edges. The performance of the proposed method is compared with several previous methods by using root mean square error (RMSE) comparison and visual comparison. Experimental results showed that our proposed method can achieve better visual effects and a lower RMSE than other methods.

Broadband circular polarizers constructed using helix-like chiral metamaterials.

In this paper, one kind of helix-like chiral metamaterial which can be realized by multiple conventional lithography or electron beam lithographic techniques is proposed to have a broadband bianisotropic optical response analogous to helical metamaterials. On the basis of twisted metamaterials, via tailoring the relative orientation within the lattice, the anisotropy of arcs is converted into magneto-electric coupling of closely spaced arc pairs, which leads to a broad bianisotropic optical response. By connecting the adjacent upper and lower arcs, the coupling of metasurface pairs is transformed into the coupling of the three-dimensional inclusions, and provides a much broader and higher bianisotropic optical response. For only a four-layer helix-like metamaterial, the maximum extinction ratio can reach 19.7. The operation band is in the wavelength range of 4.69 μm to 8.98 μm with an average extinction ratio of 6.9. And the transmittance for selective polarization is above 0.8 in the entire operation band. Such a structure is a promising candidate for integratable and scalable broadband circular polarizers, especially it has great potential to act as a broadband circular micropolarizer in the field of the full-Stokes division of focal plane polarimeters.