Snapshot linear-Stokes imaging spectropolarimeter using division-of-focal-plane polarimetry and integral field spectroscopy

The Application of Supervised Machine Learning Algorithms for Image Alignment in Multi-Channel Imaging Systems

This study presents a method for aligning the geometric parameters of images in multi-channel imaging systems based on the application of pre-processing methods, machine learning algorithms, and a calibration setup using an array of orderly markers at the nodes of an imaginary grid. According to the proposed method, one channel of the system is used as a reference. The images from the calibration setup in each channel determine the coordinates of the markers, and the displacements of the marker centers in the system's channels relative to the coordinates of the centers in the reference channel are then determined. Correction models are obtained as multiple polynomial regression models based on these displacements. These correction models align the geometric parameters of the images in the system channels before they are used in the calculations. The models are derived once, allowing for geometric calibration of the imaging system. The developed method is applied to align the images in the channels of a module of a multispectral imaging polarimeter. As a result, the standard image alignment error in the polarimeter channels is reduced from 4.8 to 0.5 pixels.

Compact single-shot multispectral polarization imager through joint spectral-polarization encoding

The technique of spectral polarization imaging (SPI) is a potent detection tool in various fields due to its ability to capture multi-dimensional information. However, existing SPI systems usually face challenges associated with architectural complexity and computational requirements, rendering them unsuitable for handheld, on-board, and real-time applications. Consequently, a compact single-shot multispectral polarization imager (CSMPI) is proposed, which employs a combined spectral-polarization encoding strategy to address the aforementioned issues. It incorporates a coded aperture for encoding multiple spectral channels together with linear polarization into a single measurement, enabling the simultaneous detection of up to nine light components with just one exposure. The resulting prototype consists solely of a color polarization detector and an imaging lens inserted with the small and easily fabricable coded aperture, which features compact dimensions of Φ5.5 cm × 21.5 cm and a light weight of approximately 670 g. This is particularly advantageous for application areas that require system miniaturization and rapid multi-dimensional detection.

Bio-inspired snapshot polarization-hyperspectral camera

The mantis shrimp is recognized to have one of the most powerful vision systems in nature, with up to 16 color-perceiving channels and the perception of linear and circular polarization detection. Inspired by its biostructure, we developed a snapshot polarization-hyperspectral camera (pHScam) to detect linear polarization in four directions and spectral signature in 21 bands of incident light, resulting in a 4D polar-spectral hypercube, represented as H(x,y,l,S→). We introduced two bio-mimetic encoding mechanisms, viz., two-tier color encoding and pixelwise mosaic polarization encoding, to efficiently compress the original 4D datacube into a 2D mosaic raw pattern. The full-resolution recovery is solved by convex optimization algorithm, utilizing known prior sparsity and experimental system calibrations.

Passive Polarized Vision for Autonomous Vehicles: A Review

This review article aims to address common research questions in passive polarized vision for robotics. What kind of polarization sensing can we embed into robots? Can we find our geolocation and true north heading by detecting light scattering from the sky as animals do? How should polarization images be related to the physical properties of reflecting surfaces in the context of scene understanding? This review article is divided into three main sections to address these questions, as well as to assist roboticists in identifying future directions in passive polarized vision for robotics. After an introduction, three key interconnected areas will be covered in the following sections: embedded polarization imaging; polarized vision for robotics navigation; and polarized vision for scene understanding. We will then discuss how polarized vision, a type of vision commonly used in the animal kingdom, should be implemented in robotics; this type of vision has not yet been exploited in robotics service. Passive polarized vision could be a supplemental perceptive modality of localization techniques to complement and reinforce more conventional ones.

Joint constraints of guided filtering based confidence and nonlocal sparse tensor for color polarization super-resolution imaging

This paper introduces a camera-array-based super-resolution color polarization imaging system designed to simultaneously capture color and polarization information of a scene in a single shot. Existing snapshot color polarization imaging has a complex structure and limited generalizability, which are overcome by the proposed system. In addition, a novel reconstruction algorithm is designed to exploit the complementarity and correlation between the twelve channels in acquired color polarization images for simultaneous super-resolution (SR) imaging and denoising. We propose a confidence-guided SR reconstruction algorithm based on guided filtering to enhance the constraint capability of the observed data. Additionally, by introducing adaptive parameters, we effectively balance the data fidelity constraint and the regularization constraint of nonlocal sparse tensor. Simulations were conducted to compare the proposed system with a color polarization camera. The results show that color polarization images generated by the proposed system and algorithm outperform those obtained from the color polarization camera and the state-of-the-art color polarization demosaicking algorithms. Moreover, the proposed algorithm also outperforms state-of-the-art SR algorithms based on deep learning. To evaluate the applicability of the proposed imaging system and reconstruction algorithm in practice, a prototype was constructed for color polarization image acquisition. Compared with conventional acquisition, the proposed solution demonstrates a significant improvement in the reconstructed color polarization images.

White-light channeled imaging polarimeter using Savart plates and a polarization Sagnac interferometer

A Stokes white-light channeled imaging polarimeter using Savart plates and a polarization Sagnac interferometer (IPSPPSI) is presented, which provides an effective solution to the problem of channel aliasing in broadband polarimeters. The expression for the light intensity distribution and a method to reconstruct polarization information are derived, and an example design for an IPSPPSI is given. The results reveal that a complete measurement of the Stokes parameters in broad band can be achieved with a snapshot on a single detector. The use of dispersive elements like gratings suppresses broadband carrier frequency dispersion so the channels in the frequency domain do not affect each other, ensuring the integrity of information coupled across the channels. Furthermore, the IPSPPSI has a compact structure and does not employ moving parts or require image registration. It shows great application potential in remote sensing, biological detection, and other fields.

Compressive space-dimensional dual-coded hyperspectral polarimeter (CSDHP) and interactive design method

A compressive space-dimensional dual-coded hyperspectral polarimeter (CSDHP) and interactive design method are introduced. A digital micromirror device (DMD), a micro polarizer array detector (MPA), and a prism grating prism (PGP) are combined to achieve single-shot hyperspectral polarization imaging. The longitudinal chromatic aberration (LCA) and spectral smile of the system are both eliminated to guarantee the matching accuracy of DMD and MPA pixels. A 4D data cube with 100 channels and 3 Stocks parameters is reconstructed in the experiment. The feasibility and fidelity are verified from the image and spectral reconstruction evaluations. It is demonstrated that the target material can be distinguished by CSDHP.

Four-quadrant retarder array imaging spectropolarimeter for the full Stokes vector spectrum

Aiming at the major demand for polarization information gap in earth observation and space exploration, we proposed a four-quadrant retarder array imaging spectropolarimeter (FQRAISP) in view of the existing technical problem of the spectral resolution degradation along with spectral aliasing crosstalk. The optical schematic diagram of the FQRAISP together with its interference model was conceptually described, and the effectiveness of the scheme was validated through the experimental simulation, which demonstrated the competitive efficiency and accuracy in the proposed FQRAISP. The FQRAISP could restore the incident Stokes vector spectrum without any errors, and the inversion accuracy was increased by seven times, avoiding the spectrum aliasing and channel filtering in the channel modulation. In order to evaluate the influences of the alignment deviation of four-partition phase retarder component, together with its thickness deviation on the reconstructed Stokes parameters, the numerical simulations were carried out, and the results showed that the alignment deviations had a relatively weak effect on the reconstructed Stokes spectra, while the thickness deviations had an obvious influence. Therefore, the alignment deviations controlled in a range of [-0.43^∘,+0.43^∘] and [-0.22^∘, + 0.22^∘] together with the thickness deviations in a range of [ - 0.03µm, + 0.03µm] were an optimal choice for the engineering implementation of the FQRAISP. This research provided a novel method for the hardware realization of the accurate acquisition of all-optical information, having broad application prospects in remote sensing (deep space exploration), biomedicine and other fields.

Division of focal plane red-green-blue full-Stokes imaging polarimeter

We calibrate and test a division-of-focal-plane red-green-blue (RGB) full-Stokes imaging polarimeter in a variety of indoor and outdoor environments. The polarimeter, acting as a polarization camera, utilizes a low dispersion microretarder array on top of a sensor with Bayer filters and wire-grid linear polarizers. We also present the design and fabrication of the microretarder array and the assembly of the camera and validate the performance of the camera by taking multiple RGB full-Stokes images and videos. Our camera has a small form factor due to its single-sensor design and the unique capability to measure the intensity, color, and polarization of an optical field in a single shot.

Optimized design, calibration, and validation of an achromatic snapshot full-Stokes imaging polarimeter

An achromatic snapshot full-Stokes imaging polarimeter (ASSIP) that enables the acquisition of 2D-spatial full Stokes parameters from a single exposure is presented. It is based on the division-of-aperture polarimetry using an array of four-quadrant achromatic elliptical analyzers as polarization state analyzer (PSA). The optimization of PSA is addressed for achieving immunity of Gaussian and Poisson noises. An extended eigenvalue calibration method (ECM) is proposed to calibrate the system, which considers the imperfectness of retarder and polarizer samples and the intensity attenuation of polarizer sample. A compact prototype of ASSIP operating over the waveband of 450-650 nm and an optimized calibration setup are developed. The achromatic performance is evaluated at three bandwidths of 10, 25, and 200 nm, respectively. The results show that the prototype with an uncooled CMOS camera works well at each bandwidth. The instrument matrix determined at the narrower bandwidth is more applicable to the wider one. The uncertainties of the calibrated instrument matrices and reconstructed Stokes parameters are improved by using the extended EMC at each bandwidth. To speed up the acquisition of high-contrast images, wide bandwidth along with short exposure time is preferable. The snapshot capability was verified via capturing dynamic scenes.

Accurate measurement for damage evolution of ceramics caused by nanosecond laser pulses with polarization spectrum imaging

Due to the interference of excitation lights and the perturbation of spattered particles, it is very difficult to detect the real-time evolution of ceramics damaged by pulsed laser. In this paper, a metrology "on-line detection of damage identification via the polarization spectrum imaging" is proposed to realize the real-time observation for damage evolution of ceramic composite irradiated by the laser. In this metrology, the detection principle is based on a mathematical model of polarization bidirectional reflectance distribution function. According to the Stokes vector analysis method, the damage law of the material surface under the continuous activating illuminations of multiple laser pulses and the increase of pulse energy is theoretically deduced and analyzed first, then the measured polarization spectra are compared with the microscopic imaging method to extract the edge texture information, and further the damage details are characterized with this metrology under the typical polarization parameters: I, Q, U, V, DOP and AOP. As a result, the damaging degree of ceramic composite irradiated by the 1064nm nanosecond pulsed laser, which is changed from the pulse power of 155.54 mJ and 14 pulses to 217.94 mJ and 1 pulse, can be identified with a series of polarization parameters in the different polarization spectrum images. These polarization parameters and their derived results reflect the physical and chemical evolutive properties including of texture orientation of the target surface that is different from other methods of damage detection. Finally, it can be concluded that this paper provides a new method for real-time detection of laser damage and lays a foundation for detection and identification under other strong light interference.

Light-guide snapshot imaging spectrometer for remote sensing applications

A fiber-based snapshot imaging spectrometer was developed with a maximum of 31853 (~188 x 170) spatial sampling and 61 spectral channels in the 450nm-750nm range. A compact, custom-fabricated fiber bundle was used to sample the object image at the input and create void spaces between rows at the output for dispersion. The bundle was built using multicore 6x6 fiber block ribbons. To avoid overlap between the cores in the direction of dispersion, we selected a subset of cores using two alternative approaches; a lenslet array and a photomask. To calibrate the >30000 spatial samples of the system, a rapid spatial calibration method was developed based on phase-shifting interferometry (PSI). System crosstalk and spectral resolution were also characterized. Preliminary hyperspectral imaging results of the Rice University campus landscape, obtained with the spectrometer, are presented to demonstrate the system's spectral imaging capability for distant scenes. The spectrum of different plant species with different health conditions, obtained with the spectrometer, was in accordance with reference instrument measurements. We also imaged Houston traffic to demonstrate the system's snapshot hyperspectral imaging capability. Potential applications of the system include terrestrial monitoring, land use, air pollution, water resources, and lightning spectroscopy. The fiber-based system design potentially allows tuning between spatial and spectral sampling to meet specific imaging requirements.

Chromatic confocal microscopy using liquid crystal display panels

A chromatic confocal microscopy system in combination with two liquid crystal display (LCD) panels is proposed and demonstrated for surface profiling. The major advantage of this system is no mechanical translation is needed for three-dimensional (3D) imaging. Axial scanning is realized thanks to the chromatic aberration in the objective, whereas lateral scanning is achieved by turning on different pixels on LCDs. Chromatic aberration of the objective lens is used to provide wavelength-to-depth coding, and decoding is realized by using a dispersion prism. System performance is validated with a 50 μm step standard and the capability of 3D imaging is demonstrated with an onion epidermis.

Compact snapshot optically replicating and remapping imaging spectrometer (ORRIS) using a focal plane continuous variable filter

In this Letter, a novel snapshot spectral imaging technique, optically replicating and remapping imaging spectrometer, is presented. It is based on the combination of shifting subimages by a specially designed lenslet array (LA) and filtering subimages by a focal plane continuous variable filter (CVF). The 3D datacube is recovered by just using a simple image remapping process. The use of the LA and the focal plane CVF makes the system compact and low in cost. A handheld proof-of-principle prototype has been built and demonstrated; it covers a wavelength range of 380-860 nm with 80 spectral channels with a spatial resolution of 400×400  pixels.

Full-Stokes temporal imaging

We developed a full-Stokes temporal imaging system which measures the Stokes vector of ultrafast signals as a function of time. The system is based on a time-lens array where each time-lens in the array projects the signal on a different state of polarization.

Methods of polarimetric calibration and reconstruction for a fieldable channeled dispersive imaging spectropolarimeter

Polarimetric calibration and reconstruction methods for a fieldable channeled dispersive imaging spectropolarimeter (CDISP) are presented. A theoretical model for the polarimetric calibration is derived first. In the polarimetric calibration for the CDISP, the alignment errors of the polarimetric spectral intensity modulation module, and the polarization effects of the optical system and phase factors of the high-order retarders at different viewing angles, are considered and determined independently. Based on the results of the polarimetric calibration, the Stokes vector of the target is reconstructed through the derived reconstruction model. Simulation results with a fieldable CDISP designed for airborne remote sensing indicate that by using the presented polarimetric calibration and reconstruction methods, the measurement accuracy at each viewing angle of the fieldable CDISP can be improved. Experimental results are summarized and analyzed to demonstrate the effectiveness of the presented methods.