Real-time measurement of the polarization transfer function

Polarization calibration assessment for a broadband imaging polarimeter based on a division of aperture architecture

This article intends to provide all the experimental insights and analyze the best polarimetric calibration method for a division of aperture polarimetric imager considering the different implications it has on the experimental set-up and its performance. Polarimetric cameras require careful calibration for the correct measurement of polarization information. The calibration methods are introduced, intermediate results are presented, and the ability of the set-up to estimate Stokes vectors and Mueller matrices of the samples in passive and active imaging modes is evaluated. Polarization information recovery achieves accuracy errors below the 10% for all polarization modes when the Data Reduction Matrix or the Eigenvalue Calibration Method are used. Such performance, however, degrades significantly when using the Polarizer Calibration Method. To the best of our knowledge, this is the first time such a detailed comparison of calibration methods is presented in the literature, and it is also the first time the Polarizer Calibration Method is applied to a division of aperture polarimeter.

Mid-wave infrared polarization detection of ship targets for small temperature difference conditions

Ship detection under small temperature difference conditions is an important research direction for infrared (IR) detection of typical targets at present. To solve the problems of low contrast and difficult recognition of ship IR imaging due to small temperature differences, the degree of polarization (DOP) images is applied to the field of low-temperature aberration imaging based on the polarization principle. Meanwhile, the misalignment problem caused by the lens jitter in the polarization calculation is solved by the proposed mutual information iterative algorithm. We demonstrate improvement in the target/background local contrast of low-temperature aberration imaging by using the difference in polarization characteristics between the target and the background. The effectiveness of the method was verified by experiments. The results show that the contrast of DOP images combined with multi-angle polarization information is about 30 times higher indoors and three times higher outdoors than IR intensity images. Therefore, the IR polarization detection technique based on DOP images can effectively deal with the problem of low imaging contrast caused by small temperature differences.

One-shot carrier fringe polarimeter in a double-aperture common-path interferometer

This paper presents a novel, simple, robust, and very stable polarimeter which is able to measure in a single-shot the state of polarization (SOP) of an optical field. In this proposal, an optical field with an unknown SOP and an optical field with a reference SOP are superposed by using a double aperture common-path interferometer (DACPI), obtaining two interferograms with adjustable carrier frequency in a single shot. Each interferogram has different visibility and phase-step where the information of the unknown SOP is contained. It is shown that under a typical Fourier analysis any SOP can be successfully measured without the necessity of knowing the carrier frequency. The present proposal is justified mathematically and probed experimentally.

Review of the emerging role of optical polarimetry in characterization of pathological myocardium

Myocardial infarction (MI), a cause of significant morbidity and mortality, is typically followed by microstructural alterations where the necrotic myocardium is steadily replaced with a collagen scar. Engineered remodeling of the fibrotic scar via stem cell regeneration has been shown to improve/restore the myocardium function after MI. Nevertheless, the heterogeneous nature of the scar patch may impair the myocardial electrical integrity, leading to the formation of arrhythmogenesis. Radiofrequency ablation (RFA) offers an effective treatment for focal arrhythmias where local heating generated via electric current at specific spots in the myocardium ablate the arrhythmogenic foci. Characterization of these myocardial pathologies (i.e., infarcted, stem cell regenerated, and RFA-ablated myocardial tissues) is of potential clinical importance. Optical polarimetry, the use of light to map and characterize the polarization signatures of a sample, has emerged as a powerful imaging tool for structural characterization of myocardial tissues, exploiting the underlying highly fibrous tissue nature. This study aims to review the recent progress in optical polarimetry pertaining to the characterization of myocardial pathologies while describing the underlying biological rationales that give rise to the optical imaging contrast in various pathologies of the myocardium. Future possibilities of and challenges to optical polarimetry in cardiac imaging clinics are also discussed.

Mueller-matrix polarimeter using analysis of the nonlinear voltage-retardance relationship for liquid-crystal variable retarders

A method for using liquid-crystal variable retarders (LCVRs) with continually varying voltage to measure the complete Mueller matrix of a general sample is presented. The LCVRs are usually employed with fixed retardance values due to the nonlinear voltage-retardance behavior that they show. For the measurement method presented here, the nonlinear voltage-retardance relationship is first measured, and then a linear fit of the known retardance terms to the detected signal is performed. For a gap of air, the measurement error in the Mueller-matrix polarimeter is estimated at 1%-10%, depending on the Mueller-matrix element. Also, we present experimental results for a Glan-Thompson prism polarizer as a test sample, and we use the measured Mueller parameters as functions of the orientation of the optical axes of the polarizer as an indication of the quality of the polarimeter. In addition, results are compared to a typical step-voltage method to measure the Mueller matrix. Both methods give good results.

Quantitative fluorescence and elastic scattering tissue polarimetry using an Eigenvalue calibrated spectroscopic Mueller matrix system

A novel spectroscopic Mueller matrix system has been developed and explored for both fluorescence and elastic scattering polarimetric measurements from biological tissues. The 4 × 4 Mueller matrix measurement strategy is based on sixteen spectrally resolved (λ = 400 - 800 nm) measurements performed by sequentially generating and analyzing four elliptical polarization states. Eigenvalue calibration of the system ensured high accuracy of Mueller matrix measurement over a broad wavelength range, either for forward or backscattering geometry. The system was explored for quantitative fluorescence and elastic scattering spectroscopic polarimetric studies on normal and precancerous tissue sections from human uterine cervix. The fluorescence spectroscopic Mueller matrices yielded an interesting diattenuation parameter, exhibiting differences between normal and precancerous tissues.

Tissue polarimetry: concepts, challenges, applications, and outlook

Polarimetry has a long and successful history in various forms of clear media. Driven by their biomedical potential, the use of the polarimetric approaches for biological tissue assessment has also recently received considerable attention. Specifically, polarization can be used as an effective tool to discriminate against multiply scattered light (acting as a gating mechanism) in order to enhance contrast and to improve tissue imaging resolution. Moreover, the intrinsic tissue polarimetry characteristics contain a wealth of morphological and functional information of potential biomedical importance. However, in a complex random medium-like tissue, numerous complexities due to multiple scattering and simultaneous occurrences of many scattering and polarization events present formidable challenges both in terms of accurate measurements and in terms of analysis of the tissue polarimetry signal. In order to realize the potential of the polarimetric approaches for tissue imaging and characterization/diagnosis, a number of researchers are thus pursuing innovative solutions to these challenges. In this review paper, we summarize these and other issues pertinent to the polarized light methodologies in tissues. Specifically, we discuss polarized light basics, Stokes-Muller formalism, methods of polarization measurements, polarized light modeling in turbid media, applications to tissue imaging, inverse analysis for polarimetric results quantification, applications to quantitative tissue assessment, etc.

Complete polarization state generator with one variable retarder and its application for fast and sensitive measuring of two-dimensional birefringence distribution

The complete polarization state generator (PSG), which consists of one rotatable polarizer and one variable retarder with a quarter-wave plate, is introduced. The orientation angle of its output polarization ellipse equals half of the retardance of the variable retarder, and the ellipticity angle corresponds to the polarizer azimuth. The PSG is employed in the quantitative orientation-independent differential polarization microscope, which uses polarized light states with the same ellipticity and different orientation angles. Image processing algorithms using three or four frames are described.

Real-time measurement of unique space-variant polarizations

A configuration for real-time measurement of unique, space-variant, polarizations is presented. The experimental results reveal that the full state of polarization at each location within the beam can be accurately obtained every 10 msec, limited only by the camera frame rate. We also present a more compact configuration which can be modified to determine the real-time wavelength variant polarization measurements.

Four-point bisensitivity velocity interferometer with a multireflection etalon

A four-point bisensitivity velocity interferometer system for any reflector (VISAR) with a renovative delay etalon is proposed and demonstrated. In this interferometer, we introduce a new film-coating strategy to accurately measure small velocity with relatively short and cheap etalon. Laser pointing to the etalon is split into two beams with different incident angles with each beam going through the etalon in different path. The beam with the smaller incident angle is reflected three times before it leaves the etalon, while the other beam with larger incident angle goes through the etalon to and forth only once. The delay time of the laser beam with smaller incident angle is almost three times longer than that of the beam with larger incident angle. In the example of the laser with a smaller incident angle, the velocity per fringe of this interferometer can be reduced by approximately three times. The etalon is optimized so that four laser beams can be penetrated in the vertical direction at the meantime. With an etalon of 200 mm in diameter and 150 mm in length, a four-point bisensitivity velocity interferometer can achieve the velocity per fringe of 100 and 350 m/s fringe. A measurement has been successfully undertaken for the steel flyer driven by the explosive where the developed interferometer applies.

Use of polar decomposition for the diagnosis of oral precancer

The Mueller matrix describes all the polarizing properties of a sample and, therefore, the optical differences between noncancerous and precancerous tissue that may be present within the matrix elements. A high-speed polarimetry system that generates 16 (4x4) full Mueller matrices to characterize tissues is presented. Feature extraction is done on the Mueller matrix elements resulting in depolarization and retardance images by polar decomposition. These are used to detect and classify early oral cancers and precancerous changes in epithelium such as dysplasia. These images are compared with orthogonal polarization images and analyzed in an attempt to identity useful factors for the differentiation between cancerous lesions and their benign counterparts. Our results indicate that polarimetry has potential as a method for the in vivo early detection and diagnosis of oral premalignancy.

Calibration method for a photoelastic modulator with a peak retardation of less than a half-wavelength

A new calibration method for a photoelastic modulator is proposed. The calibration includes a coarse calibration and a fine calibration. In the coarse calibration, the peak retardation of the photoelastic modulator is set near 1.841 rad. In the fine calibration, the value of the zeroth Bessel function is obtained. The zeroth Bessel function is approximated as a linear equation to directly calculate the peak retardation. In experiments, the usefulness of the calibration method is verified and the calibration error is less than 0.014 rad. The calibration is immune to the intensity fluctuation of the light source and independent of the circuit parameters. The method specially suits the calibration of a photoelastic modulator with a peak retardation of less than a half-wavelength.

Development and calibration of an automated Mueller matrix polarization imaging system

The high fatality rate associated with the late detection of skin cancer makes early detection crucial in preventing death. The current method for determining if a skin lesion is suspect to cancer is initially based on the patient's and physician's subjective observation of the skin lesion. Physicians use a set of parameters called the ABCD (asymmetry, border, color, diameter) rule to help facilitate diagnosis of potential cancerous lesions. Lesions that are suspicious then require a biopsy, which is a painful, invasive, and a time-consuming procedure. In an attempt to reduce the aforementioned undesirable elements currently associated with skin cancer diagnosis, a novel optical polarization-imaging system is described that has the potential to noninvasively detect cancerous lesions. The described system generates the full 16-element Mueller matrix in less than 70 s. The operation of the system was tested in transmission, specular reflection, and diffuse reflectance modes, using known samples, such as a horizontal linear polarizer, a mirror, and a diffuser plate. In addition, it was also used to image a benign lesion on a human subject. The results of the known samples are in good agreement with their theoretical values with an average accuracy of 97.96% and a standard deviation of 0.0084, using 16 polarization images. The system accuracy was further increased to 99.44% with a standard deviation of 0.005, when 36 images were used to generate the Mueller matrix.