Multi-wavelength quantitative polarization and phase microscope

Multispectral polarization microscopy of different stages of human oral tissue: A polarization study

Many optical techniques have been used in various diagnostics and biomedical applications since a decade and polarization imaging is one of the non-invasive and label free optical technique to investigate biological samples making it an important tool in diagnostics, biomedical applications. We report a multispectral polarization-based imaging of oral tissue by utilizing a polarization microscope system with a broadband-light source. Experiments were performed on oral tissue samples and multispectral Stokes mapping was done by recording a set of intensity images. Polarization-based parameters like degree of polarization, angle of fast axis, retardation and linear birefringence have been retrieved. The statistical moments of these polarization components have also been reported at multiples wavelengths. The polarimetric properties of oral tissue at different stages of cancer have been analyzed and significant changes from normal to pre-cancerous lesions to the cancerous are observed in linear birefringence quantification as (1.7 ± 0.1) × 10-3 , (2.5 ± 0.2) × 10-3 and (3.3 ± 0.2) × 10-3 respectively.

Polarimetric measurement of non-depolarizing optical systems

The use of polarization measurements has become more common in recent years, as it gives more information than pure intensity measurements. Polarimetric components such as fixed or variable retarders and polarizers must be included in optical systems to obtain the polarization parameters required, and in many cases the optical system also includes other components such as relay and/or imaging optical systems. In this work we present a simple and robust method for the polarimetric characterization of non-depolarizing polarization components and other optical elements in the system, which does not require a full polarimeter. Since there is no depolarization, we represent the components as pure retarders with diattenuation and find their parameters (transmittance for the polarization components, angle of orientation of the fast axis, and retardance), from which we can retrieve their Mueller matrix. Our results show that the proposed method is accurate when compared with results obtained with a Mueller matrix dual-rotating retarder polarimeter calibrated using the eigenvalue calibration method, considered in this work as the gold standard, and is comparatively easier than the latter to implement, particularly for imaging polarimeters.

Comparison of two configurations of a Stokes polarimeter measuring three wavelengths simultaneously

In this paper, we study two configurations of a full-Stokes polarimeter that measures three wavelengths simultaneously. A recently developed fitting calibration process is used to reduce the error in the final Stokes vector. The first configuration uses an optimized setup for the central wavelength that fixes the non-optimized setups for the other two wavelengths. The second configuration is the result of a search for the best simultaneous setups for all three wavelengths. It is found that this second setup gives smaller errors in the measured Stokes parameters before calibration, but the first configuration gives smaller errors after calibration. These results demonstrate the need for a method of finding the best polarimeter configuration for this type of measurement. It seems clear that the condition number by itself is not a metric that provides sufficient information to determine whether a particular configuration is better by giving smaller errors. It is also important to point out that the results presented here are for the particular calibration scheme used. Additionally, it may be that a different calibration method is required to give better results, but that is outside the scope of the present work.

Visible, near-infrared dual-polarization lidar based on polarization cameras: system design, evaluation and atmospheric measurements

A visible, near-infrared (VIS-NIR) dual-polarization lidar technique employing laser diodes and polarization cameras has been designed and implemented for all-day unattended field measurements of atmospheric aerosols. The linear volume depolarization ratios (LVDR) and the offset angles can be retrieved from four-directional polarized backscattering signals at wavelengths of 458 nm and 808 nm without additional optical components and sophisticated system adjustments. Evaluations on the polarization crosstalk of the polarization camera and the offset angle have been performed in detail. A rotating linear polarizer (RLP) method based on the Stokes-Mueller formalism has been proposed and demonstrated for measuring extinction ratios of the polarization camera, which can be used to eliminate the polarization crosstalk between different polarization signals. The offset angles can be online measured with a precision of 0.1°, leading to negligible measurement errors on the LVDR. One-month statistical analysis revealed a small temporal variation of the offset angles, namely -0.13°±0.07° at 458 nm and 0.33°±0.09° at 808 nm, indicating good system stability for long-term measurement. Atmospheric measurements have been carried out to verify the system performance and investigate aerosol optical properties. The spectral characteristics of the aerosol extinction coefficient, the color ratio, the linear particle polarization ratio (LPDR) and the ratio of LPDR were retrieved and evaluated based on one-month continuous atmospheric measurements, from which different types of aerosols can be classified. The promising results showed great potential of employing the VIS-NIR dual-polarization lidar in characterizing aerosol optical properties, discriminating aerosol types and analyzing long-range aerosol transportation.

Single-Shot Quantitative Polarization Imaging of Complex Birefringent Structure Dynamics

Polarization light microscopes are powerful tools for probing molecular order and orientation in birefringent materials. While a number of polarization microscopy techniques are available to access steady-state properties of birefringent samples, quantitative measurements of the molecular orientation dynamics on the millisecond time scale have remained a challenge. We propose polarized shearing interference microscopy (PSIM), a single-shot quantitative polarization imaging method, for extracting the retardance and orientation angle of the laser beam transmitting through optically anisotropic specimens with complex structures. The measurement accuracy and imaging performance of PSIM are validated by imaging a birefringent resolution target and a bovine tendon specimen. We demonstrate that PSIM can quantify the dynamics of a flowing lyotropic chromonic liquid crystal in a microfluidic channel at an imaging speed of 506 frames per second (only limited by the camera frame rate), with a field-of-view of up to 350 × 350 μm² and a diffraction-limit spatial resolution of ~2 μm. We envision that PSIM will find a broad range of applications in quantitative material characterization under dynamical conditions.

Calibration and data extraction in a Stokes polarimeter employing three wavelengths simultaneously

We present a method for calibration and data extraction for a Stokes polarimeter working with three different wavelengths simultaneously. In the Stokes polarimeter considered in this work, we use two liquid crystal variable retarders (LCVRs) combined with a Glan-Thompson linear polarizer. A recently developed fitting calibration procedure is used. We use the same calibration samples and LCVR voltages for all three wavelengths, giving simultaneous measurement and calibration. We compare the performance of the polarimeter, after calibration, using four or six calibration samples in our experiment. To generate the four known calibration beams, we use a linear polarizer oriented at 130° and 30° with respect to the horizontal, a horizontal linear polarizer followed by a half-wave plate (at 632 nm) with its fast axis at 30°, and a horizontal linear polarizer followed by a quarter-wave plate (at 632 nm) with its fast axis at 30°. For calibration with six reference beams, we add two known calibration beams by setting the fast axis of the half- and quarter-wave plates at 130°. Experimental results show good agreement with the expected results, with the fitting calibration procedure giving an approximately 50% reduction in total RMS error with four calibration samples. There is a negligible reduction in the error when six calibration samples are used compared to the case with four samples.

Unambiguous measurement range and error tolerance in dual-wavelength interferometry

In dual or multiwavelength interferometry, the traditional equivalent wavelength method is widely used for phase recovery to enlarge the unambiguous measurement range (UMR). In fact, however, this method ignores information of size and sign (positive or negative) of single wavelength wrapped phases, and the extension of the UMR is not sufficient. For the reflective measurement, the largest UMR of the dual or multiwavelength interferometry is half of the least-common multiple (LCM) of single wavelengths, called the LCM effective wavelength, which is often several times the equivalent wavelength. But why do we often use the equivalent wavelength and seldom use the larger UMR in practice? Existing research reveals that the actual UMR is related to the measurement error of single-wavelength-wrapped phases, and half of the LCM effective wavelength is only the theoretical value. But how do errors affect the UMR? We think the quantitative analysis and description are lacking. In this paper, we continue to study this problem, analyze it in a graphical method, and give quantitative descriptions. The simulation experiments are carried out and verify our analysis.

Classification of cell morphology with quantitative phase microscopy and machine learning

We describe and compare two machine learning approaches for cell classification based on label-free quantitative phase imaging with transport of intensity equation methods. In one approach, we design a multilevel integrated machine learning classifier including various individual models such as artificial neural network, extreme learning machine and generalized logistic regression. In another approach, we apply a pretrained convolutional neural network using transfer learning for the classification. As a validation, we show the performances of both approaches on classification between macrophages cultured in normal gravity and microgravity with quantitative phase imaging. The multilevel integrated classifier achieves average accuracy 93.1%, which is comparable to the average accuracy 93.5% obtained by convolutional neural network. The presented quantitative phase imaging system with two classification approaches could be helpful to biomedical scientists for easy and accurate cell analysis.

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.

Snapshot phase-shifting diffraction phase microscope

We propose a novel and simple snapshot phase-shifting diffraction phase microscope with a polarization grating and spatial phase-shifting technology. Polarization grating separates the incident beam into left and right circular polarization beams, one of which is used as the reference beam after passing through a pinhole. Four phase-shifted interferograms can be captured simultaneously from the polarization camera to reconstruct the high spatial resolution phase map. The principle is presented in this Letter, and the performance of the proposed system is demonstrated experimentally. Due to the near-common-path configuration and snapshot feature, the proposed system provides a feasible way for real-time quantitative phase measurement with minimal sensitivity to vibration and thermal disturbance.

Pathological crystal imaging with single-shot computational polarized light microscopy

Pathological crystal identification is routinely practiced in rheumatology for diagnosing arthritis disease such as gout, and relies on polarized light microscopy as the gold standard method used by medical professionals. Here, we present a single-shot computational polarized light microscopy method that reconstructs the transmittance, retardance and slow-axis orientation of a birefringent sample using a single image captured with a pixelated-polarizer camera. This method is fast, simple-to-operate and compatible with all the existing standard microscopes without extensive or costly modifications. We demonstrated the success of our method by imaging three different types of crystals found in synovial fluid and reconstructed the birefringence information of these samples using a single image, without being affected by the orientation of individual crystals within the sample field-of-view. We believe this technique will provide improved sensitivity, specificity and speed, all at low cost, for clinical diagnosis of crystals found in synovial fluid and other bodily fluids.