Hybrid spatial-temporal Mueller matrix imaging spectropolarimeter for high throughput plant phenotyping.

Many correlations exist between spectral reflectance or transmission with various phenotypic responses from plants. Of interest to us are metabolic characteristics, namely, how the various polarimetric components of plants may correlate to underlying environmental, metabolic, and genotypic differences among different varieties within a given species, as conducted during large field experimental trials. In this paper, we overview a portable Mueller matrix imaging spectropolarimeter, optimized for field use, by combining a temporal and spatial modulation scheme. Key aspects of the design include minimizing the measurement time while maximizing the signal-to-noise ratio by mitigating systematic error. This was achieved while maintaining an imaging capability across multiple measurement wavelengths, spanning the blue to near-infrared spectral region (405-730 nm). To this end, we present our optimization procedure, simulations, and calibration methods. Validation results, which were taken in redundant and non-redundant measurement configurations, indicated that the polarimeter provides average absolute errors of (5.3±2.2)×10-3 and (7.1±3.1)×10-3, respectively. Finally, we provide preliminary field data (depolarization, retardance, and diattenuation) to establish baselines of barren and non-barren Zea maize hybrids (G90 variety), as captured from various leaf and canopy positions during our summer 2022 field experiments. Results indicate that subtle variations in retardance and diattenuation versus leaf canopy position may be present before they are clearly visible in the spectral transmission.

Practical spectral photography II: snapshot spectral imaging using linear retarders and microgrid polarization cameras.

Despite recent advances, customized multispectral cameras can be challenging or costly to deploy in some use cases. Complexities span electronic synchronization, multi-camera calibration, parallax and spatial co-registration, and data acquisition from multiple cameras, all of which can hamper their ease of use. This paper discusses a generalized procedure for multispectral sensing using a pixelated polarization camera and anisotropic polymer film retarders to create multivariate optical filters. We then describe the calibration procedure, which leverages neural networks to convert measured data into calibrated spectra (intensity versus wavelength). Experimental results are presented for a multivariate and channeled optical filter. Finally, imaging results taken using a red, green, and blue microgrid polarization camera and the channeled optical filter are presented. Imaging experiments indicated that the calculated spectra's root mean square error is highest in the region where the camera's red, green, and blue filter responses overlap. The average error of the spectral reflectance, measured of our spectralon tiles, was 6.5% for wavelengths spanning 425-675 nm. This technique demonstrates that 12 spectral channels can be obtained with a relatively simple and robust optical setup, and at minimal cost beyond the purchase of the camera.

Multistatic fiber-based system for measuring the Mueller matrix bidirectional reflectance distribution function.

Bidirectionality effects can be a significant confounding factor when measuring hyperspectral reflectance data. The bidirectional reflectance distribution function (BRDF) can effectively characterize the reflectivity of surfaces to correct remote sensing measurements. However, measuring BRDFs can be time-consuming, especially when collecting Mueller matrix BRDF (mmBRDF) measurements of a surface via conventional goniometric techniques. In this paper, we present a system for collecting mmBRDF measurements using static optical fiber detectors that sample the hemisphere surrounding an object. The entrance to each fiber contains a polarization state analyzer configuration, allowing for the simultaneous acquisition of the Stokes vector intensity components at many altitudinal and azimuthal viewing positions. We describe the setup, calibration, and data processing used for this system and present its performance as applied to mmBRDF measurements of a ground glass diffuser.

Organic-based photodetectors for multiband spectral imaging.

Using organic photodetectors for multispectral sensing is attractive due to their unique capabilities to tune spectral response, transmittance, and polarization sensitivity. Existing methods lack tandem multicolor detection and exhibit high spectral cross talk. We exploit the polarization sensitivity of organic photodetectors, together with birefringent optical filters to design single-pixel multispectral detectors that achieve high spectral selectivity and good radiometric performance. Two different architectures are explored and optimized, including the Solc-based and multitwist-retarder-based organic photodetectors. Although the former demonstrated a higher spectral resolution, the latter enables a more compact sensor as well as greater flexibility in device fabrication.

Dual-beam potassium Voigt filter for atomic line imaging.

Spectrally narrowband imaging in remote sensing applications can be advantageous for detecting atomic emission features. This is especially useful in detecting specific constituents within rocket plumes, which are challenging to discern from naturally occurring sunglints. In this paper, we demonstrate a dual-beam technique, implemented with a Wollaston prism, for calibrating a Voigt magneto-optical filter for a linear polarizer's finite extinction ratio, as well as optical misalignment between the linear polarizers' transmission axes. Such a strategy would be key towards expanding the filter's field of view while maintaining its classification capabilities. Validation of the potassium Voigt filter is demonstrated using the simulation tool ElecSus in combination with a potassium hollow cathode lamp. RMS error between the filter's temperature response and that of the simulation was approximately 2%. We then demonstrate the detection of a potassium model rocket motor outdoors alongside a sunglint. Results indicate a 20-fold increase in contrast when using our dual-beam calibration strategy.

Optical crosstalk and off-axis modeling of an intrinsic coincident polarimeter.

Polarimeters have broad applications in remote sensing, astronomy, and biomedical imaging to measure the emitted, reflected, or transmitted state of polarization. An intrinsic coincident (IC) full-Stokes polarimeter was previously demonstrated by our group, in a free space configuration, by using stain-aligned polymer-based organic photovoltaics. To minimize the model's complexity, these were tilted to avoid crosstalk from back-reflections. We present a theoretical model of a monolithic IC polarimeter that considers the back-reflection's influence for on-axis light. The model was validated using a monolithic four-detector polarimeter, which achieved an error of less than 3%. Additionally, an off-axis model was produced and validated for a simpler two detector polarimeter, demonstrating an error between the TM and TE polarized components of less than 3% for angles spanning an 18° incidence cone.

Snapshot channeled imaging spectrometer using geometric phase holograms.

In this paper, we present the design and experimental demonstration of a snapshot imaging spectrometer based on channeled imaging spectrometry (CIS) and channeled imaging polarimetry (CIP). Using a geometric phase microlens array (GPMLA) with multiple focal lengths, the proposed spectrometer selects wavelength components within its designed operating waveband of 450-700 nm. Compared to other snapshot spectral imagers, its key components are especially suitable for roll-to-roll (R2R) rapid fabrication, which gives the spectrometer potential for low-cost mass production. The principles and proof-of-concept experimental system of the sensor are described in detail, followed by lab validation and outdoor measurement results which demonstrate the sensor's ability to resolve spectral and spatial contents under both experimental and natural illumination conditions.

Dual-beam cross-correlation spectrometer for radial velocity measurements.

Measuring the radial velocity of an object can be achieved by quantifying the Doppler shift of Fraunhofer lines. Measurements are typically made using high-resolution conventional spectroscopy, in which the Doppler shift is calculated numerically on a computer. An alternative technique includes cross-correlation spectroscopy, which performs an optical correlation of the incident spectrum against a reference spectrum embedded in the instrument. Many existing correlation spectrometers leverage a chrome mask and obtain a single beam measurement, making the sensors more sensitive to atmospheric turbulence without moving parts. In this paper, we present a static dual-beam polarization-based technique for acquiring cross-correlation spectra that is insensitive to atmospheric turbulence and contains no moving parts. The instrument is based on acquiring light both inside and outside of the solar Fraunhofer lines using a twisted nematic liquid-crystal spatial light modulator. Correlation spectra can be calculated as a ratio of these two components. A model of the dual-beam cross-correlation spectrometer is presented and subsequently validated with experimental observations of Venus. Radial velocity accuracies, as calculated against reference ephemerides, yielded an absolute error less than 0.24%.

Microbolometer with a multi-aperture polymer thin-film array for neural-network-basedtarget identification.

Infrared imaging spectrometers are frequently used for detecting chemicals at standoff distances. Cost, size, and sensitivity are common tradeoffs in this regime, particularly when deploying infrared imaging arrays. In this work, we develop and characterize an infrared snapshot computational imaging spectrometer that leverages a multi-aperture filtered design. A theoretical model is developed, describing the multiplexed encoding technique. The experimental system is then described, including filter optimization and fabrication. Finally, the performance of the system is tested, leveraging a neural-network-based calibration approach, for various indoor and outdoor detection scenarios involving liquid contaminants. The results of our testing demonstrate that the system can detect room-temperature liquid contaminants under cold sky downwelling radiance conditions. We achieve a false positive rate (FPR) of 0.12% at a true positive rate (TPR) of 95% for silicon oil on sand at 18°C and a FPR of 2% at a TPR of 95% for silicon oil on various substrates at 23°C. Results support the efficacy of using uncooled polymer absorption filters for infrared imaging liquid contaminant detectors.

Imaging linear and circular polarization features in leaves with complete Mueller matrix polarimetry.

Spectropolarimetry of intact plant leaves allows to probe the molecular architecture of vegetation photosynthesis in a non-invasive and non-destructive way and, as such, can offer a wealth of physiological information. In addition to the molecular signals due to the photosynthetic machinery, the cell structure and its arrangement within a leaf can create and modify polarization signals. Using Mueller matrix polarimetry with rotating retarder modulation, we have visualized spatial variations in polarization in transmission around the chlorophyll a absorbance band from 650 nm to 710 nm. We show linear and circular polarization measurements of maple leaves and cultivated maize leaves and discuss the corresponding Mueller matrices and the Mueller matrix decompositions, which show distinct features in diattenuation, polarizance, retardance and depolarization. Importantly, while normal leaf tissue shows a typical split signal with both a negative and a positive peak in the induced fractional circular polarization and circular dichroism, the signals close to the veins only display a negative band. The results are similar to the negative band as reported earlier for single macrodomains. We discuss the possible role of the chloroplast orientation around the veins as a cause of this phenomenon. Systematic artefacts are ruled out as three independent measurements by different instruments gave similar results. These results provide better insight into circular polarization measurements on whole leaves and options for vegetation remote sensing using circular polarization.

Mueller matrix polarimetry on plasma sprayed thermal barrier coatings for porosity measurement.

Yttria-stabilized zirconia (YSZ) is the most widely used material for thermal plasma sprayed thermal barrier coatings (TBCs) used to protect gas turbine engine parts in demanding operation environments. The superior material properties of YSZ coatings are related to their internal porosity level. By quantifying the porosity level, tighter control on the spraying process can be achieved to produce reliable coatings. Currently, destructive measurement methods are widely used to measure the porosity level. In this paper, we describe a novel nondestructive approach that is applicable to classify the porosity level of plasma sprayed YSZ TBCs via Mueller matrix polarimetry. A rotating retarder Mueller matrix polarimeter was used to measure the polarization properties of the plasma sprayed YSZ coatings with different porosity levels. From these measurements, it was determined that a sample's measured depolarization ratio is dependent on the sample's surface roughness and porosity level. To this end, we correlate the depolarization ratio with the samples' surface roughness, as measured by a contact profilometer, as well as the total porosity level, in percentage measured using a micrograph and stereological analysis. With the use of this technique, a full-field and rapid measurement of porosity level can be achieved.

Neural network calibration of a snapshot birefringent Fourier transform spectrometer with periodic phase errors.

Systematic phase errors in Fourier transform spectroscopy can severely degrade the calculated spectra. Compensation of these errors is typically accomplished using post-processing techniques, such as Fourier deconvolution, linear unmixing, or iterative solvers. This results in increased computational complexity when reconstructing and calibrating many parallel interference patterns. In this paper, we describe a new method of calibrating a Fourier transform spectrometer based on the use of artificial neural networks (ANNs). In this way, it is demonstrated that a simpler and more straightforward reconstruction process can be achieved at the cost of additional calibration equipment. To this end, we provide a theoretical model for general systematic phase errors in a polarization birefringent interferometer. This is followed by a discussion of our experimental setup and a demonstration of our technique, as applied to data with and without phase error. The technique's utility is then supported by comparison to alternative reconstruction techniques using fast Fourier transforms (FFTs) and linear unmixing.

Achromatic Wollaston prism beam splitter using polarization gratings.

We describe a method to achromatize a Wollaston prism beam splitter by combining it with a polarization grating. The advantage of this technique, compared to refractive methods of correction, is that only one type of birefringent crystal is needed. Additionally, the assembly can be made thinner while remaining achromatized. In this Letter, a model for the achromatized grating prism is formulated. Experimental validation is conducted by achromatizing a calcite Wollaston prism (apex angle of 5.35°) using a polarization grating with a spatial period of 253 µm. We found that the primary dispersion was reduced by approximately 6.5 times for wavelengths spanning the conventional F, d, and C Fraunhofer lines (486 to 656 nm).

Spatially heterodyned snapshot imaging spectrometer.

Snapshot hyperspectral imaging Fourier transform (SHIFT) spectrometers are a promising technology in optical detection and target identification. For any imaging spectrometer, spatial, spectral, and temporal resolution, along with form factor, power consumption, and computational complexity are often the design considerations for a desired application. Motivated by the need for high spectral resolution systems, capable of real-time implementation, we demonstrate improvements to the spectral resolution and computation trade-space. In this paper, we discuss the implementation of spatial heterodyning, using polarization gratings, to improve the spectral resolution trade space of a SHIFT spectrometer. Additionally, we employ neural networks to reduce the computational complexity required for data reduction, as appropriate for real-time imaging applications. Ultimately, with this method we demonstrate an 87% decrease in processing steps when compared to Fourier techniques. Additionally, we show an 80% reduction in spectral reconstruction error and a 30% increase in spatial fidelity when compared to linear operator techniques.

Athermalized channeled spectropolarimetry using a biaxial potassium titanyl phosphate crystal.

Channeled spectropolarimeters measure the polarization state of light as a function of wavelength. Typically, a channeled spectropolarimeter uses high-order retarders made of uniaxial crystal to amplitude modulate the measured spectrum with the Stokes polarization information. A primary limitation of these instruments is the thermal variability of the retarders, which necessitates frequent system recalibration. Past work has addressed this issue by implementing an athermalized retarder produced from two uniaxial crystals. However, reducing the complexity of an athermalized retarder is advantageous for minimizing size and weight requirements. In this Letter, a technique for producing a thermally stable channeled spectropolarimeter using biaxial retarders is presented. This technique preserves a constant phase over an appreciable temperature range. Proof-of-concept results from a KTP-based athermal partial channeled spectropolarimeter are presented from 500 to 750 nm for temperature changes up to 26°C. Spectropolarimetric reconstructions produced from this system vary by <=2.6% RMS when the retarder experiences a 13°C increase in temperature above 21°C ambient, <=5.2% for a 20°C increase, and <=6.7% for a 26°C increase.

Compact real-time birefringent imaging spectrometer.

The design and experimental demonstration of a snapshot hyperspectral imaging Fourier transform (SHIFT) spectrometer is presented. The sensor, which is based on a multiple-image FTS (MFTS), offers significant advantages over previous implementations using Michelson interferometers. Specifically, its use of birefringent interferometry creates a vibration insensitive and ultra-compact (15x15x10 mm(3)) common-path interferometer while offering rapid reconstruction rates through the graphics processing unit. The SHIFT spectrometer's theory and experimental prototype are described in detail. Included are reconstruction and spectral calibration procedures, followed by the spectrometer's validation using measurements of gas-discharge lamps. Lastly, outdoor measurements demonstrate the sensor's ability to resolve spectral signatures in typical outdoor lighting and environmental conditions.

Spatial heterodyne interferometry with polarization gratings.

The implementation of a polarization-based spatial heterodyne interferometer (SHI) is described. While a conventional SHI uses a Michelson interferometer and diffraction gratings, our SHI exploits mechanically robust Wollaston prisms and polarization gratings. A theoretical model for the polarization SHI is provided and validated with data from our proof of concept experiments. This device is expected to provide a compact monolithic sensor for subangstrom resolution spectroscopy in remote sensing, biomedical imaging, and machine vision applications.

Snapshot imaging Mueller matrix polarimeter using polarization gratings.

A snapshot imaging Mueller matrix polarimeter (SIMMP) is theoretically described and empirically demonstrated through simulation. Spatial polarization fringes are localized onto a sample by incorporating polarization gratings (PGs) into a polarization generator module. These fringes modulate the Mueller matrix (MM) components of the sample, which are subsequently isolated with PGs in an analyzer module. The MM components are amplitude modulated onto spatial carrier frequencies which, due to the PGs, maintain high visibility in spectrally broadband illumination. An interference model of the SIMMP is provided, followed by methods of reconstruction and calibration. Lastly, a numerical simulation is used to demonstrate the system's performance in the presence of noise.

Flexible sensor patch for continuous carbon dioxide monitoring.

Monitoring and measurement of carbon dioxide (CO2) is critical for many fields. The gold standard CO2 sensor, the Severinghaus electrode, has remained unchanged for decades. In recent years, many other CO2 sensor formats, such as detection based upon pH-sensitive dyes, have been demonstrated, opening the door for relatively simple optical detection schemes. However, a majority of these optochemical sensors require complex sensor preparation steps and are difficult to control and repeatably execute. Here, we report a facile CO2 sensor generation method that suffers from none of the typical fabrication issues. The method described here utilizes polydimethylsiloxane (PDMS) as the flexible sensor matrix and 1-hydroxypyrene-3,6,8-trisulfonate (HPTS), a pH-sensitive dye, as the sensing material. HPTS, a base (NaOH), and glycerol are loaded as dense droplets into a thin PDMS layer which is subsequently cured around the droplet. The fabrication process does not require prior knowledge in chemistry or device fabrication and can be completed as quickly as PDMS cures (∼2 h). We demonstrate the application of this thin-patch sensor for in-line CO2 quantification in cell culture media. To this end, we optimized the sensing composition and quantified CO2 in the range of 0-20 kPa. A standard curve was generated with high fidelity (R 2 = 0.998) along with an analytical resolution of 0.5 kPa (3.7 mm Hg). Additionally, the sensor is fully autoclavable for applications requiring sterility and has a long working lifetime. This flexible, simple-to-manufacture sensor has a myriad of potential applications and represents a new, straightforward means for optical carbon dioxide measurement.

White-light channeled imaging polarimeter using broadband polarization gratings.

A white-light snapshot channeled linear imaging (CLI) polarimeter is demonstrated by utilizing polarization gratings (PGs). The CLI polarimeter is capable of measuring the two-dimensional distribution of the linear Stokes polarization parameters by incorporating two identical PGs, in series, along the optical axis. In this configuration, the general optical shearing functionality of a uniaxial crystal-based Savart plate is realized. However, unlike a Savart plate, the diffractive nature of the PGs creates a linear dependence of the shear versus wavelength, thus providing broadband functionality. Consequently, by incorporating the PG-based Savart plate into a Savart plate channeled imaging polarimeter, white-light interference fringes can be generated. This enables polarimetric image data to be acquired at shorter exposure times in daylight conditions, making it more appealing over the quasi-monochromatic channeled imaging polarimeters previously described in the literature. Furthermore, the PG-based device offers significantly more compactness, field of view, optical simplicity, and vibration insensitivity than previously described white-light CLI polarimeters based on Sagnac interferometers. Included in this paper are theoretical descriptions of the linear (S(0), S(1), and S(2)) and complete (S(0), S(1), S(2), and S(3)) channeled Stokes imaging polarimeters. Additionally, descriptions of our calibration procedures and our experimental proof of concept CLI system are provided. These are followed by laboratory and outdoor polarimetric measurements of S(0), S(1), and S(2).