Timothy York

CCD polarization imaging sensor with aluminum nanowire optical filters

We report an imaging sensor capable of recording the optical properties of partially polarized light by monolithically integrating aluminum nanowire optical filters with a CCD imaging array. The imaging sensor, composed of 1000 by 1000 imaging elements with 7.4 μm pixel pitch, is covered with an array of pixel-pitch matched nanowire optical filters with four different orientations offset by 45°. The polarization imaging sensor has a signal-to-noise ratio of 45 dB and captures intensity, angle and degree of linear polarization in the visible spectrum at 40 frames per second with 300 mW of power consumption.

Characterization of a visible spectrum division-of-focal-plane polarimeter

The development of high resolution division-of-focal-plane polarimeters in the visible spectrum allows real-time capture of two chief properties of interest, the degree of linear polarization and the angle of polarization. The accuracy of these two parameters can be influenced by a number of factors in the imaged scene, from the incident intensity and wavelength to the lens used for image capture. The alignment, transmission, and contrast ratios of the pixel matched filters also impact the measured parameters. A system of measurements is presented here that shows how these factors can determine the quality of a division-of-focal-plane polarimeter.

Complementary fluorescence-polarization microscopy using division-of-focal-plane polarization imaging sensor

Abstract. Fluorescence microscopy offers high sensitivity for disease diagnosis. However, little structural information is revealed by this method, requiring another technique to localize the source of fluorescence. We developed a complementary fluorescence-polarization microscope. We used a division-of-focal-plane charge coupled device polarization sensor to enable real-time video rate polarization imaging without any moving parts. The polarization information provided by the microscope enabled detection of structural element and complements the fluorescence information. Application of this multimodal system for cancer imaging using a tumor selective molecular probe revealed the association of diminished structural integrity of tumor tissue with high fluorescence of the imaging agent compared to surrounding normal tissue. This study demonstrates a new paradigm to improve cancer detection and diagnosis.

Bioinspired polarization imaging sensors: from circuits and optics to signal processing algorithms and biomedical applications

In this paper, we present recent work on bioinspired polarization imaging sensors and their applications in biomedicine. In particular, we focus on three different aspects of these sensors. First, we describe the electro-optical challenges in realizing a bioinspired polarization imager, and in particular, we provide a detailed description of a recent low-power complementary metal-oxide-semiconductor (CMOS) polarization imager. Second, we focus on signal processing algorithms tailored for this new class of bioinspired polarization imaging sensors, such as calibration and interpolation. Third, the emergence of these sensors has enabled rapid progress in characterizing polarization signals and environmental parameters in nature, as well as several biomedical areas, such as label-free optical neural recording, dynamic tissue strength analysis, and early diagnosis of flat cancerous lesions in a murine colorectal tumor model. We highlight results obtained from these three areas and discuss future applications for these sensors.

Real-time high-resolution measurement of collagen alignment in dynamically loaded soft tissue

Abstract. A technique for creating maps of the direction and strength of fiber alignment in collagenous soft tissues is presented. The method uses a division of focal plane polarimeter to measure circularly polarized light transmitted through the tissue. The architecture of the sensor allows measurement of the retardance and fiber alignment at the full frame rate of the sensor without any moving optics. The technique compares favorably to the standard method of using a rotating polarizer. How the new technique enables real-time capture of the full angular spread of fiber alignment and retardance under various cyclic loading conditions is illustrated.

Differences in the Microstructural Properties of the Anteromedial and Posterolateral Bundles of the Anterior Cruciate Ligament

Background: Tissue properties of the anteromedial (AM) and posterolateral (PL) bundles of the anterior cruciate ligament (ACL) have not been previously characterized with real-time dynamic testing. The current study used a novel polarized light technique to measure the material and microstructural properties of the ACL. Hypothesis: The AM and PL bundles of the ACL have similar material and microstructural properties. Study Design: Controlled laboratory study. Methods: The AM and PL bundles were isolated from 16 human cadaveric ACLs (11 male, 5 female; average age, 41 years [range, 24-53 years]). Three samples from each bundle were loaded in uniaxial tension, and a custom-built polarized light imaging camera was used to quantify collagen fiber alignment in real time. A bilinear curve fit was applied to the stress-strain data of a quasistatic ramp-to-failure to quantify the moduli in the toe and linear regions. Fiber alignment was quantified at zero strain, the transition point of the bilinear fit, and in the linear portion of the stress-strain curve by computing the degree of linear polarization (DoLP) and angle of polarization (AoP), which are measures of the strength and direction of collagen alignment, respectively. Data were compared using t tests. Results: The AM bundle exhibited significantly larger toe-region (AM 7.2 MPa vs PL 4.2 MPa; P < .001) and linear-region moduli (AM 27.0 MPa vs PL 16.1 MPa; P = .017) compared with the PL bundle. Average DoLP values were similar at low strain but were significantly larger (ie, more uniform alignment) for the AM bundle in the linear region of the stress-strain curve (AM 0.22 vs PL 0.19; P = .036) compared with the PL bundle. The standard deviation AoP values was larger for the PL bundle at both transition (P = .041) and linear-region strain (P = .014), indicating more disperse orientation. Conclusion: Material and microstructural properties of the AM and PL bundles of the ACL differ during loading. The AM bundle possessed higher tissue modulus and failure stress, as well as more uniform fiber alignment under load. Clinical Relevance: These insights into native ligament microstructure can be used to assess graft options for ACL reconstruction and optimize surgical reconstruction techniques.

Trimodal color-fluorescence-polarization endoscopy aided by a tumor selective molecular probe accurately detects flat lesions in colitis-associated cancer

Abstract. Colitis-associated cancer (CAC) arises from premalignant flat lesions of the colon, which are difficult to detect with current endoscopic screening approaches. We have developed a complementary fluorescence and polarization reporting strategy that combines the unique biochemical and physical properties of dysplasia and cancer for real-time detection of these lesions. Using azoxymethane-dextran sodium sulfate (AOM-DSS) treated mice, which recapitulates human CAC and dysplasia, we show that an octapeptide labeled with a near-infrared (NIR) fluorescent dye selectively identified all precancerous and cancerous lesions. A new thermoresponsive sol-gel formulation allowed topical application of the molecular probe during endoscopy. This method yielded high contrast-to-noise ratios (CNR) between adenomatous tumors (20.6±1.65) and flat lesions (12.1±1.03) and surrounding uninvolved colon tissue versus CNR of inflamed tissues (1.62±0.41). Incorporation of nanowire-filtered polarization imaging into NIR fluorescence endoscopy shows a high depolarization contrast in both adenomatous tumors and flat lesions in CAC, reflecting compromised structural integrity of these tissues. Together, the real-time polarization imaging provides real-time validation of suspicious colon tissue highlighted by molecular fluorescence endoscopy.

Calibration method for division of focal plane polarimeters in the optical and near-infrared regime

Advances in nanofabrication allow for the creation of metallic nanowires acting as linear polarizers in the visible and near infrared regime. The monolithic integration of silicon detectors and pixelated nanowire metallic polarization filters allows for an efficient realization of high resolution polarization imaging sensors. These silicon sensors, known as division of focal plane polarimeters, capture polarization information of the imaged environment from ~400nm to 1050nm wavelength. The performance of the polarization sensor can be degraded by both irregularities in the fabrication of the nanowires and possible misalignment errors during the final deposition of the optical nanowire filters on the surface of the imaging sensor. In addition, electronic offsets due to the readout circuitry, electronic crosstalk, and optical crosstalk will also negatively affect the quality of the polarization information. Partial compensation for many of these post-fabrication errors can be accomplished through the use of a camera calibration routine. This paper will describe one such routine, and show how its application can increase the quality of measurements in both the degree of linear polarization and angle of polarization in the visible spectrum. The imaging array of the division of focal plane polarimeter is segmented into two by two blocks of superpixels. The calibration method chooses one of the four pixels as a reference, and then a gain and offset for each of the remaining three is computed based on this reference. The output is a calibration matrix for each pixel in the image array.

Optical characterization of a polarization imager

Polarization is a fundamental property of light for which humans have no innate detector. Numerous sensors have been devised to capture this property of light. Of these types of sensors, division of focal plane polarimeters allow for both high resolution and real-time capture of the angle and degree of polarization. This paper details a formal system of measurements designed to categorize the optical performance of one such sensor. Specifically, this paper details methods for evaluating the accuracy of both the degree and angle of polarization as functions of intensity, wavelength, and camera orientation for a division of focal plane polarimeter.

260 frames-per-second 648x488 resolution division-of-focal-plane polarimeter with structural dynamics and tracking applications

We have designed an image sensor that can capture the first three Stokes parameters at 648 by 488 spatial resolution at 260 frames per second. The sensor consists of a CCD image sensor monolithically integrated with pixel pitch-matched aluminum nanowire polarization filters. The sensor demonstrates a Malus law response over all pixels, and has a relatively uniform diattenuation over the visible spectrum. We demonstrate two potential applications for the sensor. The first uses circular polarization in transmission mode to observe high-speed stress failure in polycarbonate. The second uses polarization in reflected mode to track high speed automobile traffic.