1,720,969 research outputs found
Three-wavelength SPAD-based photoplethysmography
No full textContinuous and real-time monitoring of cardiorespiratory signals by portable and accurate instrumentation is very important for the early diagnosis of cardiovascular diseases. We aim to present a novel photoplethysmography device to assess changes in blood oxygen saturation and beat-by-beat pulse waves of finger blood volumes not affected by possibly occurring variations in oxygen saturation. For this purpose, our device works at three light wavelengths simultaneously and is based on a Single-Photon Avalanche Diode to evaluate the feasibility of using this technology in contact photoplethysmography. Our preliminary validation shows that the device is robust against movement artifacts and provides measures that reflect the physiological cardiorespiratory adaptations to the Valsalva maneuver, suggesting its overall reliability and possible use in cardiovascular monitoring
Multi-wavelength SPAD photoplethysmography for cardio-respiratory monitoring
Full text linkThere is a growing interest in photoplethysmography (PPG) for the continuous monitoring of cardio-respiratory signals by portable instrumentation aimed at the early diagnosis of cardiovascular diseases. In this context, it is conceivable that PPG sensors working at different wavelengths simultaneously can optimize the identification of apneas and the quantification of the associated heart-rate changes or other parameters that depend on the PPG shape (e.g., systematic vascular resistance and pressure), when evaluating the severity of breathing disorders during sleep and in general for health monitoring. Therefore, the objective of this work is to present a novel pulse oximeter that provides synchronous data logging related to three light wavelengths (green, red, and infrared) in transmission mode to optimize both heart rate measurements and a reliable and continuous assessment of oxygen saturation. The transmission mode is considered more robust over motion artifacts than reflection mode, but current pulse oximeters cannot employ green light in transmission mode due to the high absorbance of body tissues at this wavelength. For this reason, our device is based on a Single-Photon Avalanche Diode (SPAD) with very short deadtime (less than 1 ns) to have, at the same time, the single photon sensitivity and high-count rate that allows acquiring all the wavelengths of interest on the same site and in transmission mode. Previous studies have shown that SPAD cameras can be used for measuring the heart rate through remote PPG, but oxygen saturation and heart-rate measures through contact SPAD-based PPG sensors have never been addressed so far. The results of the preliminary validation on six healthy volunteers reflect the expected physiological phenomena, providing rms errors in the Inter Beat Interval estimation smaller than 70 ms (with green light) and a maximum error in the oxygen saturation smaller than 1% during the apneas. Our prototype demonstrates the reliability of SPAD-based devices for continuous long-term monitoring of cardio-respiratory variables as an alternative to photodiodes especially when minimal area and optical power are required
Multi-Channel FPGA Time-to-Digital Converter With 10 ps Bin and 40 ps FWHM
Full text linkWe present a novel architecture for multi-channel time-to-digital converters (TDCs) to be implemented into low-cost field-programmable gate arrays (FPGAs), achieving 10-ps least significant bit (LSB), 164- full-scale range, and good linearity both in terms of differential nonlinearity (DNL) and integral nonlinearity (INL). The conceived architecture is based on the carry chain delay line model and wave union A method: the positions of both rising and falling edges that propagate in multiple parallel carry chains are recorded each time there is an HIT input. This technique effectively subdivides the ultrawide bins improving the measurement precision and, combined with the sliding-scale technique and continuous code density calibration, improves the TDC linearity. Employing the proposed architecture, we have implemented in a Xilinx Artix-7 FPGA a TDC with 20 timestamp units and validated the device in a time-correlated single photon counting (TCSPC) setup, when connected to an array chip with single-photon avalanche diodes (SPADs)
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
SPAD Pixel with Sub-NS Dead-Time for High-Count Rate Applications
Full text linkSingle-photon avalanche diode (SPAD) exploitation in high-flux applications is often hindered by the trade-off between the SPAD dead-time and afterpulsing probability. In this paper, we present the architecture and the experimental characterization of two chips including a novel SPAD sensing, and readout scheme designed to minimize dead-time (1.78 ns and 0.93 ns respectively) and afterpulsing probability (0.14% maximum). We have coupled this architecture with high-performance SPADs obtaining an extremely stable dead-time (6.44 psrms jitter) that can be easily regulated through an external voltage. Thanks to its compact size, this novel pixel architecture can be easily integrated within high-resolution SPAD arrays for GHz applications
Multipass wide-field phase imager
Full text linkAdvances in optical imaging always look for an increase in sensitivity and resolution among other practicability aspects. Within the same scope, in this work we report a versatile interference contrast imaging technique, with high phase sensitivity and a large field-of-view of several mm2. Sensitivity is increased through the use of a self-imaging non-resonant cavity, which causes photons to probe the sample in multiple rounds before being detected, where the configuration can be transmissive or reflective. Phase profiles can be resolved individually for each round thanks to a specially designed single-photon camera with time-of-flight capabilities and true pixels-off gating. Measurement noise is reduced by novel data processing combining the retrieved sample profiles from multiple rounds. Our protocol is especially useful under extremely low light conditions as required by biological or photo-sensitive samples. Results demonstrate more than a four-fold reduction in phase measurement noise, compared to single round imaging, and values close to the predicted sensitivity in case of the best possible cavity configuration, where all photons are maintained until n rounds. We also find good agreement with the theoretical predictions for low number of rounds, where experimental imperfections would play a minor role. The absence of a laser or cavity lock-in mechanism makes the technique an easy to use inspection tool
Spatially Resolved Event-Driven 24 × 24 Pixels SPAD Imager with 100% Duty Cycle for Low Optical Power Quantum Entanglement Detection
Full text linkQuantum microscopy requires efficient detectors able to identify temporal correlations among photons. Photon coincidences are usually detected by postprocessing their timestamps measured by means of time-To-digital converters (TDCs), through a time and power-consuming procedure, which impairs the overall system performance. In this article, we propose an innovative single-photon sensitive imager based on single-photon avalanche diodes (SPADs), able to signal coincident photon pairs along with their position through a TDC-free, event-driven architecture. The result is a highly efficient detector (25.8%) with a 100% duty cycle and minimized data throughput. The modular architecture and the 330 ns readout time, independent of pixel number, pave the way to large format imagers based on the same paradigm. The detector enabled quantum imaging at extremely low, microwatt-level optical pump powers, four orders of magnitude lower than previous experiments with similar optical setups
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Non-line-of-sight snapshots and background mapping with an active corner camera
Full text linkThe ability to form reconstructions beyond line-of-sight view could be transformative in a variety of fields, including search and rescue, autonomous vehicle navigation, and reconnaissance. Most existing active non-line-of-sight (NLOS) imaging methods use data collection steps in which a pulsed laser is directed at several points on a relay surface, one at a time. The prevailing approaches include raster scanning of a rectangular grid on a vertical wall opposite the volume of interest to generate a collection of confocal measurements. These and a recent method that uses a horizontal relay surface are inherently limited by the need for laser scanning. Methods that avoid laser scanning to operate in a snapshot mode are limited to treating the hidden scene of interest as one or two point targets. In this work, based on more complete optical response modeling yet still without multiple illumination positions, we demonstrate accurate reconstructions of foreground objects while also introducing the capability of mapping the stationary scenery behind moving objects. The ability to count, localize, and characterize the sizes of hidden objects, combined with mapping of the stationary hidden scene, could greatly improve indoor situational awareness in a variety of applications
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