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The detection of impact and depth defects in Glass Fiber Reinforced Polymer (GFRP) composites has been extensively studied to develop effective, reliable, and cost-efficient assessment methods through various Non-Destructive Testing (NDT) techniques. Challenges in detecting these defects arise from varying responses based on the geometrical shape, thickness, and defect types. Long Pulse Thermography (LPT), utilizing an uncooled microbolometer and a low-resolution infrared (IR) camera, presents a promising solution for detecting both depth and impact defects in GFRP materials with a single setup and minimal tools at an economical cost. Despite its potential, the application of LPT has been limited due to susceptibility to noise from environmental radiation and reflections, leading to blurry images. This study focuses on optimizing LPT parameters to achieve accurate defect detection. Specifically, we investigated 11 flat-bottom hole (FBH) depth defects and impact defects ranging from 8 J to 15 J in GFRP materials. The key parameters examined include the environmental temperature, background reflection, background color reflection, and surface emissivity. Additionally, we employed image processing techniques to classify composite defects and automatically highlight defective areas. The Tanimoto Criterion (TC) was used to evaluate the accuracy of LPT both for raw images and post-processed images. The results demonstrate that through parameter optimization, the depth defects in GFRP materials were successfully detected. The TC success rate reached 0.91 for detecting FBH depth defects in raw images, which improved significantly after post-processing using Canny edge detection and Hough circle detection algorithms. This study underscores the potential of optimized LPT as a cost-effective and reliable method for detecting defects in GFRP composites. [ABSTRACT FROM AUTHOR]

The detection of impact and depth defects in Glass Fiber Reinforced Polymer (GFRP) composites has been extensively studied to develop effective, reliable, and cost-efficient assessment methods through various Non-Destructive Testing (NDT) techniques. Challenges in detecting these defects arise from varying responses based on the geometrical shape, thickness, and defect types. Long Pulse Thermography (LPT), utilizing an uncooled microbolometer and a low-resolution infrared (IR) camera, presents a promising solution for detecting both depth and impact defects in GFRP materials with a single setup and minimal tools at an economical cost. Despite its potential, the application of LPT has been limited due to susceptibility to noise from environmental radiation and reflections, leading to blurry images. This study focuses on optimizing LPT parameters to achieve accurate defect detection. Specifically, we investigated 11 flat-bottom hole (FBH) depth defects and impact defects ranging from 8 J to 15 J in GFRP materials. The key parameters examined include the environmental temperature, background reflection, background color reflection, and surface emissivity. Additionally, we employed image processing techniques to classify composite defects and automatically highlight defective areas. The Tanimoto Criterion (TC) was used to evaluate the accuracy of LPT both for raw images and post-processed images. The results demonstrate that through parameter optimization, the depth defects in GFRP materials were successfully detected. The TC success rate reached 0.91 for detecting FBH depth defects in raw images, which improved significantly after post-processing using Canny edge detection and Hough circle detection algorithms. This study underscores the potential of optimized LPT as a cost-effective and reliable method for detecting defects in GFRP composites.

Uncooled bolometric type thermal detectors, combined into a matrix and placed into a focal plane array have the following characteristics: low cost, operation at room temperature, compatibility with the silicon CMOS technology, and high detecting performance; therefore recently it became a hot spot in infrared or terahertz detection field. The performance of uncooled infrared focal plane detector arrays depends on the optimization of critical parameters which are determined by geometrical design and the electrical, optical, and thermal physical properties of the detector materials. We report the study of a fabrication process and characterization of two (2D) dimensional arrays of uncooled microbolometers based on silicon (α-Si) thermo-sensing films. Because these arrays substantially reduce sensor size, they are becoming the preferred format for most modern applications.

Infrared imaging sensors are frequently used in thermal signature detection applications in industrial, automotive, military and many other areas. However, advanced infrared detectors are generally associated with high costs and complexity. Infrared detectors usually necessitate a thermoelectric heater–cooler for temperature stabilization and various computationally complex preprocessing algorithms for fixed pattern noise (FPN) correction. In this paper, we leverage the benefits of uncooled focal plane arrays and describe a complete digital circuit design for Field Programmable Gate Array (FPGA)-based infrared image acquisition and pre-processing. The proposed design comprises temperature compensation, non-uniformity correction, defective pixel correction cores, spatial image transformation and registration with RGB images. When implemented on Xilinx Ultrascale+ FPGA, the system achieves a throughput of 30 frames per second using the Fraunhofer IMS Digital 17 μm QVGA-IRFPA with a microbolometer array size of 320 × 240 pixels and an RGB camera with a 1024 × 720 resolution. The maximum ratio of the standard deviation to the mean of 0.35% was achieved after FPN correction. [ABSTRACT FROM AUTHOR]

Vanadium oxide (VO X) is an excellent thermal sensitive candidate for uncooled microbolometers. However, undoped VO X prepared by atomic layer deposition (ALD) has a temperature coefficient of resistance (TCR) of ca. −2 ∼ −3%/K. For improving its TCR, our deposition approach based on the combination of ALD and rapid post-deposition annealing (RTA) is proposed. Besides, aluminum-doping into the VO X films is performed via that approach, and the number of Al 2 O 3 cycles is adjusted for varying the dopant loadings. Changes in physical, chemical, and electrical characteristics of the VO X films due to Al-doping are discussed in detail. The advantage of introducing Al3+ dopants is to hinder the thermally activated phase transition of VO 2 phases, leading to an improvement in TCR of Al-doped VO X up to −4.2%/K, which remains stable over a wide temperature range of 298–328 K. However, the excessive Al doping also carries an adverse effect on TCR. The reason for that is discussed for further understandings of doping effects. [ABSTRACT FROM AUTHOR]

This paper presents a novel and power efficient readout architecture for uncooled microbolometers that offers adequate noise performance along with a circuit-based method for self-heating compensation. The proposed method employs an event-generation scheme and a time-mode readout chain to achieve dynamic mode of operation resulting in low power. The readout chain consists of a capacitive transimpedance amplifier (CTIA) based current-to-time converter followed by a two stage time-to-digital converter (TDC). The CTIA employs a novel integration scheme for time amplification along with a modified reset mechanism to achieve self-heating compensation. We also present a model to analyze the implications of time-mode readout on imager operation. An experimental readout chip, based on this approach, has been designed using a 130 nm bulk CMOS technology. The proposed architecture offers robust frontend processing and achieves a per channel power consumption of $66~\mu \text{W}$ , which is considerably lower than the most recently reported designs, while maintaining better than 10-mK readout noise equivalent temperature difference (NETD). [ABSTRACT FROM AUTHOR]

[Display omitted] • Our Al-doped VOx thin films were grown by PEALD and crystallized by PDA. • Those approach combination has benefits for producing stable and uniform films. • The optimum Al:VO x film has low resistance (<100 kΩ) and high TCR (−3.5%/K). • It can be as a sensor film of uncooled microbolometers. Vanadium oxide (VO X) is a promising material for developing the sensor film of uncooled microbolometers. However, amorphous VO X film, which is typically formed on the Si substrate, is unstable due to ambient unwanted oxidation in air, making their resistance too high to be employed in microbolometers. Therefore, studies on improving the quality and stability of VOx film are still ongoing. This study presents our strategies based on plasma-enhanced atomic layer deposition (PEALD) and rapid thermal annealing (RTA) for manufacturing VOx films with high crystallization and stability. The VOx sensing layer for microbolometers was fabricated at a low process temperature of 370 °C using NH 3 gas, and the rapid thermal anneal was performed as the essential post-deposition annealing process for improving the VOx crystallization. Besides, in-situ deposition of an Al 2 O 3 layer on the VOx film improves its stability. Compared with a bare VOx film, our developed Al 2 O 3 /VOx film shows an improvement of −3.5%/K in the temperature range of 298–328 K. For the optimized sample, changes in the electrical, chemical, and surface characteristics of the VOx film due to Al 2 O 3 or Al-doping are studied and discussed in detail to find the mechanism of that improvement. [ABSTRACT FROM AUTHOR]

Highlights • Characterization of Si x Ge y O 1−x−y thin films for uncooled infrared microbolometers. • Electrical properties of the films were studied as function of oxygen and silicon. • The results demonstrate a high TCR and a low resistivity can be achieved. • XRD FWHM and the peak shift are related to changes in TCR and resistivity. • FTIR and Raman explain bond between Ge-Ge and Ge-Si relating electrical properties. Abstract This paper presents a detailed characterization of Silicon Germanium Oxide (Si x Ge y O 1−x−y) thin films for uncooled infrared microbolometers. The films were prepared by RF magnetron sputtering using simultaneous deposition from silicon and germanium targets in an argon and oxygen environment. The electrical properties of the films were studied as a function of oxygen (up to 10%) and silicon (up to 30%). The results demonstrated that a high temperature coefficient of resistance (TCR) and a low corresponding resistivity can be achieved using various compositions, for example, Si 0.054 Ge 0.877 O 0.069 films have achieved a TCR and a resistivity of −3.5 %/K and 629 Ω -cm, respectively. X-ray diffraction (XRD) demonstrated that the films are amorphous, and the broadening of XRD spectra (full width at half maximum, FWHM) is related to the decrease and increase of TCR and resistivity, for example, oxygen and fixed Si concentrations of 6.16% and 4% result in narrow spectra which corresponds to a higher TCR value of −3.6 %/K. The Fourier transform infrared spectroscopy (FTIR) study shows that the absorption peaks corresponding to Ge-O and Si-O are located around 800 cm−1 and 960 cm−1, respectively. The peak intensity of Si-O increases as the Si content increases while Ge-O bond disappears as the Si content increases above 6%. These results are used to explain the TCR and resistivity behavior as the Si concentration increased in the film. In addition, the chemical bonding between Ge-Ge and Ge-Si, measured using the Raman spectroscopy study demonstrates that at a fixed oxygen concentration of 4%, The peak shift of Ge-Ge and Ge-Si are relatively small which corresponds to a small change in TCR and resistivity at this Si concentration range, while at oxygen concentration of 8%, the Ge-Si bond have more effects on increasing TCR values, and the larger peak shift corresponds to a higher TCR values. [ABSTRACT FROM AUTHOR]

Mosaic focal plane arrays (MFPAs) have been designed, and a number of addressing schemes reducing the blind zone have been proposed. A precision technology for fabricating MFPAs with a total size of the blind zone of not more than 30 μm has been developed. Certain systemic issues of applying MFPAs are discussed. The designed 3072 × 576 uncooled microbolometer MFPAs based on 384 × 288 submodules with a pixel pitch of 17-51 μm provide ∼99% image conversion efficiency in the infrared range and those with a pixel pitch of 100 μm provide 100% image conversion efficiency in the terahertz range. [ABSTRACT FROM AUTHOR]

In this paper, a self-heating compensation method for uncooled microbolometers is presented. In this method, unlike the conventional use of IR-shielded (blind) references, the current from active sensors adjacent to each other are used to compensate for the undesired self-heating effect. Eliminating the need for IR-shielded references can simplify the fabrication process, resulting in lower costs. Moreover, using the adjacent sensor as the reference makes the read-out circuit insensitive to parameter mismatches in the sensor array. The thermal offset effect due to limited cooling time is also investigated in this paper. It is shown that the proposed structure can completely cancel the thermal offset effect. The proposed read-out has been designed in the standard 0.18- μ m CMOS technology considering an array with 352 × 288 pixels. The simulation results show that the proposed blind-less structure can achieve 25% more compensation than the common differential blind-based circuit in the presence of 20% resistance mismatch and 10% thermal constant mismatch. [ABSTRACT FROM AUTHOR]

It is demonstrated for the first time that the Wheatstone bridge structured metal thin film resistive uncooled microbolometer (in short, WB-bolometer) provides promising temperature sensitivity. This paper describes the design, simulation, and characterization of WB-bolometer using titanium nitride (TiN) thin film as the infrared (IR) sensing material. TiN thin film is designed into four resistors which are connected to each other in Wheatstone bridge configuration. The resistance value of each resistor changes with different rates upon IR absorption, which can be attributed to the difference in their associated thermal conductance. As a result, the bridge output voltage varies in response to the absorbed IR power. Simulation was employed to compare and characterize different designs of WB-bolometer. It was found that design with two sensing elements has the optimum performance. The proposed WB-bolometer is also capable of operating at elevated temperatures (> 250 o C) due to its adjustable and small initial offset voltage with minimum associated noise. [ABSTRACT FROM AUTHOR]

This work describes two methods for the dynamic characterization of a microbolometer infrared camera. The time constant, parameter commonly used for microbolometers dynamic performance identification, has been addressed in literature with studies mainly focused on theoretical computation and only seldom supported by testing activity. In this paper, alternative methods for the dynamic characterization of infrared sensors are presented and related uncertainty budgets are assessed. The infrared camera NEC TH7102WX has been tested with the proposed methods and results have been compared. The microbolometer behavior has proved to strongly deviate from the common first order model so that the time constant is no more a characterizing parameter. Depending on the definition of "equivalent time constants", the measured parameters changed by more than 50%, with values ranging between 30 and 50 ms. [ABSTRACT FROM AUTHOR]

It has been long known that thermal imaging may be used to detect stress (e.g. water and nutrient deficiency) in growing crops. Developments in microbolometer thermal cameras, such as the introduction of imaging arrays that may operate without costly active temperature stabilization, have vitalized the interest in thermal imaging for crop measurements. In this study, we have focused on the challenges occurring when temperature stabilization is omitted, including the effects of focal-plane-array (FPA) temperature, camera settings and the environment in which the measurements are performed. Further, we have designed and tested models for providing thermal response from an analog LWIR video signal (typical output from low-cost microbolometer thermal cameras). Finally, we have illustrated and discussed challenges which typically occur under practical use of thermal imaging of crops, by means of three cereal showcases, including proximal and remotely based (UAV) data acquisition. The results showed that changing FPA temperature greatly affected the measurements, and that wind and irradiance also appeared to affect the temperature dynamics considerably. Further, we found that adequate settings of camera gain and offset were crucial for obtaining a reliable result. The model which was considered best in terms of transforming video signals into thermal response data included information on camera FPA temperature, and was based on a priori calibrations using a black-body radiation source under controlled conditions. Very good calibration (r2>0.99, RMSE=0.32°C, n=96) was obtained for a target temperature range of 15-35°C, covering typical daytime crop temperatures in the growing season. However, the three showcases illustrated, that under practical conditions, more factors than FPA temperature may need to be corrected for. In conclusion, this study shows that thermal data acquisition by means of an analog, uncooled thermal camera may represent a possible, cost-efficient method for the detection of crop stress, but appropriate corrections of disturbing factors are required in order to obtain sufficient accuracy. [ABSTRACT FROM AUTHOR]

The high level of accumulated expertise by ULIS and CEA/LETI on uncooled microbolometers made from amorphous silicon enables ULIS to develop uncooled infrared focal plane array (IRFPA) with 17 µm pixel-pitch to enable the development of small power, small weight and power and high performance IR systems. Key characteristics of amorphous silicon based uncooled IR detector is described to highlight the advantage of this technology for system operation. A full range of products from 160 x 120 to 1024 x 768 has been developed and we will focus the paper on the 1/4 VGA with 17 µm pixel pitch. Readout integrated circuit (ROIC) architecture is described highlighting innovations that are widely on-chip implemented to enable an easier operation by the user. The detector configuration (integration time, windowing, gain, scanning direction), is driven by a standard I²C link. Like most of the visible arrays, the detector adopts the HSYNC/VSYNC free-run mode of operation driven with only one master clock (MC) supplied to the ROIC which feeds back pixel, line and frame synchronisation. On-chip PROM memory for customer operational condition storage is available for detector characteristics. Low power consumption has been taken into account and less than 60 mW is possible in analogue mode at 60 Hz. A wide electrical dynamic range (2.4V) is maintained despite the use of advanced CMOS node. The specific appeal of this unit lies in the high uniformity and easy operation it provides. The reduction of the pixel-pitch turns this TEC-less 1/4 VGA array into a product well adapted for high resolution and compact systems. Noise equivalent temperature difference (NETD) of 35 mK and thermal time constant of 10 ms have been measured leading to 350 mK.ms figure of merit. We insist on NETD tradeoff with wide thermal dynamic range, as well as the high characteristics uniformity and pixel operability, achieved thanks to the mastering of the amorphous silicon technology coupled with the ROIC design. This technology node associated with advanced packaging technique, paves the way to compact low power system. [ABSTRACT FROM AUTHOR]

This paper describes a readout integrated circuit architecture for an infrared focal plane array intended for infrared network-attached video cameras in surveillance applications. The focal plane array consists of 352 × 288 uncooled thin-film microbolometer detectors with a pitch of 25 μm, enabling ambient temperature operation. The circuit features a low-noise readout path, detector resistance mismatch correction and a non-linear ramped current pulse scheme for the electrical biasing of the detectors in order to relax the dynamic range requirement of amplifiers and the ADC in the readout channel, imposed by detector process variation and self-heating during readout. The design is implemented in a 0.35-μm standard CMOS process and two versions of a smaller 32 × 32-pixel test chip have been fabricated and measured for evaluation. The latest test chip achieves a dynamic range of 97 dB and an input-referred RMS noise voltage of 6.4 μV yielding an estimated NETD value of 26 mK with f/1 optics. At a frame rate of 60 FPS the chip dissipates 170 mW of power from a 3.4 V supply. [ABSTRACT FROM AUTHOR]

The mechanism of occurrence of sensitivity of microbolometer detector arrays based on vanadium oxide to terahertz radiation are analyzed. Experimental data are given showing the possibility of increasing the sensitivity of microbolometer detectors in the terahertz range by using an additional thin metal absorbing layer applied on the microbolometer membrane. A polarization dependence of the sensitivity of microbolometers in the terahertz and far infrared spectral ranges is found. It is shown that the sensitivity of microbolometers in the terahertz range is due to the absorption of radiation in the narrow metal runs placed on the support legs of the microbolometers and playing the role of an ohmic contact between the heat-sensitive layer and the processing curcuit. [ABSTRACT FROM AUTHOR]

In this paper, high performance read-out integrated circuits (ROIC) for infrared (IR) image sensor applications is proposed. Because an uncooled microbolometer image sensor, used in an IR image sensor, is made by a micro electro mechanical systems process, the resistance of bolometer by each process does not appear same value under same temperature condition. This resistance variation generates a different output signal for same input by each chip. In order to improve on these drawbacks, a ROIC, which compensates for the error described above, was designed. Instead of a single input mode, a differential input mode ROIC was proposed and thus, a new circuit structure was proposed. Using results from a computer simulation, improvements such as that the effect of the process error was decreased 10–12% without an additional compensation circuit was found. Based on the simulation results, a prototype device including a ROIC and a micro bolometer with 16 × 16 cell arrays was fabricated and characterized. [ABSTRACT FROM AUTHOR]

A low noise readout architecture for uncooled microbolometer focal plane arrays is described. The on-chip readout circuit contains an integration circuit in which the bolometer current is directed injected into a capacitor, and exhibits extremely low noise with no decrease in signal by using an ultra low noise capacitive transimpedance amplifier (CTIA). The simple configuration of the integration circuit makes it possible to operate more circuits in parallel, and increases the integration time and number of pixels. A 40 × 30 uncooled microbolometer focal plane array based on the low noise ROIC was implemented on silicon using a 0.5 μm CMOS technology. The total output noise voltage is 260 μV RMS. A noise at this level is so low that can loosen required TCR in the bolometer material. Experimental values of voltage responsivities of 3.98 × 105 V/W on average at 1 Hz modulation frequency have been achieved. [ABSTRACT FROM AUTHOR]

We present the first flight data using a Computational Reconfigurable Imaging Spectrometer (CRISP) system. CRISP (Sullenberger et al., 2017) is a novel hyperspectral thermal imaging spectrometer that uses computational imaging to enable high sensitivity measurements (via spectral multiplexing) from smaller, noisier, and less-expensive components (e.g., uncooled microbolometers) making it useful on small space and air platforms with strict size, weight, and power requirements. In contrast to other multiplexing hyperspectral solutions (e.g Michelson interferometer), it does not require moving parts, allowing for a robust system without aggressive engineering solutions. We discuss flight system design and calibration. Spectra from ground targets and gaseous species are compared to performance expectations. We successfully demonstrate the ability to extract airborne longwave infrared (8–12 μm) imagery and spectra from an uncooled camera-based CRISP system. • CRISP is an ultra-compact means of collecting hyperspectral imagery. • Airborne collection of longwave infrared spectra using uncooled microbolometers. • Airborne detection of gaseous species using longwave spectrometry. [ABSTRACT FROM AUTHOR]

Boron-doped hydrogenated silicon films are used in thermo-sensing layer in infrared detectors or uncooled microbolometers. Among thermo-sensing materials such as vanadium oxide and amorphous silicon and silicon diodes, amorphous silicon is the most common. Parameters such as the temperature coefficient of resistance (TCR), sheet resistance and 1/f noise are very important for the performance of these devices. One thermo-sensing material in particular, boron-doped hydrogenated silicon (BSi:H), has satisfactory TCR, sheet resistance (R sheet), and 1/f noise values. The BSi:H films are deposited using radio frequency plasma-enhanced chemical vapor deposition. The dependence of electrical, structural, and chemical properties of the BSi:H films on the plasma parameters is reported. The TCR of the films is 1.0–2.9%/K, R sheet is 1.2–37.8 MΩ/Υ and crystalline volume fraction is 10.2–68.5%. The properties of the amorphous and mixed-phase are compared. It is found that the 1/f noise of the mixed-phase film is lower than that of the amorphous phase film. These results show that the boron doped mixed-phase silicon films are suitable for use as thermo-sensing layers. • The properties of the silicon films on the plasma parameters is reported. • Mixed-phase film has appropriate thermal and electrical properties. • The 1/f noise of mixed-phase is lower than that of the amorphous phase. [ABSTRACT FROM AUTHOR]