Uncooled Microbolometer Research Articles

Long-Term Radiometric Stability of Uncooled and Shutterless Microbolometer-Based Infrared Cameras

Uncooled and shutterless microbolometer cameras are good candidates for infrared imaging systems installed on small satellites or small unmanned aerial vehicles: they are light and passive since no cooling system or mechanical shutter is required and they can be operated at ambient temperatures. However, the radiometric compensation has to be carefully performed to make the system compatible with applications where the radiometric accuracy of the images is mandatory. In this paper, we study the impact of the camera environment to the radiometric accuracy of the images. We propose and test hardware and software solutions to improve this accuracy and the quality of the radiometric images. We show that the radiometric calibration of the camera with our model is valid over a long time period— about 3 years—using in-door experiments.

Defect Detection of GFRP Composites through Long Pulse Thermography Using an Uncooled Microbolometer Infrared Camera.

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.

Plasmonic metamaterial absorber for MWIR and LWIR bispectral microbolometers

Plasmonic metamaterial absorbers (PMAs) designed for multispectral imaging in the infrared (IR) with uncooled microbolometers are investigated. The study presents Fourier transform infrared spectroscopy (FTIR) measurements of PMAs consisting of metal-insulator-metal-stacks (MIM) with square-shaped micropatches as top metal layers. The measurements reveal high absorptances of 82% to 99% for distinct wavelengths within a range from 2 μm to 9.2 μm. The spectra are evaluated with respect to the lateral dimensions of the patches and to the refractive indices of the used dielectrics SiO2, Al2O3 and Ta2O5. Numerical simulations and analytical calculations of the TM010-mode using the transmission line model (TLM) for microstrip antennas show good qualitative agreement with the measurement results. Additionally, bispectral PMAs were fabricated consisting of fields of PMAs with two different patch sizes arranged in a chessboard pattern. The individual fields of this pattern correspond to microbolometers with 12 μm pitch in shape and size. Two distinct absorption maxima can be seen in the spectra measured by FTIR. The choice of materials, deposition methods and patterning processes is suitable for the integration into the existing Fraunhofer IMS's nanotube microbolometer technology to realize multispectral infrared imaging. The fabrication process is CMOS-compatible and carried out on 8-in. wafers.

Mid-infrared passive spectroscopic imaging for visualizing tooth quality.

Although the measurement of tooth quality is necessary for precise prediction of caries formation, typical measurement methods include tooth-hardness measurements and absorption spectroscopy, which generally use infrared light irradiation. These methods are destructive or invasive, and obtaining two-dimensional information in the oral cavity is difficult. Mid-infrared emissions from the surface of an object reflect intrinsic vibrations of molecules in the object. In this study, a mid-infrared passive spectroscopic imaging system was developed using an inexpensive uncooled microbolometer array sensor with an optimized multi-slit, which eliminated the cancellation of interference intensities between two adjacent emission points, to obtain two-dimensional information from an object without external infrared light irradiation. First, the feasibility of obtaining two-dimensional information on tooth quality using the proposed system was examined, and emission spectra attributed to phosphate ions in hydroxyapatite (HAp), the main component of enamel, were successfully obtained from bovine teeth. Further, the hardness of bovine teeth was measured, and a correlation (R2 = 0.8067) between the Vickers hardness and peak area ratio of phosphate ions assigned to the crystalline and amorphous phases of a tooth was established. Additionally, tooth-hardness visualization in a non-contact manner was demonstrated as two-dimensional information using the obtained regression equation.

High-altitude demonstration of LWIR polarimetry using uncooled microbolometers

An Infrared Channeled Spectro-Polarimeter (IRCSP) was successfully demonstrated in a near-space environment on a high-altitude balloon mission out of the NASA Columbia Scientific Balloon Facility in August 2021. The compact IRCSP is sensitive to linearly polarized thermal radiation between 7-12μm and targets polarized cloud top scattering. The IRCSP comprises a series of polarization modulation components followed by a diffraction grating and lens that images a polarization modulated spectrum at the focal plane. At altitudes greater than 30 km, the IRCSP reported brightness temperatures (Tb) between 250–285 K with an uncertainty of less than 1.5K. These IRCSP Tb values overlapped the collated GOES-16 cloud top temperatures within the pointing uncertainty of the instrument. IRCSP’s inaugural high-altitude deployment demonstrated the operation of uncooled microbolometers in the low-pressure environment and resulted in the first known observations of downward-viewing thermal polarized radiation from Earth’s atmosphere. Using the Lomb–Scargle periodogram with a maximum acceptable false alarm probability of 1%, the IRCSP detected statistically significant polarized signals with average degrees of linear polarization (DoLP) ranging from 1% to 20%. The top-of-atmosphere simulations that motivated IRCSP’s development predict that LWIR polarization is highest for ice clouds of moderate optical thickness. IRCSP observed cloud-top DoLP trends with brightness temperature that are consistent with these simulations. The highest and lowest Tb values occur for no clouds and optically thick clouds, respectively. The lowest DoLP measurements by IRCSP were at these extreme conditions. The DoLP peaked at intermediate Tb values. This preliminary work motivates IRCSP’s participation in future deployments that provide independent data products of cloud microphysical properties.

Application of Uncooled Microbolometers for Detecting Pulsed Terahertz and Infrared Radiation

Application of Uncooled Microbolometers for Detecting Pulsed Terahertz and Infrared Radiation

Enhanced temperature coefficient of resistance of VOX-based uncooled microbolometers manufactured by plasma enhanced atomic layer deposition

Vanadium oxide (VOX) is a promising material for developing the sensor film of uncooled microbolometers. However, amorphous VOX 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 NH3 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 Al2O3 layer on the VOx film improves its stability. Compared with a bare VOx film, our developed Al2O3/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 Al2O3 or Al-doping are studied and discussed in detail to find the mechanism of that improvement.

Detection of Barely Visible Impact Damage in Composite Structures Using Backward Sweep Vibro-thermography Technique Utilizing Asymmetry in Local Defect Resonance

ABSTRACT In this article, local defect resonance-based vibrothermography has been studied for different sweep direction and ranges. Two different carbon fiber-reinforced polymer (CFRP) composite plate has been fabricated from vacuum assisted resin transfer molding. In the first plate, a flat bottom hole has been made and two barely visible impact damages are created in other plate. The area of delamination has been determined from phased array ultrasound testing. Laser doppler vibrometry (LDV) has been performed first for different sweep ranges and directions. The CFRP plate is excited with a piezoelectric element at 150 Vpp and the vibration over defect area is captured using a single point laser doppler vibrometer, operating in scanning mode. It has been found that vibration amplitude at local defect resonance (LDR) frequency increases in narrow sweep range compared to a wideband excitation. Again, in both cases, it has been found that backward sweep produces more amplitude compared to forward one due to softening nonlinearity. An asymmetry in LDR frequency is also been observed when the sweep range is further narrowed. An uncooled microbolometer camera is used for reception in case of vibrothermography. Backward sweep is found to be more effective as compared to the forward one and the temperature increment increases in case of narrowband excitation range.

Detection and Characterization of Artificial Porosity and Impact Damage in Aerospace Carbon Fiber Composites by Pulsed and Line Scan Thermography

Nondestructive testing (NDT) of composite materials is of paramount importance to the aerospace industry. Several NDT methods have been adopted for the inspection of components during production and all through the aircraft service life, with infrared thermography (IRT) techniques, such as line scan thermography (LST) and pulsed thermography (PT), gaining popularity thanks to their rapidity and versatility. On one hand, LST is an attractive solution for the fast inspection of large and complex geometry composite parts during production. On the other hand, PT can be employed for the characterization of composite materials, e.g., the determination of thermal diffusivity and defect depth estimation. In this study, the use of LST with an uncooled microbolometer camera is explored for the identification of artificially produced porosity and barely visible impact damage (BVID) on academic samples. The performance of LST is quantitatively assessed with respect to PT (considered the gold standard in this case) using a high-definition cooled camera through the contrast-to-noise ratio (CNR) criterium. It is concluded that, although in most cases the measured CNR values were higher for PT than for LST (as expected since a high-definition camera and longer acquisition times were used), the majority of the defects were clearly detected (CNR ≥ 2.5) by LST without the need of advanced signal processing, proving the suitability of LST for the inspection of aerospace composite components. Furthermore, the deepest defect investigated herein (z ≈ 3 mm) was detected solely by LST combined with signal processing and spatial filtering (CNR = 3.6) and not by PT (since pulse heating was not long enough for this depth). In addition, PT was used for the determination of the thermal diffusivity of all samples and the subsequent depth estimation of porosity and damaged areas by pulsed phase thermography (PPT).

Design and Performance of Amorphous Silicon Based on Uncooled Bolometer-Type Infrared Focal Plane Arrays

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 .

Comparative study of real-time terahertz imaging of concealed metallic objects, drug, wood and TNT explosive in transmission/reflection modes using uncooled microbolometer and ultrafast pulsed terahertz imaging systems

Comparative study of real-time terahertz imaging of concealed metallic objects, drug, wood and TNT explosive in transmission/reflection modes using uncooled microbolometer and ultrafast pulsed terahertz imaging systems

Design of MIR Dispersive Spectrograph System with Uncooled Microbolometer.

To make the mid-infrared (MIR) dispersive spectrograph a practical tool in industrial food processing lines, we designed a dispersive spectrograph system with an uncooled microbolometer focal plane array (FPA) detector for MIR spectral acquisition. To precisely regulate the angle of a rotatable grating to acquire the MIR spectrum, the spectral resolution and spatial resolution of the system were rigorously controlled to improve system performance. In the reflectance operation mode of the MIR dispersive spectrograph, the uncooled microbolometer FPA detector offered a maximum spectral resolution of 12 nm for the MIR, when a 300 grooves/mm blazed grating was used. Utilizing an optical parametric oscillator (OPO) pulse laser source, the wavelengths of the first-order diffraction were validated, and the system's spectral resolution limit was determined. As a line-scanning source, a Globar broadband source was installed, and the USAF 1951 Resolution Calculator was used to establish the spatial resolution of the imaging spectrograph. Using NI LabView, the logical operational technique for controlling the MIR dispersive spectrograph was encoded into system firmware. The GUI and test results are thoroughly described.

Downhole FT-MIR Spectrometer Using Perfect Metasurface Absorber

Realization of “Fourier transform infrared (FT-IR) on a chip” holds the potential of a disruptive technology for downhole chemical analysis within the oil and gas industry. One of the critical obstacles to downhole integration though has been the cooling requirements of conventional technologies. Here, we report the design and numerical analysis of an uncooled miniaturized Fourier transform mid-infrared (FT-MIR) spectrometer compatible with downhole thermal environments, enabled by a broadband mid-infrared metasurface detector/source combination derived from a geometric inversion of a set of conformal mapping contours. The metasurface is numerically found to exhibit a near-zero index metamaterial (NZIM) behavior with absorption characterized by surface plasmon resonances confined to the ultrathin ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda $ </tex-math></inline-formula> /300) metasurface plane, making the absorption properties of the microbolometer design much less sensitive to the remaining support structure than in typical designs. This feature allows the metasurface to be integrated on a single VO2 substrate operated at elevated downhole temperatures that coincide with the metal–insulator transition region. Within this transition region, the VO2 material exhibits enhanced thermometric properties, enabling an uncooled microbolometer design with predicted maximum detectivity <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${D}^{*} = 1.5\times {10}^{{10}}\,\,\text {cm} \sqrt {\text {Hz}}/\text {W}$ </tex-math></inline-formula> and noise equivalent difference temperature (NEDT) of 1 mK at a modulation frequency of 500 Hz. These parameters approach entry-level lab FT-MIR spectrometers and could represent a significant step in deploying mid-infrared spectroscopy into oilfield downhole logging applications.

Monocular thermal imaging for pedestrian detection and ranging

To react to the presence of pedestrians, an automated braking system must first find the pedestrian(s) including locations relative to the automobile both in angular position and in distance. In the daytime, cameras and radar can provide the necessary information, but this combination, which requires ambient or active illumination, fails at night. Passive thermal sensors are now being enlisted to dramatically improve imaging at night, whereas substantial effort is underway to assure proper fusing of object information from the thermal sensor with other sensors on the automobile. To simplify the acquisition of information needed to make valid automated braking decisions at night, a camera system with a thermal image sensor was developed that identifies pedestrians and labels each identified pedestrian with location and distance data. The camera utilizes a single uncooled custom microbolometer sensor and a software suite implementing artificial intelligence and machine learning capabilities, running on a combination of convolutional neural networks and fusion processing to provide the data that an automobile host computer needs to implement fast, safe, and accurate automatic braking. We present details of the system construction and operation as well as initial test results showing the potential this technology has to dramatically reduce pedestrian fatalities at night as well as augment safety across all conditions, whether day, night, fog, rain, snow, dust, sun glare, or headlight glare.

Designing dual-band athermal refractive-lens IR objectives

The possibility of achieving superior optical characteristics and passive athermalization in dual-band IR objectives of a simple design is shown. This is ensured, in particular, by using refractive lenses made of chalcogenide glasses in the optical scheme. Recommendations regarding the layout of the optical scheme and obtaining initial design parameters for the subsequent optimization are given. The reliability and effectiveness of the recommendations are confirmed by the results of designing a super-aperture refractive-lens objective operating in the mid- and long-IR subranges and forming a high-quality polychromatic image on the matrix of an uncooled microbolometer in the operating temperature range from – 40°C to + 60°C.

Self-heating compensation in uncooled microbolometer read-out without using IR-shielded (blind) references

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-μm 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.

Photothermal deflection study of V/V2O5 alternating layers structure: effect of annealing atmospheres on thermal and optical properties

High sensitivity uncooled microbolometers are necessary to meet the needs of the next generation of infrared detectors, and vanadium oxide thin films are the potential candidates for uncooled microbolometers due to their high temperature coefficient of resistance (TCR) at room temperature. It is, however, very difficult to deposit vanadium oxide thin films having a high temperature coefficient of resistance because of the process limits in microbolometer fabrication. We present a fabrication method for vanadium oxide thin films. Through the formation of a vanadium oxide VxOy thermometer thin films with thickness of 95 nm prepared by sputter-depositing nine alternative layers of V2O5 and V of thicknesses of 15 and 5 nm, respectively. Two samples of vanadium oxide mixed phase are characterized in this work and compared to a not annealed sample, the first sample is annealed at 300°C for 30 min in O2 and the second in N2 atmosphere. The results show that annealing atmosphere has an effect on microstructure, optical, and thermal properties of the mixed phase. Comparing to our previous work, we have reached in this work successful results with vanadium oxide sample having both high TCR and low resistivity.

Optimization of temperature coefficient of resistance of Al-doped vanadium oxide thin film prepared by atomic layer deposition for uncooled microbolometer

Vanadium oxide (VOX) is an excellent thermal sensitive candidate for uncooled microbolometers. However, undoped VOX 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 VOX films is performed via that approach, and the number of Al2O3 cycles is adjusted for varying the dopant loadings. Changes in physical, chemical, and electrical characteristics of the VOX films due to Al-doping are discussed in detail. The advantage of introducing Al3+ dopants is to hinder the thermally activated phase transition of VO2 phases, leading to an improvement in TCR of Al-doped VOX 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.

Ar-implanted vanadium dioxide thin film with the reduced phase transition temperature

VO2 layers with a low metal–insulator phase transition (MIT) temperature TMIT below 40 °C were prepared using magnetron sputtering, rapid thermal annealing, and Ar2+ ion implantation. The implantation-induced strains and point defects effectively lower the metal–insulator transition temperature, while the subsequent thermal cycling (cooling-heating cycles) improves the temperature coefficient of resistance (TCR) up to the values >10%/°C within the temperature range 30–40 °C. These approaches can be used to extend the range of applications of VO2 thin films, especially for uncooled microbolometers. A methodology for studying the influence of defects and strains on electrical parameters of MIT in VO2 films has been proposed.

Application of uncooled microbolometers for detecting pulsed terahertz and infrared radiation

Analytical relations for temperature response of the bolometer to periodic radiation pulses are obtained. It is theoretically shown and experimentally confirmed by the example of infrared bolometers that when detecting short radiation pulses, in contrast to the case of constant radiation, increasing the thermal conductivity of the bolometer and, accordingly, decreasing its thermal relaxation time, it is possible to significantly increase the response rate of the receiver, practically without reducing its sensitivity. The possibility of effective registration of pulsed terahertz radiation by microbolometers with a resistively coupled, thermally non-isolated antenna is considered. It is shown that such bolometers, which have increased thermal conductivity and, accordingly, reduced sensitivity to continuous-wave radiation, can be highly effective when detecting pulsed radiation with a duration shorter than the thermal relaxation time of the bolometer. On their basis, uncooled matrix detectors of pulsed terahertz radiation, characterized by a minimum detectable energy of less than 1·10-12 J and a frame rate of up to 1000 Hz, can be developed. Keywords: microbolometer, pulsed terahertz radiation, antenna.