LWIR Bands Research Articles

Multi-band compatible camouflage enabled by phase transition modulation of flexible GST films

With the rapid development of detection technology, there is an urgent need for multi-band compatible camouflage technology, particularly in the visible-infrared band. However, the conflicting requirements between camouflage across different bands and surface thermal management pose significant challenges in achieving synchronized control for multiple bands in diverse application environments and targets. Herein, we propose a spectrally selective emitter leveraging phase-change modulation of Ge2Sb2Te5, capable of achieving effective camouflage in both the visible band and two atmospheric windows over a wide temperature range. The designed emitters exhibit colorful appearances, compatible with various background environments, thereby ensuring the visible camouflage. Meanwhile, the emitters demonstrate low emissivity values of 0.1 in the MWIR band and 0.08 in the LWIR band, effectively reducing the infrared emission through atmospheric windows. The radiative temperatures of the optimized emitters in the MWIR and LWIR bands are reduced by 105°C and 140°C relative to the target surface (200°C), respectively, providing excellent radiative suppression compared to similar studies. Remarkably, the designed emitter still demonstrates robust flexibility, making it suitable for a variety of targets with complex curved surfaces. This work paves the way for designing the flexible visible-infrared compatible camouflage coatings with integrated thermal management functionality.

Metoda pomiaru minimalnej rozróżnialnej różnicy temperatury w funkcji powiększenia i rozogniskowania kamery termowizyjnej

Temperature measurement of allergic skin reactions requires meeting certain criteria regarding temperature resolution between small areas. The usual criterion of temperature resolution is NETD. However, this metric does not take into account the limitations of the spatial resolution of the optical system or the detector. In this paper, a model is presented that allows for selection of camera parameters based on the required spatial thermal resolution and size of the object being imaged. The method has been implemented and compared to results obtained in commercial software and has been applied to evaluate spatial thermal resolution of a prototype objective designed for the LWIR band.

Long wave infrared signature of swept back leading edges in aircraft frontal aspect

PurposeIn recent years, increased use of all-aspect infrared (IR)-guided missiles based on the long-wave infrared (LWIR; 8–12 µm) band has lowered the probability of aircraft survival in warfare. The lock-on of these highly sensitive missiles is difficult to break, especially from the front. Aerodynamically heated swept-back leading edges (SBLE), because of their high temperature and large area, serve as a prominent LWIR source for aircraft detection from the front. This study aims to report the influence of sweep-back angle (Λ, based on the Mach number [M∞]) on aerodynamic heating and the LWIR signature of SBLE.Design/methodology/approachThe temperature along SBLE is obtained numerically as radiation equilibrium temperature (Tw) by discretizing the SBLE length into “n” number of segments, and for each segment, emission based on Tw is evaluated. IR radiance due to reflected external sources (sky-shine and Earthshine) and radiance due to Tw are collectively used to determine the IR contrast between SBLE and its replaced background in the LWIR band (icont-SBLE,LWIR).FindingsThe results are obtained for low subsonic turboprop aircraft (Λ = 3°, M∞ = 0.44); high subsonic strategic bombers (Λ = 35°, M∞ = 0.8); fifth-generation stealth aircraft (Λ = 40°, M∞ = 1.6); and aircraft with supercruise/supersonic capability (Λ = 50°, M∞ = 2.5). The aircraft with supersonic capability (Λ = 50°, M∞ = 2.5) reports the maximum LWIR signatures and hence the highest visibility from the front. The results obtained are compared with values at Λ = 0° for all cases, which shows that increasing Λ significantly reduces aerodynamic heating and LWIR signatures.Originality/valueThe novelty of this study comes from its report on the influence of Λ on the LWIR signatures of aircraft SBLE in the frontal aspect for the first time.

Non-contact monitoring of soda-lime glass surface changes by its LWIR apparent emissivity: Possibilities and limitations

Soda-lime glass is widely used in many areas of daily life because of it is inexpensive, chemically stable, reasonably hard, extremely workable, and has excellent optical properties.In use, it is subject to weathering degradation. This causes surface changes that reduce its optical properties. After carrying out a spectral characterization of their effects, we highlight that the changes produced have a specific signature in the LWIR band due to the IR absorbing properties of the glass. Therefore, LWIR cameras used as radiometers are perfectly suited for their quantification by a non-contact measurement of their LWIR apparent emissivity. The LWIR apparent emissivity is provided in a quantitative thermographic approach. Based on the imaging capabilities of the thermal camera, it is possible to generate a 2D mapping of the surface changes in the form of an emissogram.The metrological capabilities of LWIR bolometric thermal cameras are then tested under laboratory conditions (strict control of influence temperatures) and under outdoor conditions. Under laboratory conditions, LWIR apparent emissivity is capable of quantifying specral anomalies as small as A=5%μm. The indicator is suitable for managing the effects of erosion and hydration of glass, respectively from 8 % of the eroded surface and from 100h of humid heat (2 years of natural exposure in a temperate climate). In outdoor conditions, the reflected temperature is an advantage because it improves the Type B accuracy of LWIR apparent emissivity measurements. Assuming a 3 °C non-uniformity of surface temperature, as it is the case for PV systems with active cooling, we conclude that erosion effects should be quantifiable from 48 % of the eroded surface, allowing optical transmittance losses of glass to be detected from 10 %. The effects of glass hydration remain difficult to manage due to the weakness of the spectral anomalies they produce.

Chalcogenide GRIN glasses with high refractive index and large refractive index difference for LWIR imaging.

Gradient refractive index (GRIN) materials utilize an internally tailored refractive index in combination with the designed curvature of the optical element surface, providing the optical designer with additional freedom for correcting chromatic and spherical aberrations. In this paper, new GRIN materials suitable for the second (3-5 µm) and third (8-12 µm) atmospheric windows were successfully developed by the thermal diffusion method based on Ge20As20Se60-xTex series high refractive index glasses, where the maximum refractive index difference (Δn) at 4 µm and 10.6 µm were 0.281 and 0.277, respectively. The diffusion characteristics and refractive index distribution of the GRIN glass were analyzed by Raman characterization. Furthermore, the performance of GRIN singlet and homogeneous singlet in the LWIR band (8 µm, 10.6 µm (primary wavelength), 12 µm) was compared, and the results showed that the GRIN singlet had better chromatic aberration correction and unique dispersion characteristics.

Cryogenic Refocusing of an Ultrawide FOV Long-Wave Infrared Imaging Spectrometer in a Geostationary Orbit

Imaging spectrometers in a geostationary orbit have unique advantages for various applications ranging from atmospheric to ocean remote sensing. To increase remote sensing range, we developed an ultrawide FOV LWIR imaging spectrometer. We compared several spectrometers and picked the “Offner + center single prism” form, which can simplify the system structure and increase cryogenic adaptability, making it ideal for ultrawide FOV designs. However, to reduce background radiation in LWIR bands, the imaging spectrometer was cooled to 123 K, causing the image plane to deviate from the focal plane. Therefore, we propose a cryogenic refocusing method based on an inclined slit in this paper that employs the full width half maximum (FWHM) of the spectral response function (SRF) as the refocusing evaluation function. By analyzing the effect of deviation on FWHM, the refocusing principle is well explained. The slit is tilted to find the minimal function value, so the deviation amount is calculated using differences in FOVs corresponding to the minimum FWHM at different temperatures. Experiments show that the FWHM is nearly 2.1 pixels after refocusing and that the actual spectral resolution is within 120 nm after spectral calibration. The method reduced the design time and provided references for optical systems experiencing refocusing issues in remote sensing imaging.

Experimental insights toward carrier localization in in-rich InGaAs/InP as candidate for SWIR detection: Microstructural analysis combined with optical investigation

Hyperspectral imaging has been flourished thanks to the huge investigation of the infrared spectrum from NIR to LWIR bands. The ternary InGaAs has been investigated herein in the context of studying the structural dependences of localization phenomenon by X-ray diffraction (XRD), scanning electron microscopy-energy dispersive X-ray (SEM-EDX), Raman, ultraviolet–visible (UV–vis), and photoluminescence (PL) techniques. Using metal-organic vapor phase epitaxy (MOVPE), we succeed to grow the InGaAs directly on InP substrate at 560 °C as an active layer with indium concentration exceeding the “golden” value (53%) to enlarge its cutoff absorption wavelength. X-ray diffraction proved a good crystallinity of the heterostructure with a sharp peak related to the thick substrate and another peak attributed to the thin layer of InGaAs. Moreover, an interfacial layer appeared at the logarithmic scale of XRD patterns and was confirmed by Raman analysis. The SEM-EDX revealed an average indium concentration (62%), almost the growth concentration. However, a cross-section compositional profile over the heterostructure showed an inhomogeneous distribution of the indium. This is predictable from the composition fluctuation in the indium-containing alloys and the volatility (surface segregation) of As (In). On the other side, the optical investigation of InGaAs demonstrated an anomalous behavior of luminescence versus temperature, manifested by the S-shape feature. This trend stems from the potential fluctuation induced by the non-uniform distribution of indium. A numerical simulation was developed based on the localized state ensemble (LSE) model to well-reproduce this anomaly by giving the best fitting parameters and comparing them with those calculated using the semi-empirical models (Viña and Pässler). The results reported here will help in optimizing the epitaxy design of future InGaAs/InP and further studying its surface morphology and device performance.

Whole LWIR Directional Thermal Emission Based on ENZ Thin Films

AbstractLong‐wave infrared (LWIR) of 8–14 μm is an imperative atmospheric transmission window for applications such as infrared detectors, radiative cooling, and infrared stealth. Attempts to manipulate thermal emission in the LWIR band consist of either directional emission but narrowband or LWIR emission but non‐directional. Directional thermal emission covering the entire LWIR band has remained elusive, so far. Here, a whole LWIR directional thermal emitter is introduced consisting of top epsilon‐near‐zero (ENZ) films (SiO2/SiO/Al2O3), a dielectric gap (Ge), and bottom ENZ films (TiO2/Ta2O5) on a metal. The emitter exhibits high emissivity (>0.9) in p‐polarization at specific directions (72°–82°) covering the entire 8–14 μm atmospheric window. Additionally, infrared encryption information on the emitter can only be observed in p‐polarization in the oblique direction. This approach introduces a new route toward simultaneous control of bandwidth and directionality of thermal emission and has inclusive applications such as infrared camouflage and energy management.

Infrared scene projector (IRSP) for cryogenic environments based on a light-driven blackbody micro cavity array (BMCA)

An infrared scene projector (IRSP) that can operate at an ambient temperature lower than 190 K is developed for the hardware-in-the-loop (HIL) simulation of space-borne IR detection systems. The IRSP is composed of a visible scene generator (VSG), a visible to IR converter, and an IR projection system optimized for cryogenic environments. The core component of the IRSP is a light-driven blackbody micro cavity array (BMCA). The BMCA is a photothermal conversion device. It can transform visible light images into IR images. The BMCA can operate properly in an ultralow temperature environment, and the temperature of the BMCA is the same as the ambient temperature. This property allows the BMCA to generate IR scenes with a pure low temperature background, which is crucial for the ground testing of space-borne IR detection systems. The performance of the IRSP was tested in a vacuum cold chamber. In the cryogenic environment with an ambient temperature of 187.75K, the observed highest temperature of the generated IR scene was 426.15 K, the lowest temperature was 187.75 K, the dynamic range of the IR scene was 38.69 db, and the frame rate of the IR scene reached 76 Hz. The average visible to IR conversion efficiency of BMCA was about 10.6%∼3.1% under different ambient temperatures. The radiation spectrum of the IRSP is close to the standard blackbody radiation spectrum both in the MWIR band and the LWIR band. The IRSP has been applied in a HIL simulation test of a space-borne IR detection system.

Dual-band metamaterial absorber with a low-coherence composite cross structure in mid-wave and long-wave infrared bands.

The atmospheric window in the infrared (IR) band primarily consists of mid-wave (MWIR, 3-5 μm) and long-wave IR (LWIR, 8-12 μm) bands, also known as the working bands in most of the IR devices. The main factor affecting the device capability includes the absorption efficiency, hence, the absorption material. Herein, a dual-band absorber based on the composite cross structure (CCS) in both MWIR and LWIR bands was proposed, with absorption peaks of 4.28 μm and 8.23 μm. The obtained absorber is with high scalability in the MWIR and LWIR region respectively by tuning the structural parameters. A quadrupole polarization model is proposed for further understanding of the uneven distribution of electromagnetic field that was caused by the change of the center spacing of the embedded structure. Meanwhile, it was shown that the two absorption peaks exhibited good incident angle stability. In addition, as the incident angle of the TM mode increases, a waveguide is formed between the embedded structure and the surface structure, leading to another strong absorption in the LWIR band. The results showed that absorption increases as the incident angle increases. The proposed absorber can be a good candidate for applications in thermal emission, detection and solar energy harvesting.

Broadband long wavelength infrared metamaterial absorbers

Infrared broadband absorbers are in great demand in various applications, such as integration into uncooled infrared detectors and microbolometers. In this thesis, we demonstrate and compare three different design methods: coupling multi-sized resonators, inlaying metallic structures in the dielectric layer and choosing laminated dielectric layers made of lossless and lossy materials. Due to their absorption mechanisms, we initially propose a W/ZnTe/W/Ge/W and an embedded Ti/Si/Ti absorber, both of which can achieve ultra-broadband absorption from 8 to 14 μm, with average absorptions of 88% and 92%. We further propose a Ti/Ge/Si3N4/Ti absorbers which can realise near-perfect absorption in the LWIR band, with average absorptions of 95% where all absorption curves exceed 90%. Our study provides the broadband LWIR absorbing structures in addition to providing new insight into the understanding of the interaction of propagating and localised surface plasmon resonances, with the prospect of expanding to other absorbing-structure designs from visible to THz regimes.

Range Performance Modelling of Thermal Imaging System based on Single Parameter Characterised by Ambient Temperature and Relative Humidity

Range performance of a thermal imaging system is characterised by the prevailing atmospheric condition present at that time. There are two dominant parameters that limit the range performance of any thermal imaging systems i.e. ambient temperature and relative humidity. In the present work, comparative study of acquisition range performance of thermal imaging system operating in LWIR and MWIR spectral bands has been presented as a function of absolute humidity (AH) which is responsible for attenuation of IR radiation due to water vapour molecules present in path length. Presentation of acquisition range as function of AH leads to a single range performance table/graph for thermal imaging system under consideration for predefined visibility (V), target size, ambient temperature (T), target to background temperature difference (ΔT) and relative humidity (RH). This table/graph can be used to predict detection, recognition and identification ranges for any set of combination of air temperature (T) and relative humidity (RH). The approach presented in this paper is versatile and has been illustrated through comparative performance analysis of LWIR and MWIR thermal imaging systems based on 640X512 staring focal plane array (FPA) having identical design parameters in terms of resolution (IFOV). It has been shown that MWIR performance is superior to LWIR beyond a crossover value of AH(T) even though MRTD of MWIR sensor is inferior to that of LWIR sensor at all spatial frequencies. Study has been carried out both for clear atmosphere and hazy conditions.

Infrared chameleon-like behavior from VO2(M) thin films prepared by transformation of metastable VO2(B) for adaptive camouflage in both thermal atmospheric windows

Adaptive materials active in the thermal infrared (IR) region have recently attracted intense research interest as they are suitable for adaptive IR camouflage applications, although significant challenges remain. Under external thermal stimuli, vanadium dioxide (VO2) thin films exhibit a reversible metal-insulator transition (MIT) from an IR transparent state to a reflecting state with heating. An alternative route is proposed for depositing thermochromic VO2(M) thin films on quartz substrates via the transformation of metastable VO2(B). A silicone elastomer sheet with high absorption in the IR region and high thermal conductivity was adhered to the back of a VO2/quartz structure to improve its thermochromic properties in the mid-wavelength (MWIR; 3–5 µm) and long-wavelength (LWIR; 8–14 µm) thermal atmospheric windows. The emissivity of the VO2(M)/quartz/silicone structure could change by 0.21 and 0.49 across MIT in the MWIR and LWIR ranges, respectively. This VO2/quartz/silicone structure showed IR chameleon-like behavior in the MWIR and LWIR atmospheric windows, respectively. When heated above the MIT temperature, the structure autonomously lowers its IR radiation intensity to appear to be at the same or even lower temperature on IR cameras. When the real temperature (Tr) of the object is 95 °C, the apparent temperature (Ta) on the MWIR camera is 51.9 °C close to the Ta of 51.8 °C at Tr = 65 °C; in the LWIR band, Ta is merely 43.8 °C when Tr is 90 °C, close to the Ta of 42.1 °C at Tr = 50 °C and even cooler than the Ta of 49.3 °C at Tr = 60 °C. These exciting results provide new opportunities for adaptive IR camouflage of military objects with dynamical temperature change and IR exposure for cheating IR cameras.

Detection of propane in LWIR band

Detection of propane in LWIR band

Bridging the spectral gap using image synthesis: a study on matching visible to passive infrared face images

We propose an approach that bridges the gap between the visible and IR band of the electromagnetic spectrum, namely the mid-wave infrared or MWIR (3---5 $$\upmu \hbox {m}$$μm) and the long-wave infrared or LWIR (8---14 $$\upmu \hbox {m}$$μm) bands. Specifically, we investigate the benefits and limitations of using synthesized visible face images from thermal and vice versa, in cross-spectral face recognition systems when utilizing canonical correlation analysis and manifold learning dimensionality reduction. There are four primary contributions of this work. First, we assemble a database of frontal face images composed of paired VIS-MWIR and VIS-LWIR face images (using different methods for pre-processing and registration). Second, we formulate a image synthesis framework and post-synthesis restoration methodology, to improve face recognition accuracy. Third, we explore cohort-specific matching (per gender) instead of blind-based matching (when all images in the gallery are matched against all in the probe set). Finally, by conducting an extensive experimental study, we establish that the proposed scheme increases system performance in terms of rank-1 identification rate. Experimental results suggest that matching visible images against images acquired with passive infrared spectrum, and vice-versa, are feasible with promising results.

Relative performance analysis of IR FPA technologies from the perspective of system level performance

The majority of high performance infrared systems today utilize FPAs composed of intrinsic direct bandgap semiconductor photon detectors such as MCT or InSb. Quantum well detector technologies such as QWIPs, QDIPs, and SLS photodetectors are potentially lower cost alternatives to MCT and InSb, but the relative performance of these technologies has not been sufficiently high to allow widespread adoption outside of a handful of applications. While detectors are often evaluated using figures of merit such as NETD or D∗, these metrics, which include many underlying aspects such as spectral quantum efficiency, dark current, well size, MTF, and array response uniformity, may be far removed from the performance metrics used to judge performance of a system in an operationally relevant scenario. True comparisons of performance for various detector technologies from the perspective of end-to-end system performance have rarely been conducted, especially considering the rapid progress of the newer quantum well technologies.System level models such as the US Army’s Night Vision Integrated Performance Model (NV-IPM) can calculate image contrast and spatial frequency content using data from the target/background, intervening atmosphere, and system components. This paper includes results from a performance parameter sensitivity analysis using NV-IPM to determine the relative importance of various FPA performance parameters to the overall performance of a long range imaging system. Parameters included are: QE, dark current density, quantum well capacity, downstream readout noise, well fill, image frame rate, frame averaging, and residual fixed pattern noise. The state-of-the art for XBn, QWIP, and SLS detector technologies operating in the MWIR and LWIR bands will be surveyed to assess performance of quantum structures compared to MCT and InSb. The intent is to provide a comprehensive assessment of quantum detector performance and to identify areas where increased research could provide the most benefit to overall system level performance.

Susceptibility of combat aircraft modeled as an anisotropic source of infrared radiation

This paper describes the analysis of susceptibility of a supersonic aircraft to infrared (IR)-guided missiles by considering the aircraft as an anisotropic IR source. Supersonic aircraft have been considered as point sources in previous analyses; however, this assumption results in an overestimation of susceptibility. The procedure described here addresses this overestimation problem, and a more detailed susceptibility analysis was conducted. Detailed temperature distributions, including aerodynamically heated surfaces and hot engine parts, were obtained by coupled simulations of computational fluid dynamics with a radiation and conduction solver. Using the calculated surface temperature of the aircraft, the IR signature levels were calculated in the mid-wavelength IR and long-wavelength IR bands as a function of the detection aspect. A susceptibility analysis was carried out that considered the burnout range of air-to-air missiles to determine the lethal range. The lethal range was investigated for both the MWIR and LWIR bands and for various detection angles, including the frontal and rear aspects. An effective method to decrease susceptibility using IR signature-reduction technology was identified by comparing the changes in the lethal area of each detection band and aspect.

Long-Wave Infrared Thermophotonic Imaging of Demineralization in Dental Hard Tissue

Dental caries remains the most prevalent chronic disease in both children and adults worldwide. To address this prevalence through disease prevention and management, dentists need tools capable of detecting caries at early stages of formation. Looking into the physics of light propagation in teeth, this study presents a clinically and commercially viable platform technology for thermophotonic detection of early dental caries using an inexpensive long-wavelength infrared (LWIR; 8 $$\upmu $$ m to 14 $$\upmu $$ m) camera. The developed system incorporates intensity-modulated light to generate a thermal-wave field inside enamel and uses the subsequent infrared emission of the thermal-wave field to detect early caries. It was found that the greater light absorption at caries sites shifts the thermal-wave field centroid, providing contrast between early caries and intact enamel. Use of LWIR detection band in dental samples is novel and beneficial over the conventional mid-wavelength infrared band (3 $$\upmu $$ m to 5 $$\upmu $$ m) as it suppresses the masking effect of the instantaneous radiative emission from subsurface features due to the minimal transmittance of enamel in the LWIR band. The efficacy of the LWIR system is verified though experiments carried out on nonbiological test samples as well as on teeth with natural and artificially induced caries. The results suggest that the developed LWIR technology is an affordable early dental caries detection system suitable for commercialization/translation to Dentistry.

High-Performance M/LWIR Dual-Band HgCdTe/Si Focal-Plane Arrays

Mercury cadmium telluride (HgCdTe) grown on large-area silicon (Si) substrates allows for larger array formats and potentially reduced focal-plane array (FPA) cost compared with smaller, more expensive cadmium zinc telluride (CdZnTe) substrates. In this work, the use of HgCdTe/Si for mid- wavelength/long-wavelength infrared (M/LWIR) dual-band FPAs is evaluated for tactical applications. A number of M/LWIR dual-band HgCdTe triple-layer n-P-n heterojunction device structures were grown by molecular-beam epitaxy (MBE) on 100-mm (211)Si substrates. Wafers exhibited low macrodefect densities (< 300 cm−2). Die from these wafers were mated to dual-band readout integrated circuits to produce FPAs. The measured 81-K cutoff wavelengths were 5.1 μm for band 1 (MWIR) and 9.6 μm for band 2 (LWIR). The FPAs exhibited high pixel operability in each band with noise-equivalent differential temperature operability of 99.98% for the MWIR band and 98.7% for the LWIR band at 81 K. The results from this series are compared with M/LWIR FPAs from 2009 to address possible methods for improvement. Results obtained in this work suggest that MBE growth defects and dislocations present in devices are not the limiting factor for detector operability, with regards to infrared detection for tactical applications.

Narrow-band long-/very-long wavelength two-color type-II InAs/GaSb superlattice photodetector by changing the bias polarity

We report on a narrow-band two-color photodetector using type-II InAs/GaSb superlattices (SLs) in the long-/very-long wavelength infrared (VLWIR) ranges by changing the polarity of the bias. The narrow-band photoresponse is achieved by sequentially growing the doped SL structure that has a shorter cutoff wavelength as a low-pass filter for the absorption layers that has a longer cutoff wavelength. At 77 K, the 50% cutoff wavelength of the photodiode is 10 μm when the applied bias voltage is –0.1 V and is 16 μm at +40 mV. The δλ/λ is 44% for the LWIR band and is 46% for the VLWIR band.