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The performance evaluation of infrared (IR) imaging systems plays a vital role prior to effective deployment. The minimum resolvable temperature difference (MRTD) is the primary key parameter of an IR imaging system, because it describes the overall system performance including that of a human operator. MRTD defines the minimum temperature difference required to resolve the four-bar target of defined spatial frequencies and is used to estimate the recognition ranges of a thermal imaging system before deployment for field evaluation. Conventionally, MRTD measurements use the temperature difference of a four-bar target that is exposed to the minimum resolvable condition. In current practice, subjective MRTD measurement employs a human operator to observe the minimum resolvable condition and therefore suffers from poor repeatability. It is a tedious procedure to evaluate large numbers of thermal imaging sights during production. In this work, an accurate objective MRTD measurement method is described by measuring the modulation transfer function and the noise equivalent temperature difference (NETD) of the thermal imager. Experiments were carried out to measure both subjective and objective MRTD. The results of the subjective and objective MRTD measurements are presented along with a comparison of the results. It is observed that the percentage error in objective MRTD is very close to that from a conventional subjective MRTD measurement.

We propose an all-ZnO bilayer microbolometer, operating in the long-wave infrared regime that can be implemented by consecutive atomic layer deposition growth steps. Bilayer design of the bolometer provides very high absorption coefficients compared to the same thickness of a single ZnO layer. High absorptivity of the bilayer structure enables higher performance (lower noise equivalent temperature difference and time constant values) compared to single-layer structure. We observe these results computationally by conducting both optical and thermal simulations. (C) 2017 Society of Photo-Optical Instrumentation Engineers (SPIE)

Cooled infrared detectors are typically characterized by well-known electro-optical parameters: responsivity, noise equivalent temperature difference, shot noise, 1 / f noise, and so on. Particularly important for staring arrays is also the residual fixed pattern noise (FPN) that can be obtained after the application of the nonuniformity correction (NUC) algorithm. A direct measure of this parameter is usually hard to define because the residual FPN strongly depends, other than on the detector, on the choice of the NUC algorithm and the operative scenario. We introduce three measurable parameters: instability, nonlinearity, and a residual after a polynomial fitting of the detector response curve, and we demonstrate how they are related to the residual FPN after the application of an NUC (the relationship with three common correction algorithms is discussed). A comparison with experimental data is also presented and discussed.

An uncooled infrared (IR) focal plane array (FPA) with 23.5 μm pixel pitch has been successfully demonstrated and has found wide commercial applications in the areas of thermography, security cameras, and other applications. One of the key issues for uncooled IRFPA technology is to shrink the pixel pitch because the size of the pixel pitch determines the overall size of the FPA, which, in turn, determines the cost of the IR camera products. This paper proposes an innovative pixel structure with a diaphragm and beams placed in different levels to realize an uncooled IRFPA with smaller pixel pitch (≦17 μm ). The upper level consists of a diaphragm with VO x bolometer and IR absorber layers, while the lower level consists of the two beams, which are designed to be placed on the adjacent pixels. The test devices of this pixel design with 12, 15, and 17 μm pitch have been fabricated on the Si read-out integrated circuit (ROIC) of quarter video graphics array (QVGA) (320×240 ) with 23.5 μm pitch. Their performances are nearly equal to those of the IRFPA with 23.5 μm pitch. For example, a noise equivalent temperature difference of 12 μm pixel is 63.1 mK for F/1 optics with the thermal time constant of 14.5 ms. Then, the proposed structure is shown to be effective for the existing IRFPA with 23.5 μm pitch because of the improvements in IR sensitivity. Furthermore, the advanced pixel structure that has the beams composed of two levels are demonstrated to be realizable.

Against a background of newly emerged security threats, the well-established idea of utilizing submillimeter-wave radiation for personal security screening applications has recently evolved into a promising technology. Possible application scenarios demand sensitive, fast, flexible and high-quality imaging techniques. At present, best results are obtained by passive imaging using cryogenic microbolometers as radiation detectors. Building upon the concept of a passive submillimeter-wave stand-off video camera introduced previously, we present the evolution of this concept into a practical application-ready imaging device. This has been achieved using a variety of measures such as optimizing the detector parameters, improving the scanning mechanism, increasing the sampling speed, and enhancing the image generation software. The camera concept is based on a Cassegrain-type mirror optics, an optomechanical scanner, an array of 20 superconducting transition-edge sensors operated at a temperature of 450 to 650 mK, and a closed-cycle cryogen-free cooling system. The main figures of the system include: a frequency band of 350±40 GHz, an object distance of 7 to 10 m, a circular field of view of 1.05 m diameter, and a spatial resolution in the image center of 2 cm at 8.5 m distance, a noise equivalent temperature difference of 0.1 to 0.4 K, and a maximum frame rate of 10 Hz.

We have extended our investigation of corrugated quantum well infrared photodetector focal plane arrays (FPAs) into the far infrared regime. Specifically, we are developing the detectors for the thermal infrared sensor (TIRS) used in the Landsat Data Continuity Mission. To maintain a low dark current, we adopted a low doping density of 0.6×1018 cm−3 and a bound-to-bound state detector. The internal absorption quantum efficiency (QE) is calculated to be 25.4%. With a pixel fill factor of 80% and a substrate transmission of 70.9%, the external QE is 14.4%. To yield the theoretical conversion efficiency (CE), the photoconductive gain was measured and is 0.25 at 5 V, from which CE is predicted to be 3.6%. This value is in agreement with the 3.5% from the FPA measurement. Meanwhile, the dark current is measured to be 2.1×10−6 A/cm2 at 43 K. For regular infrared imaging above 8 μm, the FPA will have an noise equivalent temperature difference (NETD) of 16 mK at 2 ms integration time in the presence of 260 read noise electrons. The highest operability of the tested FPAs is 99.967%. With the CE agreement, we project the FPA performance in the far infrared regime up to 30 μm cutoff.

Application requirements driving present IR technology development activities are improved capability to detect and identify a threat as well as the need to reduce size weight and power consumption (SWaP) of thermal sights. In addition to the development of 3rd Gen IR modules providing dual-band or dual-color capability, AIM is focused on IR FPAs with reduced pitch and high operating temperature for SWaP reduction. State-of-the-art MCT technology allows AIM the production of mid-wave infrared (MWIR) detectors operating at temperatures exceeding 120 K without any need to sacrifice the 5-μm cut-off wavelength. These FPAs allow manufacturing of low cost IR modules with minimum size, weight, and power for state-of-the-art high performance IR systems. AIM has realized full TV format MCT 640×512 mid-wave and long-wave IR detection modules with a 15-μm pitch to meet the requirements of critical military applications like thermal weapon sights or thermal imagers in unmanned aerial vehicles applications. In typical configurations like an F/4.6 cold shield for the 640×512 MWIR module an noise equivalent temperature difference (NETD) 20000, e.g., for warning sensors in 24/7 operation. The modules are available with optional image processing electronics providing nonuniformity correction and further image processing for a complete IR imaging solution. The latest results and performance of those modules and their applications are presented.

We demonstrate the suppression of the bulk generation- recombination current in nBn devices based on an InAsSb active layer (AL) and a AlSbAs barrier layer (BL). This leads to much lower dark cur- rents than in conventional InAsSb photodiodes operating at the same temperature. When the BL is p-type, very high doping must be used in the AL (nBpn + ). This results in a significant shortening of the device cut- off wavelength due to the Moss-Burstein effect. For an n-type BL, low AL doping can be used (nBnn), yielding a cutoff wavelength of ∼4.1 μm and a dark current close to ∼3 × 10 −7 A/cm 2 at 150 K. Such a device with a4 -μm-thick AL will exhibit a quantum efficiency (QE) of 70% and background-limited performance operation up to 160 K at f/3. We have madenBnnfocalplane arraydetectors(FPAs)with a 320 ×256 formatand a 1.3-μm-thick AL. These FPAs have a 35% QE and a noise equivalent temperature difference of 16 mK at 150 K and f/3. The high performance of our nBnn detectors is closely related to the high quality of the molecular beam epitaxy grown InAsSb AL material. On the basis of the temperature dependence of the diffusion limited dark current, we estimate a minority carrier lifetime of ∼670 ns. C 2011 Society of Photo-Optical Instrumentation Engineers

A new pixel structure with twice-bent beams and eaves structure, suitable for high-resolution uncooled infrared (IR) focal plane arrays (FPAs), is proposed. In comparison with previous results (FPA of 37-µm pixel pitch), the thermal conductance of the test device with the proposed pixel structure of 23.5-µm pitch is reduced about 2.5 times. The eaves structure, which is adopted to increase the fill factor of pixels, improves the responsivity by a factor of 1.3. A 640×480 bolometer-type uncooled IRFPA is demonstrated by utilizing the new pixel structure, with supplementary modification to improve thermal conductance and thermal time constant. It shows a noise equivalent temperature difference (NETD) of 50 mK for F/1.0 optics at 30 frames/sec, a thermal conductance of 0.03 µW/K, and a thermal time constant of 16 msec.

Differential absorption radiometers (DARs) using uncooled detectors are introduced as a simple method for low-cost remote sensing of chemical vapors for domestic security. A DAR consisting of a pair of uncooled LiTaO3 pyroelectric detectors integrated with bandpass filters selected to detect methanol, a simulant of many hazardous vapors, was demonstrated. At a signal-to-noise ratio (SNR) 5, the measured detection limit for methanol was 0.014 atm cm. This corresponds to a detection limit of 5.70×10–4 atm cm (31.5 mg m–2) for dimethyl methylphosphonate (DMMP). For comparison, a DAR consisting of a pair of cryogenically cooled HgCdTe (MCT) detectors was also tested. The detector-limited noise equivalent temperature differential (NETD) of the MCT detectors was measured to be 0.38 mK, whereas for the pyroelectric detector it was 115 mK. Despite the much lower detector noise, the MCT-based DAR provided a detection limit of only 0.005 atm cm for methanol, corresponding to 2.04×10–4 atm cm (11.25 mg m–2) for DMMP. The relatively poor sensitivity of the MCT-based DAR was shown to be limited by small temperature gradients of 15 mK across the noncoincident fields of view of the detectors in the DAR and by environmental fluctuations, which contributed a total NETD = 37 mK.

We explain the technologies used for high-performance long linear arrays based on HgCdTe/CMOS hybrid multiplexers with bidirectional time delay and integration (TDI) functions and describe the development of the first high-resolution IR imaging system with a superextended graphics array (SXGA) format. Long-wavelength IR photodiode arrays are fabricated using liquid-phase epitaxially grown HgCdTe on a CdZnTe substrate. Each photodiode array consists of 480x8 element n+/n-on-p diodes formed by B+ implantation. Diodes with a 10.3-μm cutoff wavelength have a typical zero-bias resistance of 10 MΩ and a shunt resistance of 1 GΩ. Four CMOS readout integrated circuits (ROICs) are used for bidirectional TDI and multiplex operations, where each ROIC sums up and multiplexes eight signals from 120 channels. The ROIC also includes pixel deselection and gain control circuits along with the corresponding memory and writing means. The IR focal plane arrays (IRFPAs) have a typical noise equivalent temperature difference (NETD) of 18 mK after TDI with F/1.55 optics and 10-μs integration. The IR imaging system adopts an interlace scan using the 480x8 element IRFPA to demonstrate a high spatial resolution of 1280 horizontal lines by 960 vertical lines (SXGA format) and an NETD of less than 30 mK. The unique algorithm for image enhancement is successfully confirmed to be efficient.

International audience; We present a new video sensor for multimeasurements in an aircraft cargo compartment called a video sensor unit (VSU). This sensor, based on CCD technology, operates in the near IR (NIR) spectral band to measure temperatures due to overheating and fire events. After a characterization of the measurement chain, a radiometric model is applied to the VSU, enabling the measurement of temperatures ranging from 350 to 900 degreesC with a noise equivalent temperature difference (NETD) lower than +/- 8 degreesC. In the same spectral band (NIR), the VSU can also detect smoke and 3-D load displacements. The geometric model of the VSU is described and the associated calibration procedure is presented.

The performance of an IR detector in an area array is studied by numerically solving the 2-D diffusion equation for thermal and photo- generated carriers. The zero-bias resistance area product R0A, quan- tum efficiencyh, and the noise equivalent temperature difference NETD for diodes of different size and junction depth are calculated for long wavelength infrared (LWIR) HgCdTe n 1 -on-p diffusion-limited diodes in the backside illuminated configuration. The 2-D calculations incorporate thermally generated- and photocarriers that originate under the junction (the ''normal'' current), as well as those that originate from around the junction (the lateral current). The calculation of the diffusion currents— both optical and thermally generated carriers—is made using a trapezoi- dal grid, which better fits the symmetry of diodes in an area array. The present results are compared with previous calculations in which a uni- form grid was used. The results of R0A and h calculated by the uniform and trapezoidal grids differ significantly, especially for diodes with deep junctions or diodes that are small compared to the center-to-center dis- tance between diodes. Both the uniform and trapezoidal grid calculations have been compared with spot scan measurements and Semicad simu- lation in a stripe diode array in the literature. © 2001 Society of Photo-Optical

An infrared imager (often referred to as a FLIR, from the term forward looking infrared) can be characterized in terms of sensitivity, resolution and human performance. Sensitivity, resolution and human performance have been classically described by the following measurable parameters: noise equivalent temperature difference (NETD), modulation transfer function (MTF) and minimum resolvable temperature difference (MRTD or MRT). These are laboratory measurable quantities that can be used to verify the performance of an infrared system. These laboratory measurements are used to evaluate expected design performance and to periodically test an imager during its life cycle. These quantities are predictable in sensor design through the use of analytical models. Both model estimates and laboratory measurements can be used in a target acquisition model to determine the field performance (probability of detection, recognition or identification) of the imager. Sensitivity, resolution, and human performance are influenced by sampling artifacts that must be characterized. First, sensitivity is no longer sufficiently described by a single-valued NETD. The 3-D noise methodology, inhomogeneity equivalent temperature difference (IETD), and correctability are noise figures that have been developed over the past decade to more adequately describe both temporal and fixed pattern noise associated with focal plane arrays. Undersampled imaging systems are not shift-invariant. The shift-invariance condition, in particular, is compromised by under-sampling so that sensor performance becomes a function of phase (relative position between the image and the detector array). Resolution depends on the target-to-imager phase, so the MTF measurement may reveal sampling artifacts that give large MTF variations with target to sensor position. Finally, the human-performance parameter is perhaps most affected by undersampling. MRT can be strongly dependent on phase, where dramatic differences in measured MRT are attributed to different phase shifts. MRT is normally measured at optimum phase, yet the authors don't feel that static MRT measured with either optimum or unknown phase relationships correlate well with field performance. In the same way, it is not clear how to write field acquisition calculations based on MRT values measured past the half-sample (Nyquist) rate. A dynamic MRT method has been developed and measured which uses motion to provide enhanced sampling.

The noise-equivalent-temperature difference (NETD) is calculated in fixed-pattern-noise-limited mercury cadmium telluride (MCT) focal-plane arrays (FPA5) for both 3- to 5-μm and 8- to 14-μm terrestrial imaging applications, on the basis of a model for MCT in which a linear two-point compensation scheme is considered. The contributions to the NETD from the fixed-pattern noise of an array-in terms of gain, offset, and residual error-are calculated and taken in quadrature to obtain the NETD of the FPA. The effect of stability of the focal-plane temperature is included, but the contribution of readout is not considered. The largest contribution to the NETD is from the residual error in the compensation scheme. Decreasing the intrascene temperature difference and including a sharp-cutoff filter are shown to decrease the NETD. The composition has been varied so as to obtain the lowest NETD in each band: at x = 0.31 in the 3- to 5-μm band, and at x = 0.22 to 0.23 in the 8- to 14- μm band (where x is the composition in the alloy Hg 1- x Cd x Te).

An improved 512 x 512-element PtSi Schottky-barrier IR image sensor (512 x 512 IRCSD) has been developed using the charge sweep device (CSD) readout architecture and 1.2-[mu]m minimum design rules. Finer pattern process technology enhances the advantage of the CSD readout architecture, enlarging the fill factor without sacrificing the saturation signal level. A large fill factor of 71% is achieved in spite of a small pixel size of 26 x 20 [mu]m[sup 2]. At the Schottky-barrier detector reset voltage of 4 V, the differential temperature response with f/1.2 optics at 300 K and saturation signal level were 3.2 [times] 10[sup 4] electrons/K and 2.9 [times] 10[sup 6] electrons, respectively. The noise equivalent temperature difference was estimated as 0.033 K with f/1.2 optics at 300 K. The improved 512 x 512 IRCSD was designed to be operated either in the field or frame integration interlace modes for versatility.

The performance of focal plane arrays of cadmium mercury telluride (HgCdTe) photodetectors when used in the 8 to 12 um spectral range is limited by the nonuniformity of the responses of the detector elements in the array. If arrays of large numbers of detectors are to be used as sensors to achieve small noise equivalent temperature differentials (NETDs), these nonuniformities will have to be reduced during the fabrication of the arrays or compensated for in operation by correcting the output obtained from each individual detector. This paper analyzes some of the requirements for compensation techniques and discusses their application to infrared focal plane detector arrays. A method of compensation is proposed that provides significantly better correction than linear two-point correction schemes. The performance of the method is analyzed, and its application is demonstrated using a HgCdTe focal plane detector array.

The fundamental question in determining radiometric accuracy in a Forward Looking Infrared (FLIR) system is "What does the detector see?" The answer is that it "sees" with thermal radiation originating from emitting surfaces in a completely enclosed surround by virtue of the various transfer mechanisms -reflection, refraction, and scattering. To achieve high radiometric accuracy, the detector ideally would "see" only the object. However, due to effects such as lens emission, lens surface reflection, mirror surface emission, calibration source reflection (emissivity less than unity), and vignetting, the detector "sees" many other points in the field of view which are interpreted as part of the target itself. Other factors could contribute, e.g., cosn falloff, distortion, and pupil aberrations. From the viewpoint of the detector, all of these are closely related to vignetting, where the detector is partially "seeing" walls and lens mounts together with reduced intensities of radiation from the object scene. The approach here is to determine radiometric accuracy across the field of view on the basis of contributions from geometrically-defined sources outside of the object. Such a spatial spread of detector view must be combined with the temperature distribution to determine the radiometric accuracy of the system as well as image effects such as shading and narcissus. What the detector "sees" is determined by a computerized backward trace of many rays from the detector. To determine the variation across the field of view, this computation is made for a number of rotations of the scan mirrors. In the case of a FLIR designed for imaging and display, a high relative radiometric accuracy is required. For a FLIR used as an absolute radiometer, absolute radiometric accuracy close to the noise equivalent temperature difference of the system is achievable.

Performance models can be used to design systems to meet specific applications, evaluate competitive designs, or direct research into areas of potentially large payoffs in system performance. This paper presents a validated performance model for thermal imaging devices, where performance is defined as static (non-time varying) detection, recognition and identification as a function of target range. The target, atmosphere, device, and subjective observer behavior are described by mathematical equations which result in predictions of system noise equivalent temperature, minimum resolvable temperature and minimum detectable temperature. Detection, recognition, and identification are then based on these quantities through experimentally determined algorithms using required signal-to-noise ratio and subjective resolution.

A 256 x 256 element PtSi Schottky-barrier 1R-CCD image sensor has been developed using a minimum design rule of 2 µm and a three-level polysilicon structure. The pixel size and chip size are 37 x 31 µm2 and 10 x 10 mm2, respectively. In spite of the small pixel size, a large fill factor of 25% has been obtained. The responsivity has been improved by use of a thin metal film and an optical cavity structure. The barrier height and quantum efficiency coefficient obtained from the array performance measurement are 0.23 eV and 0.15 eV -1, respectively. The noise equivalent temperature difference of about 0.1 K is obtained with f/1.4 optics and a 16.7 ms stare time. The noise in this case is limited by the shot noise of the detector. An infrared camera was also developed using the 256 x 256 element IR-CCD image sensor.