Second-generation infrared image sensors are becoming available in a variety of infrared detector materials, including photovoltaic HgCdTe, PtSi, and lnSb and photoconductive extrinsic silicon, PbS, and PbSe. We review the background of the evolution of infrared detector materials and summarize the formats and configurations available now in prototype production. Capsule data summaries are provided to assist the reader in understanding the operating range of spectral response, temperature, and other performance parameters such as uniformity for each detector material type.
A number of techniques are under development for creating dynamic IR scenes for testing IR imaging systems. Comparison of these techniques may be misleading if appropriate performance parameters are not used. One straightforward method is the thermal emission from microemitters fabricated in an array. The small mass leads to good response times if crosstalk between elements and to any substrate can be limited. Silicon micromachining can create good performance in arrays of this type because of the excellent thermal isolation of the elements
Fire detection systems as they are now configured in aircraft cargo bays do not always meet the current FAA requirements, including a specification that a fire be detected within 1 mm after initiation. A new FAA Airworthiness Directive requires upgraded fire detection systems on all combination passenger/cargo aircraft built by Boeing and McDonnell Douglas. A thermal monitoring/fire detection system that meets the new FAA regulations has been developed. It is based on infrared detection technology coupled with a Fresnel scanning optical system. The system uses a dual computer controller unit, which permits totally redundant thermal monitoring within the cargo bay. The system is responsive to either overheat or fire conditions, has three levels of fire alarm signal, incorporates complete optical coverage and electronic circuitry redundancy, and provides fire location information. It is highly reliable and immune to false alarm stimuli. Each sensor is designed to cover a portion of the cargo bay approximately 20 ft in diameter from 12 ft above. A system description is presented, and performance characteristics for fire/overheat and false alarm immunity are discussed.
The visual threshold criteria for the psychophysical tasks of identification (ID) and identification friend or foe (1FF) for use with infrared imaging systems are presented, including the previously unused approach of signal detection theory (SDT) for determining visual thresholds for the Johnson task of identification. The results are also interpreted using the standard high-threshold theory methods and the two methods are compared. The original task of identification friend or foe is presented and compared to the standard identification task. The experiment presented used a much larger set of stimuli than other reported contemporary experiments and the effects of set size are discussed.
The noise limitations of IR imaging systems are often described as arising from predominantly temporal sources, such as background, thermal, and readout noise. Actual systems can also be limited by structured pattern) noises and drift. This is especially true with the advent of highly integrated hybrid focal plane arrays, which consist of an infrared detector array, usually photovoltaic, mated to a silicon CMOS VLSI readout device. In these arrays the large detector count, high density, and complexity create new susceptibilities for image noise. Low natural backgrounds in some systems, and the need to save aperture area in others, often force the designer to work at low background levels and high quanturn efficiencies, eliminating options for simpler FPA architectures and making control of focal plane elements more challenging. The desire to dc couple the array output and use an infrequent detector equalization update further complicates the issue. We discuss several noise processes in low-background hybrid arrays, both observed and anticipated. These processes are described through example circuits, typical of proposed and assembled lR focal plane arrays.
The magnitude of target thermal contrast is used as a key parameter in determining whether observers using thermal-imaging systems can detect military targets. The effects of differences in target and background emissivities on thermal contrast estimates are evaluated, and the effects these differences have on the estimate of the transmittance attenuation thresholds required to obscure a target viewed with a thermal imager are examined. Target-background radiometric difference in the thermal infrared bands is first computed in terms of target-background emissivities and thermodynamic temperatures; the result is then used to compute a difference function for brightness temperature relative to thermodynamic temperature as a function of the target-background emissivity ratio. The effect of emissivity differences between targets and backgrounds on smoke/obscurant attenuation required to obscure a target viewed with a thermal imager is then examined using an expression derived from the Center for Night Vision and Electro-Optics Static Performance model. Results of the analysis show that smoke screen requirements for obscuring targets from thermal imager detection can change by factors of 10 as the ratio of target-to-background emissivity changes by less than 25%.
Thermal image quality depends on properties of hardware, atmosphere, and thermal contrast in the target plane. Weather affects both the modulation transfer function (MTF) of the atmosphere and the thermal contrast in target space. Atmospheric effects are chiefly aerosol light scatter, which causes blurring as well as reduced contrast, and absorption, which reduces contrast. Thermal contrast in the target plane is affected by wind, which tends to equalize temperatures, and by dew, which tends to equalize emissivity. Experiments were carried out over a 2-km line of sight and MTF results and weather parameters processed. Quantitative relationships have been determined relating overall thermal image quality to weather for imaging of passive targets and are suggested as a criterion for forecasting relative quality of thermal imagery according to weather forecast.
A system has been developed that reduces the effective integration time of an off-shelf RS-170 optomechanical scanning imaging radiometer by two orders of magnitude. The system can be used to image repetitive thermal events at frequencies up to 4 kHz, without aliasing. The effective integration time of the system is 125 μs/frame. Examples include in-cylinder engine combustion and heat transfer in a transistor operating at 2.5 kHz.
A novel imaging technique in which frequency-modulated retides encode different pixel locations by light modulation is presented. In this technique a reticle modulates different pixel locations at different frequencies, and photodetectors collect the resulting signals. Filters decode these signals to recreate the image on a display. The technique allows multiplexing many pixels onto a fewer number of detectors by utilizing the bandwidth of the detectors more effectively. Since frequency modulation creates an additional dimension for the detector, a single detector can function as a linear array, a linear array can function as a staring array, or the additional dimension can be used to convey spectral or other information. At wavelengths requiring expensive focal plane components, costs can be greatly reduced.
Detection of chemical vapors with a remote sensor is necessary for both military defense and civilian pollution control. The thermal imager is a natural instrument from which to build a chemical sensor since most chemical vapors of interest are spectrally active in its operating wavelength range. A system has been designed to place a chemical detection capability as an adjunct function in a military thermal imager. An additional detector array, which is spectrally filtered at the focal plane, is added to the imager. Real-time autonomous detection and alarm is also required. A detection system model by Warren, based on a Gaussian vapor concentration distribution is the basis for detection algorithms. Algorithms recursive in both time and spectral frequency have been derived using Kalman filter theory. Adaptive filtering is used for preprocessing clutter rejection. Various components of the detection system have been tested individually and an integrated system is now being fabricated.
A Scophony-configuration infrared scene projector, consisting of a raster-scanned CO2 laser and an acousto-optic (AO) modulator, was characterized for modulation transfer function (MTF) performance. The MTF components considered in the model were the Gaussian beam input to the AO cell, the finite aperture of the scan mirror, the width of the detector in the image plane, the transfer function of the amplifier electronics, and a term caused by Bragg-angle detuning over the bandwidth of the amplitude modulation (AM) video signal driving the AO cell. The finite bandwidth of the input video signal caused a spread in the Bragg angle required for maximum diffraction efficiency. In the Scophony configuration, a collimated laser beam enters the AO cell at only one particular angle, so a falloff of diffraction efficiency (and hence MTF) resulted as the modulation frequency was increased. The Bragg-angle detuning term was found to dominate the measured system MTF.
The Mosaic Array Test System (MATS) has been developed to perform radiation effects characterization of single-element infrared detectors, readout devices, and infrared flocal plane arrays (IRFPAs). MATS has been used to perform radiation effects characterization of low background IRFPAs in various radiation environments. We describe the components and capabilities of the MATS and present representative data to demonstrate the testing capabilities of the MATS.
The generation of high-fidelity simulated infrared imagery requires a unique combination of physical principles and computer image generation technology. At the Georgia Tech Research Institute infrared simulation software has been developed that couples three-dimensional geometric models with geographic databases, infrared radiance prediction models, and computer graphics techniques for image rendering to generate high-resolution synthetic infrared imagery. These features provide a large degree of flexibility to the simulation and allow it to be employed over a wide spectrum of applications. A discussion of a simulation methodology, a review of the GTVISIT scene simulation tool, and a discussion of several applications are given.
A 64-channel readout device with a 64:1 multiplexer output, designed for use in cryogenic, infrared focal plane applications, is being tested extensively in a production environment. For the test system, an existing product was modified and expanded to provide the accuracy, flexibility, and throughput needed to meet test requirements. Unique test approaches, coupled with innovative throughput enhancement techniques, streamlined operations to allow automated testing of 500 devices per day on one test set with minimum operator intervention. Confidence in the quality and reliability of the deliverable product has also been increased. Creative software modules were designed and integrated with a standard software package (developed and refined over 5 yr of device testing), forming a test program that fulfills demanding test requirements. Additional benefits derived from this development effort include automated testing of packaged parts at both ambient and cryogenic temperatures, and reduced characterization test time for similar devices. Anticipated future improvements include custom redesign of the pipeline processor already present in the system architecture, and continued development to generalize the software package to allow swift reconfiguration for testing other readouts and hybrid arrays of varying size and performance.
Various methods for the calibration and error estimation of imaging radiometers are described. The methodologies are presented in the form of specific examples of the calibration of the Naval Research Laboratory's airborne infrared (IR) radiometric measurement system. Two imaging radiometers are part of this system and are used to perform radiometric measurements on a variety of targets and backgrounds over a spectral range of 3 to 12 μm. The calibration procedures for these radiometers are presented along with an extensive error analysis that examines in detail 16 different sources of error. Such an error analysis clearly illustrates the need for proper calibration procedures.
Analytical models ofthermal imaging system performance have gradually become obsolete as thermal imaging systems (TIS) have become more complex and have improved in vertical performance. In particular, the effects of sampling and aliasing have not been included directly, but have had to be accounted for by side calculations before entering the data. An approach to modeling second-generation TIS is described in which the effects of sampling on both signal and noise are accounted for without requiring the user to make subsidiary calculations. Proper treatment of the several sources of noise in sampled systems is analyzed, including aliased noise. The model is two dimensional, using both vertical and horizontal resolution in the prediction of recognition and detection performance. A model for human perception is presented that differs slightly from matched filter models.
Impedance measurements were made on SPRITE (signal processing in the element) detectors. The resistive contributions were found to contain a contact resistance term, along with a term proportional to the semiconductor material length between terminals. The capacitive contributions were strongly frequency dependent and were largely independent of material length. This behavior would appear to be caused by nonohmic contacts at the metal-semiconductor interfaces where the SPRITE is bonded to its external electrical connections. When the SPRITEs were illuminated, the resistance decreased as expected, but the capacitance showed a sizable increase as well. A T-network model is developed that is consistent with the set of two-terminal impedance measurements. A model of this type should provide a useful starting point for electronics optimizations in SPRITE systems, including the design of the bias source, the readout mechanism, and the preamplifier.
TOPICS: Minimum resolvable temperature difference, Solids, Spatial frequencies, Eye models, Signal to noise ratio, Black bodies, Imaging systems, Electro optical modeling, Temperature metrology, Systems modeling
Discrepancies between the predicted minimum resolvable temperature difference (MRTD) and field performances are indicative of the fact that the modeling of MRTD has certain inherent problems. Several sources for MRTD error are identified. MRTD is also shown to be a special case of a more general minimum resolvable luminance difference (MRLD) measure. The MRLD measure involves absolute temperature and can be used to describe more generic targets.
Results are presented of the initial testing of a computer simulation model designed for use by persons not necessarily expert in all or any of the disciplines involved in signature analysis. The model can be utilized for evaluating sensor system performance and prediction of sensor acquisition ranges. The ATIMS III airborne turret infrared measurement system flight experiments and the IASPM simulation model are described. Analysis of preliminary results comparing experimental data with simulated data for the 2- to 5-μm and 8- to 12-μm IR bands reveals the potential of the model for simulating a multitude of sensor-observed phenomena. Model strengths and shortcomings are discussed.
For the past 12 yr, a coordinated triservice effort has been under way in the U.S. Department of Defense to provide an atmospheric effects assessment capability for existing and planned electro-optical (EO) systems. This exploratory development effort by the U.S. Navy is reviewed. An initial marine aerosol model, the Navy Aerosol Model (NAM), which transitioned into LOWTRAN 6, and a more comprehensive model, the Navy Oceanic Vertical Aerosol Model (NOVAM), are discussed. In addition to marine aerosols and their extinction properties, accurate knowledge of marine background radiances and the effects of the intervening atmosphere are needed. Because in situ measurements of relevant environmental parameters are essential for real-time EO systems performance assessment and direct measurement of slant path extinction is most desirable, light detection and ranging (LIDAR) techniques are being developed. No technique, however, has yet emerged that is ready for shipboard implementation. A shipboard real-time performance assessment system, PREOS (performance and range for EO systems), has been developed and incorporated into the Navy's Tactical Environmental Support System (TESS), and improved target and background models are under development and will be incorporated into TESS when tested and validated. A reliable assessment capability can be used to develop tactical decision aids (TDAs), which permit optimum selection or combination of sensors and estimation of a ship's vulnerability against hostile systems.
One of the principal concepts of sub-Nyquist interferometry is the use of all available knowledge about a surface to extend the available measurement range. The use of a priori information in resolving the ambiguities found in interferometric step-height measurements is explored. Information about the height of a step that has been obtained by another measurement technique or through process parameters, such as etch rate, may be used for this purpose. The theory behind this type of measurement and experimental results are presented.
New methods to determine the central position of rotation of a rotating object using laser Doppler velocimetry (LDV) are described. The principle is based on the fact that the tangential velocity of the rotating object is proportional to the distance from the center of rotation and that it can be measured by LDV with a microscope optical system. To realize this principle, three methods (angular and tangential velocity method, multiple-point method, and two-point method) are proposed. In the third method, a holographic optical element was used to play the role of multiple beamsplitter and a lens to obtain a rigid and simple optical system. From experimental results, it has been indicated that the position of the center of rotation is determined with an accuracy of 1 to 2 μm by these methods.