Determination of the performance of undersampled IR cameras by means of four-bar patterns suffers from aliasing for spatial frequencies above the Nyquist limit. An alternative method, using two parallel line sources, is described. This method avoids the aliasing effect and allows a reproducible way for performance measurements by simple variation of the spacing between the two line sources and their temperatures. The modulation depth at optimum phase is measured in the camera display or electronics, similar to the classical Rayleigh criterion. Additional benefits of the new method are the simple target construction (only one target is required) and the ease to model line sources. The result of a performance measurement is similar to the standard MRTD (Minimum Resolvable Temperature Difference) method, where sensor resolution is coupled to range for a given target size and contrast. The new method has therefore the potential to be implemented into the present STANAG4347 and 4349. Results of performance measurements, carried out with the uncooled Thermacam PM395 camera and other cameras, are shown. Predicted and measured performance agree very well.
The Low Cost Gun Launched Seeker is a component of the Navy's effort to develop effective weapons for surface fire support missions by enhancing the performance of projectiles like the Extended Range Guided Munitions with low-cost, uncooled infrared (IR) staring focal plane array terminal seekers. IR target images for validating target detection algorithms were collected using a Merlin long wave camera from Indigo Systems. This paper characterizes the camera in order to develop performance parameters for simulating the seeker and to understand features in the imagery. These parameters include temperature response, temporal noise characteristics, fixed pattern noise, and the modulation transfer function.
This paper describes test software developed for both the 480 X 12 X 4 and 256 X 256 InSb focal planes manufactured at Litton EOS, Tempe, AZ. The software controls flux sources, frame grabbers and control electronics to provide a fully automated test environment. As well as providing focal plane screening and characterization, the software is an essential diagnostic tool, whereby critical performance attributes such as R0A, quantum efficiency and noise may be displayed both in histogram form and as spatial color-coded bitmap images. These bitmap images may be scanned using the mouse, so that the performance attributes of each pixel may be directly accessed. The software incorporates an emulation mode, in which the array performance is modeled in complete detail (including parameter spreads), with the data presented in identical format to that from the measured data. This allows theoretical performance to be compared directly with measured performance. The software also has the capability to perform bad pixel identification and substitution--a variety of algorithms are available--as well as detector gain/offset correction coefficients calculation.
The traditional test pattern for end-to-end EO system performance testing in the laboratory has been the static 3- or 4-bar target. This choice was governed by linear systems approach. The introduction of under-sampled imagers such as IRFPAs (infrared focal plane array cameras) has challenged the testing community to develop an alternative test, because the occurrence of aliasing has a completely different effect on periodic targets (such as the bar target) and real, non-periodic targets. A new test should at least have the following properties: lab testing is objective and easy, the measure is representative for field performance, and modeling (sensor and human) the test should be relatively easy. Several alternative test methods and test patterns have already been proposed. An example is the TOD method that uses non-periodic test patterns. Other examples are the dynamic MRT that uses a moving 4-bar target, and the MTDP that uses the traditional static target but allows that not all four bars have to be present in the image. The development of real-time scene projection allows testing with real infrared targets under controlled conditions. The authors will discuss a large number of test patterns and methods and show their advantages and disadvantages for end-to-end EO system performance testing. They conclude that simple non-periodic spatial test patterns, such as used in the TOD, are the best choice for sensor performance characterization.
Santa Barbara Infrared is working with TNO Human Factors Research Institute to develop the equipment and procedures necessary for a new standardized test method for characterizing electro-optical system performance. This method called Triangle Orientation Discrimination (TOD), offers a statistically more accurate and less subjective method of characterizing electro-optical system performance. TOD testing provides an excellent means of performing system level tests that characterizes spatial sensitivity in conjunction with target contrast, just as MRTD and MRC tests do, but in a more statistically accurate manner with less operator to operator variability. System design parameters and options will be discussed.
Target acquisition is important especially in the military sector, with the evolution of automated systems for target acquisition there is an enormous importance to digital target detection models. Johnson's criterion is based on research with large numbers of observers. The model divides resolution into 4 stages: Detection, Orientation, Recognition and Identification. These stages are separate from one another and involve different levels of resolution. Moving from one stage to a higher stage is by increasing resolution (the number of line pairs is increased). The Johnson model was based on analog images, which are not sampled. An Image converter was the sensor in his experiments. We examined whether Johnson's criteria comply with sampled images as well as with analog ones, or if they should be changed when sampled images are used. Perception experiments with 120 observers were used to check our assumptions and the results were summarized.
The sampling limitations associated with focal plane array imagers caused an aliased signal that corrupts the image. The aliased signal is a function of pre-sample blur, sampling frequency, and post-blur or image reconstruction. Previous experiments at the U.S. Army Night Vision and Electronic Sensors Directorate have quantified the effect of aliasing on the task of infrared target identification. Based on data from these experiments, the MTF Squeeze model was developed. The degraded performance due to under- sampling was modeled as an increase in system blur or, equivalently, a contraction or `squeeze' in the MTF. This paper describes the results of numerous sampling experiments.
The standard way to characterize sensor performance is by means of the Minimum Resolvable Temperature Difference (MRTD) and Minimum Resolvable Contrast (MRC) methods. These methods are based on Fourier analysis and work reasonably well for linear (analogue) systems. However, nonlinear effects, such as sampling, are not properly accounted for. As an alternative, the Triangle Orientation Discrimination (TOD) method has been proposed, based on 4 oriented triangles, that can handle nonlinear effects. Here, we present a model that predicts the TOD-sensor performance characterization curve from the system parameters. It consists of i) a sensor model and ii) a model of the visual system of the observer. The sensor model generates display images which are fed into the visual system model. The visual system is modeled by a bank of band-pass filters which mimic the pattern of neural activity in the visual cortex (using a common stack model). The neural activity is calculated and internal noise is added. Finally, a decision is made based on a correlation with the expected neural activity of the 4 possible inputs. The model has been validated with two human observer experiments in which the TOD-curve 1) of the naked eye, and 2) of a simulated thermal staring sensor system were measured. An internal noise level could be found for which the TOD of the naked eye of the two observers could be predicted. The model gives reasonable (but somewhat optimistic) predictions of the TOD-sensor performance curve of the simulated staring camera. Although more tests and modifications are required, these preliminary results suggest that the model can be developed into a model which predicts the TOD for all kinds of sensor systems, which may include sampling effects, noise, blur and (local) image enhancement methods.
The effect of sampling, or aliasing, on target recognition performance (discriminating between armored tracked, armored wheeled, and soft wheeled classes of tactical vehicles) is investigated in this research. A recognition target set was processed with various levels of blur and aliasing. Integrated spurious response levels were set to 0, 0.3, 0.6, and 0.9. A perception experiment was conducted with U.S. Army soldiers at Ft. Benning, Georgia to determine the impact of blur and aliasing on recognition performance. The results are described in this paper.
Heterojunction HgCdTe detector chips are used in military and commercial focal plane array systems. These devices give improved performance because they are built with a wide bandgap semiconductor on top of a narrow bandgap semiconductor. Optimization FPA performance with heterojunction detectors has posed problems because many of the HgCdTe material parameters vary as a function of composition, temperature and doping concentrations. AET, with funding from the US Army Night Vision Labs, has developed a new simulator called IRSIM that automates the HgCdTe device analysis and design. This paper discusses the details of and operation of this simulator.
In pursuit of a low cost, high performance IR, Imager Litton Electro-Optical Systems has developed an Uncooled Lead Salt 320 X 240 camera. The 30 um pitch detectors consist of either Lead Sulfide or Lead Selenide photoconductors fabricated onto a CMOS Multiplexer Read-Out Integrated circuit. The multiplexer uses novel techniques to subtract the large bias current adaptively, and to integrate the residual signal current. The image is acquired in `snapshot' mode. A chopper provides a uniform reference. Although not necessary for functionality, a thermo-electric cooler is incorporated to improve performance. The signal is digitized off focal plane, and pixel substitution/correction functions are subsequently performed. The resulting video signal is either observed on a LCD display at the back of the camera or remoted to an external viewing device.
A multi-channel, high precision, high detector count, TDI real time multi-spectral infrared focal plane residing in a flight platform generates volumes of image data. Data rates in excess of 50 Mbytes/sec are typical. The design requirements of high dynamic range, low noise, server weight and package constraints place burdens on the system designers. The fielding of a complex instrument of this type with near theoretical noise performance demonstrates that when the important principles of system design such as grounding, shielding, and signal processing are understood, excellent performance may be achieved in the face of the inevitable compromises which must be made to satisfy all the system constraints. This paper discusses the architecture of a multi-channel, multi-spectral high frame rate camera. Several key signal processing philosophies will be illustrated and used to demonstrate the effect upon overall system design. Several electronics modules have been manufactured and integrated with infrared focal plane assemblies, ultimately becoming part of an infrared camera system for a reconnaissance program.
An algorithm is developed to compensate for the spatial fixed-pattern (nonuniformity) noise in focal-plane arrays, which is a pressing problem, particularly for mid- to far- infrared imaging systems. The proposed algorithm uses pairs of frames from an image sequence exhibiting pure horizontal and vertical sub-pixel shifts. The algorithm assumes a linear irradiance-voltage model for which the nonuniformity is attributed only to variation in the offset of various detectors in the array. Using a modified gradient-based shift estimator, pairs of frames exhibiting the above shift requirements can be identified and used to generate a correction matrix, which will compensate for the offset nonuniformity in all frames. The efficacy of this nonuniformity correction technique is demonstrated by applying it to infrared and simulated data. The strength of this technique is in its simplicity, requiring relatively few frames to generate an acceptable correction matrix.
The Semi-Active Laser (SAL)/Infrared Imaging (I2R) Seeker concept consists of utilizing a small single Silicon PIN Photodiode detector in conjunction with a staring midwave infrared focal plane array (FPA) and correlating the temporal laser returns obtained from the PIN detector with the spatial laser returns obtained from the FPA. The correlation of the spatial-temporal returns will allow identification of the correct laser return on the FPA. Once the correct laser return is identified, the seeker can position a track gate about the designated target identified by the correct laser return and generate guidance commands to guide a missile to a target. The concept will enable SAL, I2R, or SAL/I2R (simultaneous) processing to be accomplished. Guidance commands will be generated with the FPA detector only, using the multispectral data.
This paper outlines a generalized image reconstruction approach that improves the resolution of an Electro-Optic (EO) imaging sensor based on multiple frame exposures during a temporal window of video. Such an approach is innovative in that it does not depend on controlled micro dithering of the camera, nor require the set of exposures to maintain any strictly defined transformation. It suffices to assume such video is physically captured by a focal plane array, and loosely requires some relative motion between sensor and subject.
Recent changes in the infrared industry, specifically, the switch to next generation focal plane array infrared detectors, has increased the number and type of Thermoelectric Thermal Reference Sources (TTRS) in the market place. Until now little has been published to aid design engineers incorporating TTRSs into infrared systems. This paper will describe the optical, system, electrical, and mechanical parameters of TTRSs. It will provide infrared system design engineers with an overview of critical TTRS parameters as an aid during their design process.
An approach to determining the fidelity of synthetic noise based on the NVESD 3-D noise methodology is introduced. After reviewing the theory of 3-D noise analysis, three methods for the generation of synthetic noise are outlined: power spectrum matching, physical noise modeling, and 3-D noise parameter inversion. Real and synthetic noise samples from a staring thermal sensor were analyzed using a 3-D noise analysis. A series of simulated minimum resolvable temperature difference (MRTD) experiments using real and synthetic noise were also conducted. Results from the analysis and the MRTD indicate that modifications to the 3-D noise parameter inversion method could provide a more accurate match to real MRTDs. In addition, the MRTD method used in this research could be used to quantify the relative impact of each component of the 3-D noise model.
Current target acquisition models are for monochrome imagery systems (single detector). The increasing interest in multispectral infrared systems and color daylight imagers highlights the need for models that describe the target acquisition process for color systems (2 or more detectors). This study investigates the detection of simple color targets in a noise color background.
To ensure that an imaging system supplies images that meet the needs of the observation mission, it is important to assess the imaging system performance. In this paper we propose a new quality criterion for imaging system assessment. This method is based on three parts: end-to-end analysis of the imaging system, standardized expression of the mission by an objective definition of the observation tasks and 'a priori' knowledge of the properties of the objects to be observed. This quality criterion can also be used as a tool to aid in the design of observation systems based on the properties of the objects to be observed.
This paper discusses the model's atmospheric components, past efforts to validate a turbulence blur model, current efforts to model turbulence distortion effects, and intended future work to include additional atmospheric effects.
Perception tests have been performed by Night Vision and Electronic Sensors Directorate (NVESD) addressing the process of searching an image with the intent of detecting a target of military importance. The imagery used in the experiments was generated using NVESD's Paint-the-Night (PTN) thermal image simulation. The use of PTN simulation permits the same scene and target to be viewed with different sensor characteristics (such as resolution, noise and sampling). This allows the isolation of single variables in an experimental environment and the evaluations of their effect on probability of detection. Typical first and second generation FLIR sensor effects were applied to each of 100 synthetic images resulting in an experimental data set with identical thermal signatures and sensor fields of view. Experimental results are presented and the advantages of using synthetic imagery to evaluate differences in sensor resolution, noise, and other characteristics are discussed.
Proc. SPIE 4372, Designing and testing of optronic sensors and systems using real-time simulation, virtual prototyping, and computer image generation, 0000 (10 September 2001); https://doi.org/10.1117/12.439156
For ten years, Thales Optronique has developed its own simulation facilities to support research and development of various optronic sensors and systems such as IRST, Laser Designation Pods or Recce systems. These facilities have allowed the company to reduce the level of risks and optimize the design in order to meet the operational needs at lower costs. Simulation facilities include man-in-the- loop real-time `technico-operational simulations' to help system specification and operational validation in an interactive synthetic environment, and `technical fine simulations' to support the design and test of the system and its components.