There are several techniques for calculating apparent target-to-background differential temperatures, or apparent delta T. Apparent differential temperatures are equivalent blackbody differential temperatures that represent a target- to-background contrast with degradation due to atmospheric effects. Apparent delta T depends on a number of parameters including source emission characteristics and atmospheric transmission characteristics. Techniques for calculating apparent delta T range from broadband temperature and transmission calculations to spectral transcendental equations. Various techniques for calculating apparent delta T are reviewed in this paper. Six calculations are described and applied to four example scenarios. The four scenarios include longwave and midwave bands with humid summer and dry winter climates. Six apparent delta T's are plotted as a function of range for these scenarios. The techniques are compared and contrasted and descriptions of their corresponding errors are provided.

The design of satellite based multispectral imaging systems requires the consideration of a number of tradeoffs between cost and performance. The authors have recently been involved in the design and evaluation of a satellite based multispectral sensor operating from the visible through the long wavelength IR. The criteria that led to some of the proposed designs and the modeling used to evaluate and fine tune the designs will both be discussed. These criteria emphasized the use of bands for surface temperature retrieval and the correction of atmospheric effects. The impact of cost estimate changes on the final design will also be discussed.

A sensor system simulation has been developed which aids in the evaluation of a proposed fast framing staring sensor as it will perform in its operational environment. Beginning with a high resolution input image, a sequence of frames at the target sensor resolution are produced using the assumed platform motion and the contribution of various noise sources as input data. The resulting frame sequence can then be used to help define system requirements, to aid algorithm development, and to predict system performance. In order to assess the performance of a sensor system, the radiance measured by the system is modeled using a variety of scenarios. For performance prediction, the modeling effort is directed toward providing the ability to determine the minimum Noise Equivalent Target (NET) intensities for each band of the sensor system. The NET is calculated at the entrance pupil of the instrument in such a way that the results can be applied to a variety of point source targets and collection conditions. The intent is to facilitate further study within the user community as new mission areas and/or targets of interest develop that are not addressed explicitly during sensor conceptual design.

In FY-97 the Counter Drug Optical Upgrade (CDOU) demonstration program was initiated by the Program Executive Office for Counter Drug to increase the detection and classification ranges of P-3 counter drug aircraft by using advanced staring infrared sensors. The demonstration hardware is a `pin-for-pin' replacement of the AAS-36 Infrared Detection Set (IRDS) located under the nose radome of a P-3 aircraft. The hardware consists of a 3rd generation mid-wave infrared (MWIR) sensor integrated into a three axis-stabilized turret. The sensor, when installed on the P- 3, has a hemispheric field of regard and analysis has shown it will be capable of detecting and classifying Suspected Drug Trafficking Aircraft and Vessels at ranges several factors over the current IRDS. This paper will discuss the CDOU system and it's lab, ground, and flight evaluation results. Test targets included target templates, range targets, dedicated target boats, and targets of opportunity at the Naval Air Warfare Center Aircraft Division and at operational test sites. The objectives of these tests were to: (1) Validate the integration concept of the CDOU package into the P-3 aircraft. (2) Validate the end-to-end functionality of the system, including sensor/turret controls and recording of imagery during flight. (3) Evaluate the system sensitivity and resolution on a set of verified resolution targets templates. (4) Validate the ability of the 3rd generation MWIR sensor to detect and classify targets at a significantly increased range.

Recent developments on QWIP-LED detectors have led to Pixelless Imaging Devices. These detectors convert a thermal IR image into a near-IR image giving the possibility to image an IR scene at a higher resolution on the same detector area. Their use into a surveillance system is of great interest. The aim of this theoretical study is to compare the Signal-to-Noise ratio obtained with different spectral bands of these new pixelless sensors.

A computer model has been developed to predict the probability of recognition of particular shapes when viewed through a thermal imager employing either scanned or focal plane array detectors. This model is based on the results of a series of psychophysical trials during which human observers have considered over 120,000 images of shapes having a range of initial contrasts, and which have been degraded by various combinations of blurring and sampling. These computer generated images were presented to the observers in a random order and with a random degradation, using programs to select images and display them on a computer monitor. After each presentation the observer decided which was the most likely shape to represent the image displayed on the screen. The responses collected have been used to calculate the human recognition probability of each image. A correlation has been found between the probability of recognition of any specified degraded shape and the relative contrast between the image of that shape, and the image of a similarly degraded circle of the same area. This model has been extended to include the effects of fixed pattern noise and applied to simplified images of cars and vans.

To generate realistic synthetic IR images, required for training in mission rehearsal simulators, image acquisition by IR sensors must be reproduced. In this paper, we propose a general framework for IR sensor modeling which provides a physical basis for describing the geometric and radiometric relationship between the points in the observed scene and the corresponding pixels on the IR sensor output image. This framework is based on the combination of current camera models and draws upon both post-processing and ray tracing techniques. It thus offers more capabilities than standard IR sensor model structures based only on post-processing techniques: better accordance with sensor physics, higher modularity, more accurate sampling, computations suitable for parallelization and connection with any rendering algorithm. The framework enables development of modeling algorithms for each component of the IR image chain (optics, scanner, detector, electronics, signal and image processing) to match the system technology and the desired precision. The IR sensor model developed from this structure allows simulation of a wide range of technologies including staring and scanning systems based on thermal and photon detectors. It can also account for the variations in many physical magnitudes through spatial, spectral and temporal dimensions.

This paper documents the Matlab routines used to conduct infrared focal plane array (IR-FPA) sensor data analysis. Matlab is a commercially available software package that enables users to conduct a multitude of data analysis, file I/O, and generation of graphics with little or no computer programming skills. This effort was conducted in support of the US Army Tank-automotive and Armaments Command-Armament Research, Development and Engineering Center's (TACOM-ARDEC) 120 mm Precision Guided Mortar Munition (PGMM). PGMM's sensor included a 256 X 256 mid-band IR-FPA. This paper summarizes a primer generated to help train PGMM sensor engineers to use Matlab for conducting IR-FPA image analysis. A brief system description of the PGMM IR sensor will be presented, and follow by discussion on the Matlab IR-FPA image analysis, such as measurement of; FPA operability, Noise Equivalent Temperature Difference, temporal noise, spatial noise, as well as gain and offset calibration for non-uniformity correction.

We evaluate the contribution of the temperature-dependent emissivity to the temperature-dependent gray-body incidence detected in a wavelength interval with a thermal detector. This term, previously neglected, is shown to be significant, resulting in a 20% error for the traditional applications. The infection of vegetation with a decrease will modify its emissivity, its change of emissivity with temperature, and its temperature. The results presented here may be applied productively to monitor remotely the health of fragile crops, grown on extended areas.

The Minimum Resolvable Temperature Difference (MRTD) is the standard for measuring the performance of infrared imaging systems. Refined and validated modeling programs can accurately predict MRTD for scanning and staring Forward Looking Infrared (FLIR) imaging systems operating at video frame rates. However, there is a need to predict the MRTD performance of infrared systems that display imagery as static frames. Infrared imaging systems used for reconnaissance operate at low frame rates of about 1 to 5 Hz (framing cameras), or continuously gather imagery line by line (line scanners). Typically, each image is of a different scene and is displayed as a static image or in a waterfall display. Under normal lighting conditions, the human eye has a temporal bandwidth of approximately 10 Hz. Therefore, the perceived sensitivity, measured at MRTD, of these low frame rate systems is lower than a comparable video frame rate imaging system. The low frame rate systems do not benefit from the temporal filtering effect of the human eye as video frame rate systems do, and should exhibit a higher MRTD. This paper presents data comparing predicted MRTD performance calculated by the FLIR92 program with measured performance.

A model for Image Generation in Optronic (electro-optical) Sensor Systems (IGOSS) has been developed at the Defence Research Establishment in Sweden. The aim of this model is to study and evaluate different optronic sensor systems, and to use it as a tool in technical duel simulations. The model can operate on any input image, or on a sequence of images, and the user can see the resulting image after it has been processed by the different components in the system. It is also possible to use the model for calculations of the probability of detection as a function of distance for a target with a given size. To describe the performance characteristics of an electro-optical sensor system, IGOSS models the effects of vibrations, the optics, the detector, the processing unit, the display, the eye, and transversal movement of the sensor platform and/or a separate target in the background image. The performance characteristics is as far as possible described by modulation transfer functions (MTF) that are either calculated or read from a file. However, several effects are included in the model where MTFs can not be used. Example of these effects are: vignetting, sampling and a non-ideal fill factor of detector elements in the detector, detector generated noise, non- uniformity of the detector elements, and automatic gain control. The user can at run time decide what components or processes to use when the system is modeled. It is also possible to describe a particular component using different parameter values or type of transfer function.

Spectral response dispersion has a significant impact on the radiometric performances of infrared imaging system using detectors with a large amount of pixels. The main contributors of this dispersion are the detector itself, which contributes to the highest spatial dispersion, and the optical components and filters which contribute with smaller frequencies. This paper proposes a theoretical approach of the contribution of spectral response dispersion to the spatial noise, considering classical calibration methods with blackbodies. AEROSPATIALE has performed measurements of spectral response dispersion on large format 2D infrared detectors. The performances obtained taking into account these test results are given in order to identify the real weigh of spectral response dispersion effects on the infrared imaging system.

Current infra-red focal point arrays are limited by their inability to calibrate out component variations. Nonuniformity correction (NUC) techniques have been developed and implemented in off-board digital hardware to perform the necessary calibration for most IR sensing applications. There are two possible types of NUC that can be considered for focal-plane integration: (1) Two-point correction using calibrated images on startup and (2) Scene- based techniques that continually recalibrate the sensor for parameter drifts. The problems with the two-point methods have been well-documented in the literature (parameter drift, expense, etc.) We address the two major problems of scene-based techniques: (1) a more difficult hardware implementation and (2) ghosting artifacts in the corrected images. We have previously addressed the implementation problems by developing and demonstrating special purpose analog hardware as well as an efficient digital algorithm that incorporates the constant statistics model. The ghosting artifact occurs in all scene-based techniques when an object that does not move enough tends to `burn in' and can remain visible for thousands of images after the object has left the field of view. We have improved our model to eliminate much of the ghosting artifact that plagues all scene-based NUC algorithms. By modifying the correction update during ghosting situations, we are able to significantly remove the ghosting artifact and improve the overall accuracy of the correction procedure. We demonstrate these results on real and synthetic image sequences.

Currently, there are several kinds of infrared imaging system with 2D detector using PtSi, InSb, MCT and any others in the commercial market. The infrared imaging system with 2D detector is generally required to correct non-uniformity of response of each pixel in order to display satisfactory infrared images. This non-uniformity correction is normally performed digitally after an ADC. Furthermore, this operation should be finished within a field time, therefore it must work correspondingly fast. PtSi detector has non- linear Input/Output characteristics because of their spectral response characteristics. Therefore, two point gain/offset correction is not suitable for non-linear characteristics detector. This paper reports on three alternate methods for non-uniformity correction for PtSi detector. These methods are actually tested and then the uniformity performances after these corrections are measured. The standard deviation of the residual fixed pattern noise is used to evaluate the performance of non- uniformity correction.

A statistical model for the focal-plane array (FPA) output is developed characterizing the random nature of nonuniformity in time and space. The rationale of this method is that current and past outputs of the FPA bear information about the nonuniformity. Using a statistical algorithm, this hidden information about the random nonuniformity can be extracted and used to restore the true image. The proposed algorithm consists of two main parts. The first part involves a periodic statistical estimation of the model parameters using current data. The second part involves utilizing the estimated parameters in restoring the true image by means of a least mean square FIR filter whose coefficients remain unchanged between the rounds of parameter estimation. This model-based approach exploits the slow drift of the sensors' offset voltage, gain, and circuit noise in order to reduce the necessary computations to a minimum.

As high density staring focal plane array (FPA) detectors become available, imaging IR sensors must be constructed to display high performance images in compact packages. To make best use of engineering development efforts, the sensor packages must be designed to address multiple mission environments and to create displays for several detector types. Cincinnati Electronics has developed a 640 X 512 pixel InSb FPA and a 2-axis, micro-scanned, 256 X 256 array. These FPA alternatives have been incorporated into a new design sensor package utilizing a compact set of electronics and a miniature Stirling cryo-cooler. This camera design addresses a variety of missions ranging from an open frame OEM package to custom environmental housings. The detector interface can be electrically reconfigured to a variety of IR detectors, including medium or high-density arrays, as well as one or two dimensionally micro-scanned systems. This new packaging design demonstrates improvements over current systems in several performance areas: weight, package size, system sensitivity, system resolution, and fast Non-Uniformity Corrections. The device supports local or completely remote command operation. Rapid in-field reconfiguration is supported from an external connection. Digital and TV-standard analog display video are supported. Several examples are given of system tradeoffs in resolution and sensitivity to optimize system performance, with different FPA configurations for specific mission requirements.

This paper describes a low cost, high performance, and hand- held staring Platinum Silicide (PtSi) medium wave infrared imaging system developed by Chung Shan Institute of Science and Technology. The infrared imaging system utilizes an monolithic 256 X 244 pixels PtSi staring focal plane array sensor, cooled by a stirling cryocooler. The sensor is integrated into a miniature dewar/cooler assemblies; this approach provides unsurpassed reliability and low power consumption at affordable cost. The camera system achieves an Noise Equivalent Temperature Difference of less than 0.1 degree(s)K at 300 degree(s)K background with f/1.4 optics at 30 Hz frames rate. At a spatial frequency of 1/2 Nyquist frequency the vertical and horizontal Minimum Resolvable Temperature Difference are in the range of 0.12 degree(s)K.

One of the sub-tasks for the GOES N/O program is to redesign the assembly/test program to reduce instrument test time and to make test results more comparable to data that can be taken on orbit and at the spacecraft integrator's facility. Currently, ITT A/CD measures the instrument MTF at specific spatial frequencies using a collimator and bar targets. This gives only a few individual points on the MTF curve. To improve this production test, the MTF will now be measured by scanning a single slit target. The result of this slit scan is the Line Spread Function (LSF) and the Fourier Transform of the LSF yields the continuous frequency MTF curve. Obtaining this curve with a single target eliminates the need for repeated scans using different spatial frequency targets. A problem with scanning a slit for this application is the number of samples taken across it is very few when the GOES scanner is running at operational speed. Two methods to collect multiple scan lines and recombining the scans to get a `highly sampled' scan of the slit have been considered. This paper discusses the two approaches and presents a validation of the slit scan method for both a slow scan of the target and for an operational speed scan where multiple scan lines are recombined to give a highly sampled slit.

This paper addresses the problem of determining the amount of aliasing reduction achieved in a dithered infrared imaging system. Usually the elements of an infrared focal plane array (IRFPA) stare at fixed positions in space. Without scanning, the scene is unavoidably undersampled as long as the optical blur spot is smaller than the IRFPA elements. To increase the sampling rate, the scene can be optically dithered to allow the image to be augmented by an additional set of samples offset relative to the initial set by half the width of the detector elements. This technique is commonly referred to as microscanning. There is a need for laboratory test procedures with which to ascertain the effectiveness of the dithering technique. This paper describes a technique for determining the effectiveness of aliasing reduction through analysis of images collected using a canted unresolved slit. The procedure uses the 2D fast Fourier transform (FFT). The baseband component of the FFT yields information on the system MTF, whereas the component centered about the sampling frequency reveals the amount of aliasing. The analysis yields an estimate of the dither positioning error, where the maximum error corresponds to mere pixel replication with no effective aliasing reduction. The approach demonstrates a technique for objective assessment of imaging IR performance that has application in acceptance testing procedures.

An objective methodology that can be used to perform automatic MRTD tests on infrared imaging systems is presented. It is based on the assumption that a unique threshold function should exist between the signal-to-noise ratio measured by a computer that perform a spatio-temporal filtering on digitized images and the MRTD.

Non isoplanatism and aliasing properties of undersampled staring array cameras make their resolution measurement difficult. In this paper, two MTF assessment methods are presented, thanks to specific gray-level test targets, composed of a set of wavelet patterns distributed according to a fractal algorithm. First, a random multiscale wavelet pattern distribution allows a statistical averaged measurement of the camera point spread function, providing an averaged global MTF. The second method consists in setting the measurement free from the aliasing effects. Its aim is to reconstruct the optical and detector MTF, but also the frequency response of the reconstruction filter. This method uses a steady distribution of wavelet patterns and is based on the shape distortion analyse of the output wavelet patterns from the camera. The image processing analyse procedure provides an automatic measurement technique without mechanical adjustment requirements. The feasibility of the second method is checked by a computer simulation of a modelled camera measurement.

In the past, ad-hoc and manual testing of infrared images hasn't been a deterrent to the characterization of these systems due to the low volume of production and high ratio of skilled personnel to the quantity of units under test. However, with higher volume production, increasing numbers of development labs in emerging markets, and the push towards less expensive, faster development cycles, there is a strong need for standardized testing that is quickly configurable by test engineers, which can be run by less experienced test technicians, and which produce repeatable, accurate results. The IRWindows^TM system addresses these needs using a standard computing platform and existing automated IR test equipment. This paper looks at the general capabilities of the IRWindows^TM system, and then examines the specific results from its application in the PalmIR and Automotive IR production environments.

This paper surveys the projected advancements in technology areas applicable to imaging infrared seeker designs for tactical missiles projected to exist between the years 2010 to 2020. The results are part of a study conducted by the Signature Technology Laboratory in the Georgia Tech Research Institute under the direction of Air Force Research Laboratories. This paper reviews a wide variety of databases for published technical literature describing advances in critical technologies for tactical missile systems. The information provides forecasts of improvements in missile subsystem technologies to provide justifiable missile configurations for the given time frame. Although the paper reviews the state of the art in missile subsystems including the fuse, warhead, flight control, and propulsion systems, emphasis is on the seeker and acquisition and tracking algorithms. The seeker subsystems addressed include the dome, optics, detector, stabilization system, readout, analog-to-digital conversion, and signal processor. The study analyzes the strengths and weaknesses of current designs to identify key technology areas for improvement as well as to survey and forecast their development. Key parameters that describe the performance of the individual subsystems are identified and related to measures of effectiveness for the individual components and overall missile system. Design equations describing the performance of the advanced configurations are documented and used to predict the effectiveness of future designs. As part of the study, these predictions will then be applied to postulate the hardware and software configurations for two missile designs anticipated for the 2010 to 2020 time frame in sufficient detail to project their performance.

In many IR imagers the resolution is determined by the finite size of the detector elements rather than inherent optical limitations, resulting in undersampled images. If a number of frames are collected in which the sharply-focused image of an object has moved across the detectors, the sequence of frames will contain more information on the object than any single frame. Over the past few years there have been a number of papers reporting success at producing a single high-resolution frame from such lower-resolution image sequences. In the current study techniques are described for the real-time enhancement of aircraft images during air-to-air combat. Of particular interest is the early identification of the target aircraft during a merge: in this situation the aircraft image expands on closure and produces a relative motion across the detector elements in addition to the jitter normally experienced. Issues covered include the need for accurate determination of the target movement against the background and the handling of rotation and aspect changes of the target. One novel suggestion is the use of the intensity weighted centroid of the aircraft target for frame-to-frame matching. Examples of enhanced images are provided, including some synthesized air-combat scenarios.

The Missile Research Development and Engineering Center (MRDEC) of the US Army Aviation and Missile Command has an ongoing effort to develop trackability metrics for imaging target trackers. Experience with imaging IR imagery has shown that classic approaches to target signature or trackability metrics employing signal-to-clutter ratio expressions are inadequate for resolved targets. Analysis of real imagery suggests that these expressions are limited by the use of first-order statistics to quantify the structure of both the target and the background. In most imagery for example, the background can not be adequately described by a single Gaussian-like process and thus, violate fundamental assumptions of these approaches. This paper summarizes the results of a multi-year effort that has resulted in the development of a Gray-Level Co-occurrence Matrix based Trackability Metric. This metric has become the basis for algorithm development and performance evaluation tools at MRDEC for both imaging terminal-homing missile seekers and imaging fire control applications.

The optical fabrication, metrology, and system wavefront testing of an off-axis three mirror anastigmatic telescope will be presented. The telescope is part of a multi-band imaging system which includes a single mechanically cooled focal plane with 15 spectral bands covering a wavelength range from 0.45 microns to 10.7 microns and an on-board calibration subsystem. The imaging system is to be operated in a low earth orbit in a pushbroom scanning mode. The telescope has a 36 cm aperture, a 1.38 degree cross-track by 1.82 degree along-track field of view (FOV), near diffraction limited performance in the visible, and strictly diffraction limited performance from 1.3 microns to 10.7 microns. The primary and the tertiary mirrors are general aspheres which have undergone 80% lightweighting. The secondary mirror is a hyperbola. The primary mirror was extremely difficult to fabricate and test due to its large departure from sphericity, fast f-number, and large off axis distance. The tertiary mirror has a small departure from sphericity and is only slightly off-axis, but it has a very fast f-number also. The surface wavefront measurements for the three mirrors after final figuring and lightweighting are 0.048 waves rms at 0.6328 microns for the primary mirror and 0.025 waves rms at 0.6328 microns for the secondary and tertiary mirrors. The telescope wavefront requirement at the center of the along-track FOV is 0.178 waves rms at 0.6328 microns and at the edge of the along-track FOV is 0.677 waves rms at 0.6328 microns.

This paper describes a method for transforming measured optical and infrared filter data for use with optical systems of arbitrary f-number and angle of incidence. Although it is generally desirable to have normal incidence at the filter (i.e., collimated light where an optical filter is used), other system design considerations may take precedence. In the case of a multispectral sensor under development at Sandia National Laboratories, system constraints require optical filter placement very near the focal plane. The light rays incident on the filters are therefore converging as determined by the system f-number while the chief ray of each ray bundle varies with focal plane position. To analyze the system's spectral response at different points on the focal plane, a method was devised to transform the filter vendor's measured data to account for the optical system design. The key to the transformation is the determination of weighting factors and shift factors for each angle of incidence making up a ray bundle. A computer worksheet was developed using a popular mathematical software package which performs this transformation for 75 key points on the focal plane.

The background irradiances in different temperatures are calculated to find its relation with the measured NETD (Noise Equivalent Temperature Difference). In the nonuniformity correction, it always uses the active pixels to cover the dead pixels so that the final value of NETD is an average. Some experimental results show that the video waveform of thermal imaging depends on the setting of the level or the gain. These factors always influence the measured data of NETD. Nevertheless, NETD is a good diagnostic tool for determining the quality of the thermal imager. A NETD analysis for 1-point and 2-point correction is attached at the end of this article.

Reliability of generation II image intensifier is discussed in this paper. Based on the engineering practice, some problems in design, manufacturing and using that have an influence on the reliability of an image intensifier are analyzed.

The current standard to characterize Electro-Optical system performance is the MRTD (Minimum Resolvable Temperature Difference) for thermal imagers and the MRC (Minimum Resolvable Contrast) for visual devices. This standard has at least three serious disadvantages: (1) the standard 4-bar test pattern is theoretically and practically unsuitable for 1D or 2D spatially sampled systems such as pixel-array camera's, (2) spatial phase is not taken into account, and (3) the results depend on the observer's subjective decision criterion. We propose an adequate and easily applicable alternative: TOD (Triangle Orientation Discrimination threshold). The TOD is based on an improved test pattern, a better defined observer task, and a solid psychophysical measurement procedure. The new method has theoretical and practical advantages: it is suitable for pixel-array camera's, scanning systems and other (Electro-) Optical imaging systems in both the thermal and vision domain, it has a close relationship to real target acquisition, and the observer task is easy. The results are free from observer bias and allow statistical significance tests. The method lends itself very well for automatic measurement, and can be extended for future sensor systems that include advanced image processing. The TOD curve can be implemented easily in a TA model such as ACQUIRE. An observer performance study with real targets shows that the TOD curve predicts TA performance better than the MRC does. The method has been implemented successfully in a thermal imager field test apparatus called TIPI, and may be implemented in current MRTD and MRC test equipment with little effort.