Two designs of wavelength detector intended for field applications are presented. The first design makes use of a pair of sensors located behind a linear variable filter. The active area of one sensor increases, whereas that of the other sensor decreases, along the direction of increasing value of transmission wavelength of the filter. The second design involves the use of a pair of identical sensors, one of which is coated with a spectrally variable attenuator. For both designs, by computing the ratio of the photoresponse of one sensor to the other, the wavelength of monochromatic light is obtained from a calibration chart. Experimental wavelength detectors were constructed using superconductor and semiconductor dual sensors. The results obtained in the spectral range of 0.6 - 7 micrometer showed that sensitive, accurate detection of wavelength can be achieved for a wide range of incident angles.

Reticle systems, which are widely used in infrared (IR) missile seekers, are considered to be the classical approach for estimating the position of a target in the field of view (FOV). This paper presents a new simulation tool that gives tracking results of the concentric annular ring reticle seeker in various cases. Our simulation model is applicable to the development of the advanced seekers. While false targets such as flares are presented in the FOV, simulation results show that the existing seeker cannot determine a precise target location. In order to decrease the susceptibility to countermeasures such as flares, we propose an efficient counter-countermeasure using signal prediction. In the simulation results, we have ascertained that the reticle seekers using our technique can perform more effective target tracking than previous seekers.

A dynamic simulator of the reticle seekers has been developed to predict the tracking performance in various engagement scenarios. We can analyze the performance of the infrared (IR) reticle seekers with various positions of targets in the field of view, with several signal-to-noise ratios. We have shown that the presented simulator is a very powerful means for developing the signal processing algorithms in the presence of infrared countermeasures (IRCM). A simple and efficient algorithm for infrared counter-countermeasures (IRCCM) is proposed and simulated. With the incorporation of this IRCCM technique into the reticle seeker in the presence of IRCM, the missile can intercept its target successfully.

The detection and discrimination of man-made objects in a terrain or sky background has long been a challenge to the military. Conventional infrared (IR) systems suffer from poor spatial resolution and have a difficult time imaging targets when there is little or no thermal variation ((Delta) T) in the scene. These applications, as well as applications such as aircraft ice detection, can benefit from an imaging system that can overcome these and other limitations. In this paper an enhanced IR imaging sensor which overcomes the above shortcomings is described. Its advantages are detailed and accompanied by numerous experimental examples. The focus in this paper is on the performance of the sensor, and the benefits derived therefrom, not on sensor processing theory.

A concept for a spaceborne tactical sensor has been developed, which will have the ability to detect and geolocate targets of interest in near real time. A complement of panchromatic, multispectral and hyperspectral imaging sensors allows flexibility for high spectral and high spatial feature extraction and exploitation. The concept includes a high performance, compact optical design. The key advantage of this tactical sensor is an integrated system capability which allows simultaneous operation of the imaging sensors.

A miniaturized digital 1-band HARLID module using linear silicon detector arrays has been developed. These HARLID modules, which fit inside standard TO-8 packages, were designed to locate angularly a pulsed laser source within plus or minus 1 degree over a 90 degree field of view either in azimuth or elevation. The principle of operation of this new patented-module is based on the use of a Gray code ask to encode the angle of arrival of a laser beam. The electro- optical (E-O) performance of this new module has been evaluated in the laboratory. A laser warning receiver (LWR) demonstrator integrating two of these modules has been built and its E-O performance measured in the laboratory and in a field environment aboard a tank. A new 2-band HARLID module now under development will include a sandwich of Si and InGaAs detector arrays and will extend the spectral band of the HARLID from 0.4 to 1.7 micron while increasing significantly its responsivity at 1.064 micrometer. A study of its technical characteristics and limitations has been recently completed and future HARLID technology development plan established.

A new reflow method for indium bump of hybridized HgCdTe IRFPA is proposed using H₂ plasma. Twenty micrometer height indium bump is easily achieved with this method. In the new method, H₂ plasma makes the indium bump surface clean with removing the oxidized indium by H radical chemical reaction. Simultaneously, H₂ plasma increases the temperature of indium bump above 160 degrees Celsius. This sphere shaped bump is easily deformed plastically with relatively small force. Force of 2 g/bump changes the 20 micrometer height bump to 10 micrometer. The flip-chip bonding technique using the new reflow method is characterized with shear strain strength measurement. It is found that bonding reliability can be improved owing to increased height and smooth surface.

We have investigated the reverse current-voltage characteristics of both 3 - 5 micrometer and 8 - 10 micrometer band HgCdTe photodiodes under background illumination in the temperature range of 40 K to 120 K. The experimental results show that the differential resistance of reverse biased photodiodes decreases with an increase in the incident photon flux density. In the higher reverse biased illuminated photodiodes, breakdown occurs. The reverse differential resistance is limited by this breakdown mechanism. In order to determine the causal mechanism, we calculated the photocurrent multiplication resulting from the electron impact ionization in the photodiode depletion region. For the calculation, we applied Shockley's 'lucky electron' equation assuming that only electrons multiply the photocurrent. The calculated reverse differential resistance-voltage characteristics agrees well with the measured results. The calculated differential resistance at a low reverse bias voltage as a function of incident photon flux density corresponds to the measured results in the temperature range of 40 K to 100 K. This indicates that the photocurrent multiplication by electron impact ionization occurs in the photodiode depletion region. In the low temperature region, the measured differential resistance increases with the decrease in temperature. This is caused by the acceptor freeze-out effects. We concluded that the differential resistance-voltage characteristics of HgCdTe photodiodes are limited by the effect of photocurrent multiplication, and the photomultiplication effect is limited by the carrier freeze-out in the low temperature region.

In this paper, we report electrical and optical properties of the rapid thermal diffused (RTD) p-n junction photodiodes fabricated on LPE-grown p-type Hg_0.70Cd_0.30Te/CdZnTe substrate. In comparison with the ion implanted p-n junction on the same substrate, the reverse bias tunneling current is drastically suppressed in the RTD junction. The spectral photo-response of indium diffused HgCdTe photodiode shows the high quantum efficiency and the detectivity of 1.3 by 10¹¹ cm/Hz^1/2W. The suppression of the reverse bias leakage current, high quantum efficiency and low noise of the RTD photodiode could be explained by the suppression of the electrical active defects generation in the depletion region during the junction formation. The extracted carrier lifetime in the junction depletion region of the RTD HgCdTe photodiodes is larger than that of the ion-implanted one.

Since 1991 Cryotechnologies undertook extensive research and development on miniature pulse tube cryocoolers in order to give an outstanding alternative to the Stirling cryocoolers for up coming applications. This paper summarizes the research and engineering work done so far and describes the ability of this technology to meet current and future requirements in the military and commercial fields.

We report the results of annealing effect on the Hg_0.78Cd_0.22Te diodes fabricated by ion-implantation technique. The annealing was performed after flip-chip bonding with Si substrate. The performances of the diodes, before and after the annealing process, were investigated in detail by model fitting analyses. This model includes five current components, such as diffusion current, generation- recombination current, band to band tunneling current, trap- assisted tunnelling current, and photo current. Especially, in the view of a trap-assisted tunneling mechanism, newly developed model is proposed with the introduction of the Poole-Frenkel effect. Using this model, it is well explained that measured RoA product is much lower than their theoretical values in the ideally diffusion limited or generation- recombination limited cases. By flip-chip bonded annealing, RoA products of the diodes were increased and dark currents were decreased. From the model fitting, the improvements are explained by the change of carrier concentration profile in a p-n junction and the reduction of trap density by the annealing process.

In this paper, we propose a new unit cell, named current mirroring direct injection (CMDI) circuit for focal plane arrays. With the current mirroring effect, the circuit designed to control detector bias automatically. Its characteristics are derived analytically, and compared with those of direct injection (DI) and buffered direct injection (BDI). With the CMDI approach, we can achieve almost zero input impedance. Therefore, almost 100% injection efficiency can be obtained even for low R_DA values. It has also other advantages such as near zero detector bias, relatively small area and low power consumption.

A 256 by 256 IRCMOS array with a 35 micron pitch operating at 88 K and above 10 microns has been developed at LETI/LIR. High performances have been obtained owing on one hand to a reduced dark current detector technology and on the other hand to a new readout circuit architecture which maximizes both charge handling capacity and responsivity. We have measured a NEDT of 13 mK at 88 K for a cutoff wavelength of 10.1 micrometer. A description of the array is given and the main electro-optical characteristics of the component are presented.

Current tendencies in infrared arrays are to decrease the pitch and increase the number of detectors without degrading the electro-optical performances. It is therefore necessary to maximize the charge-handling capacity in the pixel. In this paper, a new architecture particularly-well suited to this kind of application is described. A brief review of classical readout circuits is given. The advantages and drawbacks of these architectures are emphasized. The new architecture is discussed in detail, compared to existing ones and the performance of the new readout circuit evaluated. Results measured on IRCMOS designed with the new architecture are presented.

This paper presents system trade-offs necessary to deliver the high performance available from the emerging UK second generation long-linear detector technology, in the long wave infra-red (LWIR) waveband. These trade-offs include thermal referencing methods, dynamic range, and scan configurations. Experience and measurements from the new GEC-Marconi Sensors' LOLA Imager are used to support the arguments. Also indicated are the trade-offs leading to optimized systems for roles in demanding applications such as advanced airborne targeting.

Santa Barbara Focalplane has recently added a versatile large- format MWIR camera to its product family. The camera features a dynamically moveable window ranging in size from 128 X 8 (greater than 4 KHz frame rates) up to 640 X 512 (120 Hz frame rates) with an 8 pixel resolution. This paper addresses the FPA architecture along with various performance parameters, noise, resolution issues, and some early measurements.

We have developed an 801 by 512 element PtSi Schottky-barrier FPA with low-power multiphase CSD readout for a compact high performance infrared camera. The power consumption of this detector is about a half of the conventional IRCSD and the number of pixels is 1.5 times of that. The cryocooler is a high efficiency and compact size Stirling-cycle cooler that has also been developed for this imager at the same time. The cooling capacity is just match for the new IRCSD. This paper describes the basic imager design and features.

A new gimbal-based, FLIR camera for several types of airborne platforms has been developed. The FLIR is based on a PtSi on silicon technology: developed for high volume and minimum cost. The gimbal scans an area of 360 degrees in azimuth and an elevation range of plus 15 degrees to minus 105 degrees. It is stabilized to 25 (mu) Rad-rms. A combination of uniformity correction, defect substitution, and compact optics results in a long range, low cost FLIR for all low-speed airborne platforms.

The pyroelectric thermal detectors were prepared with lead zirconate titanate (PZT) ceramics, where a signal electrode had a structure of Au/metallic buffer/(PZT ceramic). The effect of buffer layer on the voltage responsivity was investigated with a response to step signal, taken by dynamic pyroelectric measurement. Pyroelectric ceramic wafer was prepared by mixed oxide technique. Au layer (thickness: 50 nm) and metallic buffers (thickness: 0 - 20 nm) of Cr, NiCr (80/20), and Ti were prepared by dc magnetron sputtering. In order to improve the light absorptivity, an Au-black was coated on Au signal electrode by thermal evaporation. At steady state, the output voltage (V_o) was decreased with increasing chopping frequency in the range of 1 - 100 Hz. A sensor without buffer showed the severe time-drift and instability in the output signal. However, the sensors with buffer layer showed the stable outputs. For step radiations, rising time (t_p), peak voltage (V_p), and initial slope (k) of the output voltage were dependent upon the thickness and materials of buffer layer. The mechanical and electrical contacts between Au electrode and PZT ceramics were improved by inserting the metallic buffer layer. Considering the characteristics of the output voltage, the optimum thickness of buffer layer was about 15 - 20 nm, and the sensors with Ti buffer of 15 - 20 nm in thickness showed the good detectivity. Therefore, the stability and reliability of the thermal sensors could be improved by use of appropriate buffer layer.

There is widespread requirement for low cost lightweight thermal imaging sensors for both military and civilian applications. In Europe, these requirements are now being met by systems using large uncooled ferroelectric detector arrays offering performance levels which, until recently, could only be achieved by expensive cryogenically cooled systems. The uncooled technology is the result of collaboration between the UK Defence Research Agency (DRA) and GEC-Marconi Electro Optics (GMEO) under a 'dual use technology program' (DUTP). This has resulted in the design of an uncooled module suite which constitute the core elements of the European uncooled thermal imaging sensors. This module suite has been adopted by UK MoD to meet the requirements of the UK Stairs A demonstrator program producing a lightweight thermal surveillance and weapon sight. A European consortium including Delft Instruments and Signaal Usfa (DISU) in Holland have also adopted the modules to meet the requirements of the lightweight infrared observation nightsight (LION). This product is being marketed in collaboration with Thomson CSF Optronics, of France. This paper discusses the current status of the underlying detector and processing technology including enhancements already in development. In addition, the designs and benefits offered by the Stairs A and LION products are addressed.

Lockheed Martin IR Imaging Systems is developing low cost, high performance, uncooled infrared imaging products for both military and commercial applications. These products are based on the microbolometer technology, a silicon micromachined sensor that combines wafer level silicon processing with a device structure capable of yielding excellent imaging performance. Here we report on the latest technical improvements and performance of an uncooled sensor as measured through laboratory and field testing. The performance of our uncooled sensor has been measured to determine sensor capabilities for insertion into both military and commercial products. Linearity of the sensor over a scene temperature range of 95 degrees Celsius is less than 0.5%. Our sensors typically have temporal NETDs of less than 70 mK as well as spatial NETDs of less than 50 mK. MRTD performance is less than 0.4 degrees Celsius at spatial frequencies more than 20% beyond Nyquist. Sensor stability over time has been measured and found to meet both commercial and military requirements. Spatial noise over a wide scene temperature range is reported as well as other test results. Video is used to demonstrate sensor performance capabilities in a variety of applications.

A 128 by 128 pixel bolometer infrared focal plane array using thin film titanium has been developed. The device is a monolithically integrated structure with a titanium bolometer detector located over a CMOS circuit that reads out the bolometer's signals. Since the thermal conductance of the bolometer detector is minimized, the temperature of the detector itself is increased by applying the bias current. Under the present operating conditions of the titanium bolometer, this temperature increase becomes about 30 degrees Celsius. The influence of this bias heating on device destruction and degradation was experimentally investigated and is discussed. The noise equivalent temperature difference obtained with the device is 0.07 degrees Celsius. Since the fabrication process is silicon-process compatible, costs can be kept low.

The bi-material concept for room-temperature infrared imaging has the potential of reaching an NE(Delta) T approaching the theoretical limit because of its high responsivity and low noise. The approach, which is 100% compatible with silicon IC foundry processing, utilizes a novel combination of surface micromachining and conventional integrated circuits to produce a bimaterial thermally sensitive element that controls the position of a capacitive plate coupled to the input of a low noise MOS amplifier. This approach can achieve the high sensitivity, the low weight, and the low cost necessary for equipment such as helmet mounted IR viewers and IR rifle sights. The pixel design has the following benefits: (1) an order of magnitude improvement in NE(Delta) T due to extremely high sensitivity and low noise, (2) low cost due to 100% silicon IC compatibility, (3) high image quality and increased yield due to ability to do offset and sensitivity corrections on the imager, pixel-by-pixel; (4) no cryogenic cooler and no high vacuum processing; and (5) commercial applications such as law enforcement, home security, and transportation safety. Two designs are presented. One is a 50 micrometer pixel using silicon nitride as the thermal isolation element that can achieve 5 mK NE(Delta) T; the other is a 29 micrometer pixel using silicon carbide that provides much higher thermal isolation and can achieve 10 mK NE(Delta) T.

Software neural nets hosted on a parallel processor can analyze input from an IR imager to evaluate the likelihood of a buried object. However, it is only recently that low weight, staring LWIR sensors have become available in uncooled formats at sensitivities that provide enough information for useful man-portable helmet mounted applications. The images from the IR are presented to a human user through a see-through display after processing and highlighting by a neural net housed in a fanny-pack. This paper describes the phenomenology of buried object detection in the infrared, the neural net based image processing, the helmet mounted IR sensor and the ergonomics of mounting a sensor to head gear. The maturing and commercialization of uncooled focal plane arrays and high density electronics enables lightweight, low cost, small camera packages that can be integrated with hard hats and military helmets. The head gear described has a noise equivalent delta temperature (NEDT) of less than 50 milliKelvin, consumes less than 10 watts and weighs about 1.5 kilograms.

Several prototypes of individual VO₂-based bolometric detectors and their arrays consisting of 64 by 64, 128 by 128 and 240 by 320 pixels are presented. The fabrication method and the device characterization results are described. Three types of readout integrated circuits for the arrays are also presented. A custom vacuum package for the IR bolometric detector arrays is descried.

We have developed hybrid 256 by 256 focal plane arrays (FPAs) using MBE grown HgCdTe(MCT) layers on Si substrates for 10 micrometer-wavelength band detection and successfully demonstrated infrared images for the first time. The characteristics of MCT-on-Si-substrate FPAs have been compared with those for MCT-on-GaAs-substrate FPAs. MCT epilayers grown on 3-inch Si substrates used in FPAs were found to have almost the same characteristics as MCT epilayers on GaAs, including etch pit density of 1 - 2 X 10⁶cm^-2 and p-type carrier concentration of 1 - 2 X 10¹⁶ cm^-3. The 256 by 256 photodiode array consists of n⁺-on-p junctions formed by boron-ion implantation and ZnS films for surface passivation. It was hybridized on a silicon readout circuit with an indium bump array. The mean value of R_OA for the diode array was measured and found to be 80 (Omega) cm² with a cutoff wavelength of 8.7 micrometer at 77 K; this is comparable to the typical value for a diode array using MCT grown on GaAs substrates. A diode array with 95% operability was placed in a camera system with which infrared images were taken, and high image sensitivity was found to be obtained.

We propose a new concept for long wavelength infrared imaging using a pixelless up-conversion device together with a CCD camera. The concept is applicable to wavelengths longer than the CCD response range (longer than about 1.1 micrometer). We present experimental results which support the scheme for infrared imaging. Our first device consists of a long wavelength p-type GaAs/AlGaAs quantum well infrared photodetector (QWIP) on top of which is grown a shorter wavelength InGaAs/GaAs light emitting diode (LED). Upon long wavelength infrared excitation of the QWIP, near infrared light is generated by the LED whose output is directed towards a commercial CCD camera where the up-converted image of the long wavelength infrared source object is formed.

Chemical imaging has tremendous potential as a quantitative imaging modality for chemical analysis. Chemical imaging integrates focal plane array technology and novel schemes for obtaining high spectral and spatial resolution imagery in the visible, near-infrared and mid-infrared. We have applied chemical imaging techniques to the characterization of a diverse array of materials, including human tissues, polymers, semiconductors, thin films, and corrosion products. This report describes recent technological advances in the development of liquid crystal tunable filter (LCTF) near- infrared imaging spectrometers suitable for high performance Raman microscopy. This technology is anticipated to have broad impact in a wide variety of commercial applications.

The TIRIS is a pushbroom long wave infrared imaging spectrometer designed to operate in the 7.5 to 14.0 micrometer spectral region from an airborne platform, using uncooled optics. The focal plane array is a 64 by 20 extrinsic Si:As detector operating at 10 K, providing 64 spectral bands with 0.1 micrometer spectral resolution, and 20 spatial pixels with 3.6 milliradians spatial resolution. A custom linear variable filter mounted over the focal plane acts to suppress near field radiation from the uncooled external optics. This dual- use sensor is developed to demonstrate the detection of plumes of toxic gases and pollutants in a downlooking mode.

A 9 micrometer cutoff 256 by 256 hand-held quantum well infrared photodetector (QWIP) camera has been demonstrated. Excellent imagery, with a noise equivalent differential temperature (NE(Delta) T) of 26 mK has been achieved. In this paper, we discuss the performance of this portable long- wavelength infrared camera in quantum efficiency, NE(Delta) T, minimum resolvable temperature difference (MRTD), uniformity, etc., and its applications in science, medicine and defense.

Infrared imaging of the breasts for breast cancer risk assessment with a second generation amber indium antimonide focal plane staring array system was found to produce images superior to a first generation Inframetrics scanning mercury cadmium telluride system. The second generation system had greater thermal sensitivity, more elements in the image and greater dynamic range, which resulted in a greater ability to demonstrate asymmetric heat patterns in the breasts of women being screened for breast cancer. Chi-square analysis for independence of the results from 220 patients with both the scanning and focal plane infrared imaging systems demonstrated that the results from the two systems were strongly associated with each other (p equals .0001). However, the improved image from the second generation focal plane infrared imaging system allowed more objective and quantitative visual analysis, compared to the very subjective qualitative results from the first generation infrared imaging system. The improved image also resulted in an increase in the sensitivity for asymmetric heat patterns with the second generation focal plane system and yielded an increase in the percentage of patients with an abnormal asymmetric infrared image of the breasts from 32.7% with the scanning system to 50.5% with the focal plane system. The greater sensitivity and resolution of the digitized images from the second generation infrared imaging system has also allowed computer assisted image analysis of both breasts, breast quadrants and hot spots to produce quantitative measurements (mean, standard deviation, median, minimum and maximum temperatures) of asymmetric infrared abnormalities.

A Small Business Innovative Research (SBIR) contract was recently awarded to a start up company for the development of an infrared (IR) image based combat casualty care system. The company, Medical Thermal Diagnostics, or MTD, is developing a light weight, hands free, energy efficient uncooled IR imaging system based upon a Texas Instruments design which will allow emergency medical treatment of wounded soldiers in complete darkness without any type of light enhancement equipment. The principal investigator for this effort, Dr. Gene Luther, DVM, Ph.D., Professor Emeritus, LSU School of Veterinary Medicine, will conduct the development and testing of this system with support from Thermalscan, Inc., a nondestructive testing company experienced in IR thermography applications. Initial research has been done with surgery on a cat for feasibility of the concept as well as forensic research on pigs as a close representation of human physiology to determine time of death. Further such studies will be done later as well as trauma studies. IR images of trauma injuries will be acquired by imaging emergency room patients to create an archive of emergency medical situations seen with an infrared imaging camera. This archived data will then be used to develop training material for medical personnel using the system. This system has potential beyond military applications. Firefighters and emergency medical technicians could directly benefit from the capability to triage and administer medical care to trauma victims in low or no light conditions.

A novel mid infrared (IR) endoscopic imaging system (patent pending) is described with applications as both a medical and industrial diagnostic tool. The endoscopic and borescopic IR imaging systems can be configured to work in the range of 2 to 15 microns, according to the application.

The OBER 2 is an infrared light eye movement measuring system and it works with IBM PC compatible computers. As one of the safest systems for measuring of eye movement it uses a very short period of infrared light flashing time (80 microsecond for each measure point). System has an advanced analog-digital controller, which includes background suppression and prediction mechanisms guaranteeing elimination of slow changes and fluctuations of external illumination frequency up to 100 Hz, with effectiveness better than 40 dB. Setting from PC the active measure axis, sampling rate (25 - 4000 Hz) and making start and stop the measure, make it possible to control the outside environment in real-time. By proper controlling of gain it is possible to get high time and position resolution of 0.5 minute of arc even for big amplitude of eye movement (plus or minus 20 degree of visual angle). The whole communication system can also be driven directly by eye movement in real time. The possibility of automatic selection of the most essential elements of eye movement, individual for each person and those that take place for each person in determined situations of life independently from personal features, is a key to practical application. Hence one of conducted research topic is a personal identification based on personal features. Another task is a research project of falling asleep detection, which can be applied to warn the drivers before falling asleep while driving. This measuring system with a proper expert system can also be used to detect a dyslexia and other disabilities of the optic system.

In this paper, the new application of IRCCD camera is introduced. This application is to detect a presence and location of natural gas such as methane or propane gas. In order to detect the presence and location of the gas, an infrared light source and IRCCD camera are used. The natural gas has the special absorption within 3 to 5 micron. When the infrared light from background surroundings goes through the gas, its intensity is reduced by this gas absorption. If the intensity which goes through the gas is very weak, it is difficult to observe the difference. The performance without infrared light source was evaluated before. It was difficult to apply for normal environmental monitoring. Therefore, the infrared light source to increase an intensity from background surroundings was used. To increase the intensity, irradiation to the background surroundings from an observing site, then the reflected light from the background surroundings goes through the gas into the observing site. Therefore, the reflected light which is added to emitted light from the background can be observed. It can make the performance of detecting the gas higher.

This paper describes the design of a DFOV objective intended for use with a focal plane array in the MIR waveband. The seven-elements' arrangement with binary and aspheric surfaces yields a low distortion, high quality and lightweight objective. Focusing and athermalization mechanisms are evaluated. In addition, the implications of different configurations for the detector calibration subassembly are discussed.

Future airborne surveillance systems require high performance optics to provide high spatial resolution over wide field of view. The system for 10 km altitude needs to resolve a ground target of 10 cm over 6 degree FOV. The spectral range covers from 0.4 to 10.0 micrometer. The spectral bands of interest include the visible, near infrared, MWIR (middle wavelength infrared), and LWIR (long wavelength infrared). This paper describes the design and the experimental results of the off- axis three mirror optical system which fulfills these requirements. The three mirror anastigmat consists of two aspherical concave mirrors and a spherical convex mirror. The design is configured for a telecentric flat focal plane.

The evolution of an unobscured all-reflective zoom optical system for utilization in the infrared spectrum is presented. The system is based upon earlier investigations by one of the authors with the objective of developing a system which has a flat image surface, wide field-of-view, spatially remote entrance pupil, 2:1 zoom range, and is spatially compact. The optical system comprises three aspheric mirrors sharing a common optical axis where the primary mirror is spatially fixed with respect to the entrance pupil and secondary and tertiary mirrors move during zoom. The field-of-view ranges from 2.2 degrees by 2.2 degrees to 4.4 degrees by 4.4 degrees. The focal ratio varies from F/4 to F/8. The inherent characteristics of this type optical system are discussed as are design methods to control aberrations, distortion, and anamorphic error over the zoom range. The resultant design and optical performance of the zoom optical system is presented and discussed.

Infrared lenses, simultaneously color corrected at multiple wavelengths from 1.0 to 5.0 micrometer, can be designed using several different technologies such as color corrected lens triplets and three mirror anastigmats (TMA) telescopes. In this paper, several different broadband reflective and refractive lens design solutions are presented and compared. In addition, the problem of designing and producing broadband AR coatings for such a wide wavelength region will be considered. An assessment of the advantages and disadvantages for each of the lens design forms will be provided together with a discussion of producibility limitations.

The requirement for multi-sensor systems continues to expand in many areas of electro-optics. For defence applications, an increasing need to combine visual and infrared (IR) imagery in compact configurations drives advances in material processing and coating technology. This paper reviews the materials required for such systems, highlights the importance of optical coatings, and describes coating developments at Pilkington Optronics St. Asaph site.

An arsenic trisulphide refractive plus diffractive hybrid element is both achromatic and athermal across the 3 - 5 micrometer waveband when set in an aluminum mount. We present two possible ways of manufacturing this element: using a photodarkening process to realize the diffractive structures, and by diamond turning, initially not thought to be viable. We demonstrate that the required depths in the diffractive surface are obtainable by both methods, and consider the advantages of each. Above all, however this lens is fabricated it offers, a very low cost, light weight solution to athermal singlet imaging in the 3 - 5 micrometer waveband.

We designed an electro-optical relay system to perform nonmechanical beam switching in the mid-infrared waveband and built a proof-of-concept prototype to verify the system performance. The prototype is a scalable building block that can be used to fabricate a two-dimensional system to scan a large field of regard at high resolution, low power, and high speed. We designed, fabricated, and tested the major components and then assembled the components into the electro- optical relay system, which demonstrated 8 kHz switching between two fields of view. In order to implement the system, we fabricated mid-wave infrared polarizing beamsplitter cubes that provided excellent polarization separation over a 600 nm waveband and a range of angles of incidence. Additionally, we demonstrated 90 degree polarization rotation in the mid-wave infrared waveband using ferroelectric liquid crystals.

On May 17, 1987 two EXOCET missiles hit and crippled the frigate USS STARK. Thirty seven sailors lost their lives due to the inability of the ship to defend itself against a sea- skimming cruise missile attack. In 1991, as a result of this incident, Congress mandated the establishment of a Program Executive Office for Ship Self Defense. The purpose of the legislation was to preclude another incident by placing a high priority on the combat system engineering process used to design and field the anti-ship cruise missile (ASCM) defense capability of surface ships. Over 35 countries now have sea- skimming ASCMs and this type of threat continues to proliferate. The use of IRST is a critical element of ship self defense, providing early and reliable detection of sea- skimming cruise missiles. This paper describes the contribution of IRST in providing self-defense and the current status of the United States Navy (USN) shipboard IRST development program.

Key aspects in the development of a generic electro-optic system model are reviewed and a model structure identified. Components within the model are described and examples of parametric analysis, image processing and design optimization components are presented. For design optimization, the use of various search methodologies including heuristic techniques, artificial neural networks and genetic algorithms are reported and compared.

This paper describes the results of an experiment to measure the correlation between the atmospheric turbulence induced intensity fluctuations in the mid and long IR bands for transmission paths appropriate to the IRS&T. A DF laser operating at 3.8 microns and a carbon-dioxide laser operating at 9.24 microns served as the mid and long IR sources. The experiments were conducted on a 16 km transmission path over the Chesapeake Bay. Data was obtained under a variety of turbulence strengths including the transition between weak and strong turbulence. The experimental correlations values were compared to the predictions of a weak turbulence model. Dispersion between the indices of refraction at 3.8 microns and 9.24 microns appears to be responsible for significant disagreement between the measured and predicted results. Possible applications of the correlation for the IRS&T are briefly discussed.

High resolution infrared imaging system calls for very long scanning arrays with several thousands of detectors and high performance. This paper presents the recent technological developments carried out at LETI/LIR on long butted arrays and gives the results obtained on a 1500 detector linear HgCdTe array with a 30 micrometer pitch and a 5.5 micrometer cut-off wavelength. This very large array (length approximately equals 50 mm) has an indirect hybrid architecture composed of 5 butted HgCdTe PV detection circuits and 5 Si CMOS readouts hybridized on a mechanically close-matched fanout substrate. Defect free dicing and butting, respecting the detector pitch, is made by accurate and non damaging techniques. A detailed description of the array and the main electro-optical performances are presented.

GEC-Marconi infra-red has developed a sensor technology based on lateral collection CdHgTe photodiode arrays mounted on custom designed CMOS multiplexer integrated circuits. The availability of submicron silicon processes has enabled a very high degree of functionality to be integrated within the detector thereby simplifying the overall system design. This paper describes a generic architecture that finds particular application for advanced infrared search-and-track, surveillance and high performance imaging applications. These applications require the highest possible performance and are therefore based on time-delay and integration (TDI) to enhance the signal-to-noise ratio, and detector element redundancy with defective element deselection (DED) to give resultant arrays with no dropouts. The detectors have fully variable integration period control, selectable integration capacitors, and a signal-to-noise enhancement capability at low infrared flux levels. The overall power consumption is low rendering the detectors suitable for engine cooling. The architecture is based on a number of unit cell designs and is readily adaptable to a wide range of configurations. The unit capacitor sizes within the design being rescaled to accommodate the required signal levels. In this way the numbers of elements in TDI and the number of TDI channels can be matched to the end application requirements. The architecture is applicable to both long and medium wave detectors. TDI channels are typically composed of 8 or 10 elements and in excess of 700 channels have been demonstrated. The results obtained from a number of prototype detectors are presented.

NSWCDD has developed a new nonuniformity correction (NUC) technique that has been demonstrated to significantly reduce both fixed pattern and temporal noise in sensors using high quantum efficiency (QE) infrared (IR) staring focal plane arrays (FPA). Sensors using this technique have been shown to have good response in every pixel, i.e., there are no dead or anomalously noisy pixels anywhere in the field of view (FOV). This technique will also enable development of sensors with very small apertures as well as those which can dynamically trade off sensitivity, resolution and frame rate. In addition, effective yield of detector production will be enhanced, since these benefits can be obtained using arrays that would be rejected for most applications, were conventional NUC used. This technique has been demonstrated to work as specified through analysis of real time data. A high performance, concept demonstration sensor, is in the final stages of acceptance testing, with delivery planned for April 1997.

To realize the potential of modern staring IR technology as the basis for an improved IRST, one requires better algorithms for detecting unresolved targets moving at fractions of a pixel per frame time. While available algorithms for such targets in white noise are reasonably good, they have high false alarm rates in non-stationary clutter, such as evolving clouds. We review here a new class of temporal filters which have outstanding signal to clutter gains in evolving clouds and still retain good signal to temporal noise sensitivity in blue sky or night data. The generic temporal filter is a damped sinusoid, implemented recursively. Our final algorithm, a triple temporal filter (TTF) based on six parameters, consists of a sequence of two damped sinusoids followed by an exponential averaging filter, along with an edge suppression feature. Initial tests of the TTF filter concept demonstrated excellent performance in evolving cloud scenes. Three 'trackers' based on the TTF operate in real-time hardware on laboratory IR cameras including: an empirical initial version; and two recent forms identified by an optimization routine. The latter two operate best in the two distinct realms: one for evolving cloud clutter, the other for temporal noise- dominated scenes such as blue sky or stagnant clouds. Results are presented both as specific examples and metric plots over an extensive database of local scenes with targets of opportunity.

In this paper, we approach the problem of point target detection in infrared image sequences by investigating the application of the continuous wavelet transform to temporal pixel profiles. Observations made on the resulting time-scale images suggest that a multiscale temporal filtering algorithm is suitable for this application. Such an algorithm would exploit the fact that those clutter pixels that are the cause of false alarms when using a medium scale filter, respond to fine and coarse scale filters differently than the target pixels. To demonstrate the effectiveness of a multiscale approach, a first-cut 3-scale approach is tested on actual infrared image sequences featuring targets of opportunity and evolving cloud clutter. Results indicate that the 3-scale approach exhibits improved clutter suppression and target detection, when compared to a single scale filtering approach.

Real-time embedded image processing applications are being implemented on commercial multicomputers. The use of programmable signal processing provides increased adaptability to higher performance robust algorithms and flexibility for future modifications. Efficient implementation on these architectures are a technical challenges to the development of cost effective solutions. This paper describes the implementation of a real-time infrared search and track (IRST) algorithm on a commercial off the shelf (COTS) multicomputer. It provides information about problem partitioning onto the processor including designing for scalability to varying numbers of processors. It provides lessons learned about implementation of the algorithm onto the processor including efficiency issues. It discusses portability of software while optimizing the implementation for a specific microprocessor and making use of vendor proprietary algorithm libraries.

The objective of the infrared search and track (IRST) program was to develop a demonstration system capable of long range detection and tracking of air targets in an airborne environment. This paper describes each of the major subsystems of the IRST equipment, which comprises a pointing and stabilization system, a thermal imaging system and a signal processing unit. The various modes of operation are outlined which provide the capability to search for, detect and track multiple targets; to track and display imagery of a selected target and to provide passive ranging information. A brief discussion of the installation and trials is given. Finally, a discussion of future system capabilities is given. The equipment is being flown by the Defence Research Agency in an experimental Tornado aircraft and further details are given in the paper titled 'Optimization of IRST algorithms,' P. N. Randall and A. J. Seedhouse, Defence Research Agency (Farnborough), UK.

The optimization of algorithms for an airborne infra-red search and track (IRST) demonstrator is described. Models of the detection and tracking algorithms have been produced and evaluated using real trails data. An automated process, using a genetic algorithm, was used to optimize the performance and significant improvements have been achieved. Various performance metrics have been developed to quantify performance of the algorithm.

A naval infra red search and track (IRST) system is a passive surveillance device capable of detecting and tracking air and surface threats in the region where electromagnetic sensors are less efficient, typically within a few degrees around the horizon. The evolution in anti ship sea skimming missiles performances has outlined the benefit that can be gained from infrared systems in ship self-defense. The complementary nature of radar and IRST systems can also be exploited to full advantage in the field of multisensor data fusion. The combined use of radar and infrared secures the detection by redundancy of data; it substantially enhances target tracking, classification and identification and reduces the combat system's reaction time. Designing an IRST implies matching technical choices with operational requirements, and this under increasingly stringent cost constraints. This paper first reminds the benefits that can be obtained with an IRST system in the context of modern naval warfare, then retraces the evolutions from the first generation IRST systems, such as VAMPIR, to the second-generation systems now entering service. A general presentation of the current SAGEM SA IRST family is also made for naval, air and ground-based applications.

The U.S. Navy Office of Naval Research (ONR) has developed an infrared search and track (IRST) demonstrator system named the infrared sensor system (IRSS). This technology-base sensor was successfully developed and tested both in the laboratory and at-sea. IRSS now is being transitioned to the Naval Sea Systems Command (NAUSEA) IRST Engineering and Manufacturing Development (E&MD) Program, where it will serve, with appropriate modifications, as the engineering development model (EDM) and will be fielded aboard a U.S. Navy ship. This paper summarizes the process of developing and fielding IRSS, describes test results accomplished at sea during 1996, and discusses the technical and engineering lessons associated with design, development and testing of IRSS. Results are presented covering the areas of sensor component and overall system radiometrics (e.g., sensitivity and dynamic range), channel uniformity, stabilization, and optical, electrical and information (i.e., signal processing/track) resolution.

Sirius is a long range infra red search and track system (LR- IRST) and intended to be used in an anti air warfare (AAW) multisensor suite on board of modern frigates. This Dutch/Canadian development program started 1/1/95 and includes also the evaluation of the system in warm and cold water scenarios. The operational requirements were drafted by both the national navies. The primary task is automatic detection, tracking and reporting of seaskimming missiles at long range. The design is based on recent experiences with IRSTs and the latest technological achievements in the areas of processing capabilities and IR-detectors. In this presentation design drivers and main technical choices are discussed.

A Rockwell staring infrared panoramic sensor (SIRPS) system will support the naval surface ship combatant against various threats such as anti-ship cruise missiles, sea skimming missiles, and various POI (points of interest) over the horizon. An ultra wide-angle shipboard electro-optical system that provides continuous 360 degree area surveillance can be deployed as a radar adjunct to detect threats where current radar systems have difficulty against low flying targets with multi-path reflection and sea surface clutter. The sensor provides detection of nonresolved targets over a panoramic 360 degree horizon field of view, and operates in a continuously staring mode providing positional and coordinate mapping of potential threats. Full azimuthal coverage with high angle resolution is achieved by using a distributed array of only four 640 by 480 HgCdTe focal plane arrays (FPAs) responsive over the mid-wavelength infrared (MWIR) waveband of 3.8 to 4.8 microns. A spectral band of 3.8 to 4.2 microns may instead by incorporated by using a 4.2 micron cutoff detector material. Rockwell has produced 4.2 cutoff FPAs for other applications. Final selection of these detection wavebands will optimize system performance for various potential target's exhaust spectral content. Current analysis has been done for the 3.8 to 4.8 micron waveband for preliminary system trades. Considerable cost reduction can be accomplished through the use of split-aperture optics to diplex two different azimuthal fields of view onto separate halves of each FPA. Optical diplexing reduces by one-half the required number of FPA, dewar and cryo-cooler subassemblies.

The parameter values for a Navy's IRST (infrared search and track) sensor are determined by (1) the operational requirements, (2) the available best technology, and (3) past experience with IRST system design. The selection process for the parameter values is illustrated by use of a set of operational requirements representative of the North Sea. Initial values of the optics parameters are determined by use of a design diagram. The approximate sensor parameter values are input to the TEDIS design computer algorithm for optimization. The effectiveness of the false-alarm reduction algorithms will depend on some of the sensor parameter values. The final assessment of these values can therefore be made only after performance testing of the algorithms. The design of a combined radiometer -- IRST testbed, the SIRDAS, is outlined. This system is used for measuring high quality target and clutter data in the potential battlefields. This data is essential for testing and fine-tuning of the all- important false alarm reduction algorithms.

Infrared search and track (IRST) system is a wide field of view surveillance system, meant for autonomous search, detection, acquisition, and cue of potential targets. The first and second generation IRSTs utilized detectors with multiple elements followed by discrete preamplifiers for signal read-out. They have many performance limitations. With the advent of infrared focal plane array (IRFPA) sensors, the present trend is to build IRSTs based on line FPA sensors to achieve higher sensitivity and resolution. However, due to system limitations of line IRFPA sensors, scanning mode of IRST cannot be stopped at any desired position to scan a small sector of interest. They also suffer from more false alarms in target detection. In future, it may be desirable to reduce false alarms, and also to use an IRST system for closed-loop- tracking of a potential target, in addition to its surveillance mode. IRST based on area array sensors may be a better option for this purpose, but it may pose some problems when used in a surveillance mode. This paper addresses this issue. Design considerations of all sub-systems of an IRST based on line/area array sensors, such as scanner assembly, interface electronics with the sensor, nonuniformity correction, signal processor, and the display methodology to cover 360 degrees are also discussed.

A physical model for the prediction of the radiant statistics over thermal images of ground desert terrain landscapes and their temporal behavior had been fully established by Ben Yosef et al. This model can be further developed in order to formulate the joint radiant statistics of reflective and thermal infrared images over the same type of landscapes. However, it fails to predict the actual measured correlation between the images in the two bands, and hence, a modification of the joint radiant density function in order to consider the influence of the local ground topography over the scene is introduced. The prediction of the modified joint density function and correlation coefficient is consistent with the experimental data acquired over a rough desert landscape. The effect of local scene topography on thermal image properties is not negligible, especially when sun's elevation angle is low. In such cases, shaded areas are generated in the scene, occupying a substantial portion of it. Analysis of the temporal dynamics of the correlation coefficient between thermal and reflective images can infer about the relative importance of the topography contributed variance over the thermal image and as well as the clutter characteristics of the thermal image. Scene topography also introduces errors in the production of thermal inertia maps for remote sensing applications and limits its utility.

This paper describes a novel approach of generating radiometric scenes of natural backgrounds that will serve as an input for simulating E-O sensor scene outputs in the thermal band. The methodology is based on segmentation of a measured scene (in any spectral band) into elements that have similar thermal behavior. The thermal radiance value for each thermal element is calculated using a set of four semi empirically determined coefficients that relate the surface temperature to the local meteorological parameters such as solar radiation, longwave sky radiation, air temperature and wind speed. The thermal coefficients are determined using a theoretical model and an experimental data base. The diurnal variations of the scene are thus easily predicted by knowing the meteorological parameters and the individual set of thermal coefficients for the various thermal elements of the scene. Since the approach is based on a real scene image and an experimental database the generated images have a realistic appearance including realistic clutter properties. The generated thermal scene will serve as the input to a sensor model that will calculate the expected image of a thermal camera viewing the scene. The paper describes the methodology of the scene generation, the sensor model and demonstrates the approach by giving some examples.

Remote sensing is based on the ability to measure accurately the spectral radiance of remote objects in the object plane. This ability is limited by the measuring system (resolution and sensitivity) and by the atmospheric transmittance, especially when long distances are involved. As a result, the need to enhance S/N led us to develop new measurements techniques and analysis methods. This presentation deals with two different techniques of modern radiometry -- point spectroradiometry with moderate spectral resolution and spatial radiometry (imaging systems) with low spectral resolution. This presentation will address three issues related to advanced analysis methods of radiometric measurements: (1) The effect of the exact shape of the slit- function of the point radiometer on the results of the spectral analysis, (2) the optimal calculation of a signature from radiometric imager, and (3) the correcting factor that must be introduced into the analysis of a spatial picture of point target which is much smaller than the IFOV of the imaging system (star detection). The experience and knowledge gained by IMOD and EORD in the area of radiometric analysis was implemented in a user friendly software (TIRAS) that is used for the radiometric (and not temperature) analysis of various spatial radiometers. The radiometric data was measured for various applications of IMOD such as data bases of targets and backgrounds, and study of radiometric behavior of IR scene elements.

Studies of blooming effects in InSb focal plane array (FPA) detectors, are presented. Two blooming test devices are described, which have allowed to isolate optical, charge- diffusion and electronic blooming mechanisms. It is demonstrated that when a spurious illumination due to optical scattering is eliminated, then no extended blooming occurs, and only normal cross-talk mechanisms cause signal offset in elements adjacent to the hot target image. Cross-talk data are analyzed in terms of the signal decay versus element position, and the lateral carrier diffusion length is derived. Susceptibility of different diode structures to blooming, is discussed. It is also shown that an FPA signal processor may cause an extensive electronic blooming.

Ricor reports about a new, high performance miniature split Stirling cryocooler model K529H with integral dewar/cold finger, consisting of an electrodynamically driven linear piston compressor and pneumatically driven expander. The cryocooler was optimized to lift 1 W heat load, applied to a 80 K cold finger, with a reject temperature of 300 K and net power consumption of 25 W ac 60 Hz. High cryocooler performance, reasonable pricing, decreased total weight and dimensions were achieved due to the simplified compressor design of unbalanced single piston type. To meet the requirement of low vibration export from the cryocooler to its environment (less than 1 N rms at operational frequency) the compressor unit was suspended on the intermediate frame by means of the soft, lightly damped all-metal planar resilient elements. For the operational safety of cryocooler and its surrounding, the compressor excessive axial motion, originated by the high-level external vibration, was limited by elastomer bumpers with controllable dissipation of impact energy. Bumpers were installed on the intermediate frame with a certain axial clearance permitting impactless compressor operation in any space orientation while the cryocooler was not exposed to the external vibration. The bumped vibroisolation arrangement was optimized to minimize the impact acceleration Peak and rms levels.

The transition to second generation backside-illuminated dense LWIR FPAs requires consideration of issues not previously relevant in first generation modules: unlike in front illuminated arrays, the MTF (or effective area) of a pixel is no longer close to the ideal sinc function. The cutoff wavelength, quantum efficiency and crosstalk depend on the thickness and composition grading of the epitaxial layer. The tradeoff between resolution and sensitivity demands extensive engineering and optimization of the array configuration. The transition was accomplished by comparisons of simulations with experimental results. Expectations of performance indicators, such as MTF, quantum efficiency and crosstalk were obtained by detailed Monte-Carlo simulations. The results were used to configure the focal plane array. This paper discuses the basic assumptions and simulation results and compares them with the performance of actual detectors and various test structures.

'TADIR' is an El-Op's new second generation thermal imager based on 480 by 4 TDI MCT detector operated in the 8 - 10.5 micrometer spectral range. Although the prototype configuration design of TADIR is aimed toward the light weight low volume applications, TADIR is a generic modular technology of which the future El-Op second generation FLIR applications will be derived from. Beside the detector, what put the system in the second generation category are the state of the art features implemented in every component. This paper describes the system concept and design consideration have been taken during the development of its components.

Simulation of scenarios for modern seekers involves the generation of multiple targets, electro-optical counter measures and textured backgrounds, all with realistic physical characteristics. True intensity, spectral distribution, real angular size and velocity are essential. The optical and radiometric design approach is based on imaging the seeker entrance pupil on different positions on a scene generation table. This paper presents some of the novel system characteristics. The optics, comprising mostly reflective surfaces and uncoated beam combiners, provides wide infrared and visible spectral range. The system is designed for high resolution over a large 10 degree field of view and is optimized for maximum intensity. Gimbaled mirror axes states are transformed into real world line of sight (LOS) motion. The whole scene may be tilted with respect to the seeker axis at large angles with derotation compensation. Targets, flares and thermal backgrounds are implemented using diversified types of thermal radiators, and their intensity and size closing effects are controlled by opto/electro-mechanical assemblies.

The sensitivity of staring and scanning infrared seekers responding in the 3 - 5 micrometer band is assessed. The signal-to-noise ratio (SNR) for an aircraft target at near head-on aspect is calculated. Results are presented at the moment of lock on, as well as during the free flight of the missile. Noise increase associated with dome heating is well taken into account. Staring seekers are superior in sensitivity compared to scanning seekers, allowing the missile to be launched from longer distance. But in some cases the noise signal grows at faster rate than the target signal along part of the missile route. This is further aggravated by the residual fixed pattern noise (RFPN), which grows in proportion to the dome heating, and eventually becomes the dominating noise. With sapphire-like dome, the focal plane array (FPA) of the staring seeker eventually saturates due to dome heating, unless a narrow spectral band is used, or the integration time is shortened. Reducing the integration time does not substantially affect the SNR as long as the RFPN is the dominating noise. Dome heating may cause rapid increase in the detector dc current, so the usual non-uniformity correction (NUC) procedures may no longer be adequate. One may have to consider correction in real time along with other methods that rely on target motion. These issues are, however, out of the scope of this paper.

We study the effect of high energy proton, alpha, oxygen ion, and gold ion radiation on the performance of GaAs/AlGaAs quantum well infrared photodetectors. The particle energies of proton, alpha and oxygen ions ranged from 0.8 MeV to 30 MeV and the fluences varied from 10¹⁰ to 10¹⁶ cm^-2. The energy of Au ions was 1.5 GeV and the fluence ranged from 10⁶ to 10⁹ cm^-2. Dark current and spectral response of irradiated devices were measured. For one set of samples, dark current noise and detectivity were also measured. A device operability defined by the fractional reduction of detector responsivity was used to evaluate the performance degradation. Device operability degrades with fluence for all particles. It also degrades with the mass of the ion and with the decrease in the energy of the particle.

A new light coupling geometry for quantum well infrared photodetectors (QWIP), which is referred as the corrugated- QWIP or the C-QWIP, has been demonstrated. The coupling scheme is based on the total internal reflection at a number of slanted sidewalls created within a detector pixel. The structure was found to be able to couple normal incident light efficiently into the detector with concomitant reduction in the dark current. For a C-QWIP with thinned substrate, the background photocurrent to the dark current ratio is improved by a factor of 4.9 compared with the edge coupling. The coupling scheme does not show significant wavelength and size dependence, and is therefore suitable for multi-color or high resolution thermal imaging. With these characteristics, a C- QWIP behaves as a detector with normal incident absorption. Because of its simple fabrication procedures, the manufacturability of the detector arrays can be greatly improved.

Due to the well established GaAs material and processing technology QWIPs are viable candidates for high resolution (greater than 128 by 128 pixels), low cost LWIR (8 - 12 micrometer) focal plane arrays (FPAs). Usually n-doped AlGaAs/GaAs QWIPs are used since, at least to date, these have been shown to provide the highest performance. Fabrication and evaluation of 320 by 240 pixels QWIP arrays have also been done. The fabrication involves hybridizing GaAs chips consisting of detector mesas to specially designed CMOS readout chips. The hybridization is effected by indium bump flip-chip bonding. Optical coupling into the detectors is performed by using optimized, etched, two-dimensional gratings combined with GaAs substrate thinning down. The advantages of substrate removal are: (1) reduction of thermal mismatch between materials and thus permitting large array sizes, (2) enhancement of absorptance, and (3) elimination of optical cross-talk between pixels. The intended operating temperature range is 70 to 73 K, achievable by a miniature Stirling cooler. Excellent wafer uniformities resulting in responsivity uniformities of 3.3% across an array are found, and a temperature resolution NETD (noise equivalent temperature difference) equals 40 mK is achieved. Finally, the presence of fixed-pattern noise and its influence on the image performance are discussed.

A 9 micrometer cutoff 640 by 484 hand-held quantum well infrared photodetector (QWIP) camera has been demonstrated. Excellent imagery, with a noise equivalent differential temperature (NE(Delta) T) of 43 mK has been achieved. In this paper, we discuss the development of this very sensitive long wavelength infrared (LWIR) camera based on a GaAs/AlGaAs QWIP focal plane array (FPA) and its performance in quantum efficiency, NE(Delta) T, uniformity, and operability.

Quantum well infrared photodetectors do not respond to normal incident light due to the quantum mechanical selection rules associated with intersubband transitions. Thus alternate light coupling systems, such as gratings are required in order to deflect the incoming light away from the normal. The resolution of the photolithography and accuracy of the etching become key issues in producing smaller grating feature sizes especially in shorter wavelengths. An enhancement factor of three due to 2D periodic grating fabricated on a QWIP structure was observed. Variation of the enhancement factor with groove depth and feature size of the grating can be theoretically explained.

During the last decade, the QWIPs technology has improved from start to an undeniable maturity level. High performance focal plane arrays have already been realized (ATT, Lockheed-Martin, JPL, . . .) with a spectacular format increase ranging from 128 by 128 up to 640 by 480, and images from bicolor 256 by 256 arrays have been shown last year. All these devices illustrate the high potential of the QWIP technology. In the same time, the modeling of detection mechanism has advanced to permit the present design of specific detectors and their optimization in given operating environments (near 77 K detector temperature for instance). In this communication, we summarize our recent technological studies leading to the next generation of very large infrared detector arrays. We present the QWIP ultimate performances allowed by the standard dual III - V technological processes developed at THOMSON CSF, in terms of pixel size, array filling factor or connectics. The influence of the pixel size for the grating optical coupling is analyzed. We finally include in this analysis our results for more complex devices like multispectral infrared detectors.

A very high performance two-stack, two-color, high strain (HS- ) quantum well infrared photodetector (QWIP) has been demonstrated. The sample was grown on a semi-insulating (100) GaAs by molecular beam epitaxy (MBE). It consists of two stacks of MWIR and LWIR QWIPs as the active region with a 100 nm thick highly doped contact layer grown between the two stacks. Each stack is designed to have detection in one of the two atmospheric windows, 3 - 5 micrometer (MWIR) and 8 - 12 micrometer (LWIR), respectively. The MWIR stack consists of 20 periods of 300 angstrom Al_0.38Ga_0.62As barrier and 24 angstrom doped In_0.35Ga_0.65As well sandwiched between two 5 angstrom GaAs, and the LWIR stack is composed of 20 periods of 500 angstrom Al_0.27Ga_0.73As barrier and 55 angstrom GaAs well. In this work, a 35% of indium has been employed in the MWIR-stack which not only shifts the peak wavelength to 4.3 micrometer, but also enhances the responsivity greatly in this wavelength band. This is due to the fact that higher indium concentration in the InGaAs QW reduces the electron effective mass and increases the intersubband absorption. Despite of the large strain induced by the high indium concentration, the device is highly uniform with very low dark current. For the MWIR stack, a peak responsivity of R_p equals 0.65 A/W and D* equals 1.9 by 10¹¹ cm-Hz^1/2/W at 4.3 micrometer, 3 V bias, and 77 K were obtained, while for the LWIR stack, R_p equals 0.55 A/W and D* equals 2.7 by 10¹⁰ cm-Hz^1/2/W at 9.4 micrometer, 2 V bias, and 77 K were obtained using 45 degree light coupling. Normal incidence without grating coupling also has high responsivity with about 50% for the MWIR stack and 40% for the LWIR stack respectively, compared with the 45 degree incidence coupling. The BLIP temperature was found to be 125 K for the MWIR stack with cutoff wavelength of lambda_c equals 4.6 micrometer and 70 K for the LWIR stack with (lambda) _c equals 10 micrometer.

As second generation FLIR systems become a reality the need for a reliable, high performance, moderately priced matrix arrays in the 8 - 12 micron atmospheric window becomes a real demand. At the same time, the remarkable advancement of QWIP technology over the past few years makes it one of the best candidates for such applications that suffice in the resolution provided by 320 by 256 pixel arrays. According to its fast advancement it could be expected that in the near future also the requirements of high-end applications will be met by QWIP technology. In light of this potential, the QWIP program in EL-OP was recently started in order to develop in- house QWIP technology and demonstrate 320 by 256 pixel image. Additionally we take part in scientific activities in a joint project with leading Israeli university groups. Preliminary results are presented, including the fabrication of QWIP arrays and measurement of single detectors. Measurement results show D* greater than 5 X 10¹⁰ Jones and responsivity approximately equals 0.5 A/W. Additionally, an optimization method for quasi-random scattering arrays is briefly presented.

Highly p-doped Si_1-xGe_x quantum well (QW) layers have been grown by molecular beam epitaxy (MBE) on <100> silicon (Si) for detectors in the mid-infrared regime (3 (mu) - 5 (mu) , 8 (mu) - 12 (mu) ). The 5 nm - 10 nm thick SiGe QW layers were boron doped up to 5 10²⁰ cm^-3 with Ge contents of 0.4 less than or equal to x less than or equal to 0.5 and have been pseudomorphically deposited on undoped Si. The principle of detection is by hetero-internal photoemission (HIP) across the Si/SiGe valence band barrier. This sequence has been repeated up to ten times and the structure has been terminated with a p-doped SiGe contact layer on top. The structure of the grown samples have been extensively analyzed by secondary ion mass spectroscopy (SIMS), x-ray diffraction (XRD), Rutherford backscattering (RBS) and absorption spectroscopy. Mesa detectors of varying diameters have been fabricated using standard Si processing techniques, and the photocurrent and dark current have been measured at 77 K. A maximum quantum efficiency of (eta) _ext equals 1.4% has been achieved (at 4 (mu) and 77 K) with dark current densities of 10^-5 A/cm², the spectral dependence of the photoresponse showed a broad maximum between 3 (mu) and 5 (mu) . Different layer designs with repeated quantum wells and varying doping levels and Ge content in the well have been studied theoretically and experimentally to optimize the structure with respect to high responsivity and low dark current.

This paper introduces a new approach to measuring infrared radiant flux density. Thermal sensors featuring a significant metering area from 3 mm² to 25 cm² can be achieved by way of distributing the sensing surface upon an appropriate plated-planar thermopile. Despite a low figure of merit regarding bimetallic structures, low noise, rugged, thin and even flexible devices are made in the laboratory. Such sensors have neither to be covered with a protective widow nor to be placed in an insulating gas, thanks to their inherent immunity against convection afforded by the differential behavior of their structure. Hence wide spectrum infrared measurements, and experiments undergoing a wide range of pressure, are allowed with distribution-patterned radiometers. Current techniques of manufacture are reviewed together with the philosophical arguments concerning the distributed layout of monolithic thermopiles. Since such devices can be directly deposited upon various dielectric materials, many an application in military and space research can be expected. As regards industrialization, those multipurpose sensors meet the necessary requirements of self-calibrating ability, good reproducibility, fast response (#20 ms), ruggedness, and low cost. It is expected that the versatility of the device will result in a wide number of industrial applications.

Recent advances in multiple disciplines have lead to the development of low cost, portable radiometers with outstanding thermal sensitivity, resolution, range and true 'point and shoot' capabilities. These new systems use technologies such as: infrared focal plane array (FPA), integral dewar/cooler, hybrid optics, embedded processor, digital processing/storage and thermal image analysis. The integration of these new technologies into a camera system is described and related to imaging and radiometric performance. Specific applications in traditionally difficult radiometric areas are discussed such as low temperature sensitivity and variable integration times for high temperature radiometric capability. A camera system is broken down into specific technology blocks. Discussion of these technologies shows how each was chosen to produce a system which covers a broad temperature range while maintaining ease of use in many different radiometric applications. These choices include FPA selection criteria (readout architecture and detector material), optical design (f/# and bandpass), dynamic range management and radiometric analysis/display features.

There is a trend today towards a reduction in target signatures, the signatures becoming increasingly adapted to the background in which the targets operate. In addition, new types of countermeasures are making the task for optical seekers increasingly difficult. One way to increase the capability of detecting low-signature targets in a countermeasure environment is to utilize not only the magnitude of the signature but also its distribution over the spectrum. For collection of information regarding the spectral signatures of targets, countermeasures and backgrounds, a multispectral imaging MWIR sensor has been developed by us. This device utilizes the high frame rate made possible by modern FPA arrays. Such an array has been combined with a rapidly rotating filter wheel, thereby producing images of 128 by 128 pixels in six wavelength bands in the 2 - 5 micrometer region at a frame rate exceeding 30 Hz in each band. The sensor has a field-of-view of 3.7 degrees and a pixel resolution of 0.5 mrad. The sensor has the capability to perform two point correction in real time, thereby compensating for the different dynamic ranges in each spectral band. An extensive measurement program is in progress for gathering data for targets, countermeasures and backgrounds. Selected results from this program are presented.

Custom-made Ge and InGaAs photodiodes were tested for high sensitivity dc and ac radiometric applications. Equal size, large-area photodiodes were selected and used as optical sensors in NIST's near-infrared (NIR) standard radiometers. The dc electronic characteristics of the Ge and InGaAs radiometers were measured versus photodiode temperature. At minus 30 degrees Celsius, a limit-sensitivity of 22 fA and a dark-current stability of 0.2 pA/16 hours was achieved with the InGaAs radiometer, which was three times better than the results obtained with the Ge radiometer. The Ge radiometer was used for dc signal measurements only, because at frequencies higher than 0.3 Hz the noise boosting effect decreased the photocurrent sensitivity. The frequency dependent gain characteristics were calculated and compared for the two types of radiometer. The InGaAs radiometer could measure optical radiation with a chopping frequency of 10 Hz without any response or limit-sensitivity degradation.

Undertaken by the European Space Agency (ESA-ESTEC), HIRIS (high resolution imaging spectrometer) instrument study program includes signal processing and instrument overall data handling design concept. HIRIS is an hyperspectral imager to be implemented on a low earth orbit type satellite. The instrument is a pushbroom operating around 800 km altitude sun-synchronous orbit with 40 m sub satellite spatial sampling, 30 km swath variable by plus or minus 30 degrees and a spectral coverage from 450 nm to 2350 nm at 10 nm average spectral sampling. One of the major critical areas is therefore to develop, manufacture and integrate a full detection chain, including front-end electronics, preamplifiers and video chains (analog processing), and radiometric correction devices and compression algorithm implementation (digital processing). Taking into account imaging mission and system constraints, a data handling architecture has been selected, based on the video processing standardization, what ever the focal plane design is. The off- line processing architecture leads to an optimization of the budgets while integrating the overall digital processing imposed by mission performances fulfillment. With an input data flow near 320 Mbps, the overall data handling delivers through the output formatter a 100 Mbps data flow to the ground after having performed the required on board processing (gain correction, offset correction, needed compression and encoding implementation).

Novel methods for radiometric calibration of InSb FPA camera were developed and tested. An advanced weighting function method is used in order to transform a pixel gray level (14 bit digitized and corrected by the non uniformity correction - - NUC electronics) into the integral radiance (over the camera spectral bandwidth) on the target plane. The study can be applied for various InSb FPA cameras with controlled integration times, high stability of their NUC-electronics and large dynamic range of its data acquisition board.

The electronics signal processing system (preamplification through digitization) used to capture signals from advanced visible and infrared focal planes requires careful design and component selection to insure focal plane limited performance. We describe the design of a multi-channel signal processing system and its detailed electronic characterization in terms of gain, offset, noise (rms, spectral, and coherent), integral nonlinearity, differential nonlinearity, frequency response, step response, and electrical crosstalk. The test system, test methodology, and test issues are also discussed.

The design for a staring focal plane array with on focal plane A/D was developed to support space based sensor applications. Readout interface requirements and noise analysis were completed for an HgCdTe LWIR detector 256 by 256 array with 13.9 micron cutoff operating at 40 K with background of 10¹¹ or less. The Amain developed MOSAD (multiplexed oversample A/D) technology was applied as the readout and focal plane A/D converter with a requirement for 12 bits of conversion accuracy at 100 frames per second, a pixel pitch as small as 30 microns and heat load lower than analog readout. In the analysis of the readout requirements, consideration for SNR, dynamic range, linearity, well capacity, heat dissipation and component total dose drift were included. Conclusions are that greater than 12 bits dynamic range can be supported and that commercial grade microelectronics can be used for the digital readout, requiring only periodic gain calibration to compensate for component aging due to space environmental effects. This work was sponsored by the U.S. Air Force Phillips Laboratory, Albuquerque, New Mexico.

Current infra-red focal point arrays (IRFPAs) are limited by their inability to calibrate out component variations. Typically, off-board digital calibration is used to correct nonuniformities in these detector arrays; special calibration images are used to calibrate the system at startup. One-time calibration procedures such as these do not take into account other operating points and will fail to recalibrate for any drift in the parameters. Using clues from neurobiological adaptation, we have developed the constant-statistics (CS) algorithm for nonuniformity correction of IRFPAs. Gain and offset variations are successfully calibrated using simple assumptions of the scene under view. We give results for calibration of 1D and 2D images using a digital implementation. We also show that the constant-statistics algorithm compares favorably to an existing LMS-based nonuniformity correction algorithm by Scribner in terms of convergence rate and computational complexity. Finally, we review the results of analog circuitry that was designed and fabricated with a 2 micrometer CMOS technology. Measured results from our test-chip show that the system achieves invariance to gain and offset variations of the input signal. This hardware is targeted for eventual use for in- and behind- the focal plane implementations.

IR imagery tends to have a higher dynamic range then typical display devices such as a CRT. Global methods such as stretching and histogram equalization improve the visibility of many images, but some information in the images stays hidden for a human operator. This paper reports about the possibility to represent more information by using local adaptive techniques. Based upon methods found in literature, a new multi-scale method is presented.

Recent reports indicate that cooled and uncooled IR focal plane array sensors are progressing to a field-worthy level for commercial and defense applications. They offer higher sensitivity, amenability to signal processing and mechanical simplicity. However these sensors contain large detector-to- detector dark current (offset) and responsivity (gain) variations. These variations result in a severe problem called fixed pattern noise that can mask/distort the image obtained from the sensor. The correction process is generally termed as nonuniformity compensation. Conventional two-point compensation techniques are accurate enough, but require built-in controllable temperature references along with mechanical and electro-optical shutters. Therefore this compensation technique detracts the mechanical simplicity of using IR focal plane arrays. Scene-based nonuniformity techniques dispenses with the requirement of temperature references and shutters, but are not accurate enough for certain applications. This paper discusses two-point and scene-based nonuniformity compensation algorithms and proposes an empirical formula to automatically calculate the scene constants, which is an essential step towards practical applications. This paper reports the analyzed results of testing the algorithms on a number of IR images. A practical problem of 'artifacts' which arise when using scene-based nonuniformity compensation is also discussed. A common hardware scheme to implement both the algorithms is also presented in this paper.

Fossil fuel power plant boilers operate continuously for months at a time, typically shutting down only for routine maintenance or to address serious equipment failures. These shutdowns are very costly, and diagnostic tools and techniques which could be used to minimize shutdown duration and frequency are highly desirable. Due to the extremely hostile environment in these boilers, few tools exist to inspect and monitor operating boiler interiors. This paper presents the design of a passively cooled, infrared borescope used to inspect the interior of operating boilers. The borescope operates at 3.9 micrometer, where flame is partially transparent. The primary obstacles overcome in the instrument design were the harsh industrial environment surrounding the boilers and the high temperatures encountered inside the boilers. A portable yet durable lens system and enclosure was developed to work with a scanning radiometer to address these two problems by both shielding the radiometer from the environment and by extending the optical train into a snout designed to be inserted into access ports on the sides of the boiler. In this manner, interior images of the boiler can be made while keeping the radiometer safely outside the boiler. The lens views a 40 degree field of view through any 2.5' or larger opening in a foot thick boiler wall. Three of these borescopes have been built, and high resolution images of boiler interiors have been obtained.

A family of 2 dimensional detection modules based on 256 by 256 and 486 by 640 platinum silicide (PtSi) focal planes, or 128 by 128 and 256 by 256 mercury cadmium telluride (MCT) focal planes for applications in either the 3 - 5 micrometer (MWIR) or 8 - 10 micrometer (LWIR) range was recently developed by AIM. A wide variety of applications is covered by the specific features unique for these two material systems. The PtSi units provide state of the art correctability with long term stable gain and offset coefficients. The MCT units provide extremely fast frame rates like 400 Hz with snapshot integration times as short as 250 microseconds and with a thermal resolution NETD less than 20 mK for e.g. the 128 by 128 LWIR module. The unique design idea general for all of these modules is the exclusively digital interface, using 14 bit analog to digital conversion to provide state of the art correctability, access to highly dynamic scenes without any loss of information and simplified exchangeability of the units. Device specific features like bias voltages etc. are identified during the final test and stored in a memory on the driving electronics. This concept allows an easy exchange of IDCAs of the same type without any need for tuning or e.g. the possibility to upgrade a PtSi based unit to an MCT module by just loading the suitable software. Miniaturized digital signal processor (DSP) based image correction units were developed for testing and operating the units with output data rates of up to 16 Mpixels/s. These boards provide the ability for freely programmable realtime functions like two point correction and various data manipulations in thermography applications.

Using a recently developed PtSi IR focal plane array imaging system, ZAE Bayern develops new IR measurement techniques for industrial and medical applications. In this paper we present three examples for the analysis of material nonuniformities and a buried heat source. A new method for detecting the surface moisture of porous solids has been developed. The water content at the surface is determined by using an infrared filter tuned to the water-specific spectral line at (lambda) equals 2.92 micrometer. With a numerical simulation of the water transport we determined the effective diffusion coefficient. Surface temperature transients are recorded after a short infrared irradiation excitation at a frame rate of 50 Hz to localize voids or blisters in solid materials. The method is tested for a glass substrate. The analysis is in quantitative agreement with the test configuration. Furthermore we show that it is possible to localize leaks in heat pipelines a few meters below the earth surface by dynamical infrared imaging techniques.

Heterostructures HgCdTe/CdTe/GaAs grown by molecular beam epitaxy were used for LWIR FPA fabrication. The technology was developed and 32 by 32 and 128 by 128 photodiode arrays with indium bumps of 15 micrometer height in each pixel were fabricated. Mean NEP is 1.7 by 10^-13 W/Hz^1/2 and 1.1 by 10^-14 W/Hz^1/2 for 128 by 128 photodiode arrays with (lambda) _c value of 10.4 micrometer and 5.2 micrometer correspondently. The technology of hybrid assembling with continuous control of cold welding on the measuring stand was demonstrated on the example of 32 by 32 LWIR FPA. Mean NEP value of 5.4 by 10^-14 W/Hz^1/2 with (lambda) _c equals 10.6 micrometer at 80 K operation were obtained. using an infrared camera system the infrared image was successfully demonstrated. The NETD value of 0.077 K was obtained under 293 K background condition.

Temperature variable Hall measurement system has been setup to measurement both the carrier mobility and the carrier concentration for the CMOS devices. The measured Hall mobility show a little bit higher value than the conductance mobility. The measured carrier concentration at low gate bias region show a reverse trend as expected, and this may be due to the error in the estimation of the channel depth of the CMOS devices. In general, care has to be taken to the measurement procedure to ensure the accuracy of the Hall voltage.

The construction of superconductor focal planes for infrared or millimeter wave imaging requires that the substrate of superconductor films be micromachined into thermal isolation structures or horn cavities. Wet etching was used to create cavities in the MgO substrate of high T_c BiPbSrCaCuO films. Processes for lithography of metal patterns on superconductor films were also devised. It was found that cavities with a wall angle of 55 - 60 degrees could be formed in (100) MgO using solutions of HNO₃:CH₃COOH or H₃PO₄. The MgO normal etch rates of these solutions were found to be respectively 117 and 27 micrometer/hour. Thermal evaporation and magnetron rf sputtering were used to prepare Au and Ag films on BiPbSrCaCuO and MgO; however, only the sputtered films showed adequate film adhesion. Electric contacts and dipoles made of Au or Ag could be created by wet etching in a solution of KI-I without apparent degradation of the superconductivity of BiPbSrCaCuO.

ICC presents a new addition to their integrated detector assembly product line with the announcement of their type II large format staring class FPA units. A result of internally funded research and development, the ICC type II detector assembly can accommodate all existing large format staring class PtSi, InSb and MCT focal planes, up to 640 by 480. Proprietary methodologies completely eliminate all FPA stresses to allow for maximum FPA survivability. Standard optical and cryocooler interfaces allow for the use of BEI, AEG, TI SADA Hughes/Magnavox and Joule Thompson coolers. This unit has been qualified to the current SADA II thermal environmental specifications and was tailored around ICC's worldwide industry standard type IV product. Assembled in a real world flexible manufacturing environment, this unit features a wide degree of adaptability and can be easily modified to a user's specifications via standard options and add-ons that include optical interfaces, electrical interfaces and window/filter material selections.

TDI thermal imaging systems are a new generation in the 8 - 12 micrometer range. This device enables the correction of non- uniformity among detector elements by sampling the internal non-uniformity correction target at each scanning cycle. As compared with previous FLIRs generations, these new devices place greater importance on the correction of non-uniformity among the detector elements because of the higher required quality rendered by these devices. This quality is attributed to the improvement in the S/N ratio by the TDI method, imaging of the pupil into the system's cold shield, and the fact that since the systems are not dc-coupled, improper correction of the non-uniformity accepted as an information from the scene. The general analysis relates to all types of optical systems, and not necessarily to thermal systems only. The technical literature referring to this topic, and in particular to thermal issues, is not unequivocal, and there are contradictory estimates regarding the principles underlying distribution calculations. This is surprising in light of its importance to electro-optical systems as a whole, and thermal systems in particular, in terms of the distribution of image brightness on the display, and the device's dynamic range. The present report is divided into two main sections: (1) General physical analysis of the distribution of image illumination according to several types of optical systems, e.g., plane-to- plane imaging, infinity-to-plane imaging, and a scanning system which images infinity-to-plane. (2) Calculation of the distribution of TDI device image illumination at five (5) telescope zoom positions, and for various horizontal scanning positions of the scanner, as compared with detector elements illumination when performing a non-uniformity correction among detector elements by imaging a thermal reference target to the detector' plane. It has been customarily accepted that the distribution of illuminance in an optical system behaves primarily according to a function of cos (omega) , where (omega) equals the FOV angle. Some sources define (omega) as the chief ray angle of the beam (i.e., central ray), relative to the optical axis. A physical analysis will prove such a statement inaccurate. The chief ray angle in optical systems is a function of the location of their pupil, and may even change a sign, but the amount of light incident upon each point on the image's plane depends on the system's effective F/# as a function of FOV, and is not necessarily identical to the chief ray angle. Radiometric calculations based on our theoretical analysis (presented hereunder) showed that pupil distortions and aberrations have a significant impact on the distribution of the detectors' illumination, sometimes even more than the cos (omega) itself.

Backscatter gas absorption imaging (BAGI) has been demonstrated as a useful technique for visualizing gas leaks. BAGI uses active imaging in the infrared to generate a laser- illuminated video image of a scene. A dark cloud in the image is formed when a gas absorbs the illuminating laser radiation in the vicinity of the plume. The sensitivity of the technique is limited by the ability of the operator to distinguish scene contrasts from gas contrasts. To improve its performance, we have developed a differential absorption system to subtract off scene contrasts that can obscure a gas plume. This system is essentially an imaging differential absorption LIDAR (DIAL) that allows one to focus on contrast in a scene due to absorption from a gas plume instead of contrast due to variations in the reflectivity of the target. Practical aspects of this system are presented along with results taken in real-world settings. The noise floor for a differential image is shown to be dominated by uncorrelated speckle fluctuations -- not contrasts in the scene.

The results of field tests of an active backscatter absorption gas imaging (BAGI) system and a passive imager based on a Ga:Si infrared focal-plane array are presented. Both imagers allow real-time video imaging of gas emissions. The former system images gases through their attenuation of backscattered laser illumination; the latter images gases through temperature or emissivity differences. The results represent the first side-by-side comparison of an active and passive imager and the first BAGI field trial involving the imaging of plumes of controlled concentration and dimension.

A new type of interferometer, a differential rotationally shearing interferometer, differs from the traditional rotationally shearing interferometer in that the angle of rotation is infinitesimal or, at least, very small. A differential rotationally shearing interferometer may be used for the detection of wavefront asymmetry even when its asymmetry is very small. We present an implementation concept for the differential rotationally shearing interferometer, based on the Mach-Zehnder interferometer. One of the important advantages of the new interferometer is that the polarization problems do not arise do the small angle of rotation between the two wavefronts.

We evaluate the general wavefront function of an optical system that does not possess a rotational symmetry, subject to the infinitesimal rotational shear. The interference pattern of an astigmatic wavefront generated in the differential rotationally shearing interferometer is equal to that in the Twyman-Green interferometer, but rotated by 45 degrees. The interferometric pattern of the 45 degree astigmatism changes into that of the 90 degree astigmatism. The interference pattern of a comatic wavefront generated in the differential rotationally shearing interferometer is equal to that in the Twyman-Green interferometer, but rotated by 90 degrees. The coma along the x-axis changes to the coma along the y-axis.