The concept of remote (stand-off) detection of chemical and biological agents was suggested at least as far back as the early fifties; the DoD, (primarily the Army) has had R&D programs going back at least that far. Concepts have included active and passive sensors, infrared and ultraviolet lasers, spectral and imaging systems and all types of combinations that have some potential to meet the Army's very demanding requirements for detection of vapors, aerosols and ground contamination. The spectral principles and early history -- particularly of passive IR systems -- are emphasized.
The calibrated infrared ground/air radiometric spectrometer (CIGARS) is a new high performance, multi-purpose, multi- platform Fourier transform spectrometer (FPS) sensor. It covers the waveband from 0.2 to 12 micrometer, has spectral resolution as fine as 0.3 cm-1, and records over 100 spectra per second. Two CIGARS units are being used for observations of target signatures in the air or on the ground from fixed or moving platforms, including high performance jet aircraft. In this paper we describe the characteristics and capabilities of the CIGARS sensor, which uses four interchangeable detector modules (Si, InGaAs, InSb, and HgCdTe) and two optics modules, with internal calibration. The data recording electronics support observations of transient events, even without precise information on the timing of the event. We present test and calibration data on the sensitivity, spectral resolution, stability, and spectral rate of CIGARS, and examples of in- flight observations of real targets. We also discuss plans for adapting CIGARS for imaging spectroscopy observations, with simultaneous spectral and spatial data, by replacing the existing detectors with a focal plane array (FPA).
The lightweight standoff chemical agent detector (LSCAD) is an infrared Michelson interferometer operating in the 8 - 13 micron band and is designed primarily for military contamination avoidance and early warning applications. The system is designed to be operated autonomously from a vehicle while on the move and provide 360 degree coverage. The first group of prototypes were delivered in 1994 and have undergone integration into several platforms including the HMMWV, the M2 Bradley Fighting Vehicle, the M109 self- propelled Howitzer and the Pioneer and Hurricane unmanned air vehicles (UAVs). Additional vehicles and platforms are planned. To meet the restrictions of military applications, the prototype interferometer subsystem has a weight of about 10 lbs and is approximately 0.20 cu fit in size. The full system size and weight depends upon the particular platform and its operational requirements. LSCAD employs onboard instrument control, data collection, analysis and target detection decision software, all of which are critical to real-time operation. The hardware, software, and test results are discussed.
The U.S. Army has been involved for over 30 years in the development of remote passive chemical detection sensors. With one exception these sensors have all been based on the Fourier transform spectrometer (FTS). The one exception was an early sensor called LOPAIR (long path infrared) which was a circular variable filter spectrometer. LOPAIR provided low spectral resolution and low sensitivity. Because of the limited sensitivity and resolution the Army quickly started using FTS for all their subsequent passive sensors. The first FTS spectrometer used for chemical detection was built by Block Engineering and was called COIN or correlation interferometer. Since that first FTS, the Army has been involved in the development of more advanced versions and currently has the M-21 in production. Following the M-21 was the development of the lightweight standoff chemical agent detection sensors (LSCAD) which were intended to provide detection on the move. Eight LSCAD sensors were built by the contractor to be used by the Army in either armored vehicles or the Pioneer UAV. Currently Block Engineering is under contract to modify several of the existing LSCAD sensors. The new sensor resulting from this program are referred to as the improved lightweight standoff chemical agent detection sensors (I-LSCAD). The objective of this paper is to describe the I-LSCAD sensor, which represents the state- of-the-art for remote passive chemical detection sensors.
We present concept and design considerations of a novel optical sensor based on the distributed Bragg reflector (DBR) laser. The operation of this sensor relies on the shift of the lasing wavelength upon exposure of the chemically sensitive element to the gas species to be detected. We discuss physics of the proposed device, show the compromises in its design, and estimate potential sensitivity. Compactness and simplicity of this DBR laser- based optical sensor may play a crucial role in its mass production for various applications.
A versatile high power Q-switched Cr:LiSAF laser system has been developed using fiber optics and a low power cw oscillator to remotely control the operating wavelength. This type of system has the advantage of eliminating lossy frequency control elements inside the high power oscillator.
An efficient, compact, tunable ultraviolet laser system has been constructed by frequency tripling a Q-switched, tunable near-infrared Cr:LiSAF laser source. The nonlinear conversion from the fundamental to the third harmonic was accomplished through second harmonic generation in lithium triborate (LBO) followed by sum frequency mixing of the fundamental and the second harmonic in beta-barium borate (BBO). Third harmonic output with this combination of nonlinear crystals has yielded 5 mJ of energy at 280 nm with an overall conversion efficiency of 7%. The Cr:LiSAF master oscillator has been modeled in detail to optimize performance. System specifications of tuning range, linewidth, output energy, temporal behavior, and repetition rate were determined by the end use application as a UV excitation source for a chemical and biological stand-off detection system.
The detection of gaseous ammonia with open-path Fourier transform spectrometry furnishes a means of accessing fugitive emissions from various industrial production processes by stack and site monitoring. This study relies on direct interferogram analysis of passive infrared data from open-air industrial and controlled laboratory environments. Direct interferogram analysis permits the identification of ammonia emissions by detection of the (nu) 2 spectral bands in the interferogram time domain.
The objective of this effort is simulate realistic interferograms for passive remote detection of hazardous vapors. Radiant power spectra are the starting point from which fast Fourier transforms (FFT) are used to create interferograms. The characteristics of the various noise sources, detector and background, are discussed. In an example detector noise is added to the synthetic interferogram and the double-sided spectrum. The spectrum and the interferogram are repeatedly transformed and the noise levels compared.
Passive infrared is an emerging method for the remote detection of hazardous vapors particularly where warning is the primary consideration. Detection of gases, vapors and aerosols is based on the difference, (Delta) T), between the temperature of the target cloud and the effective radiometric temperature of the background. Computer simulation of spectra has been used to predict the detection limits for several target gases with a low angle sky background. The simulation is based on a 3 layer model that uses MODTRAN, which includes 6 standard atmospheric models, to compute background radiance and atmospheric transmittance. The detection limits, at 2 cm-1 resolution, for sulfur hexafluoride (simulant), Sarin, trichlorethylene, methyl isocyanate (the Bhopal gas), mustard gas, methyl chloride, and sulfur dioxide are discussed for selected cases with the U.S. Standard, and the sub-arctic winter and the tropical models. In this paper the method is illustrated with methyl isocyanate.
Passive remote determination of the presence and quantity of gaseous chemicals is of interest for a variety of applications, but has been technically difficult to accomplish because of challenging measurement and analysis difficulties. This paper describes progress in the development of infrared plume analysis software. To unravel the atmosphere, background, and thermal contrast relationships it has been found necessary to obtain both spatial and spectral information and to have a set of codes which take these phenomena into account. Measured data were used along with model analyses to develop advanced plume analysis software (APAS) applicable to calculating how accurately amounts of gas ben be determined from overhead with current and possible future sensor technology. The APAS suite of codes incorporates sensor noise, plume contrast and atmospheric effects modules to arrive at a comparative measure of gas plume detection accuracy. The application of the APAS codes to prediction of detection accuracy for several candidate sensors is shown.
We present details of an electronically tunable, narrow bandpass optical filter for applications in optical spectroscopy and spectral imaging. It is based on the combination of an acousto-optic tunable filter and a Fabry- Perot, which we call an acousto-optic Fabry-Perot filter (AOFPF). It offers advantages of a broadband tuning range (several hundred nanometers) and a narrow optical bandpass (subangstrom resolution in visible/NIR). A prototype AOFPF which operates in the visible/NIR wavelength region has been designed, fabricated and tested. Detailed experimental results are presented.
Two separate technical developments are described: a long wave (7.5 to 11.2 micron) multi-spectral (5 bands) second generation thermal imager (HgCdTe, 288 by 4) and real time algorithm implementation using a datacube maxvideo image processing system. When combined these two technologies provide an enhanced imaging sensor with several unique features for improved multi-spectral performance; including scene based histogram correction, spatial filtering, and frame averaging. Image processing options are adjusted by the operator to adapt and optimize imagery for different scene conditions via real time feedback. The paper describes system hardware, software development, and laboratory measurements and field observations which demonstrate the impact of different processing strategies against dynamic scene backgrounds. Advantages of real time processing include an NETD/MRTD performance improvement by a factor of 3. The TAS 4X FW/datacube system provides the ability to observe the migration of small quantities of gas (SF6) from stand-off ranges of several kilometers. Filtered imagery when combined with real time processing can provide a new level of imaging performance that can observe scene details unresolvable by other techniques.
Remote spectral sensing in the infrared to detect and identify chemical species, and IR imaging of remote targets are mature technologies that have been developed and used successfully for decades. Combining the two technologies represents a new opportunity to collect spatial and spectral information simultaneously (hyperspectral imaging). This allows the user to record target and background information at the same time, to observe multiple targets in the field of view independently, and to obtain spectral data on distinct portions of an extended target. Several approaches to imaging spectroscopy are being developed currently. This paper discuses imaging spectroscopy using a Michelson Fourier transform spectrometer (FTS) coupled with a focal plane array (FPA) for airborne observations of targets such as the effluents from industrial smoke stacks. This approach offers high sensitivity, and fine spectral resolution for specific chemical detection and identification, and rapid data acquisition, all in the compact, light-weight package needed for airborne sensors. The objective of the work reported here is to evaluate performance trade-offs and define the sensor performance that we can achieve with today's technology in the areas of FTS, FPA, and signal electronics, and data collection and storage. We present the spectral and spatial resolution, sensitivity, and temporal resolution that can be achieved in the near-term future.
The agile bandpass tunable filter (ABTF) is a new instrument with variable resolution and center wavelength that allows MWIR and LWIR infrared cameras to recognize spectral images that are characteristic of specific materials, including gases. Benefits of this new technology include the ability to: (1) Task infrared camera-based sensors to search for specific materials, and (2) Transform infrared camera data into signatures easily understood by untrained users. The application of a working prototype hyperspectral ABTF is described. The prototype is capable of rapidly selecting bandpasses from among several spectral resolutions, and is easily tunable over the MWIR spectral band. The application of the ABTF to signature imaging of gases is shown.
Intellitec, a division of Technical Products Group, Inc., is developing and testing a ruggedized commercial spectrometer for stable platform applications. This spectrometer design incorporates the key components utilized in the M21 remote sensing chemical agent alarm (RSCAAL). The spectrometer, which has a resolution of 4 wavenumbers (cm-1), can be used to remotely sense fugitive vapors with spectral features in the 8 - 12 micrometer region. Intellitec has initiated a parallel effort to complete the development of personal computer (PC) based vapor identification software. The spectrometer's detection capability was tested by placing small amounts of anhydrous hydrazine vapor in a cell positioned in front of a black body reference. The data collected from the spectrometer clearly showed characteristic spectral features of hydrazine vapor. A developmental set of hydrazine coefficients was generated for use with the vapor identification software utilized in the M21. The coefficients were programmed into the M21. The effectiveness of the coefficients in the M21 was then tested by attempting to detect hydrazine vapor contained in a cell positioned in front of a black body. The developmental coefficient set successfully detected the hydrazine vapor. Further testing is required to improve detection sensitivity and confirm the spectrometer's ability to detect these vapors in an open path environment.
Interferometry concept, that considers separately the phases: dynamical, Fresnel and topological has been elaborated. These phases have different origin. They are informative on specific properties of a sample, that is studied with an interferometer and a polarized light. Preliminary experiments have shown the confirmation of the role of topological phases in the interferogram formation. The concept has been applied to the micro-surface monitoring in real time.