Electronic Military & Defense Annual Resource

6th Edition

Electronic Military & Defense magazine was developed for engineers, program managers, project managers, and those involved in the design and development of electronic and electro-optic systems for military, defense, and aerospace applications.

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Techniques Passive Remote Identification Of Gases And Liquids With Hyperspectral Imaging Hyperspectral imagers can serve on the battlefield as mobile or stationary early warning systems for chemical warfare agents. By René Braun W hen attempting to mitigate risk in a complex, hostile environment, reliable information on the presence of potentially hazardous chemicals is vital. Gathering the information before entering the scene is the potential of remote sensing Fourier transform infrared (FTIR) spectroscopy. Hyperspectral imaging allows the detection and mapping of both airborne and liquid compounds without the requirement of an active source of radiation. It provides information about the position, distribution, and propagation of a target compound. Investigating dangerous chemicals using passive infrared spectroscopy is a well-established technique for the detection of airborne compounds. 1,2,3,4 Bruker, whose instrumentation was used to provide this article's examples, is among the manufacturers offering both scanning imaging devices and their successors, hyperspectral imagers. Such scanning and imaging devices are routinely used by emergency responders for the analysis of toxic gas clouds. Both mobile devices are designed for field use and can be operated from vehicles and helicopters. By interpreting the brightness temperature contrast between a target and a background measurements can be performed from distances of up to several kilometres. This safe-distance operation provides a broad overview of the situation while exposing personnel to as little risk as possible. The hyperspectral imager is a newer device that renders scanning unnecessary, thus allowing the massive parallelization of the analysis routines. This provides a visualization of present chemicals in real time. In addition to mobile use, such devices can be deployed stationary, allowing permanent surveillance. The system used in the context of this example can be operated fully automated. With a total analysis and measurement time of roughly two seconds (for a standard library with approximately 40 compounds and standard PC hardware) and a field of view of 10 degrees (using standard optics), all-around surveillance can be performed within seconds. This setup enables its use as an early warning system, especially for chemical warfare agents (CWAs). Instrumentation All of the measurements presented in this article were performed with Bruker's hyperspectral imaging system HI 90 (Figure 1). The optical system is based on a Michelson interferometer in combination with a focal- plane array (FPA) detector. The Michelson interferometer is equipped with plane mirrors, while the moving mirror is actuated by voice-coil drives. In order to prevent a loss of efficiency due to a tilt of the mirror with regard to the optical axis, the moving mirror is actively aligned. The detector is a 256 x 256-pixel FPA detector. The standard spectral range is 870 - 1450 cm -1 (app. 11.5 - 6.9 µm). The system allows simultaneous exposure (integra- tion) and read-out (integrate while read) enabling high frame rates without reduction of the integration time. Each pixel of the system's horizontal and vertical fields of view is approximately 0.69 mrad. The system's noise equivalent temperature difference is NEDT = (160±45) mK with no spatial filtering. This corresponds to a noise equivalent spectral radiance of approximately 20nW/(cm ² sr cm -1 ). 4 Spatial filtering reduces the noise equiva- lent temperature difference. The identifica- tion of a known compound is based on the pixel-wise com- parison between the sample's measured spectra and the reference spectra. No illu- Electronic Military & Defense Annual Resource, 6th Edition 30 Figure 1: The hyperspectral sensor HI 90 (top) and System overview (bottom).

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