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Remote Sensing to the Next Level

Wed, 04/24/2013 - 12:12pm
Dick Merk and Thomas J Tague Jr., PhD, Bruker Optics, Billerica, Mass.

Enhancements in hyperspectral imaging are improving response and increasing efficiency.

HI 90 Remote Chemical Imaging system with pan and tilt head.The 1984 Bhopal, India gas tragedy killed more than 3,000 people and injured tens of thousands more living in the area surrounding the Bhopal Union Carbide pesticide plant. Victims were exposed to a leaking gas cloud created when water mixed with methyl isocyanate (MIC). The invisible poisonous gas cloud flowed through the adjacent residential areas; and in the critical hours following the accident, few details were made available to the local population to help identify the severity or origin of the situation.

Today, chemical weapons, industrial processing facilities, pipelines, and agricultural sites remain potential sources of hazardous and environmentally damaging chemical compounds. But recent advances in ground-based remote sensing imaging technology, early warning, and real-time monitoring systems can now detect, identify, and map the movement of hazardous gas clouds automatically providing time-sensitive information to system operators, allowing the quick identification and tracking of airborne compounds.

Remote sensing employs a technique known as “stand-off detection”, in which chemicals, typically in the form of gas clouds, can be identified, located, and monitored from a safe distance of up to several kilometers. Hyperspectral imaging systems using the stand-off detection technique have been developed to meet these demands and currently offer high performance levels necessary in critical detection scenarios.

Bruker Corporation, Billerica, Mass., has coupled highly efficient interferometer technology and proprietary chemometric methods for automatic identification and imaging of chemical species present. The HI 90 hyperspectral imager rapidly detects molecules over a large field of regard (FOR) in seconds and provides both spatial and spectral analysis of the FOR. As chemical identification is carried out automatically at distances of up to several kilometers, visual and audio alarms are activated when one or more of the library compounds are detected and identified.

System operation
The mid-infrared (MIR) region of the electromagnetic spectrum contains useful “fingerprint” information that permits chemical identification through spectral analysis. A plane-mirror Michelson interferometer using active alignment, designed for maximum efficiency in the MIR, is the heart of the HI 90. Plane mirrors used in the interferometer provide higher efficiency than corner cubes used in alternative designs, as only one reflection is involved instead of three with the corner cubes. The moving mirror is adaptively aligned during motion to prevent loss of efficiency due to tilt. And the adaptive alignment is actuated by several voice coil frictionless drives. An infrared laser diode serves as a reference to move and adapt the mirror, as well as to trigger the detector. The MIR detection is performed with a Stirling-cooled mercury cadmium telluride focal plane array detector (256 by 256 pixels), which records interferograms over the MIR wavelength region simultaneously for all pixels within the FOR. Radiometric calibration of the imager is carried out automatically during the automated internal calibration process.

Alignment of the FOR to the target is accomplished by using an integrated video camera operating continuously in real time. With the FOR clearly defined on the live video image, the chemical images are overlaid and updated as each image is processed. Chemicals targeted for immediate detection are selected from a library of over 400 compounds. During detection, spectral and spatial information from the interferograms of each pixel is processed using proprietary advanced algorithms. This information is processed against the system library, yielding values at the individual pixel level for identification confidence, correlation of measured versus reference spectra, signal-to-noise, brightness temperature, and other characteristics for each compound detected. The image capture and measurement times vary as a function of the spectral resolution and the number of pixels used, both of which are user selectable. Analysis times vary with the number of compounds selected from the library.

Given that target compounds analyzed in remote sensing applications often generate small signals, an important parameter of stand-off detection systems is signal-to-noise ratio, which is achieved through the HI 90’s optimized signal collection system, interferometer, and MIR-sensitive focal plane array.

Packaging and computer connectivity are critical considerations for field-deployable measurement systems, as exposure to a wide range of thermal conditions and varying degrees of humidity are common. In many measurement scenarios, complete protection from adverse environmental conditions may not be possible. The sealed housing of the Bruker HI 90 is water resistant, allowing deployment in many environments. The standard HI 90 operates over a temperature range of 0 to 35 C, with extended temperature range operation available as an option. With the electronic heat management air circulation system enclosed as well, the HI 90 is designed with field use in mind. A single Ethernet cable handles the operation, pan and tilt, and gigabit Ethernet data transfer between the system and computer at distances of up to 100 meters. The system operates on standard 110 to 240 V line cord power and is available with a remote battery power supply.

Long-range airborne chemical detection with the HI 90 is acquired using passive detection, in which the advanced remote sensing algorithm extracts the spectral information from the gas cloud.

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