Passive Standoff FTIR Detectors as Transducers. Background From radiation to information

Transcription

1 Slide 1 of 27 Passive Standoff Fourier-Transform Infrared Detectors as Transducers Richard J. Green, PhD, Advanced Technology Branch, West Desert Test Center, Dugway Proving Ground, UT Slide 2 of 27 Page 1 1

2 Passive Standoff FTIR Detectors as Transducers Background From radiation to information Infrared radiation and the atmosphere Hardware Signal transduction Information Conclusions Slide 3 of 27 Passive Standoff FTIR Detectors as Transducers Transducer: A device for converting energy from one form to another for the purpose of measurement of a physical quantity or for information transfer Passive Standoff FTIRs convert infrared energy from the environment into information about chemical hazards such as chemical warfare agents (CWAs) There are other uses, but this use is the focus Slide 4 of 27 Page 2

3 Passive Standoff FTIR Detectors Passive - They do not shoot out a beam, rather they receive energy that is in the environment (cameras also do this) Standoff They are designed to detect at a distance FT Fourier-Transform Method of acquiring frequency-dependent signal IR Infrared type of radiation that they detect Slide 5 of 27 The US Military, Developers, Testers, and Dugway The US Military is interested in detecting chemical warfare agents at a distance to protect soldiers Developers include General Dynamics (JSLSCAD, M21) Northrop Grumman (MCAD) Bruker (RAPID) Block Engineering (Block 100) Micro Engineering Software and Hardware (MESH) Inc. Aerospace Corporation Physical Sciences Inc. Mention of manufacturers does not constitute endorsement Testing Dugway s role is to test the effectiveness and suitability of the detectors We do not make the detectors; we measure their performance Slide 6 of 27 Page 3

4 West Desert Test Center Dugway Proving Ground Dugway is the DoD reliance area for CB testing Existing Licenses and EAs for CB simulants State and federal agency support for mission 1300 square miles of area With true no encroachment status Expertise with field and chamber CB testing Slide 7 of 27 Controlled Chamber Testing DPG Chamber facilities allow controlled testing indoors with actual CWAs The Multipurpose Chamber has a state-ofthe-art fixture for testing passive FTIRs Slide 8 of 27 Page 4

5 Outdoor Testing with Simulants West Downwind Road F5 F4 F6 Road 945 F3 F2 32 m TOWER F1 750 M Stacks Juliet Road 5 KM NO ROAD ACCESS Hotel Road Highway 101 Foxtrot Road Delta Road Bravo Road M Highway 102 Base Road Infrared cameras scanning FTIR FIXED FTIR 2 EACH ONE LOS. ONE AT ANGLE 32 M TOWER MET DCA JSLSCAD AERO SPACE JHU East Downwind Road CP TRAILER SODAR 2 KM Dugway has a state-of-the art test grid for outdoor testing with simulants Simulants released through stacks or explosively Ground-truth box is instrumented to verify cloud characteristics Wireless ethernet and GPS allow time- and positiondependent results to be acquired. Slide 9 of 27 From Radiation to Information Slide 10 of 27 Page 5

6 Electromagnetic Radiation Shorter wavelengths are higher frequencies and higher energies. Infrared (IR) is electromagnetic radiation between 0.7 and 100 µm in wavelength Slide 11 of 27 Blackbody Radiation Most materials emit and absorb in the infrared An ideal body that can emit and absorb all wavelengths is called a blackbody As the temperature of a blackbody rises: it emits more radiation at shorter wavelengths (higher energies) 60x10-6 Radiance (W/cm*sr) C 100 C 0 C -100 C Energy (cm -1 ) 1500 Slide 12 of 27 Page 6

7 Atmospheric IR is mainly from water, carbon dioxide and ozone Atmospheric Windows at 8-12 µm and 3-4 µm Many compounds of interest have finger prints in the 8-12 µm range Atmospheric Radiation Slide 13 of 27 Different chemicals have different features in the IR Structure of the spectrum is like a finger print Can quantify the amount of the chemical Infrared Spectra Absorbance TEP GB AA HD TEP - nerve simulant GB - nerve agent AA - blister simulant HD - blister agent SF 6 - tracer gas SF 6 Energy (cm -1 ) Slide 14 of 27 Page 7

8 Atmospheric Radiation Radiation arriving at a detector depends on: The temperature difference between the cloud and the background The nature of the background (sky, mountains, etc.) The amount and type of chemicals present in the cloud The analytes can absorb or emit radiation Cloud Temperature = T amb Background IR source Temperature = T bb T= T bb -T amb Slide 15 of 27 Hardware Collect, Separate and Detect Radiation Collect: Optics must either transmit or reflect in the infrared.» Zinc Selenide, Germanium, Thorium Fluoride Separate: Information is contained in the wavelength dependence.» Hardware must separate by wavelength Detect: Many existing and emerging detector technologies» Thermal Type: e.g. bolometers» Quantum Type: photoconductors: e.g. HgCdTe Slide 16 of 27 Page 8

9 Basically two ways to separate radiation based upon wavelength Dispersive optics act like a prism Interferometer uses constructive and destructive interference of waves Hardware Slide 17 of 27 Radiation takes two paths which interfere Reference laser (monochromatic source) can track mirror spacing Resulting signal is an interferogram Interferometers freiburg.de/spectroscopy/time- Resolved/michelson%20interferometer.gif Slide 18 of 27 Page 9

10 Signal Transduction Convert interferogram into a spectrum (Fouriertransform) Average multiple spectra Remove instrument effects (radiometric calibration) Remove atmospheric effects and clutter (background correction or modeling) Convert spectrum to information (chemometrics, expert systems, neural networks etc.) Slide 19 of 27 Fourier Transform Convert interferogram into a spectrum Based upon sampling rate and reference laser frequency Some additional processing:» apodization and phase correction The resulting spectrum includes instrumental effects Energy (cm -1 ) Interferogram Uncalibrated Spectrum 1500 Slide 20 of 27 Page 10

11 Radiometric Calibration Instrumental Effects Self-Radiance (the instrument radiates in the IR). Responsivity (the instrument is more sensitive to some wavelengths than others). Calibration based on acquiring black body spectra at different temperatures A black body spectrum should be a Planck function Uncalibrated Radiance Uncalibrated Calibrated Energy (cm -1 ) 20x Radiance (W/cm*sr) Slide 21 of 27 Background Removal 2.0x10-5 Radiance (W/cm*sr) Background Signal Energy (cm -1 ) Slide 22 of 27 Page 11

12 Feature Extraction Need to know the background Measure or model Many methods to correct for known background Division Subtraction Other methods Extracted features can be used to identify and quantify Ratio Spectrum Energy (cm -1 ) Slide 23 of 27 From Data to Information Identify the spectrum: compare with spectral library Quantify the spectrum Peak height can be used to extract the concentration*pathlength product (CL) Slide 24 of 27 Page 12

13 Taking the Human out of the Loop CWA detector needs to make a decision on whether a threat is present Needs to balance low false alarm rate with high probability of detection Decision is not based merely on intensity of a band at a given wavelength Band shape and secondary features matter Slide 25 of 27 Possible Approaches Chemometrics: linear algebra with known target, interferent sets Rules-based systems Neural networks etc. Slide 26 of 27 Page 13

14 Conclusions Traced story from radiation to information Real-life problems are difficult to solve Additional complications are possible Slide 27 of 27 Backup Slides Slide 28 of 27 Page 14

15 Structure of the Atmosphere Troposphere: H 2 O, CO 2, O Stratosphere: O Stratospheric Ozone emits against a cold sky background Altitude (km) Mesosphere Thermosphere Tropospheric interferences may emit or absorb 50 0 Stratosphere Troposphere Temperature (K) Slide 29 of 27 Temperature Effects on Passive Spectra As T changes, so do passive spectra A given signal level can be reached by increasing CL or T Need to measure detector response as a function of CL and T Ratio Spectrum Τ Energy (cm -1 ) Slide 30 of Page 15

16 Detectors Slide 31 of 27 Page 16