Adaptive InfraRed Imaging Spectroradiometer

System Concept and Basis of Operation

AIRIS is PSI's patented multispectral infrared imaging technology, U.S. Patent 5,461,477. In contrast to other approaches to multispectral and hyperspectral infrared imaging, e.g., pushbroom spectrometers and FTIR spectrometers, AIRIS may be commanded to collect data at only those wavelengths which facilitate target detection. This capability can reduce data volume and data processing requirements for many multispectral imaging applications.

AIRIS Tunable Filter Module

Each AIRIS is comprised of an IR focal plane array (FPA) which views the far field through a tunable Fabry-Perot interferometer (etalon). The tunable etalon is the critical enabling technology. It was developed by PSI and acts as a continuously tunable bandpass filter which selects the wavelength which illuminates the FPA. The wavelengths transmitted by the etalon are determined by its mirror spacing, which is maintained by high speed piezoelectric-based actuators coupled with a closed-loop, digital feedback system. The etalon be operated in conjunction with an IR camera or as a stand-alone device.

AIRIS optical system concept.

FWHM and peak transmission of LWIR tunable etalon.

Features

• Continuous coverage of MWIR (λ~3 µm to 5 µm) or LWIR (λ~8 µm to 12 µm) atmospheric transmission windows
• High spectral resolution (λ/δλ >100) imagery
• Random wavelength access
• Etalon tuning time is 10 to 20 ms
• Etalon easily integrated with commercial IR cameras or operated as a stand-alone device
• Windows NT software interface
• Supported commercial IR cameras include Santa Barbara Focalplane, SE-IR, Amber, Cincinnati Electronics
• Hyperspectral data processing software for ATR

Operation of AIRIS from an airborne platform was demonstrated in 2002 in collaboration with U.S. Army SBCCOM and the U.S. Army Redstone Technical Test Center. The sensor was integrated into a gyro-stabilized, gimbaled instrumentation pod on a UH-1 helicopter and used to monitor releases of a non-hazardous atmospheric tracer gas from above. The experiments further demonstrated the potential for using hyperspectral infrared imaging to survey wide areas for hazardous chemical vapors.

UH-1 helicopter with gimballed, gyro-stabilized instrumentation pod.

Photograph of instrumentation pod with aerodynamic skins off showing AIRIS (upper right), an IR camera for tracking (lower left) and intensified visible camera for tracking (lower right).

Photograph of UH-1 cabin with AIRIS equipment mounted to the left. Shown from top to bottom are the monitor, keyboard, computer, power supply, and an uninterruptable power supply.

The figures below show AIRIS images of an atmospheric tracer release as seen from the UH-1. The first image indicates the location of two roads, the vapor plume, and a section of terrain typical of the background against which the plume is viewed. In this image, the plume location is apparent by virtue of its thermal contrast with the background, ~5°C; however, it is the emission spectrum of the tracer gas, not simply the thermal contrast, which enables the plume to be distinguished from warm objects on the ground.

Unprocessed AIRIS image of atmospheric tracer release viewed from above.

Our proprietary data processing algorithm enables visualization of the plume. The processed images below depict the detected plume location as a function of the correlation between the measured IR spectrum of each pixel and the laboratory reference spectrum of the tracer gas. (PSI's plume detection algorithm utilizes Spectral Angle Mapping. The pixels overlayed in red in the three images correspond to |cosθ| ≥ 0.50, |cosθ| ≥ 0.55, and |cosθ| ≥ 0.60, respectively.)

Processed AIRIS image showing detected tracer plume location. Spectral Angle-based detection threshold: |cosθ| ≥ 0.50.

Processed AIRIS image showing detected tracer plume location. Spectral Angle-based detection threshold: |cosθ| ≥ 0.55.

Processed AIRIS image showing detected tracer plume location. Spectral Angle-based detection threshold: |cosθ| ≥ 0.60.

PSI has developed a physics-based system model to quantify the trade-offs between probability of detection and probability of false alarm. The AIRIS data processing goal is to maximize the probability of detection while minimizing data processing time and keeping the false alarm rate below a specified level. Band selection (number of bands), band wavelength, spectral resolution, and field-of-view are important considerations when designing an instrument and defining its mode of operation. PSI can deliver AIRIS units for general use or customized for specific applications.

Applications

The latest generation LWIR AIRIS produces 64 × 64 pixel "chemical images" of the scene under study [C.M.Gittins, W.J.Marinelli, and J.O.Jensen, "Remote Sensing and Selective Detection of Chemical Vapor Plumes by LWIR Imaging Fabry-Perot Spectrometry," Proc. SPIE 4574, pp. 63-71 (2001).] The AIRIS functions as a chemical vapor sensor by comparing the spectrum of each pixel in the hyperspectral datacube against library reference spectra of target chemical species. The figure below depicts a chemical image generated using AIRIS data of a controlled chemical vapor release and PSI"s proprietary datacube processing algorithm. Pixels whose spectra exhibit a high degree of correlation with the target chemical are highlighted in red; pixels exhibiting lower correlation are highlighted in yellow. Passive standoff chemical vapor detection requires a thermal contrast between the plume and the background. The plume in the processed image was detected in emission near the release point where it was hotter than the background and in absorption further downwind after it thermally equilibrated with the atmosphere. (It 'disappears' over the region where it matches the background temperature.) AIRIS viewed the chemical vapor release from 1.5 km standoff. An image of the plume release area recorded from 0.5 km with a conventional (visible) digital camera is shown for comparison.

Additional applications:

• Tunable bandpass filter for differential absorption LIDAR receivers
• Signature analysis and ATR for missiles and aircraft
• Camouflage defeat
• Fire science and management

Broadband IR image of chemical vapor plume release with detected plume location shown with false color overlay.

Visible wavelength image of chemical plume release area.

IR Camera Compatibility

PSI's most recent AIRIS systems have utilized Santa Barbara Focalplane and SE-IR cameras. PSI added etalon control capabilities directly to the camera vendor-supplied Windows NT-based camera control software. PSI has also integrated tunable etalons with Amber Radiance, Amber Galileo, and Cincinnate Electronics cameras.

Etalon Specifications (typical)

William J. Marinelli Physical Sciences Inc. 20 New England Business Center Andover, MA 01810-1077 Telephone: (978) 689-0003 Fax: (978) 689-3232 marinelli@psicorp.com