A high-speed passive FTIR imaging spectrometer has been developed and tested in airborne flight tests on both fixed wing and helicopter platforms. This sensor was developed and flown from 2000 to 2005 in conjunction with various organizations, and is known as the Turbo FT. The Turbo FT is a laser-less rotary high speed Fourier
Transform Infra-Red (FTIR) spectrometer capable of very high speed, spectral resolution to 1 cm<sup>-1</sup>, and operation in rugged environments. For these tests, the sensor was run at 8 cm<sup>-1</sup> resolution and 50-100 scans per second with either a single element or a 2x8 element LWIR detector. An on-board auto-calibrating blackbody accessory was developed and automated chemical detection software was developed. These features allow in-flight calibration, facilitated detection of target gas clouds, and reported detections to an on-board targeting computer. This paper will discuss the system specifications, sensor performance, and field results from various experiments. Current work on development of an 8x8 pixel Turbo FT system will also be presented.
A new rugged rotary Fourier Transform Spectrometer (FTS) has been developed. It can be used for environmental remote sensing and monitoring of chemical processes. Both single pixel and mosaic imaging configurations have been built and tested. The continuous rotary scan of the 'Turbo FT' allows operation without the laser reference of a conventional FTS, and it has been demonstrated to deliver 30 to 360 spectral scans per second and 1 cm<sup>-1</sup> resolution, with excellent lineshape. A new 'space frame' version of the interferometer, with excellent mechanical and thermal stability, was field tested in both airborne and ground systems during the year 2000, with good results. The interferometer for this instrument is palm sized, and weighs 20 oz. It is totally sealed from the environment, and can be mounted, with its drive electronics, into a temperature stabilized enclosure for outside remote sensing applications For industrial applications, it can be used on-line or off-line, in conjunction with fiber optics, to measure and/or control multiple process lines. With the appropriate optics and detector set, the wavelength range can be adjusted from 1.0 to 25.0 micrometers. The resolution is variable from 1 to 8 cm<sup>-1</sup>. Various processors, data acquisition boards, and software have been used in the development, including the Labview package from National Instruments. Custom software for acquisition, display, and storage is being developed in summer 2001. The data acquisition system can be tailored for the speed and number of pixels required for the application. The current commercially available hardware being used can support up to 16 pixels at up to 100 scans per second.
The Chemical Imaging System (CIS) is a small, high-speed long-wave infrared (8 - 12 micrometers ) imaging spectrometer which is currently under development by the United States Army. The fielded system will operate at 360 scans per second with a large format focal-plane-array. Currently, the CIS uses the TurboFT FTS in conjunction with a 16-pixel direct-wired HgCdTe detector array. The TurboFT spectrometer provides high-speed operation in a small, lightweight package. In parallel to the hardware development, an algorithm and software development effort is underway to address some unique features of the CIS. The TurboFT-based system requires a non-uniform sampling Fourier transform algorithm in order to preserve signal fidelity. Also, the availability of multiple pixels can be exploited in order to improve the interference suppression capabilities of the system by allowing the detection and identification algorithm to adapt its parameters to the changing background. Due to the enormous amount of data generated, the signal processing must proceed at very high rate. High-speed computers operating with a parallel architecture are required to process the data in real time. This paper describes the current CIS bread box system. It includes some field measurement results followed by a discussion of the issues and challenges associated with meeting the design goals set for the program.
Designs & Prototypes has developed a small, very fast, rugged rotary Fourier transform (FT) spectrometer, which can be used in conjunction with fiber optics to monitor industrial processes. When used in an imaging configuration with a mosaic detector array, multiple processes (or multiple locations in a process) can be monitored simultaneously. The rotary scan allows operation without the laser reference of conventional FT spectrometers, and can easily yield scan rates from 30 to more than 300 scans per second. Single pixel operation at 360 scans per second and 1 cm <SUP>-1</SUP> resolution, with excellent lineshape, has been demonstrated. Multiple pixel operation has also been demonstrated using a 3X3 HgCdTe PC mosaic detector. Field testing of a hand portable version with 8 cm<SUP>-1</SUP> resolution will be performed in late summer 1998. The unit can measure spectra in the NIR, SWIR, and Thermal IR, with the appropriate optics and detector sets. The optical bench for 8 cm<SUP>-1</SUP> resolution is 2.5X3.5 inches, and 2.5 inches high. It weighs 11.5 oz., and is totally sealed from the environment. Electrons for servo and detector channel functions can be packaged into an enclosure 6X4X2 inches. For very fast real time processing, single or multiple DSPs can be used. The development work has been supported by contracts from the U.S. Army, a large U.S. aerospace company, and a major Australian mining company.
A new type of imaging spectrometer is being developed for remote sensing and chemical detection. It is based on a rotary Fourier Transform Spectrometer design. The rotary nature of the scan allows both high speed operation and laserless sampling of the detector signal. Its small size also allows cryogenic operation for enhanced detection capability and stable calibration. A 360 scan per single pixel prototype was built and demonstrated at ERDEC in April 1996. The single scan time at this speed is just under 1 millisecond. The optical sensor measures 6' X 4' X 2', and weighs approximately 3 lbs. Spectral resolution is 2 wavenumbers (cm<SUP>-1</SUP>) maximum, and coverage is 12 - 14 microns using Zinc Selenide (ZnSe) optics. Power consumption is less than 1 watt at steady state, slightly more at spinup. Electronics and software package used COTS 12 bit, 10 MHz data acquisition and DSP board in conjunction with Labview software for acquisition and analysis on a desktop PC. Following this demonstration, the scan linearity was studied by passing a 3.3 micron laser signal through the IR channel using different rotor materials, and using laserless sampling to look at the resulting lineshape. The results of that testing is the subject of this paper. Subsequently, a 3 by 3 HgCdTe (MCT) detector array is being mated to the optical sensor. An enhanced data channel with 16 simultaneous 12 bit inputs is also being fitted, running at 1/10 of original speed. Image quality and pixel crosstalk are being analyzed at this lower speed. Field measurements will be made in summer 1997.
Fourier transform spectrometers are considered, and emphasis is placed on resolution and the speed of data acquisition. Detector linearity requirements are discussed as well as trading the multiplex advantage for the throughput advantage. Radiometric accuracy is considered, and focus is placed on instrument design and operational parameter optimization. Issues of calibration are presented, and it is noted that using black bodies for calibration can saturate the detector with long-wavelength radiation, before sufficient signal-to-noise ratio for the short-wavelength radiation is reached. It is suggested to look for secondary calibration sources and to refrain from calibrating a narrow-band emission spectrum with a black-body radiator.