Standoff detection, identification and quantification of chemicals require sensitive spectrometers with calibration capabilities. Recent developments in LWIR focal plane arrays combined with the mastering of Fourier-Transform Spectrometer technology allow the realization of an imaging spectrometer specifically designed for chemical imaging. The spectral and radiometric calibration of the instrument enables the processing of the data to detect the chemicals with spectral signatures in the 8-12 μm region. Spectral images are processed and the contrast between different pixels is used to map the chemicals.
Telops has built a field-portable instrument. This paper presents some details about the design of this state-of-the-art sensor. Performance and test results are also presented along with results from a field test.
Advancements in Mercury Cadmium Telluride (MCT) focal plane arrays (FPA) in recent years have allowed high performance longwave infrared imagers to prosper. In particular molecular and gas/chemical spectroscopy applications can be vastly advanced with these new products. However, for the transition from single pixel spectrometers to FPA base imaging spectrometers to succeed, a couple of parallel advancements must be made as well. Most Fourier transform spectrometers currently available are designed specifically for a 1 mm single pixel detector. Scientists who try to convert these systems into imaging spectrometers quickly run into throughput issues when FPAs reach sizes of up to 12.5mm, thus limiting the performance and greatly impacting the detection capabilities. Furthermore, for large FPAs the readout time can be significantly longer than the integration time. In turn, this requires slower sweep speeds with a higher degree of control of the scanning mechanism. The benefit of these new technologies in spectroscopy can only be demonstrated with a system optimally designed for imaging spectroscopy. This paper will address the issues of imaging spectroscopy and will show how an instrument designed for specifically imaging applications can dramatically improve the performance of the system and quality of the data acquired.
Standoff detection, identification and quantification of chemicals require sensitive spectrometers with calibration capabilities. Recent developments in LWIR focal plane arrays combined with the mastering of Fourier-Transform Spectrometer technology allow the realization of an imaging spectrometer specifically designed for chemical imaging. The spectral and radiometric calibration of the instrument enables the processing of the data to detect the chemicals with spectral signatures in the 8-12 μm region. Spectral images are processed and the contrast between different pixels is used to map the chemicals. Telops is building the field-portable instrument. This paper presents the requirements for chemical detection in the LWIR, how the system is broken down into different modules and the details of each of these modules: calibration, interferometer, datacube acquisition and processing, and the main controller. The system has real-time processing capabilities of the measured data. Performance prediction is presented as well.
For a complex remote sensor like the NPOESS Crosstrack Infrared Sounder (CrIS), the process of requirements flowdown is extremely important to the success of the project. When there is both an algorithm and a sensor, the task of allocating requirements between the sensor and the algorithm becomes a challenge. This is where the use of system models and simulations has been an invaluable tool. Complex requirements such as radiometric uncertainty and Instrument Line Shape (ILS) uncertainty have utilized system models and simulations for the allocation of requirements. For radiometric uncertainty the sensor model in conjunction with the algorithm which handles the calibration of the sensor was used to assess the contribution of parameters such as component and detector temperature stability on radiometric uncertainty. Variation of the parameter values within the sensor model allowed us to compute the impact on radiometric uncertainty and allocate requirements appropriately. Examples of how the model and simulations were used to develop requirements for the CrIS radiometric uncertainty will be presented. For the assessment of ILS uncertainty a model for predicting the ILS of a Michelson interferometer was employed. The model calculates the ILS and associated spectral shift based upon a set of input parameters. By varying the input parameters the sensitivity of the ILS to the specific parameters could be determined and used to allocate the requirements from a top level down to the module level. A description of the model, the input parameters and results for the CrIS requirements development will be presented.
It is important in any remote sensing radiometer to identify and characterize the noise and error sources of the radiometer. At ITT, we have produced a number models to characterize noise and its impacts. The latest noise model is for the Cross-track Infrared Sounder (CrIS) instrument which is part the National Polar-orbiting Operational Satellite System (NPOESS). The required accuracy of the instrument demands identifying and characterizing the noise and random error sources to lower the risk of poor instrument performance. This paper lists the sources of noises and random errors identified in the CrIS sensor and compares model predictions to measurements from the first CrIS Engineering Development Unit (EDU).