20 May 2011 Understanding the influence of turbulence in imaging Fourier transform spectroscopy of smokestack plumes
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Abstract
A Telops Hyper-Cam Fourier-transform spectrometer (IFTS) was used to collect infrared hyper-spectral imagery of the smokestack plume from a coal-burning power facility to assess the influence of turbulence on spectral retrieval of temperature (T) and pollutant concentrations (Ci ). The mid-wave (1.5-5.5 μm) system features a 320x256 InSb focal-plane array with a 326 μrad instantaneous field-of-view (IFOV). The line-of-sight distance to the 76mtall smokestack exit was 350m(11.4 x 11.4 cm2 IFOV). Approximately 5000 interferogram cubes were collected in 30 minutes on a 128x128 pixel window corresponding to a spectral resolution of 20 cm-1. Radiance fluctuations due to plume turbulence were observed on a time scale much shorter than hyper-spectral image acquisition rate, suggesting scene change artifacts (SCA) would be present in the Fourier-transformed spectra. Time-averaging the spectra minimized SCA magnitudes, but accurate T and Ci retrieval requires a priori knowledge of the statistical distribution of temperature and other stochastic flow field parameters. A method of quantile sorting in interferogram space prior to Fourier-transformation is presented and used to identify turbulence throughout the plume. Immediately above the stack exit, T and CO2 concentration estimates from the median spectrum are 395 K and 6%, respectively, which compare well to in situ measurements. Turbulence is small above the stack exit and introduced systematic errors in T and Ci on the order of 0.5 K and 0.01%, respectively. In some plume locations, turbulent fluctuations introduced errors in T and Ci on the order of 8 K and 1%, respectively. While more complicated radiance fluctuations precluded straightforward retrieval of the temperature probability distribution, the results demonstrate the utility of additional information content associated with multiple interferogram quantiles and suggest IFTS may find use as a tool for non-intrusive flow field analysis.
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Jennifer L. Massman, Kevin C. Gross, "Understanding the influence of turbulence in imaging Fourier transform spectroscopy of smokestack plumes", Proc. SPIE 8048, Algorithms and Technologies for Multispectral, Hyperspectral, and Ultraspectral Imagery XVII, 80480A (20 May 2011); doi: 10.1117/12.883197; https://doi.org/10.1117/12.883197
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