Hyperspectral quantitative imaging of gas sources in the mid-infrared

Abstract

An imaging Fourier transform spectrometer operating in the medium infrared (1800-5000 cm(-1)) has been used to image two gas sources: a controlled CO2 leak at room temperature and the exhaust of a combustion engine. Spectra have been acquired at a resolution of 0.5 cm(-1) using an extended blackbody as the background. By fitting them with theoretical spectra generated with parameters from the High-Resolution Transmission Molecular Absorption database, quantitative maps of temperature and gas column density (concentration·path product) for the gas plumes have been obtained. Spectra are related to gas plume parameters by means of a radiometric model that takes into account not only gas absorption, but also its emission and the atmospheric absorption, as well as the instrument lineshape function. Measurements for the gas leak show very good agreement between retrieved and nominal values of temperature and CO2 column density. This result has direct application to obtain quantitative imaging of exhaust emissions from automobiles and other mobile sources, as shown here with measurements of exhaust gases in a diesel engine.

Validation of Emission Spectroscopy Gas Temperature Measurements Using a Standard Flame Traceable to the International Temperature Scale of 1990 (ITS-90)

“…The UC3M hyperspectral imager, described in detail in [28,29], is a FTIR hyperspectral Imaging System (Telops Hypercam IFTS) operating in the extended MIR region, from 1800 cm −1 to 5000 cm −1 . This system has a Michelson interferometer coupled to an InSb focal plane array (FPA), with 320 × 256 pixel resolution, with an instantaneous (pixel) field of view of 0.35 mrad and a maximum resolution of 0.25 cm −1 .…”

“…Additionally, low-frequency fluctuations in the raw interferograms due to flame flickering were corrected for [30]. Interferogram processing included triangular apodization, phase correction to remove small sampling asymmetries in the interferogram, and off-axis wavenumber correction [28,29]. To enable the system to measure in radiance units, it was radiometrically calibrated at Centro Español de Metrología (CEM).…”

“…Uncertainty estimation has been divided in two parts [28]: (1) the uncertainty in the measured radiance, and (2) the propagation of this uncertainty into the retrieved values of temperature, T and species column density Q , which determines the respective sensitivity coefficients. This approach is essentially the same as that in [32].…”

Measurement of temperature and gas concentration in a flame using a FTIR imager

“…A radiometric model [2] that takes into account background emission, atmospheric absorption, flame emission and flame absorption has been used to generate simulated radiance spectra.…”

Sensitivity limits on optical gas imaging due to air turbulence

“…This three-layer model is quite general in that it does not require any assumptions on the spectrum L b ðλÞ of light behind the gas cloud. 18 For a pixel in the scene observed by the camera, we can write the radiative transfer equation of a ray along the line of sight to give an at-pupil radiative flux M of E Q -T A R G E T ; t e m p : i n t r a l i n k -; e 0 0 1 ; 6 3 ; 4 3 5…”