A computational study of tomographic measurement of carbon monoxide at minor concentrations

In order to develop a chemical species tomography system, this paper carries out various computational steps to address the problem of measuring minor species concentration using single-pass, short path-length absorption techniques in the mid-infrared. It focuses on the imaging of carbon monoxide (CO) in combustion exhaust as a case study, with an average concentration of 10 ppm over a 50 mm diameter cross-section, taking account of the presence of other absorbing species. CO absorption transitions R6, R7, R9 and R10 are identified as possible measurement targets. The joint effects of spectral absorption linewidth and laser source linewidth are considered at length, resulting in recommendations for laser linewidth to achieve appropriate levels of CO absorption signal purity. Measurement strategies are considered for achievement of the necessary sensitivity, noise and bandwidth performance. A feasible beam arrangement for tomographic imaging is discussed, providing 48 measurements of path concentration integral. Representative phantom reconstructions are presented, with encouraging results for application to such dynamic gaseous subjects.