Tomographic reconstruction of tracer gas concentration profiles in a room with the use of a single OP-FTIR and two iterative algorithms: ART and PWLS

Computed tomographic (CT) reconstructions of air contaminant concentration fields were conducted in a room-sized chamber employing a single open-path Fourier transform infrared (OP-FTIR) instrument and a combination of 52 fla t mirrors and 4 retroreflectors. A total of 56 beam path data were repeated ly collected for around 1 hr while maintaining a stable concentration gradi ent. The plane of the room was divided into 195 pixels (13 x 15) for recons truction. The algebraic reconstruction technique (ART) failed to reconstruct the orig inal concentration gradient patterns for most cases. These poor results wer e caused by the "highly underdetermined condition" in which the number of u nknown values (156 pixels) exceeds that of known data (56 path integral con centrations) in the experimental setting. A new CT algorithm, called the pe nalized weighted least-squares (PWLS), was applied to remedy this condition . The peak locations were correctly positioned in the PWLS-CT reconstructio ns. A notable feature of the PWLS-CT reconstructions was a significant redu ction of highly irregular noise peaks found in the ART-CT reconstructions. However, the peak heights were slightly reduced in the PWLS-CT reconstructi ons due to the nature of the PWLS algorithm. PWLS could converge on the ori ginal concentration gradient even when a fairly high error was embedded int o some experimentally measured path integral concentrations. It was also found in the simulation tests that the PWLS algorithm was very robust with respect to random errors in the path integral concentrations. T his beam geometry and the use of a single OP-FTIR scanning system, in combi nation with the PWLS algorithm, is a system applicable to both environmenta l and industrial settings.