Tomographic reconstruction of air contaminant concentration maps using an open path Fourier transform infrared spectrometer. (1996)

Air pollutant concentration mapping techniques using computed tomography (CT) and optical remote sensing have found increasing applications in workplaces and environmental monitoring. These techniques can be very useful for rapid detection of air pollutant releases, source identification, evaluation of the performance of control measures, tracing air contaminant dispersion and transport, and estimation of emission rates. The goal of this dissertation was to explore realistic, improved systems for application of computed tomographic reconstructions of air pollutant concentration maps in environmental and industrial air monitoring. For better quality CT reconstructions, large numbers of intersected beam paths and projection angles are necessary, which require an increase in the number of optical remote sensing instruments. However, in field practice, it is desirable to use only one optical remote sensing system. Thus, it is clear that the optimal system must satisfy these two conflicting goals. Three beam geometries were developed and tested with the use of a single beam steerable OP-FTIR. The network of intersected beam paths was obtained through the use of an arrangement of flat mirrors and retroreflectors. Although reconstructed maps showed some unrealistic artifacts, it was concluded that these geometries can be applied in many industrial and environmental situations under carefully prescribed conditions. One of the significant positive features of the geometries that use a single OP-FTIR is that it is very economical and simple compared to those suggested in previous studies. The main cause of artifacts found in CT reconstructions with these geometries was due to highly underdetermined conditions. To remedy this problem, a new iterative CT algorithm, the penalized weighted least-squares (PWLS) method was applied. In the simulation tests, the quality of CT reconstructions was greatly improved when using the PWLS method compared to those reconstructions obtained when using the standard algebraic reconstruction technique (ART) algorithm. A new geometry called the overlapped multiple emitting ray (OMER) geometry was also developed. The OMER geometry is created by using two or more OP-FTIR instruments and retroreflectors. A steerable mirror can be used to replace multiple OP-FTIR systems. Thus, the OMER geometry can be created with the use of only a single OP-FTIR. This use of a single OP-FTIR system to create a virtual multi-OP-FTIR system makes this approach practical. The OMER geometry has many unique advantages including good practical applicability, high flexibility and expandability, excellent performance, and ease of beam alignment. Various OMER geometries were evaluated with a wide range of concentration gradient profiles. OMER geometries performed fairly well even with the use of only two ray emitting sources. The CT reconstructions were not significantly changed with variation in the positions of the ray emitting sources. Various sets of OMER geometries were generated while varying the adjacent distances between ray emitting sources. Valid CT reconstructions were obtained even with an OMER geometry in which ray emitting sources were placed very close together. This resulted in a high degree of flexibility and field applicability. The OMER geometry was designed based on consideration of characteristics and performance of the OP-FTIR and retroreflectors. Therefore, it can be directly applied in many conceivable environmental and industrial settings.