<title>Evaluation of beam-path configurations for use with a monostatic OP-FTIR capable of rapid beam movement for tomographic reconstruction of gas and vapor concentrations in workplaces</title>

Abstract

Beam path average data from an open path Fourier transform infrared (OP-FTIR) spectrometer can be used to reconstruct 2D concentration maps of the gas and vapor contaminants in workplaces using computed tomographic (CT) techniques. However, a practical limitation arises because many source and detector units are required to produce a sufficient number of intersecting beam paths in order to reconstruct concentration maps. A monostatic OP-FTIR system which is capable of rapid beam movement can be used to eliminate this deficiency. Instead of many source and detector units, a number of the intersecting folded beam paths can be obtained using many flat mirrors and retro-reflectors. We conducted tests of several beam configurations generated for a single scanning FTIR system using 54 flat mirrors and 56 retro-reflectors mounted along the perimeter walls of a typical sized 24 foot by 21 foot test room. The virtual source CT configurations were tested using concentration maps created from tracer gas concentration distributions measured experimentally in a test chamber. Computer simulations of different beam configurations were used to determine the optimal beam geometry. We found that high concentration areas and the general concentration gradient pattern could be resolved from tomographic reconstructions calculated based on 102 folded beam paths. However, the reconstructions showed some effects from noise and peak-smearing artifacts. The noise level could be reduced and the quality of reconstruction maps were improved by using a spline interpolation method to correct for the influence of folded rays. We refer to this approach as a virtual source CT geometry.