EVALUATION OF VIRTUAL SOURCE BEAM CONFIGURATIONS FOR RAPID TOMOGRAPHIC RECONSTRUCTION OF GAS AND VAPOR CONCENTRATIONS IN WORKPLACES

Beam path average data from an Open Path Fourier Transform Infrared (O P-FTIR) spectrometer can be used to reconstruct two-dimensional concen tration maps of the gas and vapor contaminants in workplaces and the e nvironment using computed tomographic (CT) techniques. However, a prac tical limitation arises because in the past, multiple-source and detec tor units were required to produce a sufficient number of intersecting beam paths in order to reconstruct concentration maps. Such a system can be applied to actual field monitoring situations only with great e xpense and difficulty. A single monostatic OP-FTIR system capable of r apid beam movement can eliminate this deficiency. Instead of many sour ce and detector units, a virtual source arrangement has been proposed using a number of flat mirrors and retroreflectors to obtain intersect ing folded beam paths. Three virtual source beam configurations genera ted for a single-beam steerable FTIR system were tested using 54 flat mirrors and four retroreflectors or 54 flat mirrors and 56 retroreflec tors mounted along the perimeter walls of a typical 24- x 21-ft test r oom. The virtual source CT configurations were numerically evaluated u sing concentration maps created from tracer gas concentration distribu tions measured experimentally in a test chamber Synthetic beam path in tegral data were calculated from the test maps and beam configurations . Computer simulations of different beam configurations were used to d etermine the effects of beam geometry. The effects of noise and peak-r educing artifacts were evaluated. The performance of the tomographic r econstruction strategy was tested as a function of concentration and c oncentration gradients.