Beam path average data from an Open Path Fourier Transform Infrared
(OP-FTIR) spectrometer can be used to reconstruct two-dimensional
concentration maps of the gas and vapor contaminants in workplaces
and the environment using computed tomographic (CT) techniques.
However, a practical limitation arises because in the past, multiple-source
and detector 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 expense and difficulty. A single monostatic
OP- FTIR system capable of rapid beam movement can eliminate this
deficiency. Instead of many source and detector units, a virtual
source arrangement has been proposed using a number of flat mirrors
and retroreflectors to obtain intersecting folded beam paths.
Three virtual source beam configurations generated for a single-beam
steerable FTIR system were tested using 54 flat mirrors and four
retroreflectors or 54 flat mirrors and 56 retroreflectors mounted
along the perimeter walls of a typical 24- x 21-ft test room.
The virtual source CT configurations were numerically evaluated
using concentration maps created from tracer gas concentration
distributions measured experimentally in a test chamber Synthetic
beam path integral data were calculated from the test maps and
beam configurations. Computer simulations of different beam configurations
were used to determine the effects of beam geometry. The effects
of noise and peak-reducing artifacts were evaluated. The performance
of the tomographic reconstruction strategy was tested as a function
of concentration and concentration gradients.