OPTICAL TOMOGRAPHY

Optical tomography provides spatially resolved measurements of the densities of specific species which has applications in fluid flow and combustion studies. Computed tomographic reconstructions of absorption measurements provide a two dimensional map of specific species density in a planar cross section of a region. A 28 cam radius fan beam geometry optical tomographic scanner is used to demonstrate the imaging capability of tomography and to verify theoretical calculations of statistical noise in the images reconstructed with the convolution back projection algorithm.; The noise in a computed tomographic image scales linearly with the noise in the measurements of the projections. A theoretical analysis of noise for both fan and parallel beam geometries is presented including the effects of distance of the reconstructed point from the center of the fan source circle, correlations of the noise within each projection, and spatial averaging. Spatial averaging is shown to effectively reduce the noise in the image.; Experimental measurements are used to verify the detailed predictions of the noise theory and the imaging capability and spatial resolution of tomography. An iodine vapor plume is imaged with 1 cm spatial resolution. The noise in the absorption data is primarily caused by laser speckle. The noise-absorption length product for the experiment is 6 x 10('-4) in 2 cm which corresponds to a 6 ppm iodine concentration. A sixteen fold improvement in the noise sensitivity is due to the averaging effect of the different fans. A quantitative analysis of the noise in tomograms made without absorption (null tomograms) verifies the results of the theory including the dependence of the noise on the number of fans per revolution, correlations of the noise within each fan of the absorption data, and the position of the reconstructed point within the image.