We have developed an analytic expression to describe the shape of interference fringes produced by transparent shells using plane-wave front dual-beam interferometry. Using this expression, one can calculate the wall thickness of the shell from a few simple measurements of the curvature or separation of fringes relative to its size. ft is feasible to calculate wall thickness to within a few percent, the limit being the ability to accurately locate the center of the interference fringes. One of our approaches has been incorporated into an image analysis program. We will discuss the usefulness of this approach as a first step toward an automated image analysis program.