The computation of a continuous map of isochromatic fringe order from an isochromatic phase map or relative retardation based on a photoelastic fringe pattern is a difficult task, particularly when the direction of the principal stress is ambiguous. This happens in most experiments and introduces abrupt changes in the slope of the computed relative retardation map. We present a novel regularized phase-tracking method that at each pixel chooses the unambiguous relative retardation value. This unambiguous relative retardation map is wrapped, however the unwrapping is straightforward and fast using the already known techniques. With the presented method we have been able to process successfully complex experimental data with several isotropic points, high fringe density and low resolution, as is shown in a number of examples.
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