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The application of phase-stepping and image processing to photoelastic stress analysis is restricted by the periodic nature of the birefringence data generated and the interaction between the isoclinic parameter and relative retardation. These difficulties have been investigated using signal analysis and fuzzy set theory. A new strategy for processing phase-stepped data from birefringent materials is proposed and involves demodulation of intensity signals and the classification of periods of data using fuzzy sets. The procedure works well in a wide range of applications.