Abstract: A method for photoelasticity examination of a stress or strain field comprises successive steps. A two dimensional image of a test-piece is formed which has isochromatic fringes in the form of an array of pixels and the grey level of each pixel is stored as a digital value. From the isoclinals, the orientation of the main stresses .sigma.I and .sigma.II is determined at each pixel and the angle of orientation is stored for obtaining each pixel; from the stored pixels, the law of variation of brightness is worked out along at least three lines parallel to an arbitrary direction chosen by the operator and passing completely through the test-piece; from a condition at the limits and by finite differences then integration, a value is determined representative of .sigma.I and .sigma.II at each point of the median line among the three lines and .sigma.I and .sigma.II are derived from the stored digital values and from the computed values of .sigma.I and .sigma.II.