An overdeterministic least-squares phase-stepping method for automated photoelasticity is ,described. Problems associated with isochromatic-isoclinic interaction are solved by use of a three-wavelength method to calculate the value of the isochromatic parameter, and the isoclinic angle. The ramped isoclinic phase map can now be unwrapped to give the orientation of the principal stresses with respect to. a reference axis of the polariscope unambiguously. A three-wavelength approach to determination of the absolute value of the isochromatic parameter is shown to give reliable results also. # 1997 Optical Society of America
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