The oblique incidence method of photoelastic principal stress separation is reconsidered and presented in a form that allows the existence of negative fringe orders to be identified. The normal incidence isoclinic angle, two oblique incidence isoclinic angles and two oblique incidence isochromatic fringe orders are required for the new method. However, by allowing negative fringe orders to be identified, significant uncertainty relating to the separated principal stresses is removed and confidence in the calculated results may be improved.
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