Multiple-reflection effects in photoelastic stress analysis

The interpretation of fringes observed in photoelastic stress measurements made with coherent well-collimated optical radiation such as a laser beam a nd slab specimens with parallel surfaces is affected by multiple internal r eflections of Light within the sample, which are usually negligible when in coherent Light is used. An analysis of the multiple-reflection effects in p hotoelastic measurements involving the plane polariscope configuration is p resented. The results show that the isochromatic fringes are modified by th e interference of multiply reflected waves. The multipass differential phas e accumulations that display oscillatory magnitudes as functions of the mod el thickness and the optical wavelength result in a shifted and altered int ensity profile across the isochromatic fringes. It is shown that for large values of reflectivity, as in the case of samples with reflective coating o r partial mirrors, the bright fringes split into multiple peaks. (C) 2001 O ptical Society of America.