This paper shows how isochromatics can be eliminated in double exposure photoholoelasticity so that sharp unmodulated and doubled isopachics are obtained for the unambiguous determination of principal stress sums. A quartz rotator is used to rotate by 90° the polarization of the beam emerging from the model. The rotated beam is redirected again through the model in a reverse direction and finally impinges on the hologram plate. Analytical and experimental analysis shows that the reconstruction of a double exposure hologram by this method yields isopachics only
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