Optical differential strain gauge

An optical strain gauge has a first birefringent optical module secured to a specimen bar in a manner transferring tensile strain from the bar to the module and has a similar second birefringent module secured to the bar in a manner transferring shear strain from the bar to the module. The two modules are disposed in series in a light path along which polarized light is directed. A polarization rotator is situated in that path between the two modules and causes the phase shift between two polarized components of the light to optically subtract. The optical subtraction arrangement conduces to making the gauge insensitive to environmental effects because stresses imposed by those environmetal effects equally upon both modules result in optical cancellation of the effects of those stresses on the modules. Those two polarized components, after passing through both modules, are separated to obtain a measure of the difference between the transferred tensile strength and the transferred shear strain.