Photoelasticity evaluates stress of models or components made of transparent birefringent plastics by polarisation optics. The loaded model will be investigated between crossed polarizing filters. In the transmitted light two systems of lines appear as result of birefringence. The "isochromatics" (lines of the same colour) are a measure of the principal stress difference. The black lines are called "isoclines”. They depend on the direction of the tension.
Such a fringe pattern with “linearly polarized light" can be obtained even without the often annoying isoclines when “circularly polarized light" is used. This can be achieved by placing additional quarter wavelength plates in front of and behind the model in the optical path of the polariscope.