The motional Stark effect ( MSE ) diagnostic on the International Thermonuclear Experimental Reactor will need to guide the light through a labyrinth of mirrors to provide neutron shielding. Knowledge of how the mirrors change the polarization is essential for accurate determination of the q profile. The optical properties of the plasma facing mirror are also expected to change with time due to deposition/erosion. For the purpose of examining this experimentally a detector system, identical to the JET MSE system, using twin photoelastic modulators was constructed. Measurements have been performed on freshly prepared mirrors, on mirrors after exposure to plasmas in Tore Supra, and labyrinth designs. The result shows a significant effect on the optical properties and demonstrate the need for in situ monitoring. The measured properties of the labyrinth closely follow the Mueller matrix formalism. With a correct choice of material the angle change introduced by the four mirrors furthest away from the plasma will be below 1°.