Two complementary techniques, picosecond ultrasonics (PU) and Brillouin light scattering (BLS), have been performed to measure the mechanical properties of layers involved in an optical stack. One of the advantages of these two techniques is their ability to analyze samples whose thickness varies from a few tens of nanometers (the anti-reflective coating) to several millimeters (the polymer substrate). Since all the layers constituting the optical stack have proved to be isotropic, they are completely elastically characterized thanks to the two techniques. Moreover, as the two independent elastic coefficients are measured by PU and BLS, c 33 and c 44 , respectively, the complete stiffness matrix and, hence, the Poisson's ratio and the Young's modulus can be deduced.