The symmetry of a vibrational mode is deduced from the depolarization ratio ρ, which is the ratio of the Raman scattering with polarization orthogonal to the incident laser and the Raman scattering with the same polarization as the incident laser: \rho = \frac{I_r}{I_u} Here I_r is the intensity of Raman scattering when the analyzer is rotated 90 degrees with respect to the incident light's polarization axis, and I_u the intensity of Raman scattering when the analyzer is aligned with the polarization of the incident laser. When polarized light interacts with a molecule, it distorts the molecule which induces an equal and opposite effect in the plane-wave, causing it to be rotated by the difference between the orientation of the molecule and the angle of polarization of the light wave. If \rho \geq \frac{3}{4}, then the vibrations at that frequency are depolarized; meaning they are not totally symmetric.
