“…Concerning P 3D , it contains all contributions (linear, circular, directional) to polarimetric purity and can thus be considered a proper degree of polarimetric purity. The fact that the inequality P 3D 1/2 holds for 2D states [2,8,32] is nothing else than a natural consequence of the fact that they are characterized by P d = 1. Obviously, when assumed that a given polarization state is a 2D state (P d = 1), the conventional 2D degree of polarization P 1 = P e = P 2D is a consistent and appropriate measure of the contributions to polarimetric purity, beyond the one due to the deterministic directionality.…”