Many properties of metallic glasses, especially mechanical ones, result from the motion of defects or free volume. However, the main difficulty in defining defects or free volume is to choose an appropriate reference. We have modified, in the sense of signal processing, the global method proposed by P. J. Steinhardt et al. to detect bond-orientational order. The resulting local method allows us to quantify the distortion of a given polyhedron in relation to chosen references. Such a method allows us consequently to solve the problem of reference in the definition of defect and free volume.
