[′tīm ¦av·rij ¦häl·ə¦graf·ik ‚in·tər·fə′räm·ə·trē] (optics) The study of holograms of a vibrating surface which have been averaged over time; illumination of such a hologram yields an image of the surface on which are superimposed interference fringes which are contour lines of equal displacement of the surface, enabling vibrational amplitudes of the surface to be measured with precision.