The structural intensity gives the information on the vibrating fields of structures, which is different from that provided by the modal decomposition. The divergence of the structural intensity is used for locating injected and dissipated power by external elements such as mechanical excitations, damping, radiation regions. Both the structural intensity and the divergence are expressed in terms of high- order spatial derivatives with respect to the variables of a coordinate system. Two methods, the finite difference approximation and the spatial Fourier transform, have been used to evaluate the structural intensity and its divergence. The limitation of the number of sensors makes it impossible to use the first method in the measurements of the high-order derivatives. The technique of processing in the wavenumber domain, based on the spatial Fourier transform of 2D-vibrating fields has not such a limitation. It has been already used in the near-field acoustic holography and laser vibrometry measurements. It makes it possible to process the massive data measured by the holographic interferometry. However, performing directly the Fourier transform usually results in large distortions if a discontinuity occurs in spatial periodicity (leakage effect) as it the case for a free plate. In this paper new algorithms--mirror processing, are developed. The structural intensity, its divergence and the force distribution in a planar plate are evaluated by processing the normal velocity data measured using the holographic interferometry or laser vibrometry. It is demonstrated that the distortions caused by leakage effects can be removed by using advanced algorithms.