Nevertheless, for certain distances, the transmitted beam provides more intensity (up to a 1.5-fold increase as in Figure 5f) at its maximum than the incident beam even if the total whole transmission coefficient is less than one (here, \(T=0.81\)). This means that the \(\lambda/2\) anisotropic metasurface behaves as a focusing lens for distances estimated as \(Z_{0}\in[20p;70p]\) approximatively. The same behavior is obtained within the \(\lambda/4\) plate except that the transmission coefficient is smaller (see Figure 5i,n). For the two plates, and for distances larger than \(Z_{0}=70p\), the transmitted beam presents more than one maximum resulting from an interference pattern between all the transmitted plane waves after being affected differently (in amplitude) by the structure. Note that the intensity distributions of both incident and transmitted beams were normalized for each value of the distance in Figure 5.  
Figure 5: Incident (**a**) and transmitted electric field intensities in a transversal plane located at \(Z_{0}=30p\) from the anisotropic metasurface in the case of \(\lambda/2\) (**b**) and \(\lambda/4\) (**c**) plates. Figures (**a,b**) to (**g,h**) refer to the \(\lambda/2\) plate while figures (**i,j**) to (**o,p**) are attributed to the \(\lambda/4\) plate. The total dimension of each figure is \(32p\times 32p\). The waist of the beam is located at \(Z_{0}=0\), and its value is fixed to \(W_{0}=2p\). In conclusion, we have developed an original tool that allows us to test the robustness of a metasurface with respect to its transmission, reflection, anisotropy, etc., by adapting the definition of the \(FOM\). This tool is very versatile and can be used to study the effect of any physical parameter such as the angle of incidence, the spatial shape of the beam or the azimuthal angle. The cases of an anisotropic metasurface made in a perfect electric conductor behaving as a \(\lambda/2\) or \(\lambda/4\) plate are studied to point out the robustness of the transmission properties (efficiency and anisotropy) with respect to beam size. Fortunately, we find that these anisotropic metasurfaces are suitable to operate in both free-propagation and guided-propagation regimes because only a few periods (\(6\times 6\equiv 5\times 5\lambda^{2}\)) are needed to recover the full properties of a plane-wave illumination. The FOM can be adapted to the properties of the studied metasurface, replacing, for example, the transmission \(T\) by the reflection of the absorption (if energy harvesting is studied) or replacing the phase change \(\xi\) by beam steering or chirality (polarization direction angle). The methodology presented here is general and can be applied to beams with arbitrary polarizations and angular spectrum distributions, and it is essential for the design of structures before manufacturing. [PERSON] was involved in numerical simulations, validation and writing of the original draft. [PERSON] was concerned by the visualization, writing and review editing while [PERSON] made formal analysis, methodology and he supervised the study. This research received no external funding. We acknowledge financial support by the EIPHIGraduate School (contract \"ANR-17-EURE-0002\"). We are grateful to [PERSON] for a fruitful discussion and help in the implementation of the monomodal method. The authors declare no conflicts of interest.