A possible method to characterize damages in composite plates is the use of interaction of Lamb waves with defects. Nevertheless, the understanding of this interaction is not trivial and the reconstruction of the exact shape of the damage needs to develop models able to interpret the experiments. A two-dimensional image showing the diffraction figure is probably the richest experimental information. This paper presents a model to interpret global images of this interaction produced by shearography. The results presented are limited to the case of the modes Ao and So in an aluminum plate, case which is much smaller than what occurs in a composite but clearly explains the possibilities of the method. Experimentally, a piezo-transducer bonded on one face of the plate is used to generate the modes Ao and So while a specific optical configuration allows the separation of the normal and the two in-plane components of the displacement. A finite element model of the plate leads to the evaluation of its response b...