Figure 1: Two optical elements fully characterized by their transmission matrix, which relates the incident wave front to the transmitted one. In the case of a thin lens, the transformation of the wave front is described by a $2×2$ matrix operating on a vector describing the wave front curvature [27]. For more complex elements such as a sugar cube the transmission matrix operates in a basis of transversal modes, which is very large. Full knowledge of the transmission matrix enables disordered materials to focus light as lenses.