“…In contrast, the latter provides a distributed estimate of the refractive index n 2 by applying regularization techniques. We just mention the adjoint-based method [17,26,33], recursive linearization (for multi-frequency data) [6], Gauss-Newton method [15,20,21], contrasted source inversion [2] and level set method [18] for an incomplete list. Generally, the estimates by a method from the latter group can provide more details of inclusions/inhomogeneities, but at the expense of much increased computational efforts, especially when the forward model is the full threedimensional Maxwell system.…”