“…By reviewing the state-of-the-art literature, several techniques have been successfully developed in the frequency domain under the assumption of monochromatic illuminations [14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29]. Despite their success, such inversion methods often present some limitations since only a reduced amount of information is actually retrievable from a single frequency measurement [30] and the exploitation of higher frequencies to enhance the spatial resolution [30,31] generally causes an increase of the complexity of the measurement system [31].…”