Abstract: A method for characterizing a structure etched in a first substrate surface, the structure extending along a longitudinal direction, z, into the substrate, the method implemented by a system including a light source emitting an illumination beam with a wavelength transmitted through the substrate, and an imaging device positioned to face a second substrate surface opposite the first surface, the method including illuminating at least one structure with the illumination beam, subsequently positioning an object plane of the imaging device at at least two different longitudinal positions; acquiring at least one image of the structure at each of the longitudinal positions, the images being acquired through the substrate; measuring data relating to a lateral dimension of the structure from each acquired image at each of the longitudinal positions; and determining longitudinal data relating to a longitudinal shape of the structure from the lateral data of at least two longitudinal positions.