(a) Closeness index between the identified and the reference fields for different z_obj values (1 cm, 5 cm, 10 cm, and 15 cm) and for a object width l_obj = 15 cm. The reference field is the same for each comparison, i.e. the direct computation without rigid object. (b) Closeness index between the identified field obtained for z_obj = 1 cm and the reference field obtained without or with the presence of the rigid object.