The invention relates to a polishing device for polishing a metal product, in particular a continuously cast slab, billet, or block. In the present invention, a material having a smaller work function than a cathode material is used to form an inorganic conductive layer between the cathode and an organic compound layer. The opening of the cavity is closed by a cap defining a light transmission window. As the substrate moves over the table surface, it contacts the first and second grinding apparatuses. An adjustment system permits the optical structure to be reoriented relative to the frame assembly. In an embodiment, a unit of a collection of units includes a functional area and a preventive structure configured to prevent cracks from propagating into the functional area.