Lobre s.r.l. carries various models of interferometric surfaces in its catalogue, that is: a glass disc with one or both perfectly polished and flat surfaces. The interferometric surfaces are used as a reference for controlling the planarity of other surfaces. By placing the surface to be checked in contact with that of the interferometric surfaces, air zones are created where the contact between the two surfaces is imperfect. If the two surfaces are perfectly flat, parallel lines are seen.