Flatness-measuring roller for a continuously manufactured strip by analyzing the birefringence caused by the application of lateral force to an optical fiber wrapped around the roller

A roller is rotatably mounted about its axis and comprises a hollow, cylindrical roll face, a ring and two hollow shafts. The roller is provided with a force measuring fiber optical device which uses the double refraction in an optical fiber subjected to a lateral force. The optical fibers of the measuring device form coils, each having at least one turn and the cylindrical roll face.