A flatness roller includes a body having at least one cavity opening onto an outer surface of the body through a plurality of slots, two successive slots defining a lamella between them, each lamella being connected to the body by two connection portions. The flatness roller also includes at least one optical fibre having at least one strain sensor having a measurement axis. Each strain sensor is housed in a cavity and attached at a connection portion of a lamella, the measurement axis forming an angle less than or equal to 20° with a plane orthogonal to the axis of revolution of the body. Each strain sensor is configured to send an optical response wave representative of a strain of the strain sensor according to the measurement axis thereof.