Abstract: An apparatus (1) used for detecting flatness defects of a web (2) comprises a rotatable roller bearing assembly (4) comprising a roller unit (6) mounted on a central arbor (8), a light carrying medium (11) that extends through the roller bearing assembly (4), a light source (10) for transmitting a light signal through the light carrying medium (11), and optical sensing means (12) for measuring strain forces in the roller unit (6) caused by the web (2) passing over the roller unit (6). The optical sensing means (10) is in optical communication with the light carrying medium (11) and is disposed along a portion of the longitudinal length of the roller unit (6). Means are provided for receiving and analysing the light signal that has passed through the light carrying medium (11).