An angle sensor for detecting an out-of-round condition includes a disc supporting angle coder rigidly attached to a rotating shaft and at least one fixed optical system including a coherent light source emitting a beam interfering with the angle coder of the support disc in order to code its angular position, a collimator of the said beam before interference and a photo detector of the modulated beam after interference. In one embodiment, two optical systems arranged 90° from each other are employed. An alternative embodiment includes one optical system and the disc includes a circular track radially centred in at least one of its angular positions relative to the incident collimated light beam, and a radially orientated photo detector.