The bend line of a turbine shaft is determined by measuring the height if each bearing ram (2) of the turbine shaft. The run time difference between at least two partial beams (R1,R2) characteristic of the bearing ram height is determined. A reflector is used to generate the run time difference between a number of partial beams (T1,T2). The reflector includes prisms (14,16) which are tilted w.r.t. each other.
