The disclosure relates to a process for measuring the rotation angle of two objects rotating in relation to each other, with a transmitter assigned to one of the objects, and with an element which influences the direction of polarization, where the transmitter and the element rotate relative to each other, and where the luminous intensity passing through, or reflected by, the element is measured by a receiver and then evaluated as a signal dependent on the rotation angle, and where the receiver groups of receiver elements sensitive to polarization, and where the polarization planes of the receiver elements in each group are rotated in relation to each other, and where the reception signals of at least two of the receiver elements are evaluated independent of each other in a monitoring mode. The disclosure also relates to an apparatus for implementing the process.