An optical sensor for examining value documents, such that at a point in time before the check of the value documents, a self-test of the optical sensor is carried out, during which the light sources thereof are switched on, and, with the aid of monitor elements, the respective light intensity of the light source assigned to the respective monitor element is detected which impinges on the respective monitor element at the time of the self-test. During the check of a value document following the self-test, the light sources illuminate the value document, and measured values are recorded. The recorded measured values are then corrected with the aid of the light intensities detected by the monitor elements at the time of the self-test to take into account a change in the light intensity emitted by the light sources that occurs in the course of the service life of the light sources.