Monitoring is provided, for a passageway through which objects on carrier (12) are transported, such that the passageway is guarded by at least one light grid (16) with a plurality of light rays running one above the other, where the light grid (16) is divided into a first, lower area (16a) through which the carriers (12) pass and a second, upper area (16b) through which the objects pass, which light grid (16) disposes over an evaluating circuit which emits a signal if a light ray (22) is interrupted in the first, lower area (16a) and provides information on the height of the object. A current value for the height is stored height as last measured value. Readouts are taken of a lower area (16a) and upper area (16b) of the light grid (16), and the readouts are used to determine a current value for the height of the object, and a determination is made whether an object flag has been changed.