Light from a light source is transmitted through an original at a scanning station to a light detector. A rotating scanning disk located in the path of this light is provided with equiangularly spaced radial scanning slots. A light-blocking plate located between the light source and the scanning disk defines a scanning slot extending transverse to the transport direction of the original. Light passing through both the original and the transverse scanning slot is projected as a slot image onto the surface of the scanning disk. The radial slots and the transverse slot cooperate to define a scanning spot which sweeps across the original, and thereby performs one line-scan operation, as one radial slot sweeps across the slot image on the scanning disk. The angular span between adjoining radial slot is greater than that of the slot image so that, during intermediate intervals between successive line-scan operations, light transmitted through the original does not reach the light detector. During the intermediate time intervals, reference light from the light source is directed onto the light detector along a reference light path not passing through the original. An evaluating circuit receives both the signal produced by the light detector during a line-scan operation and the signal produced during the preceding or subsequent intermediate time interval, and in dependence upon the values of both signals produces a signal whose value indicates the density of the extreme-density scanned spot on the scanned original.