source for generating light beams along optical paths guided so as to impact the surface of the storage windings wound on the drum a plurality of sensors are arranged so as to receive a related light beam along a respective light path with a light intensity varying in proportion with the number of storage windings a plurality of light guiding blocks are each arranged on a respective one of the optical paths between the storage windings and the sensors application_number US 94593897 A description according to the invention the apparatus comprises a scanning unit and a signal processing device the scanning unit comprises a plurality of optical paths 33 generated by a related light source 3 each of such light sources comprise one or more ir diodes with a limited radiation angle 6 the light beam of which is directed substantially radially towards the inner side of the storage winding drum 1 which at least on the surface on which the windings 2 rest is transparent to the light the light sources furthermore comprise a related light sensor 4 which is preferably arranged outside the storage winding drum as well as a light guiding block 5 which is arranged between the usually available storage winding and the sensor the task of the light guiding block 5 is to provide through its light window 9 which is tuned to the maximum width of the storage winding area to be scanned for the exact optical light conditions i e to exclude stray light external light and the like and above all to keep free the light stroke between the upper surface 2 of the storage winding which it has to scan and the sensor 4 from undesirable disturbances as for instance floating fibers paraffin deposits and dust to this end the.