camera 183 a counting process for the pulses generated by the pulse generator 92 is started after a period of time which depends on the rate of advance of the plies w5 w6 and which corresponds at least to the shortest possible run through time of the scanned surface section between the scanning points 190 and 191 the rear line scan camera 184 is turned on which thereupon produces in the region of the surface section previously scanned by the front line scan camera 183 a first snapshot of the surface of ply w5 the resulting analog signal values are transformed into binary video signals in the same manner as for the front line scan camera 183 and are entered in the associated shift register 197 the binary signal profile stored in the shift register 197 in 1728 memory positions is now divided by the microprocessor 199 into three overlapping blocks of 1024 memory positions corresponding to a division of the ccd image sensor 182 into three blocks c1 c2 c3 whose length corresponds exactly to the length b of the ccd image sensor 181 the microprocessor 199 now performs successively an addition of the three signal profile blocks of the shift register 197 with the complementary value of the signal profile of the shift register 196 where for the binary addition of 1 1 10 only the value 34 0 34 is retained there is no.