respective counting direction each of the pulse series briefly named signal in the first and second columns under 2 and 5 represents the brightness distribution in the measuring area in the direction transverse to the yarn path the output signals of the circuits 2 5 and 6 and the pulses stored in counter 7 are simply plotted by the envelopes of the single coherent pulse trains in view of the condensed time scale line i shows the condition with the empty measuring area i e when no yarn is present the digital signals under 2 and 5 are of equal shape the one bit one signals indicate a bright zone in the middle of the measuring area and the one bit zero signals show dark zones at both edges of the measuring area this corresponds with the assumption that the marginal zones of the measuring area are covered with dirt and the intermediate middle zone of the measuring area is transparent or bright thus a digital zero signal is present at the output of logic circuit 6 which signal represents the difference of the equal signals under 2 and 5 this means that the ground signal which is modified by dirt deposition in the empty measuring area is completely eliminated the initial zero count in counter 7 is maintained line ii shows the signal condition immediately.