cell 18 is achieved notice must be taken that in the case of this preferred embodiment of the invention the light beams 14 of the individual light sources 7 will never impinge simultaneously at the point of intersection s and interference can therefore never occur the triggering of the light sources 7 occurs in a pulsed manner so that one individual light source each of a transmitter unit 6 will be active at a given time with the knowledge of the transmitter unit 6 which is active just now and the signal supplied by the photoelectric cell 18 the control unit 10 will decide whether the blow out nozzle 11 associated with said transmitter receiver unit 6 8 should blow out or not in order to deflect the foreign body 2 to the mentioned waste container 12 the decision is based on the fact that a predetermined voltage threshold value obtained from the light intensity on the photoelectric cell 18 is fallen below or not fig3 shows a schematic top view of a possible further preferred arrangement of light sources 7 behind the optically transparent material chute 4 as can be clearly seen the light sources 7 are aligned in two planes e.