Detection apparatus for finding holes in webs

Detection apparatus for finding holes in webs and having a transmission device including a laser arranged to direct a light curtain at a web, a detection device including photoelectric detection means, disposed to receive light transmitted through holes in the web, and an electronic processing device for processing signals from said photo-electric detection means;

• said transmission device comprising a row of individual stepped mirror strips, each said strip being displaced sideways relative to the preceding strip of the row and being inclined to the direction of the row, and a scanning device for directing a beam from said laser in the direction of the row to scan the individual mirrors which are inclined relative to said beam one after the other thereby deflecting the light through substantially 90.degree. to form said light curtain, said transmitting device further comprising optical means for introducing a degree of divergence into the beams deflected from said stepped mirror strips and forming said light curtain whereby said light curtain scans across said web without gaps, and said detection device comprising a row of individual Fresnel lenses, each said Fresnel lens having an operative section of substantially rectangular form for receiving light, and said Fresnel lenses being arranged edgewise one after the other in a substantially abutting relationship and extending parallel to the row of strips, together with a number of photoelectric converters defining said photoelectric detection means and being positioned to receive light from said Fresnel lenses.