Photoelectric scanner with light-reflector or light barrier

A photoelectric sensor for a moving web of material, comprising:

• an upper and a lower housing disposed on opposite sides of a plane of said web and straddling a location at which an edge of said web is to be positioned as said web is moved along a path between said housings;
• a light source in a first of said housings directing a light ray along an optical axis of said source onto said path at an inclination to said plane of said web so that said ray is selectively reflected and intercepted iin dependence upon the material of said web;
• a first light detector in said first of said housings having an optical axis inclined to said plane of said web and positioned on the same side as said source of another plane perpendicular to the plane of said web at said location for receiving the light ray upon reflection from said web, said optical axis of said source and said first detector lying in a plane substantially perpendicular to said other plane;
• a cover permeable to said light ray on said first of said housings extending over said source and said first detector;
• and
• a second light detector on one of said housings positioned on an opposite side of said other plane and to receive said light ray from said source when the same is not reflected or intercepted by said web.