Abstract: In an opto-electronic detector apparatus, particularly of a yarn storage and feeder apparatus, for detecting at least one yarn or yarn section moving transversely of its longitudinal direction in a detection field, said apparatus comprising a light source, a receiver surface, a photoelectric cell for generating an output signal, and an optical system, the optical system is an optical imaging system (A) having a focus depth defining a predetermined depth (T) of said detection field, and a field of vision of limited dimensions at least in the direction of its width (B), the receiver surface (16) being disposed in the vicinity of the focal plane of the optical imaging system (A), so that said imaging system (A) produced a clear image on the receiver surface (16).

Abstract: Disclosed is a method for marking out drill holes in accordance with a predetermined drill hole pattern. A direction reference beam (c) is projected in a direction defining the direction of the tunnel and part of a beam (e) is deflected from the reference beam, the part of the beam is passed to a beam derivating means (d.sub.2) by means of which the part beam is directed in accordance with a predetermined drill hole pattern onto the rock surface (4) to present thereon images of the part beam at locations of intended drill holes. Correspondingly, a device for executing said method comprises means (d.sub.1) for deflecting at least part of a direction reference light beam (c) to form part of a beam (e), and means (d.sub.2) to direct the part of the beam onto locations of drilling holes for explosives in accordacne with a predetermined pattern.