An optical fault seeking apparatus has a light scanning apparatus (21) and a light line receiving apparatus (14) which receive the light reflected from the material to be investigated and direct it onto a photoelectric converter. The light beam which forms the light line 11 impinges at an acute angle (.alpha.) onto the surface (13) of the article or web (18) under investigation. The image forming optical light line receiving apparatus (14), effectively consists of two light receivers (14, 14') positioned away from the normally reflected beam (23) and arranged to receive light from respective ones of two spaced apart strip-like regions (11', 11") extending parallel to and partly overlapping the line of light (11). In this way the device is able to detect faults (e.g. lumps or depressions) in fleece-type webs with irregular surface structures which are normally very difficult to monitor with optical fault seeking apparatus (FIG. 1).