Abstract: Optical fault seeking apparatus for material webs uses an optical scanning device to scan the material webs point by point line by line. The optical scanning device consists of a laser light source (17) which projects a light beam via a micro-objective (19), a cylindrical lens (20), and a further objective onto a mirror wheel arrangement (14) which is located substantially at the focus of a strip-like concave mirror (23). A light beam (24) is produced in the image space of the strip-like concave mirror (23) and generates a scanning light bead in a scanning plane through which the material web passes. The mirror wheel arrangement (14) has at least first and second alternately operating types of mirror surfaces which serve to alternately project light beads of different size and/or shape onto the material web thus enabling different types of fault to be reliably detected.