Abstract: A surface testing device, operating by reflection, for linearly moving, tape-like materials, for example paper or polymer films, is suitable for automatic online registration of linear irregularities transverse to the direction of movement. The testing device includes a suitable test distance, at least two optical inspection units and at least one electronic circuit arrangement for detecting both the individual signals of the inspection units and the coincidence signals. In addition, a magnetic information medium wound in the form of a roll on a hub is tested by the surface testing device and carries a machine-readable and/or visually readable quality identification.

Abstract: It has been found that a magnetic tape which may be used in particular for video recording and which has excellent abrasive and cleaning properties for all those parts of the cassette or rather the recording and play-back unit which come into contact with the magnetic layer can be obtained by addition of a non-magnetic powder having a Mohs' hardness of at least 9 and a primary particle size of 0.3 to 2 .mu.m to the magnetic pigment, which has a BET surface of at least 26 m.sup.2 /g, in the magnetic dispersion, the magnetic layer of the magnetic tape having an Ra value of less than 0.02 .mu.m. The best results were obtained with cubic Al.sub.2 O.sub.3 particles added in a quantity of 1.1 to 4.4% by weight, based on the magnetic pigment.