Abstract: The invention relates to a three-layer, biaxially oriented polyester film which has very good electromagnetic properties compared with films of the prior art in combination with improved abrasion properties. The film is composed of at least one base layer B and outer layers A and C applied to both sides of this base layer, these outer layers having a defined number of protuberances with a defined height and having a defined gas flow in the roughness profile. The invention furthermore relates to a process for the production of the film and its use as magnetic tape film in which no backing coating is applied.

Abstract: The orientation state of a film material is determined from the double refraction during the manufacturing process. A light source irradiates the film material to measure the path difference corresponding to the anisotropy of the film material. Interference patterns, created by one or more compensating wedges, are recorded by a photosensitive diode matrix. The individual electrical signals generated by the diode matrix are subjected to level discrimination by a discriminator and generated as binary images, each of which has a single interference stripe, which is evaluated. Interference patterns from different irradiation directions are simultaneously imaged next to each other and/or above one another. If an interference stripe cannot be found in a diode line, the system switches to the diode lines of the diode matrix which are above or below it. The thickness of the film material is measured and the double refraction calculated from it along with the path difference.