Abstract: The present invention relates to a three-layer film formed of polyesters, which is preferably produced in a coextrusion process. The film is suitable for embossing and does not tend to stick to the embossing tool, even at elevated temperatures. Under a thermal load which is necessarily applied in the further processing of the film, the depth of embossing is not changed or is only negligibly changed. The three-layer film formed of polyesters has external layers with higher melting points than the intermediate layer. Preferably, the external layers comprise polyethylene terephthalate and the intermediate layer of copolyester.

Abstract: Disclosed is a process for the qualitative and quantitative measurement of irregularities and impurities on and in transparent or semitransparent flexible sheet materials, comprising the steps of producing interference fringes by obliquely directing a light beam onto the surface of a sheet to be examined to produce two partial beams; placing the sheet to be examined on a reflecting reference surface and forming the partial beams by reflection from this reference surface and from the surface to be examined; superposing the two partial beams, whereby an interference field is obtained which is equivalent to the surface profile of the sheething to be examined; evaluating the interference field to determine the extension of the surface profile; additionally irradiating the sheet by a polarized light beam; passing the polarized light beam through a second polarizer; and evaluating the polarized light beam after passing through the second polarizer. Also disclosed is an apparatus for carrying out this process.