Abstract: An interference image of a biaxially stretched film strip 4 is generated optically. Streaks in the film strip are areas of different orientation and/or thickness d which are distinguished from the streakfree areas in the interference image by clearly different intensities. In order to generate the interference image, a light-source 1, a diffuser screen 2 and a polarizer 3 are arranged on one side of the film strip 4, and an analyzer 5 and a filter 6 are arranged on the other side of the film strip 4. The light beam of the light source 1 is polarized by the polarized 3 and, within the film (whose main refractive indices n.sub.1 and n.sub.2 have an angle of 45.degree. or 135.degree. respectively to the polarization plane) is split into two beams, oscillating perpendicularly to one another, which have a path difference .GAMMA.=(n.sub.1 -n.sub.2)d after emerging from the film.