laser as light source 6 furthermore the shorter its wavelength the more narrowly can the beam 8 be focussed it can therefore be expedient to use a laser diode with a shorter wavelength than the abovementioned ir laser diode since a laser diode emitting within the visible spectral range as light source 6 could interfere with the image scanning at the sensor line 5 on account of scattered light such a shorter wavelength should expediently lie within the ultraviolet spectral range according to a modification the polarization planes of polarizer 9 and analyser 12 are oriented at right angles to one another light passing through a perforation hole 3 or film material spaced apart from an edge of a perforation hole 3 maintains its polarization and is suppressed in the analyser 12 only light which experiences a change in polarization at a hole edge can pass through the analyser 12 and arrives at the scanning sensor 7 the output signal of the scanning sensor 7 thus predominantly exhibits zero level a short pulse is registered only while a perforation hole edge passes through between light source 6 and sensor 7 connected to the output of the scanning sensor 7 is a discriminator for example a schmitt trigger 13 which outputs a standard counting pulse to a frequency divider 14 for each intensity dip in the case of parallel orientation of polarizer 9 and analyser 12 or for each pulse in the case.