Abstract: In a coated or uncoated film, variations in thickness as well as all possible casting errors may be dependably determined and evaluated, in that two light sources transmit modulated IR light wherein the modulation frequencies are different, wherein one light source transilluminates the film, while the other light source is arranged on the other side of the coating carrier, illuminates the latter obliquely and both the transilluminating and the reflected light bundles are conducted to a photo-receiver. The current signal arising in the photo-receiver is transformed into a current signal, amplified and feeds two electronic filters circuited in parallel, which free the signals from their modulation frequency, after which both signals are separately rectified. Subsequently the signal originating from the transmitted light portion is delogarithmized and forms a statement concerning the thickness of coating, while the light originating from the reflected portion evaluates surface errors.