A method and a device are provided for detecting defects in coils, such as rollable ultra-thin glass, rollable glass films, rollable laminated ultra-thin glasses or films, rollable solar cells, and rollable organic light emitting diodes (OLEDs). The method includes: providing a glass ribbon in a coil with a shape of a hollow cylinder with two end faces and one ribbon surface; introducing light into a first of the two end faces so that the light is directed from a light entrance side at the first of the two end faces to a light exit side at a second of the two end faces and exits from the light exit side; detecting the light at the light exit side with a first optical imaging detector so that, if a crack is present, the crack is seen as a shading in an image captured by the first optical imaging detector.