An apparatus for illuminating monocrystalline and polycrystalline substrates in order to image micro cracks, pinholes and inclusions in the substrates is disclosed. A first illumination system illuminates a first line of light on the front surface of the substrate. A second illumination system illuminates a second line of light on the front surface of the substrate. The first line of light and the second line of light are oriented transverse to the transport direction (T). The second illumination system is arranged within a 3-dimensional coordinate system such that the substrate is illuminated under a second angle. The arrangement of the second illumination system is comparable to a street lamp. A camera, preferably a camera, is arranged such that it faces the back surface of the substrate. The camera captures light which is transmitted through the substrate from the first illumination system and the second illumination system.