A quality control station (2) for a sheet element processing machine, having at least one camera (6) arranged for capturing images of sheet elements (4) transported through the quality control station (2), and further having an illumination unit (5) with at least one light emitter (16) and two reflectors (12, 14), the illumination unit (5) directing light onto a viewing area of the camera (6) such that the illumination intensity is constant despite changing media thickness. An illumination unit for such quality control station is disclosed.

A multi-resolution spectrometer, includes: an incident slit (10) configured to receive an incident light beam; a collimating device (20) configured to collimate the light beam from the incident slit; a dispersing device (30) configured to disperse the light beam collimated by the collimating device (20) so as to form a plurality of sub-beams (61, 62) having different wavelengths; an imaging device (40) and a photon detector array (50), the imaging device (40) being configured to image the plurality of sub-beams (61, 62) on the photon detector array (50) respectively, the photon detector array (50) being configured to convert light signals of the plurality of sub-beams (61, 62) imaged thereon into electrical signals for forming a spectrogram, wherein the incident slit (10) has a first slit portion (11) and a second slit portion (12), and the second slit portion (12) has a greater width than the first slit portion (11).

A surface inspection system (10) for inspecting the surface of sheet elements (2) present in an inspection area (5): The system includes an image evaluation unit (18), two light sources (12, 14) arranged adjacent each other on opposite sides of an illumination plane (O.sub.11) and oriented for illuminating the inspection area (5), and a camera (16) for capturing line images (I.sub.12, I.sub.14) of the inspection area (5) along a viewing plane (O.sub.16). The illumination plane (O.sub.11) and the viewing plane (O.sub.16) are arranged on opposite sides of a median plane (M) which is perpendicular to an inspection plane. The angle () between the illumination plane (O.sub.11) and the median plane (M) is the same as the angle () between the viewing plane (O.sub.16) and the median plane (M). Also, a method of inspecting the surface of sheet elements (4) by using the surface inspection system (10) as defined above, wherein a first of the two light sources (12, 14) directs light onto the sheet element (4) to be inspected, and the camera (16) captures a line image (I.sub.12; I.sub.14) of the inspection area (5), and then a second of the two light sources (14, 12) directs light onto the sheet element (4) to be inspected, and the camera (16) captures a line image (I.sub.14; I.sub.12) of the inspection area (5). Then an image evaluation unit (18) compares the captured line images (I.sub.14; I.sub.12) with each other, and in particular subtracts the images from each other.

A quality control station (2) for a sheet element processing machine, having at least one camera (6) arranged for capturing images of sheet elements (4) transported through the quality control station (2), and further having an illumination unit (5) with at least one light emitter (16) and two reflectors (12, 14), the illumination unit (5) directing light onto a viewing area of the camera (6) such that the illumination intensity is constant despite changing media thickness. An illumination unit for such quality control station is disclosed.