The invention relates to an arrangement for detecting marks on reflective material for regulating registration on printing machines, making use of a camera and an illuminating element.
The principle is known in the printing industry of using cameras with illuminating devices for detecting marks for regulating registration.
The arrangements known hitherto for detecting marks for regulating registration conduct the light at a specific angle to the optical axis of the camera, such that, if the surface of the web material is glossy, the light source is not reflected directly into the camera. With such an arrangement, the printed mark has an adequate contrast due to the preponderantly diffuse reflection of the web material, and can therefore be detected by the camera. With strongly reflective surfaces, however, in accordance with the law of regular reflection, the light is almost completely reflected past the lens, such that the camera, due to lack of diffuse reflection, detects a dark image, in which no marks are identifiable. Solutions are also known from industrial image processing which function with a ring lamp formed from individual light-emitting diodes (LED's) around the camera lens. With reflective surfaces, however, the LED ring will be reflected into the camera by such an illuminating element, and therefore superimposed with the marks which are detected. This accordingly impedes an unambiguous evaluation of the marks. An arrangement for detecting marks on reflective material is known from DE 103 39 651 B3.
The invention is based on the object of providing an arrangement which allows for the detection of marks on reflective surfaces by a camera.
This object is achieved by claims 1 to 3. In particular, it is achieved in that a plate (2) is arranged in front of the camera (1), said plate functioning as a reflector and containing an opening (3), and that behind the opening the lens (4) of the camera is arranged in such a way that the camera can detect an image of the registration marks through the opening (3), that the dimensions of the flat plate are selected to be at least big enough that the plate edges (8) reflected on the web material (9) lie outside of the mark region (7) to be recorded, that a light protection element (5) is located at the edges of the flat plate, and that the illuminating element (6) is arranged inside the light protection element. In order to obtain a uniform brightness distribution on the reflective surface, the plate (2) is expediently designed as a flat plate. If it transpires to be necessary, however, for the dimensions of the reflector to be kept as small as possible, the plate (3) can also be designed in a curved shape, e.g. as a parabola, with the opening (3) located at the lower vertex of the curved shape. The distance interval between the reflector walls (5) can in this case be reduced to the mark region to be recorded plus a safety interval. The curved plate then causes an inhomogeneous brightness distribution on the reflective surface, with higher brightness values at the edge of the image detected by the camera in comparison with the middle of the image. It is also expedient for the surface of the plate (2) and the surface of the light protection element (5) to be provided with a white diffusely reflecting color on its sides which face towards the reflective web material.
The advantage achieved with the invention consists in particular of the fact that marks on reflective surfaces can be detected by a camera with a very good contrast between mark and reflective surroundings.
The invention is explained in greater detail below with reference to
Number | Date | Country | Kind |
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10 2010 007 421.7 | Feb 2010 | DE | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/DE11/00112 | 2/5/2011 | WO | 00 | 8/8/2012 |