METHOD FOR DETERMINING THE POSITION OF SPECTRAL MEASUREMENTS

Description

[0001] The invention relates to a method for evaluating spectral data of a printed image applied to a substrate, wherein the printed image is mapped using digital image data.

[0002] Various sensors are used in printing presses to monitor the quality of the printed image. These include sensors for checking the registration of the printed image, meaning the correct positioning of the primary colors used in multicolor printing relative to each other. They also include image monitoring systems that capture the entire printed image on the substrate and detect and analyze deviations in the print, such as defects, toning, or contamination.

[0003] Furthermore, monitoring the color of the printed image is of crucial importance, and there are several methods and therefore several sensors known from the prior art for this purpose.

[0004] To monitor consistent color reproduction, devices known as color density meters or densitometers are used. These devices determine the color density values ​​of the primary colors used to create a color printed image—usually cyan, magenta, yellow, and black. Color density is a measure of the amount of ink applied to the substrate, adapted to the requirements of printing technology, and is calculated from the remission level. However, color density values ​​cannot provide information about color perception or actual color perception, nor can they confirm adherence to specific color values.

[0005] Therefore, when necessary, spectrophotometers are used to record spectral data, from which colorimetric data, such as LAB data, can be derived. This data allows for the representation of the actual color, or rather, the color perception of a printed product by the viewer. Thus, spectrophotometers are used in printing presses where the precise reproduction of one or more specific colors is of paramount importance. These devices typically record specific defined pixels or sections of the printed image during production. In the case of a web press, this occurs at full production speed. The recording of the pixels or image sections to be captured is controlled by time intervals, sensor signals, or similar signals.

[0006] Spectrophotometers have the property of measuring the wavelengths of light reflected from the image point being captured. Based on the resulting spectral data, the color of a captured image point can be determined very precisely. However, due to this operating principle, spectrophotometers have the disadvantage that they are not imaging sensors and therefore do not capture the captured image area visually, meaning it cannot be reproduced.

[0007] Furthermore, in printing presses, deviations between the theoretical position and the actual position of the printed image relative to a reference point, such as a sensor for quality detection, occur due to substrate stretching, web path in and / or perpendicular to the web direction, imprecise registration, etc.

[0008] US Patent 5,774,225 A1 discloses a method in which the target location of a spectral measurement is determined, wherein an imaging sensor additionally detects this location to be measured alongside a spectral sensor, thus capturing both the position of the target location of the spectral measurement and the position of the actual measurement location, whereby, in case of deviation of the measurement location from the target location, the determined color values ​​are nevertheless used for color control.

[0009] EP 2 537 667 A1 discloses a railway monitoring system with an integrated sensor for spectral measurement, wherein the measurement location of the color value is optically displayed in the monitor of the railway monitoring system, so that an operator can at least visually recognize the location of the color measurement and, if necessary, change it.

[0010] DE 10 2018 219 138 A1 discloses the acquisition of color values ​​in an inkjet printing process, wherein the part of the printed image to be acquired is exclusively captured by a sensor for determining the color values.

[0011] Due to the limited image acquisition capabilities of spectrophotometers and the potential positional variations of the printed substrate, it is impossible to verify whether the intended pixel or image section was actually captured by the spectrophotometer. Therefore, in the event of deviations or fluctuations in the actual spectral values ​​of the captured pixel or image section of the printed image, it is not possible to determine, without considerable additional effort, whether these deviations or fluctuations are due to color variations or purely spatial differences between the intended pixel or image section and the actually captured pixel or image section.

[0012] The object of the invention is to determine the actual position of the image point or image section actually detected by a spectrophotometer - hereinafter referred to as the first detection area.

[0013] The task is solved by a procedure that includes the following procedural steps: a) Defining at least one target area within the printed image, namely either defining a first target area with a position or defining a first target area with the position and a second target area with the position, b) Capturing a first detection area within the printed image corresponding to the first target area using a spectrophotometer and determining actual spectral data of the first detection area, c) Simultaneously or sequentially capturing the second detection area corresponding to the first target area within the printed image, or, in the case of determining the second target area, simultaneously or sequentially capturing a second detection area within the printed image corresponding to the second target area using an imaging sensor and determining actual image data of the second detection area.d) Comparing the actual image data acquired by the imaging sensor with the image data and determining the position of the second detection area in the printed image, e) Determining the position of the first detection area in the printed image based on the position of the second detection area in the printed image and a defined position difference between the first detection area and the second detection area, characterized in that if a limit value of a position deviation (xa,ya) between the position of the first target area and the position of the first detection area is exceeded, the actual spectral data are rejected as invalid.

[0014] This solution has the advantage that it allows reliable determination of whether the measurement was actually taken at the intended pixel or image section, since if there is at least a large geometric deviation between the first detection area and the first target area, a different pixel or image section with a very likely different specified color would be measured, which would result in a deviation between the spectral data recorded by the spectrophotometer and the color values ​​determined from it.

[0015] Determining the geometric location of the detection area is therefore particularly, though not exclusively, advantageous when the detection of spectral data is carried out directly in the printed image and not on printed marks or print control strips, since the detection areas for specific colors in a printed image not affected by marks or print control strips are usually limited in terms of geometric spread.

[0016] For example, an area scan camera or a line scan camera can be used as the imaging sensor; with regard to the technology used, for example a CCD camera or a CMOS sensor in the sense of a CMOS camera can be used for the imaging sensor.

[0017] According to the invention, if a limit value for positional deviation between the position of the first target area and the position of the first detection area is exceeded, the actual spectral data are considered invalid and discarded. This prevents such actual spectral data from being used for color evaluation or for correcting the color output of the relevant printing press, since excessive geometric deviation between the first detection area and the first target area would, in most cases, result in inaccurate color values. If such spectral values ​​were nevertheless used for color correction, the entire color output would be distorted due to the geometrically incorrect measurement.

[0018] In the case of sheet-fed printing presses or roll-to-roll printing presses, it is advantageous according to one embodiment of the invention to perform the above-mentioned process steps b) to e) not only once or sporadically, but to repeat them for at least one subsequent printed image.

[0019] For consistent quality control and quality management, it is advantageous to repeat the above-mentioned process steps b) to e) for each printed product. This allows deviations of the actual color from the specified color to be detected very early, enabling timely corrective action. Furthermore, it allows for continuous documentation of the quality of the printed products.

[0020] According to a further embodiment of the invention, in process step a) a plurality of first target areas or a plurality of first target areas and a plurality of second target areas are defined, and process steps b) to e) are carried out for the plurality of target areas and the respective associated detection areas.

[0021] This design has the advantage that the coloring can be carried out on a plurality of different pixels or image sections and thus, if necessary, for different colors, so that a quality check distributed relatively evenly across the printed image is possible.

[0022] Preferred embodiments of the invention are described in the dependent claims and the following description. Various exemplary embodiments of the invention are explained in more detail with reference to the drawings, without being limited thereto. These show: Fig. 1 An exemplary setup of a detection device with one detection area. Fig. 2 An exemplary setup of a detection device with a first detection area and a separate second detection area. Fig. 3 An exemplary printed image with only one target area and associated printed products with a detection area corresponding to the target area. Fig. 4 An exemplary printed image with a first target area and associated printed products with a geometric deviation of the detection area corresponding to the target area. Fig. 5 An exemplary printed image with a first and a second target area and associated printed products with a first and second detection area corresponding to the first and second target areas.

[0023] Since several geometric possibilities can be implemented due to the combination of a spectrophotometer 7 with an imaging sensor 4, such a detection device 8 will first be explained below.

[0024] Fig. 1 Figure 8 shows a detection device 8 in which a spectrophotometer 7 and an imaging sensor 4 are installed such that, at a defined distance of the detection device 8 to the substrate 1 printed with a print image 2, which is moved below the detection device 8 at the substrate speed v during color measurement within a printing machine, the spectrophotometer 7 and the imaging sensor 4 can detect the side of the substrate 1 facing the detection device 8.

[0025] The spectrophotometer 7 has a first detection range 3-1, such that pixels or image sections lying within this first detection range 3-1 are detected by the spectrophotometer 7 and their actual spectral data are determined. The term "first detection range 3-1" used below encompasses both the geometric area detected by the spectrophotometer 7 and / or the pixels or image sections of the printed image 2 located therein.

[0026] The imaging sensor 4, which is designed, for example, as a CCD camera or a CMOS sensor, has a second detection area 3-2, such that pixels or image sections lying within this second detection area 3-2 are detected by the imaging sensor 4 and their actual image data, which in the broadest sense corresponds to a digital photograph, are determined. The term "second detection area 3-2" used below encompasses both the geometric area detected by the imaging sensor 4 and / or the pixels or image sections of the printed image 2 located therein.

[0027] The in Fig. 1 The depicted detection device 8 is designed such that the first detection area 3-1 of the spectrophotometer 7 on the substrate 1 is identical to the second detection area 3-2. The spectrophotometer 7 and the imaging sensor 4 are arranged relative to each other in such a way that both the spectrophotometer 7 and the imaging sensor 4 have the same detection area 3 on the substrate 1.

[0028] If the spectrophotometer 7 and the imaging sensor 4 simultaneously detect the printed image 2 or a section thereof, the first detection area 3-1 on the substrate 1 is identical to the second detection area 3-2 on the substrate 1. However, if the spectrophotometer 7 detects the substrate 1 at a time offset from the imaging sensor 4, then, assuming the substrate 1 is moving at the substrate velocity v, the spectrophotometer 7 detects a first detection area 3-1 that has a different geometric position on the substrate 1 than the second detection area 3-2 detected by the imaging sensor 4.

[0029] Although in Fig. 1 If the spectrophotometer 7 and the imaging sensor 4 are arranged in a detection device 8 under a housing not shown in detail, this configuration is not mandatory, since the spectrophotometer 7 and the imaging sensor 4 can still be aligned in this way without a common device.

[0030] Although in Fig. 1 If no lighting device is shown with which the substrate 1 is either pulsed or continuously illuminated, such a lighting device can either be integrated into the detection device 8 or arranged geometrically independently therefrom.

[0031] Fig. 2 shows a different configuration of a detection device 8 than in Fig. 1 depicted, although the components used are the same.

[0032] Fig. 2 Figure 1 shows a second exemplary arrangement of a spectrophotometer 7 and an imaging sensor 4. Both the spectrophotometer 7 and the imaging sensor 4 each have a detection area 3, which is directed perpendicularly to the substrate 1. Due to the distance between the spectrophotometer 7 and the imaging sensor 4, the spectrophotometer 7 thus has a first detection area 3-1, which is spaced apart from the second detection area 3-2 of the imaging sensor 4 by the position difference yd. Although in Fig. 2 If not shown, it is also possible that the second detection area 3-2 is additionally spaced in the direction and / or alternatively perpendicular to the substrate velocity v with a position difference xd from the first detection area 3-1.

[0033] If there is only an offset yd in the direction of the substrate velocity v between the first detection area 3-1 and the second detection area 3-2, then the first detection area 3-1 can be identical to the second detection area 3-2 on the substrate 1 if the spectrophotometer 7 is triggered at a time offset from the imaging sensor 4.

[0034] However, if the spectrophotometer 7 is triggered simultaneously with the imaging sensor 4, the first detection area 3-1 is not geometrically identical on the substrate 1 to the second detection area 3-2.

[0035] Fig. 3 shows in Fig. 3a ) an exemplary print image 2 defined using image data 12, as well as in Fig. 3b ) the corresponding printed image 2, which is printed sequentially one after the other onto a web-shaped substrate 1.

[0036] The in Fig. 3a The exemplary printed image 2 represents the subject to be printed, which is typically represented by, or defined by, digital image data 12. The image data 12 can originate from a digital prepress stage, where the subject to be printed is defined. Alternatively, the image data 12 can be determined using a print inspection system, which either photographs the entire substrate 1 or scans it sequentially. This results in digital image data 12 covering the entire printed image 2, which depicts the printed image 2.

[0037] Fig. 3a ) represents the print image 2 and thus the subject to be printed, which is specified or depicted based on digital image data 12. In order to be able to define a position for the target area 13-1, in Fig. 3a An example coordinate system is shown, so that the location of the first target area 13-1 can be defined, for example, using coordinates. However, it is also possible to define the location and thus the position using image data 12, pixels, or, for example, the grid of the image data 12. In the following examples, however, the positions are defined exclusively using the coordinate system that originates at the lower left corner of the printed image 2.

[0038] Fig. 3b ) in contrast, symbolically represents the printed image 2 printed onto the substrate 1. Fig. 3b Figure 1 shows an exemplary top view of a web-shaped substrate 1, which is conveyed through a printing press or a processing device at the substrate velocity v.

[0039] According to the present invention, at least one target area 13 is defined in the printed image 2, which is available as image data 12, or directly in the image data 12, with reference to the example according to Fig. 3a A single first target area 13-1 is defined in the image data 12. This first target area 13-1 has the position x13-1,y13-1 and includes the filled dot shown as an example in the lower third of the printed image 2. This dot can be a print mark for capturing the mark, but preferably the target area 13-1 lies in any area of ​​the printed image 2 to avoid the additional space required for print marks or print control strips.

[0040] This target area 13-1 is used in a printing press or, for example, in a finishing unit, and therefore also outside of a printing press, by the in Fig. 3b ) the detection device 8 (not shown) detects, whereby the positioning and / or activation of the spectrophotometer 7 and the imaging sensor 4 can be carried out on the basis of the image data 12 or, for example, on the basis of data from the drive control or on the basis of encoder data from the pressure cylinders, etc.

[0041] The image area corresponding to the target area 13-1 is recorded by the spectrophotometer 7 and its associated first detection area 3-1, whereby the spectrophotometer 7 determines the actual spectral data from the first detection area 3-1.

[0042] Simultaneously (for example, with a configuration according to Fig. 1 ) or at a time delay (for example, with a configuration according to Fig. 2 The image section corresponding to the target area 13-1 is captured by the imaging sensor 4 and its associated second detection area 3-2. The imaging sensor 4 thereby determines the actual image data of the second detection area 3-2.

[0043] The actual image data acquired by the imaging sensor 4 are processed either by the imaging sensor 4 itself, by a computing unit associated with the imaging sensor 4, or by another device not integrated into the system. Fig. 3 The computer device shown is compared with the image data 12, so that the position x3-2,y3-2 of the second detection area 3-2 in the printed image 2 can be determined based on this comparison.

[0044] Due to the known and defined position difference xd,yd of the first detection area 3-1 to the second detection area 3-2 (in the example according to Fig. 1 The position difference is equal to 0, in the example according to Fig. 2 The position difference yd) can be used to determine the exact position x3-1,y3-1 of the first detection area 3-1 in the printed image 2 based on the determined position x3-2,y3-2 of the second detection area 3-2, for example by adding or subtracting the position difference xd,yd to the coordinates of the position x3-1,y3-1 of the first detection area 3-1.

[0045] According to the in Fig. 3b In the example shown, the position x3-1,y3-1 of the first detection area 3-1 corresponds to the position x3-2,y3-2 of the second detection area 3-2, as can be seen from the identical detection areas 3, so that in the Fig. 3b ) shown in the example, it can be ensured that the actual spectral data were indeed captured from the area which was defined as target area 13-1.

[0046] To accurately determine the first detection area 3-1, an exact calibration of the first detection area 3-1 to the second detection area 3-2 and thus of the spectrophotometer 7 to the imaging sensor 4 is required, which, however, is possible with simple means, for example, by exactly aligning the spectrophotometer 7 to the imaging sensor 4 and / or aligning the associated optics.

[0047] Fig. 4a ) shows the same definition of a first target area 13-1 in the printed image 2 as under Fig. 3a ) described, which is why a repetition regarding the definition of the exemplary first target area 13-1 is omitted here and instead the description of the Fig. 3a ) is referred to.

[0048] Fig. 4b ) shows how Fig. 3b ) a section of the printed substrate 1 moving at substrate velocity v, such that with regard to the relationship between the Fig. 4b ) to Fig. 4a ) to the relevant part of the description Fig. 3 is referred.

[0049] In summary, it can be said that Fig. 4 It is noted that in the printed image 2 and / or in the image data 12 a first target area 13-1 for determining the color scheme was defined with the position x13-1,y13-1.

[0050] This first target area 13-1 is produced in a printing press or in a finishing unit by the Fig. 4b ) the detection device 8 (not shown) detects, whereby the positioning and / or activation of the spectrophotometer 7 and the imaging sensor 4 can be carried out on the basis of the image data 12 or, for example, on the basis of data from the drive control or on the basis of encoder data from the pressure cylinders, etc.

[0051] The image area corresponding to the first target area 13-1 is attempted to be detected by the spectrophotometer 7 and its associated first detection area 3-1, for example, using data from the drive control, sensor data from the printing press, etc., whereby the spectrophotometer 7 determines the actual spectral data of the first detection area 3-1 actually detected.

[0052] Simultaneously (for example, with a configuration according to Fig. 1 ) or at a time delay (for example, with a configuration according to Fig. 2 The image section corresponding to the first target area 13-1 is also attempted to be captured by the imaging sensor 4 and its associated second detection area 3-2, using data from the drive control or sensor data from the printing press, etc. The imaging sensor 4 thereby determines the actual image data of the second detection area 3-2 that is actually captured.

[0053] The actual image data of the second detection area 3-2, acquired by the imaging sensor 4, are processed either by the imaging sensor 4 itself, by a computing unit associated with the imaging sensor 4, or by another device not included in the Fig. 4 The computer device shown is compared with the image data 12, so that the position x3-2,y3-2 of the second detection area 3-2 in the printed image 2 can be determined based on this comparison.

[0054] Due to the known and defined position difference xd,yd of the first detection area 3-1 to the second detection area 3-2 (in the example according to Fig. 1 The position difference is equal to 0, in the example according to Fig. 2 The position difference yd) can be used to determine the exact position x3-1,y3-1 of the first detection area 3-1 in the printed image 2 based on the determined position x3-2,y3-2 of the second detection area 3-2, for example by adding or subtracting the position difference xd,yd to the coordinates of the position x3-1,y3-1 of the first detection area 3-1.

[0055] According to the in Fig. 4b In the example shown, the position x3-1,y3-1 of the first detection area 3-1 corresponds to the position x3-2,y3-2 of the second detection area 3-2, as can be seen from the identical detection areas 3. However, in Fig. 4b ) an exemplary case is shown in which the position x3-2,y3-2 of the second detection area 3-2 and thus the position x3-1,y3-1 of the first detection area 3-1 differs from the position x13-1,y13-1 of the first target area 13-1, which can be seen from a geometrically different position of the first detection area 3-1 and thus also of the second detection area 3-2 in the printed image 2 compared to the first target area 13-1.

[0056] Such a geometric deviation of the first detection area 3-1 to the first target area 13-1 may be due, for example, to an elongation or stretching or outgrowth of the substrate 1, to slippage of the substrate 1, to lateral movement or displacement of the substrate 1 from the intended position or to similar reasons.

[0057] Although in Fig. 4b ) if, for example, only a deviation of the position x3-1,y3-1 of the first detection area 3-1 and the position x3-2,y3-2 of the second detection area 3-2 to the first target area 13-1 in the extent of the substrate velocity v is shown, it is also possible that the deviation of the position x3-1,y3-1 of the first detection area 3-1 and the position x3-2,y3-2 of the second detection area 3-2 is perpendicular to the substrate velocity v or perpendicular to and in the extent of the substrate velocity v.

[0058] Since according to Fig. 4b ) the spectrophotometer 7 has recorded a first detection area 3-1 that differs noticeably from the first target area 13-1, and since the spectrophotometer 7 has thus recorded a different section of the printed image 2 than intended, it would be a subject-related coincidence if the actual spectral data determined from the first detection area 3-1 were to match the target spectral data, since a different part of the printed image 2 was recorded and the actual spectral data determined from this than intended.

[0059] If the positional deviation xa,ya between the position x13-1,y13-1 of the first target area 13-1 and the position x3-1,y3-1 of the first acquisition area 3-1 exceeds a certain limit, which can be determined, for example, depending on the image data 12 and the structure of the subject and / or depending on the direction of the positional deviation xa,ya, then the actual spectral data acquired by the first acquisition area 3-1 are declared invalid and are not used in statistical quality evaluations or in the adjustment of color settings, since the deviation of the acquired actual spectral data from the target spectral data specified for the first target area 13-1 is highly likely due to the acquisition of a different image section.

[0060] Furthermore, it is possible to calculate colorimetric target data and / or spectral target data for the first acquisition area 3-1 from the image data 12 based on the position x3-1,y3-1 of the first acquisition area 3-1. Particularly in the case of a systematic deviation of the first acquisition area 3-1 from the first target area 13-1 and a positional deviation xa,ya that remains essentially constant over a print job, it is useful to determine the target spectral data and / or the colorimetric target data and / or the spectral target data for the actually acquired first acquisition area 3-1 in order to be able to perform quality control despite the positional deviation xa,ya.

[0061] For such quality monitoring, the actual spectral data of the first detection area 3-1 are compared with the spectral target data and / or the colorimetric target data. In the case of no or only a small positional deviation xa,ya between the first detection area 3-1 and the first target area 13-1, the spectral target data and / or the colorimetric target data can be used. Alternatively, in the case of a correspondingly significant positional deviation xa,ya between the first detection area 3-1 and the first target area 13-1, the spectral target data and / or the colorimetric target data can be used from the actually detected first detection area 3-1. For the comparison, the actual spectral data must, if necessary, be converted into colorimetric data according to procedures known from the prior art.

[0062] If the comparison described above between the actual spectral data and the spectral target data and / or the colorimetric target data is carried out and a predetermined difference is exceeded, further steps or at least one further step can be initiated.

[0063] Such a step could, for example, involve logging color deviations in a production report or quality report and / or correcting the settings required for color reproduction, such as opening or closing color zones for one or more printed colors in the case of an offset printing press. Another necessary step could be to open the waste paper filter, so that printed products with excessively deviating colors are rejected as waste and not processed further or sold.

[0064] As in the Figuren 3 and 4As shown, it is advantageous to perform the steps of capturing a first detection area 3-1 with the spectrophotometer 7 and determining the actual spectral values, capturing the second detection area 3-2 with the imaging sensor 4 and determining the position of the second detection area 3-2 as described above, not only for one printed image 2 printed on a substrate 1, but for a plurality of printed images 2, preferably for all printed images 2 printed on the substrate 1, in order to be able to perform the optional subsequent steps described above for each printed image 2.

[0065] Fig. 5 shows in Fig. 5a ) essentially the same exemplary print image 2 as in the Figuren 3a ) and 4a ) presented and described, and shows in Fig. 5b ) a substrate 1 printed with the repeating print image 2, using the example of a web-shaped substrate 1 as shown below Figuren 3b ) and 4b ) presented and described.

[0066] Fig. 5a ) however, shows a correlation with the Figuren 3a ) and 4a ) different configuration, namely that in the printed image 2 and / or in the associated image data 12 not only is a first target area 13-1 defined which is to be detected by the spectrophotometer 7 and the imaging sensor 4, but Fig. 5a ) shows a configuration in which a first target area 13-1 and a second target area 13-2 have been defined.

[0067] The first target area 13-1 is the target area 13 that is to be detected by the spectrophotometer 7, while the second target area 13-2 is the target area 13 that is to be detected by the imaging sensor 4.

[0068] Separating the first target area 13-1 from the second target area 13-2 is particularly advantageous if the first target area 13-1 is ideally suited for capturing the actual spectral data due to its color and / or geometric extent, but the exact position x3-2,y3-2 of the second acquisition area 3-2 can only be determined with difficulty or inaccurately using the first target area 13-1, for example, if the first target area 13-1 lies in a larger homogeneous area.

[0069] The inventive method is then carried out in principle as described below. Fig. 3 and 4As described, but with the following special features: According to this embodiment, at least one first target area 13-1 and at least one second target area 13-2 are defined in the print image 2, which is available as image data 12, or directly in the image data 12. This first target area 13-1 has the position x13-1,y13-1 and includes the filled dot shown as an example in the lower third of the print image 2. This dot can be a print mark for capturing the mark; however, preferably the first target area 13-1 lies in any area of ​​the print image 2 in order to avoid the additional space required for print marks or print control strips. The second target area 13-2 has the position x13-2,y13-2 and includes the unfilled circle shown as an example.This circle can be a print mark for capturing the mark, but preferably the second target area 13-2 lies in any area of ​​the print image 2 to avoid the additional space required for print marks or print control strips.

[0070] The first target area 13-1 is located in a printing press or in a finishing unit by the Fig. 5b The spectrophotometer 7 attempts to acquire the image area corresponding to the first target area 13-1. The spectrophotometer 7 and its associated first acquisition area 3-1 acquire the image area corresponding to the first target area 13-1, whereby the spectrophotometer 7 determines the actual spectral data from the first acquisition area 3-1.

[0071] Simultaneously or with a time delay (depending on the geometric assignment of the spectrophotometer 7 to the imaging sensor 4 and on the geometric deviation of the first target area 13-1 from the second target area 13-2), the image section corresponding to the second target area 13-2 is acquired by the imaging sensor 4 and its associated second acquisition area 3-2. The imaging sensor 4 thereby determines the actual image data of the second acquisition area 3-2.

[0072] The actual image data acquired by the imaging sensor 4 are processed either by the imaging sensor 4 itself, by a computing unit associated with the imaging sensor 4, or by another device not integrated into the system. Fig. 5 The computer device shown is compared with the image data 12, so that the position x3-2,y3-2 of the second detection area 3-2 in the printed image 2 can be determined based on this comparison.

[0073] Based on the known and defined position difference xd,yd of the first detection area 3-1 to the second detection area 3-2, the exact position x3-1,y3-1 of the first detection area 3-1 in the printed image 2 can be determined, for example, by a time-shifted detection of the first detection area 3-1 to the second detection area 3-2 at a known substrate velocity v, by adding or subtracting the position difference xd,yd and, if necessary, the distance traveled by the substrate 1 due to the time offset to the coordinates of the position x3-1,y3-1 of the first detection area 3-1.

[0074] Although not in the Figuren 3 , 4 or 5As shown, it is also possible to define a plurality of first target areas 13-1 or a plurality of first target areas 13-1 and second target areas 13-2 in the printed image 2 or in the image data 12, and to detect the plurality of the first acquisition areas 3-1 corresponding to the first target areas 13-1 by the spectrophotometer 7 and to detect the respective actual spectral data, as well as to detect the plurality of the second acquisition areas 3-2 corresponding to the first target areas 13-1 or, in the case of defining a plurality of second target areas 13-2, to the second target areas 13-2 with the imaging sensor 4, so that for all second acquisition areas 3-2 the respective position x3-2,y3-2 of each second acquisition area 3-2 can be determined by comparison with the image data 12, and that from each position x3-2,y3-2 of the majority of the second detection areas 3-2 for each associated first detection area 3-1 the respective position x3-1,y3-1 is determined.

[0075] It is also possible to carry out all optional subsequent steps as described above for the majority of the first recording areas 3-1 and for the majority of the second recording areas 3-2. Reference symbol list

[0076] 1 Substrate 2 Printed image 3 Detection area 4 Imaging sensor 7 Spectrophotometer 8 Acquisition device 12 Image data 13 Target area vSubstrate velocity x,yPosition xd,ydPosition difference xa,yaPosition deviation

Claims

1. A method for evaluating spectral data of a printed image (2) applied to a substrate (1), wherein the printed image (2) is represented by digital image data (12), comprising the following steps: a) Defining of at least one target area (13) located within the printed image (2) printed onto the substrate (1), namely either defining a first target area (13-1) with a position (x13-1, y13-1) or defining a first target area (13-1) with the position (x13-1, y13-1) and a second target area (13-2) with the position (x13-2, y13-2), b) Detecting a first detection area (3-1) corresponding to the first target area (13-1) and located within the printed image (2) on the substrate (1) by means of a spectrophotometer (7), and determining actual spectral data of the first detection area (3-1), c) Simultaneous or time-delayed detection of a second detection area (3-2) corresponding to a first target area (13-1) located within the printed image (2) on the substrate (1), or, in the case of determining the second target area (13-2), simultaneous or time-delayed detection of a second detection area (3-2) corresponding to the second target area (13-2) positioned within the printed image (2) by means of an imaging sensor (4), and determination of actual image data for the second detection area (3-2), d) Comparing the actual image data captured by the imaging sensor (4) with the digital image data (12) and determining the position (x3-2,y3-2) of the second detection area (3-2) in the printed image (2) printed onto the substrate (1), e) Determining the position x3-1,y3-1 of the first detection area (3-1) in the printed image (2) on the substrate (1) on the basis of the position (x3-2,y3-2) of the second detection area (3-2) in the printed image (2) and a defined positional difference (xd,yd) of the first detection area (3-1) relative to the second detection area (3-2), characterised in that, if a threshold value for a positional deviation (xa,ya) between the position (x13-1,y13-1) of the first target area (13-1) and the position (x3-1,y3-1) of the first detection area (3-1) is exceeded, the actual spectral data are discarded as invalid.

2. The method according to claim 1, characterised in that process steps b) to e) are repeated for at least one subsequent print image (2) printed onto the substrate (1).

3. A method according to any one of claims 1 to 2, characterised in that, in process step a), a plurality of first target areas (13-1) or a plurality of first target areas (13-1) and a plurality of second target areas (13-2) are defined, and that process steps b) to e) are carried out for the plurality of target areas (13) and the respective associated detection areas (3).

4. A method according to any one of claims 1 and 3, characterised in that a CCD camera or a CMOS camera is used as the imaging sensor (4).

5. A method according to any one of claims 1 to 4, characterised in that the method steps are carried out inside or outside a printing machine and / or simultaneously or at a later time with respect to the production of a printed product.