Method for examining a printed color texture

Abstract

Method for examining a multicolor print (101) of a printed color texture on a surface (103) of a substrate, in an ongoing printing process of a printing system, comprising the following steps: - Capturing the first produced multicolor print (101) of the printed color texture on the surface (103) of the substrate in the ongoing printing process of the printing system with an image sensor (109) as a reference of the multicolor print to be printed, - Providing the reference for the multicolor print to be printed (101), - Illuminating the printed surface (103) of the substrate with visible light of a first light color and with visible light of a further light color, wherein the visible light of the first light color and of the further light color corresponds to a defined wavelength or a defined wavelength range; - Recording an image of the surface (103) of the substrate when illuminated with the visible light of the first light color and recording another image of the surface (103) of the substrate with visible light of the other light color of an area of ​​the multicolor print (101) using an image sensor (109) with at least one image sensor row (109a), - Evaluation of light reflection in the image recording of the first light color as well as the other light color and detection of an anomaly in the printed color texture, in particular a faulty or missing color application, based on the respective light reflection of the first light color as well as the other light color in a comparison to the reference of the multicolor print to be printed (101) by means of a processor (111); - Detection of a defective nozzle of the printing system by detecting the anomaly in the printed color texture, whereby the reference of the multicolor print to be printed (101) of the color texture to be printed is only captured once on the basis of the first printed multicolor print (101).

Description

[0001] The present invention relates to the field of examining a paint layer.

[0002] Printing systems, especially digital printing systems, often include a large number of print nozzles for applying a layer of ink to a surface. These print nozzles are usually arranged in at least four rows or print units above the surface, with each row containing print nozzles for printing on the surface, for example, in one of the four colors cyan, magenta, yellow, or black. The surface can pass through the print units sequentially to apply the ink layer.

[0003] A print nozzle failure can lead to a faulty application of the color assigned to that nozzle. However, detecting such a defect in the color application and subsequently attributing the defective application to a specific print nozzle within the printing system is difficult.

[0004] Document US 2014 / 0139579A1 describes a device for the colorimetric inspection of a CMYK ink layer on a surface. The device comprises a lighting unit and an image sensor array for detecting light reflections from a test pattern. A missing ink layer of color C, M, Y, or K can be detected, thus identifying a malfunctioning nozzle. Nozzle functionality can only be checked before or between print jobs and only using a defined test chart.

[0005] Document US 2010 / 0253982 A1 discloses a discharge error detection device and a discharge error detection method. In the method, an image is read that has been generated on a medium by discharging a fluid with nozzles, wherein the reading resolution is lower than the resolution of the image data in the relative direction of motion. The nozzles are moved relative to the medium, generating standard data with the same resolution as the reading resolution.

[0006] From US patent 2014 / 0294248A1, an image processing device for detecting a high number of missing dots is known. Missing dots are identified by comparing an image generated from print data with the actual printed image, based on the print data.

[0007] It is therefore the object of the present invention to create an efficient concept for examining a color layer or a printed color texture on a surface, which enables control within an ongoing printing process.

[0008] This problem is solved by the features of the independent claim. Advantageous further developments are the subject of the dependent patent claims, the description, and the drawings.

[0009] The present invention is based on the finding that the above problem can be solved by taking an image of the surface when illuminated with visible light of a light color, wherein a defective coating in a light color matched to the printing inks, in particular the complementary color, in a color layer on the surface can be detected by means of a light reflection of the visible light.

[0010] According to a first aspect, the invention relates to a method for investigating a multicolor print of a printed color texture on a surface of a substrate in an ongoing printing process of a printing system, comprising the following steps: - Providing a reference of the multicolor print to be produced - Illuminating the printed surface of the substrate with visible light of a first light color and with visible light of a second light color, wherein the visible light of the first light color and the second light color corresponds to a defined wavelength or a defined wavelength range; - Capturing an image of the substrate surface when illuminated with visible light of the first light color and capturing another image of the substrate surface with visible light of the second light color in an assigned area of ​​the multicolor print using an image sensor with at least one image sensor line, - Evaluation of light reflection in the image recording of the first light color as well as the other light color and detection of an anomaly in the printed color texture, in particular a faulty or missing color application, based on the respective light reflection of the first light color as well as the other light color in a comparison to the reference of the multicolor print to be printed using a processor. - Detection of a defective nozzle in the printing system by identifying the anomaly in the printed color texture.

[0011] This offers the advantage of particularly efficient detection of a defective primary color application within the printing process, on the printed product, and without the additional effort of defined test areas or test charts.

[0012] Within a reproducible printing process, the first multicolor print produced serves as the reference for the multicolor print to be printed. In this case, the reference for the multicolor print to be printed is determined based on this first multicolor print. The multicolor print to be printed is thus recorded only once and provided as a reference for the process of examining a multicolor print.

[0013] As a reference, parameters (parametric description of the color texture) of multicolor printing are recorded, preferably the description of visual characteristics of multicolor printing. The determination of these parameters can be carried out using either statistical models or machine learning methods.

[0014] According to the invention, the reference for the multicolor print to be produced can be provided as a digital print file before the first multicolor print is produced. For this purpose, a visual transformation is calculated from a digital template of the multicolor print to a printed multicolor print, independent of the subsequent printing process, based on learned or predefined parameters of the printing process. Based on this transformation, the parametric description of the multicolor print required for its analysis is determined. This process step only needs to be determined once per digital template of the multicolor print to be produced; that is, in a subsequent printing process with analysis of the printing process, the already determined parametric description can be reused as a digital print file.The characteristics of the print image to be printed can therefore be learned “off-line” and thus in advance, and then reloaded at the time of examining the multicolor print.

[0015] A combination of both variants is also possible - the reference as a digital print file of the multicolor print to be printed and the reference that is captured on the basis of the first multicolor print to be printed.

[0016] This offers the advantage that inspection can be carried out during an ongoing printing process of a printing system, i.e., during the production of multicolor prints on the surface of a substrate. Furthermore, human perception can be taken into account during error assessment, depending on the specific printed image of the multicolor print.

[0017] According to a second aspect, the invention relates to the device for examining the ink layer on the surface according to the first aspect of the invention. This achieves the advantage that a malfunction of a printhead of the printing system can be efficiently detected by means of the device.

[0018] By using visible light, optical components, in particular lighting devices, lenses and image sensors, for the non-visible range, especially the near-infrared range, can be dispensed with, thus enabling simple and cost-effective manufacturing of the device.

[0019] The color texture of the multicolor print to be inspected can be applied to the surface of a substrate, in particular a paper, wood, or plastic substrate. The substrate can be positioned beneath the device for surface examination. In particular, the substrate can be positioned so that it can be moved beneath the device to allow for the examination of different surface sections of the substrate. For example, the substrate is mounted on rollers or cylinders, or arranged on a conveyor belt, and, after the ink layer has been applied to the surface, is passed beneath or past the device. The surface can be largely smooth and / or have high reflectivity. However, it is also conceivable to examine relief-covered or textured surfaces, i.e., surfaces that have a three-dimensional structure.

[0020] Furthermore, the device can be moved across the surface.

[0021] The device for examining a multi-color print on the surface of a substrate is arranged to be movable relative to the substrate or fixed relative to the movable substrate in order to enable the examination of different surface sections of the substrate in an ongoing printing process of a printing system.

[0022] The device comprises a lighting device configured to illuminate the surface with visible light of a first light color and with visible light of a further light color, wherein the visible light of the first light color and of the further light color corresponds to a defined wavelength or a defined wavelength range.

[0023] Furthermore, the device comprises an image sensor with at least one image sensor line, wherein the image sensor line is configured to take an image of the surface in an associated area of ​​the multicolor print when illuminated with the visible light of the first light color and to take another image of the surface with visible light of the other light color.

[0024] Each spatial position of the multicolor print of the printed color texture is preferably captured in the first and the subsequent light color.

[0025] Preferably, the entire printed image of the multicolor print of the printed color texture in all light colors is illuminated with the lighting device and captured with the image sensor.

[0026] A storage unit is provided to supply a reference for the multicolor print to be produced. In addition to the respective reference textures—the digital print file or a photograph of a printed product—the storage unit can also provide the determined texture description.

[0027] The reference can be the color texture of the multicolor print or a captured and therefore already printed multicolor print. The characteristics of the print image to be printed in the multicolor print can be learned "offline" and thus in advance, and then reloaded at the time of inspection.

[0028] Furthermore, the device comprises a processor configured to detect light reflections in the image captured by the first and subsequent light colors. Upon detecting these reflections, the processor compares them to the reference color of the multicolor print stored in the memory unit. This allows the processor to identify defective ink application in a specific area of ​​the multicolor print associated with the image sensor array, thereby detecting an anomaly in the printed color texture relative to the reference color. The resulting malfunction can be attributed to one or more nozzles of the printing system based on the detection of the defective or missing ink application.

[0029] Multicolor printing can consist of multiple printed color layers. Multicolor printing can include a variety of printed color lines. Furthermore, multicolor printing can include colors from the CMYK color model, which comprises cyan (C), magenta (M), yellow (Y), and black (K). Each area of ​​the multicolor print can contain one or more colors, typically consisting of C, M, Y, or K, or a mixture of these colors. The mixture can result from applying multiple colors to a single area of ​​the surface. The light colors of the lighting device can correspond to the complementary colors of the printing system or to another defined range of the visible wavelength spectrum.

[0030] The lighting device can be configured to emit visible light in the form of a flash. The flash can be emitted at periodic intervals, and the image sensor can be configured to capture a corresponding image with each flash.

[0031] The processor can be a microprocessor and may include memory. The processor's memory may contain the reference for the multicolor printout to be produced. Furthermore, the processor may be integrated into a data processing device connected to the apparatus, such as a desktop computer or laptop. The processor may also be connected to a visualization device, such as a display.

[0032] Based on the images taken under illumination in the defined wavelength ranges - corresponding to the printing inks used in the printer - the defective ink application can be detected in relation to the respective reference texture, and the affected nozzles of the respective printhead can be assigned to the corresponding printing ink.

[0033] In one embodiment, the processor is configured to detect light reflection in the image when the image brightness exceeds one or more threshold values. This achieves the advantage of efficient light reflection detection.

[0034] According to one embodiment, the processor is configured to generate a pixel image of the surface based on the image acquisition from at least one image sensor row and / or the image acquisition from at least one further image sensor row, wherein each image pixel of the pixel image is assigned an area of ​​the surface, and wherein the processor is configured to detect a defective ink application based on the brightness at each image pixel and the defined light color in an area of ​​the multicolor print assigned to the pixel. This achieves the advantage of efficient detection of light reflection.

[0035] According to one embodiment, the processor is configured to compare the image acquired by the image sensor array with the subsequent image acquired by the same image sensor array. Furthermore, the processor is configured to detect a defective ink application of another printing ink in the multicolor print by responding to the detection of light reflection in the image acquisition and the subsequent image acquisition. This additional light color can be of a defined wavelength or range relative to a broader printing ink, or it can consist of a mixture (additive) of the first and second light colors. This achieves the advantage of efficiently detecting defective applications of all possible mixtures of the light color and the additional light color.

[0036] According to one embodiment, the light color and the other light color comprise one of the colors red, green or blue.

[0037] The processor can detect the absence of cyan in multicolor printing by responding to the detection of light reflection when the surface is illuminated with red light and by detecting the absence of light reflection when the surface is illuminated with green and blue light, where cyan is the complementary color of red.

[0038] Furthermore, the processor can detect the absence of yellow in the multicolor print by responding to the detection of light reflection when the surface is illuminated with blue light and, in the absence of light reflection when the surface is illuminated with red and green light, by detecting the absence of yellow color, where yellow is the complementary color of blue.

[0039] Furthermore, the processor can respond to the detection of light reflection when the surface is illuminated with blue and green light, and, in the absence of light reflection when the surface is illuminated with red light, detect the absence of magenta in the multicolor print, where magenta is a complementary color of a mixture of blue and green.

[0040] Furthermore, the processor can detect the absence of the color black in the multicolor print by responding to the detection of light reflection when the surface is illuminated with light of the color blue, the color green and the color red, where black is a complementary color of a mixture of blue, green and red.

[0041] According to one embodiment, the image sensor has a plurality of image sensor lines. This achieves the advantage that efficient image acquisition by the image sensor is possible.

[0042] According to one embodiment, the image sensor rows are arranged in a row along a transverse axis of the surface. This achieves the advantage of efficient image acquisition by the image sensor.

[0043] The image sensor lines can be arranged parallel to the printed color lines of the multicolor print. In particular, each image sensor line can be assigned at least one color line of the multicolor print.

[0044] According to one embodiment, the lighting device comprises a plurality of light sources, in particular LEDs, wherein the light sources are oriented to illuminate the surface below the image sensor. This achieves the advantage that efficient illumination of the surface with the light color and / or the other light color is possible.

[0045] According to one embodiment, the light sources are arranged in at least one row parallel to the image sensor arrays, with each image sensor array being assigned at least three light sources with different light colors. This achieves the advantage of efficient illumination of the surface with one light color and / or another light color.

[0046] Each image sensor row can be assigned at least one LED with the light colors red, green, and blue. Furthermore, each image sensor row can be assigned two red, two green, and two blue LEDs, with LEDs of each light color potentially located on one side and on the other side of the image sensor.

[0047] According to one embodiment, the device comprises a reference surface, in particular a mirror, which can be arranged below the image sensor to capture a reference image of the reference surface when illuminated with visible light. This achieves the advantage of enabling efficient reference capture of the first and / or other light colors produced by the light sources.

[0048] According to one embodiment, the processor is configured to detect an image parameter, in particular image brightness or image sharpness, for each image sensor row of the image sensor based on the reference image, and to determine a spectral shift of the image sensor row or of a light source associated with the image sensor row based on the image parameter. This achieves the advantage that the processor can efficiently detect spectral differences between the image sensor rows and / or the light sources of the illumination device.

[0049] The processor can also be configured to determine the first and / or the second threshold based on the reference recording.

[0050] According to one embodiment, the image sensor includes a closable aperture for capturing an additional reference image. This additional reference image can be a dark frame. The closable aperture can be a dark frame shutter.

[0051] The processor can be configured to perform an image sensor calibration of the image sensor, in particular the image sensor lines of the image sensor, based on the reference image and / or the further reference image.

[0052] According to one embodiment, the processor is configured to detect, upon identifying a defective application of a complementary color to the light color in an area of ​​multicolor printing, a defect in a printhead of a printing system associated with the area of ​​multicolor printing and the complementary color. This achieves the advantage that a malfunction of a printhead in the printing system can be efficiently detected.

[0053] According to a second aspect, the invention relates to the printing system, with the device for examining the multicolor printing on the surface according to the first aspect of the invention. This achieves the advantage that a malfunction of a printhead of the printing system can be efficiently detected.

[0054] Exemplary embodiments of the invention are shown in the drawings and are described in more detail below.

[0055] They show: Fig. 1 a schematic representation of a device for examining a multicolor print according to one embodiment; Fig. 2 a schematic representation of a device for examining a multicolor print to detect the reference of the multicolor print to be printed according to an embodiment and Fig. 3 A schematic representation of a device for examining a multicolor print with a reference surface according to one embodiment.

[0056] Fig. Figure 1 shows a schematic representation of a device 100 for examining a multicolor print 101 on a surface 103 according to one embodiment.

[0057] The device 100 for examining the multicolor printing 101 on the surface 103 of a substrate in an ongoing printing process of a printing system comprises a lighting device 105, 107, which is configured to illuminate the surface 103 with visible light of a first light color and with visible light of a second light color. The visible light of the first light color and of the second light color corresponds to a defined wavelength or a defined wavelength range.

[0058] Furthermore, the device 100 comprises an image sensor 109 with at least one image sensor line 109a, wherein the image sensor line 109a is configured to take an image of the surface 103 in an associated area of ​​the multicolor print 101 when illuminated with the visible light of the first light color and to take another image of the surface with visible light of the other light color.

[0059] A storage unit 123 is provided for the provision of a reference of the multicolor print 101 to be printed.

[0060] Furthermore, the device 100 comprises a processor 111, which is configured to detect a light reflection in the image recording of the first light color and the other light color and, responding to the detection of the light reflection, to determine a defective application of a primary color based on the first light color and of another primary color based on the other defined light color in an area of ​​the multicolor print 101 assigned to the image sensor line 109a in a comparison to the reference stored in the storage unit 123, and thus to detect an anomaly in the printed color texture related to the reference, in particular a defective or missing color application of a nozzle of the printing system.

[0061] The surface 103 can be the surface of a substrate, in particular a paper-wood or plastic substrate. The substrate can be arranged under the device 100 for examination of the surface 103. In particular, the substrate can be arranged to move under the device 100 to allow examination of different surface sections of the substrate or an examination of the printed color texture over the entire surface of the substrate under investigation. For example, the substrate is supported on rollers or cylinders, or arranged on a conveyor belt, and, after the multicolor print 101 has been applied to the surface 103, is passed under or past the device 100. The surface 103 can be largely smooth or textured and / or have high reflectivity. Furthermore, the device 100 can be moved over the surface 103.

[0062] The multicolor print 101 can encompass a variety of printed color lines. Furthermore, the multicolor print 101 can include colors from the CMYK color model, which comprises the colors cyan (C), magenta (M), yellow (Y), and black (K), or colors similar to these. Each area of ​​the multicolor print 101 can contain a color C, M, Y, or K, or a mixture of these colors. The mixture can result from applying multiple colors to a single area of ​​the surface. The color of the multicolor print 101 can be the complementary color of the light color or include it.

[0063] The lighting device 105, 107 can be configured to emit visible light in the form of a flash of light. The emission of the flash of light can occur at periodic time intervals, and the image sensor 109 can be configured to take an image each time the flash of light is emitted.

[0064] The processor 111 can be a microprocessor and can include the memory unit 121. The memory unit 121 can also be provided separately from the processor 111. Furthermore, the processor 111 can be integrated into a data processing device, such as a computer or laptop, connected to the device 100. The processor 111 can be specifically optimized for vectorized data processing or integrated into a graphics processing unit. The processor 111 can be connected to a visualization device, such as a display.

[0065] A defective job or a failure of a printing ink can be detected in the area of ​​the multicolor print 101 assigned to the image sensor line 109a under illumination with the light color corresponding to the respective printing ink.

[0066] Fig. Figure 1 further shows four printhead rows 115, 117, 119, 121 for printing the surface 103 of the substrate with the colors black (K), yellow (Y), magenta (M) and cyan (C), each printhead row 115, 117, 119, 121 comprising a plurality of printheads 115a-c 117a-c 119a-c 121a-c. The printheads 115a-c 117a-c 119a-c 121a-c of the printhead rows 115, 117, 119, 121 are part of a printing system.

[0067] The printhead lines 115, 117, 119, 121 can be arranged over the surface 103 for applying the multicolor print 101, the surface 103 being able to move past the printhead lines 115, 117, 119, 121.

[0068] The image sensor 109 in Fig. 1 includes further image sensor lines 109b-c. At least one further image sensor line 109b-c can be assigned to further areas of the multicolor printing 101.

[0069] According to one embodiment, the at least one image sensor line 109a and the at least one further image sensor line 109b-c are identical.

[0070] The image sensor lines 109a-c can be CCD cameras or CMOS cameras. The image sensor lines 109a-c of the image sensor 109 can be arranged in a row along a transverse axis of the surface 103 and parallel to a printed color line of the multicolor print 101. Each image sensor line 109a-c can be assigned at least one color line of the multicolor print 101.

[0071] The lighting device 105, 107 can comprise a large number of light sources, which can be arranged in two rows parallel to the image sensor lines 109a-c.

[0072] The light sources can include LEDs or be designed as LEDs. Each image sensor line 109a-c can be assigned light sources in defined wavelength ranges and thus of different light colors, or one or more image sensor lines can be illuminated by different light sources with a time delay. In particular, each image sensor line 109a-c can be assigned an LED with the light colors red, green, and blue. Furthermore, each image sensor line 109a-c can be assigned two red, two green, and two blue LEDs, with LEDs of each light color being arranged on one side and on the other side of the image sensor 109.

[0073] According to one embodiment, the lighting device 105, 107 is designed to generate the light beam as a flash of light.

[0074] The processor 111 can be configured to detect light reflection in the image capture when the image brightness exceeds a first threshold.

[0075] The processor 111 can further be configured to detect light reflection in the image capture when the image brightness of the image capture of the image sensor row 119a exceeds the image brightness of an image capture of at least one further image sensor row 119b-c by a second threshold.

[0076] According to one embodiment, the image sensor 109 is configured to form a joint image of the multicolor print 101 based on the image acquisition of the at least one image sensor row 119a and the image acquisition of the at least one further image sensor row 119b-c. The brightness difference between the image acquisition of image sensor row 109a, which depicts one area of ​​the multicolor print 101, and the image acquisition of the further image sensor row 109b-c, which depicts another area of ​​the multicolor print 101, can be a contrast of the joint image acquisition. According to another embodiment, the processor 111 is configured to detect light reflection in the image acquisition when the image brightness of the image acquisition exceeds a third threshold value and the average image brightness of the images of the at least one image sensor row 109a and the at least one further image sensor row 109b-c.

[0077] According to a further embodiment, the processor 111 is configured to generate a pixel image of the surface 103 based on the image acquisition of the at least one image sensor line 109a and / or the image acquisition of the at least one further image sensor line 109b-c, wherein each image pixel of the pixel image is assigned an area of ​​the surface 103 and wherein the processor 111 is configured to detect, based on the brightness at each image pixel, a defective application of a primary color of the printer in the image acquisition under the light color assigned to the primary color or a combination of light colors in different wavelength ranges in an area assigned to the pixel of the multicolor print 101 in a comparison with the reference of the multicolor print (101) to be printed stored in the memory unit 123.

[0078] The processor 111 can be configured to parameterize the image pixels of the pixel image. The processor 111 can further be configured to apply an image processing method, in particular an edge filtering operation, to the pixel image in order to efficiently detect those image pixels whose brightness exceeds a threshold.

[0079] Furthermore, the processor 111 can be configured to classify a region of pixels based on the reference, in particular the parameters stored in the memory unit 123. Additionally, the processor 111 can be configured to assess texture anomalies relative to the reference object, enabling the efficient use of machine learning methods for texture and defect detection.

[0080] According to a further embodiment, the processor 111 is configured to compare the image acquisition of the further image sensor line 109b-c with the further image acquisition of the further image sensor line 109b-c, wherein the processor 111 is further configured to detect a defective application of a printing ink of the printing system in the area of ​​the multicolor print 101 assigned to the further image sensor line 109b-c in response to the detection of the light reflection in the image acquisition of the further image sensor line 109b-c and the further image acquisition of the further image sensor line 109b-c, wherein the further light color is for the detection of a mixture color of the first and the second light color.

[0081] The light color and the other light color can be one of the colors red, green or blue.

[0082] The processor 111 can respond to the detection of light reflection when the surface 103 is illuminated with red light and when there is no light reflection when the surface 103 is illuminated with green and blue light, and can detect the absence of the color cyan in the multicolor print 101, where cyan is the complementary color of red.

[0083] The processor 111 can also detect the absence of the color yellow in the multicolor print 101 by responding to the detection of light reflection when the surface 103 is illuminated with blue light and when there is no light reflection when the surface 103 is illuminated with red and green light, where yellow is a complementary color of blue.

[0084] The processor 111 can also respond to the detection of light reflection when the surface 103 is illuminated with light of the color blue and the color green, and, in the absence of light reflection when the surface 103 is illuminated with the color red, detect a lack of the color magenta in the multicolor print 101, where magenta is a complementary color of a mixture of blue and green.

[0085] Furthermore, the processor 111 can detect the absence of the color black in the multicolor print 101 by responding to the detection of light reflection when the surface 103 is illuminated with light of the color blue, the color green and the color red, where black is a complementary color of a mixed color of blue, green and red.

[0086] Fig. Figure 2 shows a schematic representation of the device 100 for capturing the reference of the multicolor print to be printed according to one embodiment.

[0087] The device 100 in Fig. 2 includes the print pattern of the multicolor print 101 as reference 209, which is printed on the surface 103 of a substrate and which can be arranged under the image sensor 109.

[0088] Reference 209 is a first multicolor print 101 to be printed. If the print pattern of the multicolor print 101 is positioned below the image sensor 109, the sensor can capture a reference image of the multicolor print 101 to be printed as reference 209 using the illumination device 105, 107. This reference 209 and the model parameters calculated on the basis of reference 209 for describing the color texture to be printed are subsequently stored in the memory unit 123.

[0089] The processor 111 is further configured to detect an image parameter for each pixel or for a region of pixels of the image sensor row 109a-c of the image sensor 109 on the basis of the reference 209, and to store these image parameters in the memory unit 123.

[0090] Fig. Figure 3 shows a schematic representation of the device 100 for examining a multicolor print 101 with a reference surface according to a further embodiment.

[0091] The device 100 in Fig. Figure 3 includes a reference surface 205, which can be arranged under the image sensor 109. For the sake of simplicity, the processor 111 and the memory unit 123 have been omitted from the illustration.

[0092] The image sensor 109 in Fig. 3 includes an aperture 207. The aperture 207 can be closed and, when closed, can prevent light, especially the reflection of visible light, from entering the image sensor 109.

[0093] The reference surface 205 can also be a mirror of a reference white surface or defined structures and / or elements for camera and system calibration for further reference image acquisition. The image sensor 109 can be configured to acquire a further reference image when the aperture 207 is closed. This further reference image acquisition can be a dark frame. The closable aperture 207 can be a dark frame shutter.

[0094] The processor 111 can be configured to determine the first and / or the second threshold based on the reference recording and / or the further reference recording.

[0095] Furthermore, the processor 111 can be configured to perform an image sensor calibration of the image sensor 109, in particular of the image sensor lines 109a-c of the image sensor 109, on the basis of the reference image and / or the further reference image.

[0096] The processor 111 is further configured to acquire an image parameter for each pixel or for regions of pixels of the image sensor row 109a-c of the image sensor 109 on the basis of the reference image and / or the further reference image, and to store these image parameters in the memory unit 123.

[0097] Preferably, a spectral shift of the image sensor line 109a-c or of a light source assigned to the image sensor line 109a-c can be determined by the processor 111 on the basis of the image parameter.

[0098] According to another embodiment, the processor 111 is configured to detect a temporal change in the reflection in each image sensor line 109a-c of the image sensor 109.

[0099] The device in the Fig. 2 and Fig.3 further comprises the illumination device 105, 107, which is arranged on two opposite sides of the image sensor 109. On each side, the illumination device 105, 107 comprises a red 201a, 203a, a green 201b, 203b and a blue 201c, 203c LED. The LEDs 201a-c, 203a-c are oriented towards the surface 103.

[0100] According to one embodiment, each LED 201a-c, 203a-c of the lighting device 105, 107 comprises a focusing lens to focus the light beam on the surface 103.

[0101] According to another embodiment, the reference image acquisition of the reference surface serves to calibrate the image sensor 109. The reference surface can include reference markers for this purpose. Calibration can be performed pixel by pixel for each pixel of the image. In particular, pixel-by-pixel color calibration is required due to the chromatic aberrations of the optics. Furthermore, the image sharpness can vary considerably across the image. For this reason, an optical transfer function that varies across the image field can be taken into account. The calibration procedure required for this can be based on a model that calculates an individual shift in the color space for each color of light per image pixel.

[0102] According to one embodiment, the image sensor 109 is a camera system, in particular a camera array consisting of n image cameras, which are operated side by side and / or one behind the other.

[0103] Light reflection from color layer 101 in an area of ​​high reflectance may appear white or nearly white in the image. Light reflection from the multicolor print 101 in an area of ​​low reflectance may appear black or nearly black in the image.

[0104] Each section of the multicolor print 101 can be assigned at least one printhead 115a-c, 117a-c, 119a-c, 121a-c of the printing system. If a printhead 115a-c, 117a-c, 119a-c, 121a-c fails, the substrate background, i.e., the base color of the surface of the substrate to be printed, can appear, for example, as a white vertical area in the corresponding image area on white paper. The contrast can be maximized when the printing ink is black. Each printhead 115a-c, 117a-c, 119a-c, 121a-c can be configured as a nozzle.

[0105] The inline measurement of the color stability of a color across n printheads 115a-c 117a-c 119a-c 121a-c with n image sensor lines 109a-c requires stable illumination, calibration of the n image sensor lines 109a-c, and independence from external boundary conditions, especially ambient light and temperature, or the detection and compensation of deviations.

[0106] According to one embodiment, the printing system is a digital printing system, in particular a laser printer, an ink-jet printer or a drop-on-demand system.

[0107] According to one embodiment, the processor 111 is configured to detect deviations and / or shifts of a color in the multicolor print 101 over time. These deviations and / or shifts in the multicolor print 101 over time can be detected and evaluated based on a color-reconstructed image. This detection is based on the selection of the light colors used by the illumination device 105, 107.

[0108] The processor 111 can detect color defects in the ink layer 101 when the surface 103 is illuminated with a narrowband light of red, green, or blue. Furthermore, the processor 111 can detect a color image of the multicolor print 101 when the surface 103 is illuminated with a broadband light of a combination of red, green, or blue.

[0109] According to one embodiment, the multicolor print 101 on the surface 103 is examined inline by the device 100, or the device is integrated into the printing system for printing the surface 103 with the multicolor print 101.

[0110] The following procedural steps are carried out for this purpose: - Providing a reference of the multicolor print to be printed 101 - Illuminating the printed surface 103 of the substrate with visible light of a first light color and with visible light of a further light color, wherein the visible light of the first light color and of the further light color corresponds to a defined wavelength or a defined wavelength range; - Recording an image of the surface 103 of the substrate when illuminated with the visible light of the first light color and recording another image of the surface 103 of the substrate with visible light of the other light color to an assigned area of ​​the multicolor print 101 by means of an image sensor 109 with at least one image sensor row 109a, - Evaluation of light reflection in the image recording of the first light color as well as the other light color and detection of an anomaly in the printed color texture, in particular a faulty or missing color application, based on the respective light reflection of the first light color as well as the other light color in a comparison to the reference of the multicolor print to be printed (101) by means of a processor (111). - Detection of a reference-related texture anomaly of the printed product by identifying a faulty or missing ink application from one or more nozzles of the printing system.

Claims

[1] Method for investigating a multicolor print (101) of a printed color texture on a surface (103) of a substrate, in an ongoing printing process of a printing system comprising the following steps: - Capturing the first produced multicolor print (101) of the printed color texture on the surface (103) of the substrate in the ongoing printing process of the printing system with an image sensor (109) as a reference of the multicolor print to be printed, - Providing the reference for the multicolor print to be printed (101), - Illuminating the printed surface (103) of the substrate with visible light of a first light color and with visible light of a further light color, wherein the visible light of the first light color and of the further light color corresponds to a defined wavelength or a defined wavelength range; - Recording an image of the surface (103) of the substrate when illuminated with the visible light of the first light color and recording another image of the surface (103) of the substrate with visible light of the other light color of an area of ​​the multicolor print (101) using an image sensor (109) with at least one image sensor row (109a), - Evaluation of light reflection in the image recording of the first light color as well as the other light color and detection of an anomaly in the printed color texture, in particular a faulty or missing color application, based on the respective light reflection of the first light color as well as the other light color in a comparison to the reference of the multicolor print to be printed (101) by means of a processor (111); - Detection of a defective nozzle of the printing system by detecting the anomaly in the printed color texture, whereby the reference of the multicolor print to be printed (101) of the color texture to be printed is only captured once on the basis of the first printed multicolor print (101). [2] Method for examining a multicolor print (101) according to claim 1, characterized by , that the light reflection in the image capture is only detected by the processor (111) if an image brightness of the image capture exceeds a first threshold value. [3] Method for examining a multicolor print (101) according to any of the preceding claims, characterized by, that the image acquisition of the image sensor line (109a) is compared with the further image acquisition of the image sensor line (109a) and, responding to the detection of the light reflection in the image acquisition and the further image acquisition, a defective ink application within the multicolor print (101) to be examined is detected, wherein the further light color can be at a defined wavelength or in a defined wavelength range which depends on the primary colors used in the printing system, or consists of a mixture of the first and second light color. [4] Method for examining a multicolor print (101) according to claim 3, characterized by that the light color and the other light color include one of the colors red, green or blue.

Images