Procedure for online monitoring of the operating mode of at least one printhead
Patent Information
- Application Number
- ES2023152886T
- Authority / Receiving Office
- ES · ES
- Patent Type
- Patents
- Current Assignee / Owner
- Filing Date
- 2023-01-23
- Publication Date
- 2026-09-14
- Estimated Expiration
- 2043-01-23
Smart Images
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Abstract
Description
Method for online monitoring of the operating mode of at least one print head The invention relates to a method for online monitoring of the operating mode of at least one print head. In the printing industry, one of the essential goals is to produce image and color prints on printed materials that are as uniform as possible. This is known as color constancy, meaning that even when printing different batches on different substrates, the final product should always look as consistent as possible. This is particularly important when establishing a corporate identity. The color or colors chosen by a company must always look the same, regardless of whether it is printed on glass, paper, plastic, or another substrate. The aim is for printed products to look as identical as possible, and this is therefore a key objective of the printing industry. The printing process, the printer, the printing ink, and, above all, the substrate material, have a significant influence on the appearance of the printed result, which is produced from a print file. Printing ink refers to any printing medium, such as ink or toner, used to apply colored pigments to the substrate material. During industrial printing, whether in single-pass operation with a fixed printhead and moving support material or in multi-pass operation with a moving printhead and fixed support material, it is necessary to check the operating mode of at least one printhead, in particular all printheads used, to maintain the specified color constancy. A local change in the printing system, where an entire printhead or a partial area (segment) of a printhead has changed its printing behavior, results in a print image with stripes. This striping in the print image is also called banding. A change in the printer settings, and therefore a potential correction of the print image, cannot be corrected by changes to the print file, either automatically or manually. This is because these changes have a global effect on the entire image area, as it is necessary to modify the output profile (ICC profile) or the input file for color management. Therefore, it makes sense to check the printer at regular intervals to see if each color can be printed evenly across the entire print width and, if necessary, correct for banding. To correct banding, special test templates are printed and analyzed. These templates are structured so that for each individual printing ink at different intensity levels, areas are included that extend evenly across the entire print width. This allows for the quick and easy detection of banding-related striping and the implementation of appropriate countermeasures (cleaning, adjusting printhead or printhead segment tensions). If countermeasures have been taken, the test template is reprinted to verify their effectiveness. This process is repeated until the required quality is achieved. Therefore, banding correction is complex and interrupts the continuous printing process. Consequently, the described banding correction is only performed at longer intervals, sometimes excessively long. Document DE 102018 201 785 B3 discloses a procedure for detecting and compensating for defective print nozzles in an inkjet printing machine, where a test pattern in the form of a compressed print image is used. US 2016 / 0031252 A1 discloses a procedure for monitoring a printing system in which a test pattern is used as a verification data set based on the printing data and which has different spatial resolutions in a first and second direction. EP 3578939 A1 discloses a procedure for online quality control of decorative prints on substrate materials, in which a hyperspectrally measured reference image is first generated and stored as a THEORETICAL image in digital format. During the ongoing printing process, ACTUAL digital images of the print image are then generated using a conventional capture technique and compared with the THEORETICAL image. Therefore, the present invention is based on the technical problem of providing a procedure for online monitoring of the operating mode of at least one print head, thereby at least partially solving the problems mentioned above. According to the invention, the aforementioned technical problem is solved by a procedure for online monitoring of the operating mode of at least one printhead with the features of claim 1, which has a series of procedural steps described below, including preferred designs. First, a design to be printed is provided as a source image file in a chosen color space. This is done either by generating the source image file itself or by transmitting a pre-generated source image file. The source image file is typically represented in the RGB color space (R - red, G - green, B - blue), which is used in most image processing programs. Several versions of the RGB color space exist, such as sRGB, Adobe RGB, Adobe Wide Gamut RGB, etc. Other color space representations are also possible, such as the YUV color model with luminance and chrominance indication, CMYK (cyan, magenta, yellow, key (black)), or color separation as in analog printing. Next, a color management system with an input profile transfers the starting image file to an L*a*b* color space and an L*a*b* image file is generated. Color management ensures the most accurate reproducibility possible so that a print template can be faithfully reproduced throughout the printing process using any output device. In this regard, the correct conversion of data to the print image file is crucial. To achieve this, a color management system is generated within which device profiles—tables containing the device's color characteristics—can convert the respective device's own color space into an independent exchange color space, typically an L*a*b* color space such as CIELab. With the help of so-called ICC profiles (ICC - International Color Consortium), data can be exchanged between different devices so that, depending on the physical limitations of the input and output devices, for example, a printout can be created with colors as identical as possible to those displayed on a monitor. In particular, color management can utilize the PCS-CIELab exchange color space, a special design of the CIELab color space called a Profile Connection Space (PCS). Therefore, in this case, an ICC profile is preferably used as the input profile to convert the RGB image file to the L*a*b* color space. The Color Management System (CMS) ensures communication between elements of the graphic processing chain by using a common language. The standard for this is the L*a*b* color space, which is integrated into each ICC profile. L*a*b* is the color space that most closely approximates human vision and defines colors in absolute values within a coordinate system. Therefore, at each processing stage, color management transforms the L*a*b* values integrated into the respective ICC profile of all peripheral devices in the chain, up to the final print. The L*a*b* color mode is the cornerstone of the ICC architecture. The L*a*b* color space is a color space that encompasses the full range of perceptible colors. It is described using a three-dimensional coordinate system. The L* axis describes the lightness (luminance) of a color, with values from 0 (black) to 100 (white). The a* axis describes the proportion of green or red in a color, with negative values representing green and positive values representing red. The b* axis describes the proportion of blue or yellow in a color, with negative values representing blue and positive values representing yellow. The scales of the a* and b* axes cover a numerical range of -128 to +127. However, other scales are also known. When colors are represented in L*a*b* space, the distance between two colors is determined by the E00 value (CIEDE 2000) in difference analysis. E00 is a measure of color difference, where "" in this context is the symbol for difference.This allows for comparison of color printing results and quantification of distances. If the distance between two color locations in the L*a*b* color space is to be determined, the E00 (CIEDE2000) of the two color locations is calculated according to the formula in Annex B of ISO 13655:2008. In the next procedural step, the L*a*b* image file is transferred from the L*a*b* color space with an output profile to a print color space applicable to the printer, and a print image file is generated. For this purpose, an ICC profile is preferably used again as the ICC output profile. The printing color space takes into account the actual printing colors used, with the CMYK color space (cyan, magenta, yellow, key, or black) being the most common. However, in principle, the color space can also contain other printing inks or even more printing inks. Since the printing color space is limited and not all color tones are contained within it, the color management system ensures that the design is reproduced as accurately as possible within the available printing color space. Next, the color management system preferably applies a dithering procedure, RIP (Raster Image Processing), in which a pixel surface is generated from the print file using a plurality of ink droplets or toner dots, uniformly distributed across the pixel surface. The RIP procedure can be performed, for example, with software from Colorgate, Ergosoft, or Caldera. Next, a printer with at least one print head prints the design onto a substrate in a pixel grid using at least one printing ink, based on the print image file. The grid consists of rows running perpendicular to the printing direction and columns running in the printing direction. Inkjet printing is often used as the printing technique, but other printing techniques, such as laser printing, can also be used. Thus, for example, one can know the mapping between a column of the print image and the coordinate of the print nozzle within the corresponding print head in the respective row. Suitable substrate materials include paper, foil, wood, plastic, ceramic, or mineral materials. Pigment-based inks are generally used for printing, and the use of CMYK colors is widespread in this regard. In principle, the procedure described can also be applied to printers with a single print head with a print width of 3 cm. However, the preferred application of the described process is in the industrial sector, where relatively wide substrate materials are printed. For example, the process is used to print decorative papers used to cover wood-based panels. In this respect, widths of up to 1 or 2.3 meters are produced. For these widths of substrate materials to be printed, the printer has multiple printheads that are controlled synchronously, but individually. Furthermore, the individual printheads can have individually controllable segments. When this printing process is used frequently, the printer or printheads are stationary, and the media is conveyed beneath them, so the printing direction corresponds to the transport direction. This printing process is called single-pass printing. Therefore, the printed image or decoration is printed in rows perpendicular to the printing direction and in columns parallel to it. Consequently, if a malfunction occurs in one of the printheads or in a segment of one of the printheads, deviations in the printed image will occur in the columns. In a single-pass printing machine, the decoration is printed in a single pass across the entire print width. For example, an image can be printed across the width using 6 printheads in 4 colors (CMYK), meaning a CMYK printer has 4 rows of 6 printheads. The print image is then measured, at least in sections, but preferably across its entire width, using an optical measurement system with a spatial resolution (measured, for example, in dots per inch, dpi) within a few pixels of the print image, specifically with a spatial resolution of one pixel, row by row in the printing direction. A measurement image file composed of rows and columns is then generated. This measurement image file presents multichannel color information for each measured image element. The optical system is configured as a spectral measurement system. The spatial resolution of a spectral measurement system is typically in the range of 90 to 200 dpi, while color cameras have a higher resolution. Preferably, an inline color measurement system (ICMS) from ipac, known for its prior art, is used as the spectral measurement system for evaluating the color of multi-colored surfaces. The system features an inline spectral scanner that can be used for various substrate materials (paper, film, wood, plastic, ceramic, mineral material) and different printing processes (etching, digital printing, flexography, offset printing, screen printing). The ICMS is a spatially resolved spectral scanning technology for measuring a printed image and is capable of mechanically recreating an optical color impression perceived by a healthy, well-trained human eye, as well as subjecting the color print to a comprehensive objective evaluation. The spectral measurement system is, for example, a multispectral camera with 12 image channels per captured image element, which generates color information per image channel. This creates a color spectrum of approximately 10 to 12 image channels for each captured image point (pixel). A common sensor technology involves equipping individual pixels of a CMOS sensor with different color filters, so that a single image capture can record a multitude of spectral data from a captured image area. The spectral measurement system can also be a hyperspectral camera, in which the light is spectrally split by image element using optical equipment, such as a prism, and each element is measured separately using individual spectral ranges. This increases the spectral resolution compared to a multispectral camera with approximately 20 to 250 or more image channels. Unlike an RGB camera, this system captures not just one color per image element, but a spectral distribution with considerably greater information depth. Standardized lighting conditions are maintained during measurement; for example, the measurement geometry of the image generation system is 45°:0° (input:output). The next procedural step involves transferring the measurement image file to an L*a*b* measurement image file. The L*a*b* measurement image file is calculated from the image information of a spectral measurement image file, for example, according to CIE-15, preferably using CIE D50 illuminant and a 2° standard observer (CIE-15 ASTM 308E, ISO 13655 Annex I). The L*a*b* measurement image file is calculated from the image information of an RGB measurement image file, for example, using a transformation specifically created for the respective measurement conditions, which is obtained through calibration with a spectrophotometer. Next, the L*a*b* measurement image file is transferred to the printing color space using the output profile, and a comparison image file is generated. The same output profile used before printing the design is used for this process when converting the L*a*b* image file from the L*a*b* color space to the printing color space. The CMYK color space is again preferably used for printing. Applying the same ICC output profile ensures optimal comparability of the data to be compared later. Furthermore, the comparison image file and a specified reference image file are aligned to perform a comparison between the theoretical and actual values. Therefore, prior to this comparison, a reference image file is generated containing the theoretical state of the print in at least one printing ink, but preferably in all printing inks. The reference file can be generated in various ways. On the one hand, the reference image file can be generated from the print image file, particularly for the CMYK color space, for at least one print ink, preferably for all print inks. This is because the intensities of the individual print inks contained in the print image file define the print result to be achieved in digital format, and therefore, the print ink intensities can be used as a reference image file to determine the theoretical value. On the other hand, the reference image file can be generated from a comparison image file created from a previous measurement of a print or decoration image, as explained above, for at least one printing ink, preferably for all printing inks. In this case, the data in the reference image file is not theoretically generated from the original image file, but rather through a practical optical measurement performed beforehand. Before a comparison can be made between the reference image file and the comparison image file, the two image files are aligned with each other. This is done by shifting, rotating, or distorting the pixel arrays. This achieves the most accurate possible match of the pixels or groups of pixels in both image files. In the comparison stage, a difference image file is generated from the comparison image file and the reference image file, and the difference values in the difference image file are determined row by row. If the reference image file and the comparison image file match, the difference values are theoretically equal to zero at the order of magnitude of individual pixels or groups of pixels, but in practice they are affected by noise and fluctuate around zero. The determined difference values can be directly assigned to the corresponding printhead nozzles across the width. Since both the print image output file and the comparison image file are preferably generated with the same output profile (ICC profile) to convert the CIELAB color values to the printer's CMYK color channels, the cause of the striping in the print image can be uniquely associated with a physical printhead. In principle, the comparison can be done row by row, but it is advantageous to average the difference values by column across a predetermined number of rows; preferably, this averaging is performed continuously during the printing process. Since printing speed is generally relatively high and expected fluctuations occur with a small time constant, such averaging can lead to better results. If the values in the difference image file for the printing inks in each case fluctuate essentially around zero, this can serve as a measure that the print result has no deviation or only an insignificant deviation in this printing ink. However, if the difference values exceed a predetermined threshold below or above a certain value, a malfunction of the printhead assigned to the deviating pixels can be identified. This threshold value can be selected differently or the same for each printing ink. For each row range in the difference image file where the threshold value is exceeded, a malfunction of the assigned printhead or an assigned segment of the printhead can then be identified. Since the optical measurement resolution can be performed with pixel-level precision, even individual ink nozzle malfunctions can be detected. With the procedure described above, color deviations less than E00= 1 can be measured in the L*a*b* color space. If the decorated surfaces are created using a digital printing process, optical color measurement technology can be used to verify the color fidelity of the printed image. If color deviations occur in a color printout reproducing a design, these can be measured using the described procedure. Therefore, at least one printhead of the printer can be adjusted by modifying the control parameters within the printer's control system to minimize the difference values in the difference image file. Printer correction can be performed in this respect during continuous printing operation without interrupting printing, since the changes that occur and are recognizable by the described procedure are so small that they are almost imperceptible to the human eye in the printed image. Preferably, at least one printhead of the printer can be adapted by modifying the control parameters within the printer driver to minimize the difference values in the difference image file. This allows the control system to correct errors generated by at least one printhead without interrupting the printing process. Therefore, by combining the color management system with the printer control, automatic correction can be achieved. Even more precise color correction can be achieved, according to the procedure described above, if the entire printing system is characterized, i.e., calibrated, in particular linearized and profiled with the same optical color measurement system. The invention is then explained with the aid of embodiment examples with reference to the drawing. The drawing shows Fig. 1 a first installation for carrying out a procedure according to the invention, Fig. 2 a second installation for carrying out a procedure according to the invention, Fig. 3 a third installation for carrying out a procedure according to the invention, Fig. 4 a flowchart describing a procedure according to the invention and Fig. 5 a printed design whose color components are analyzed, Fig. 6a and b two combined representations of a printing ink to explain the procedure according to the invention, Fig. 7a-d Representation of the difference image file for four printing inks for a correctly printed design and Fig. 8a-d Difference image file representations for four printing inks for a design incorrectly printed in the magenta M color channel. In the following description of the various embodiments of the invention, components and elements with the same function and mode of action are provided with the same reference symbols, even though the components and elements may differ in their dimensions or shape in the various embodiments. Fig. 1 shows a first installation 2 for carrying out the automated characterization procedure of a continuous digital printing system. First, the support material 4 is unwound from a first roll 6, guided under the digital printer 8 and the optical measuring system 10, and then wound again onto a roll 12. In this respect, one can speak of an endless support material 4 with which the continuous printing and characterization are carried out. In this regard, it is evident that the endless tape has a finite, but large, length. The results of the optical measurements performed by the optical measurement system are transmitted to a control and characterization unit 14, which in turn transmits not only the print files or other control files, but also the characterization files to the digital printer 8. Figure 2 shows a second installation 2 for carrying out the automated characterization procedure of a continuous digital printing system. Compared to the installation in Figure 2, the support material 4 is not unwound from a roll, but is produced by an extrusion process. An extrusion tool 16 is shown schematically, from which a bar is extruded for the production of, for example, edge banding for use on furniture panels. For simplicity, the calenders and cooling stations, which are essential, are not shown here. Instead of the extrusion die 16, a continuous casting device can also be used to generate a continuous bar of support material. Figure 3 shows a third installation 2 for carrying out the automated characterization procedure of a continuous digital printing system.In contrast to the installations in Figures 2 and 3, the substrate material 4 is not configured as a continuous material, but rather consists of a plurality of contiguous elements 18, such as plates or sheets. Therefore, the substrate material 4 is made up of individual elements 18 that are presented separately before and after printing. The continuous printing and measurement of the color graphics are then performed on the substrate material 4 composed of these individual elements 18. Fig. 4 shows an example of implementing a procedure according to the invention for online monitoring of the operating mode of at least one print head. After starting at symbol 100, step 102 provides a design 104 to be printed as a source image file 106 in an RGB source color space. This can be done by generating your own file or using a previously generated and saved source image file. The source file 106 is subsequently processed using a color management system 110. In step 112, the source image file 106 is linked to an ICC input profile 114, and in step 116 it is converted to an L*a*b* color space in the form of a PCS-CIELAB color space. This results in an L*a*b* image file 118 that represents the design to be printed in a separate color space. In step 120, the PCS-CIELAB color space L*a*b* image file is linked to an ICC output profile 122, transferred to a printer-applicable CMYK print color space, and a print image file 124 is generated from it. In step 126, the print image file 124 for the inkjet printer 8 (see Figs. 1 to 3) is then converted into a control file 128 by means of a raster image processor (RIP). The RIP 128 calculates a distribution of a plurality of drops of the respective print ink for each pixel to be printed and for each print ink. In step 129, the inkjet printer 8 prints the design with at least one printhead based on either the print image file 124 or the control file 128 onto a paper web as support material 4 (see Figs. 1 to 3) in a pixel grid composed of rows running crosswise to the print direction and columns running in the print direction, using at least one CMYK print ink. Due to the width of the paper web 4, the printer 8 has multiple printheads with individual nozzle segments for each separate print ink. The measurement and evaluation of the printed design is recorded and evaluated below with a measurement system 130 and an evaluation system 143. First, using a spectral measurement system, indicated in Figures 1 to 3 with the reference symbol 10, the print image is measured row by row in step 132 with a spatial resolution of one pixel in the print direction, and a spectral measurement image file 134 composed of rows and columns is generated. The ICMS measurement system from ipac is used for this purpose. The lighting conditions are standardized during the measurement process, and the measurement geometry of the image generation system is 45°:0° (input:output). In step 136, the spectral measurement image file 134 is then converted to an L*a*b* measurement image file 138. This conversion is preferably performed in accordance with CIE-15, preferably using CIE D50 illuminant and a 2° standard observer (CIE-15 ASTM 308E, ISO 13655 Annex I). In the following step 140, the L*a*b* measurement image file 138 is converted to the CMYK print color space using output profile 122, which was already applied in step 120, and a comparison image file 142 is generated. Therefore, the same ICC output profile 122 used to create the CMYK print image file 124 is applied. In the evaluation system 143, a reference image file 146 is first generated in a separate step 144 for use in the subsequent procedure, either once or repeatedly at predetermined time intervals. The reference image file 146 can be generated, in this respect, either directly from the print image file 124 of step 120 for CMYK print inks, or from a previous comparison image file 142' for CMYK print inks. A previous comparison image file 142' is, in this respect, a comparison image file 142 from a previous measurement cycle that took place before the current measurement process. This could be an initial measurement process or, if applicable, a repeated measurement process. Next, the reference image file 146 is fed into the evaluation system 143, where in the next stage of procedure 148, the comparison image file 142 and the previously established reference image file 146 are aligned with each other, i.e., they are aligned with each other by shifting, rotating, or distorting the pixel arrays. The procedure then continues in step 150, and a difference image file 152 is generated from the comparison image file 142 and the reference image file 146, thus producing a pixel-accurate array of difference values. The difference values in the difference image file 152 can be determined by averaging the row values of a column across a predetermined number of rows. In step 154, an evaluation of the difference image file 152 is then performed. If the difference values exceed a predetermined threshold value 156, a malfunction of the printhead of the inkjet printer 8 assigned to the pixels exhibiting deviations is determined. This may involve a modification of a range of pixels in a row or an averaged row of the difference image file 152, thus identifying a malfunction of a printhead or a segment of the printhead—that is, only a part of the printhead. This can then be noted and corrected. A large difference in only a few pixels or in a single pixel indicates that one or a few nozzles of the printhead are dirty or damaged.Such deterioration cannot be remedied by selective printhead control, but can be issued as an error message with printhead identification. In decision step 158, it is determined whether the procedure ends with step 200 or whether it should continue. A jump back (represented by arrow 160) to the printing system then occurs to continue printing the design. If the threshold value has not been exceeded for any of the difference values, or if no error has been detected that can be resolved by the color management system, the procedure continues without modifying the printing system. However, if during the difference measurement an error is detected in an entire printhead or in a segment of one of the printheads, which can be reduced or corrected by the color management system, for example, by changing a specific control voltage, then the corresponding information is transmitted to the printing system by jumping back (arrow 160). This information is used in the printing system, for example, to modify the control voltage of one of the printheads in use, or of individual segments of one of the printheads in use, so as to minimize the difference between the theoretical and actual values. After a modification to the printing system, the procedure described in steps 129, 132, 140, 148, 150, and 154 then continues. In this way, an error that can be resolved by the color management system can be corrected continuously without interrupting the printing process, thus improving print quality. This error resolution can also be performed for color deviations in the L*a*b* color space below E00 = 1 that are not visible to the human eye. This means that the errors only occur within the range imperceptible to the human eye and can be corrected. Figures 5 to 7 show example images of a test performed using the flowchart shown in Figure 4 with a spectral measurement system. Figure 5 shows a grayscale image of a reproduction of a decorative wood surface with grain. The predominant color tone of the design is light brown with darker brown grain. In the test, starting with a well-adjusted printing system, an error was intentionally induced in the magenta (M) color channel. To do this, a print job was first performed using a well-adjusted printing press (sample 1 M), and then the tension on a magenta printhead was slightly modified (sample 2 M). The tension variation caused a shift in the color image across this printhead, resulting in a color difference of E00 = 1 as measured. Figures 6a and 6b show square sections of the printed images of samples 1M and 2M, as well as the corresponding reference images, also for magenta (M). The printed images 1M and 2M are aligned pixel by pixel with the reference image M in each case. The respective grayscale value images for the printing ink are shown. The mathematical symbols minus and equals represent the equation used to subtract the images from each other with pixel-level precision. The calculated differences can take on positive or negative numerical values. Therefore, the differences are summed in each case to a predetermined standard pixel value, for example, a mean value of 50% within the representable intensities of the printing ink between 0% and 100%. Thus, the gray values of the difference image have approximately the same tone as the source images, even though the difference values are lower than the absolute pixel values of the source images. Figures 6a and 6b also show a graph representing the pixel-by-pixel difference values across the rows. The pixel difference values in each row are averaged across the entire height of the columns in the displayed images. The x-axis shows the number of pixels in a row, and the y-axis shows the gray levels of the averaged difference values. Initially, a line fluctuating around the zero line (noise) can be observed. Only with a pixel value of approximately 550 can a spike in the difference values be seen in both graphs, caused by a single-nozzle error. However, this error cannot be resolved using the described procedure. Nevertheless, this fault may be indicated by the measurement and evaluation system 130 in the form of an error message. Furthermore, the graph in Figure 6a shows no noticeable fluctuations, so the underlying printing process appears to be in order and no correction of the printing system is necessary. The graph in Fig. 6b, on the other hand, shows a noisy section in the pixel range between approximately 710 and 980, but otherwise with a consistently lower value than the zero line. This variation is caused by a printhead malfunction deliberately introduced during this test. Therefore, this section corresponds to the incorrectly controlled printhead, and the cause of the failure can be resolved by modifying the printhead control. As mentioned earlier, deliberately modifying the printhead control causes a color difference of approximately E00 = 1, which is not visually perceptible in the grayscale values of the print image from sample 2M. Therefore, the procedure can also identify errors occurring below the visibility threshold. In contrast, a dark vertical band is visible in the difference image of sample 2, which also stands out visually from the adjacent areas of the graph. This is essentially because the difference image contains averaged values across multiple columns, and thus smaller fluctuations are more visible in the more uniform image. Figures 7a to 7d and 8a to 8d show square sections of the difference images for samples 1 and 2, as well as the corresponding graphs, for the four color channels M (magenta), C (cyan, blue), Y (yellow, yellow) and K (key, black). For sample 1, with the exception of a failure in individual pixels for samples 1M and 1Y, a curve is obtained that fluctuates around the zero line, as previously described. For sample 2, according to Fig. 8a, the behavior corresponding to Fig. 6b is initially observed. The drop in the difference curve corresponds to the forced error in the printhead during the test. The evolution of the difference values in Figs. 8b to 8d for the other printing inks also shows a change in the row range, which can be clearly seen in Fig. 8a, even though their assigned printheads had no modified settings. While Fig. 8b shows only a slight increase in difference values and Fig. 8c a slight decrease, Fig. 8d for the K channel shows a noticeable drop, although not as pronounced as in the M channel. These changes in the difference values are due to the fact that, in the spectral measurement of sample 2, a different color effect occurs compared to sample 1 because of the modified proportion of magenta (M). The spectral measurement values are converted, as described above, through the CIELab color space and then with the ICC output profile to the CMYK color space. This mathematical representation is not unambiguous in all cases and can lead to the described changes in the difference values in color channels that have not changed in themselves. However, since the fluctuation in difference values is greater in the color channel where the error occurs—in this case, the magenta (M) channel—the resulting error can be attributed to the corresponding channel. Correcting this color channel will then reduce all difference values. Another rule is to not initially correct the key (K) (black) channel if fluctuations occur in all color channels.
Claims
1. A method for online monitoring the operating mode of at least one printhead, wherein a design to be printed is provided as a source image file in a source color space, wherein the source image file is transferred from a color management system with an input profile to an L*a*b* color space and an L*a*b* image file is generated, wherein the L*a*b* image file is transferred from the L*a*b* color space with an output profile to a print color space applicable to the printer and a print image file is generated, wherein a printer with at least one printhead prints the design onto a support material, based on the print image file, in a pixel grid of at least one print ink, composed of rows running crosswise to the print direction and columns running in the print direction,- wherein the print image is measured row by row, at least segment by segment, in the print direction with an optical spectral measurement system having a spatial resolution in the range of a few pixels, in particular with a spatial resolution of one pixel, and a measurement image file composed of rows and columns is generated, - wherein the measurement image file is converted to an L*a*b* measurement image file, - wherein the L*a*b* measurement image file with the output profile is converted to the print color space and a comparison image file is generated, - wherein the comparison image file and a predetermined reference image file are registered against each other, - wherein a difference image file is generated from the comparison image file and the reference image file, - wherein the difference values of the difference image file are determined row by row, and - wherein,1. If the difference values exceed a predetermined threshold value, a malfunction of the printhead assigned to the pixels exhibiting deviations is determined, and wherein the procedural steps are carried out in online operation in continuous printing mode without interrupting printing.
2. A method according to claim 1, wherein a reference image file is generated from the print image file for at least one print ink.
3. A method according to claim 1, wherein a reference image file is generated from a previous comparison image file for at least one print ink.
4. A method according to any one of claims 1 to 3, wherein the difference values are averaged by columns across a predetermined number of rows.
5. A method according to any one of claims 1 to 4, wherein color deviations in the L*a*b* color space of less than E00 = 1 can be observed.
6. A method according to any one of claims 1 to 5, wherein at least one printhead of the printer is controlled by the color management system so as to minimize the difference values in the difference image file.