Patent Publication Number: US-2021178754-A1

Title: Method and device for determining faulty print nozzles of a printing device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This patent application claims priority to German Patent Application No. 102019134721.1, filed Dec. 17, 2019, which is incorporated herein by reference in its entirety. 
     BACKGROUND 
     Field 
     The disclosure relates to a method and a device for determining faulty print nozzles of a printing device with the aid of a test image detected by an image detector. 
     Related Art 
     Faulty print nozzles of an inkjet printing device reduce the print quality of a printed print image. In particular, the print image may have an optically visible white streak due to a failed print nozzle. An additional object is the determination of faulty print nozzles of a printing device in order to ensure a high print quality. 
     DE 10 2016 120 753 A1 describes a method for determining the state of at least one print nozzle of an inkjet printing device. In the known method, a test image is printed with the aid of the printing device. The print nozzles are thereby activated so that a predetermined pattern of lines is printed over 2032 μm of total length onto a recording medium in the transport direction, wherein each print nozzle prints precisely one line. The test image is subsequently detected with the aid of an image detector. Starting from a defined print nozzle of one or more print heads, the line associated therewith is determined in order to determine a state of this defined print nozzle. For this, at every position at which a print nozzle was activated to print a line, a greyscale value of this line is determined and compared with a threshold. A malfunction is established depending on the comparison. 
     However, the problem exists that, due to the length of the predetermined pattern, respectively only print nozzles of a print bar of one primary color on a page may be checked with respect to their state. Given a typical printing device having four primary colors (CMYK), the print nozzles of each primary color may thereby be checked only every four pages. Given occurring print nozzle errors, this leads to a delayed determination of these, and therefore to a reduced print quality and/or to an increased waste. Furthermore, the checking per line, in which each line is checked by means of threshold analysis, is time-consuming and inefficient. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES 
       The accompanying drawings, which are incorporated herein and form a part of the specification, illustrate the embodiments of the present disclosure and, together with the description, further serve to explain the principles of the embodiments and to enable a person skilled in the pertinent art to make and use the embodiments. 
         FIG. 1  is a schematic side view of a printing device according to an exemplary embodiment. 
         FIG. 2  is a schematic plan view of the printing device according to  FIG. 1 . 
         FIG. 3  is a schematic side view of an image detector and of a recording medium for detecting print images printed onto the recording medium according to an exemplary embodiment. 
         FIG. 4A  shows a recording medium with the printed test image according to an exemplary embodiment. 
         FIG. 4B  shows a recording medium with an incorrectly printed test image according to an exemplary embodiment. 
         FIG. 5  is a schematic detail view of a test image according to an exemplary embodiment. 
         FIG. 6A  shows a detected test image according to an exemplary embodiment. 
         FIG. 6B  shows a detected test image having an incorrect region according to an exemplary embodiment. 
         FIG. 7  is a flowchart of a method for determining faulty print nozzles of a printing device according to an exemplary embodiment. 
     
    
    
     The exemplary embodiments of the present disclosure will be described with reference to the accompanying drawings. Elements, features and components that are identical, functionally identical and have the same effect are—insofar as is not stated otherwise—respectively provided with the same reference character. 
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth in order to provide a thorough understanding of the embodiments of the present disclosure. However, it will be apparent to those skilled in the art that the embodiments, including structures, systems, and methods, may be practiced without these specific details. The description and representation herein are the common means used by those experienced or skilled in the art to most effectively convey the substance of their work to others skilled in the art. In other instances, well-known methods, procedures, components, and circuitry have not been described in detail to avoid unnecessarily obscuring embodiments of the disclosure. The connections shown in the figures between functional units or other elements can also be implemented as indirect connections, wherein a connection can be wireless or wired. Functional units can be implemented as hardware, software or a combination of hardware and software. 
     An object of the present disclosure is to reduce the reaction time for determining faulty print nozzles of a printing device. The present disclosure advantageously improves the method known from DE 10 2016 120 753 A1. 
     According to embodiments of the disclosure, a detection of a print image printed as a defined test image takes place per dot, wherein a plurality of print nozzles of at least one primary color are activated so that they print dots in each print line on a recording medium in a plurality of line rows that are successive as viewed in the printing direction and travel in the line direction, and thus form lines as viewed in the printing direction. Image data are provided in this way, wherein an image pattern processing evaluates homogenized image data and determines defective image regions. 
     Upon homogenization of the image data, brightness values of the detected image regions are smoothed. This smoothing, or leveling, by means of preferably digital filtering, leads to the situation that missing or faulty lines distinctly stand out from properly printed lines. Such flaws may be quickly and simply detected using homogenized image data. 
     Furthermore, the method allows the detection of the printed test image with lower resolution than the resolution of the printing device in the line direction. At the same time, the method allows the denser arrangement of the printed lines on the recording medium so that the test image on the recording medium is shortened in the printing direction, for example is 800 μm to 1000 μm, in particular 900 μm. In spite of the dense arrangement of the lines and the short length of the test image on the recording medium, with the aid of the homogenization it is possible to print the test image with a resolution of 1200 dpi, for example, and to detect this test image by means of an image detector with a reduced image resolution of 600 dpi, for example, and nevertheless to determine faulty image regions and therefore faulty print nozzles. This reduced space requirement allows all print nozzles of each primary color to be checked with a separate test image on each printed page. This reduces the reaction time in the event of failure of print nozzles, whereby an increase in the print quality is possible with simultaneous reduction of waste. 
     In particular, the homogenized image data of the detected test image are analyzed, and image regions are detected that have a color property deviating from the average of at least one region of the test image, and the respective associated print nozzles is determined. The homogenization in particular encompasses the smoothing of the image data with the aid of a smoothing mean value algorithm. Print nozzles may thereby be determined that exhibit a malfunction, in particular print nozzles that do not print, or that print incompletely and/or at an angle on the recording medium. 
     According to a further aspect of the disclosure, a device for generating print images is disclosed that comprises a printing device, an image detector, and a controller. The technical advantages achieved with this device coincide with those that are explained in conjunction with the method according to the disclosure. 
       FIG. 1  shows a schematic side view of a printing device  10  for printing to a recording medium  12  in the form of a web. In the exemplary embodiment, the printing device  10  is executed as a known inkjet printing device. Such a printing device is known from the document DE 10 2014 106 424 A1, for example. 
     In an exemplary embodiment, the printing device  10  has, per primary color, at least one print bar  18  through  24  having one or more print heads  26 , shown in  FIG. 2 , which are arranged transverse to a transport direction T 1  of the continuously drivable recording medium  12  in the form of a web. The transport direction T 1  therefore also corresponds to a printing direction T 1 . The recording medium  12  may be produced from paper, paperboard, cardboard, textile, a combination thereof, and/or other media that are suitable and can be printed to. 
     As an alternative to continuously supplied recording media  12  in the form of a web, recording media in the form of sheets may also be supplied to the printing device  10  for printing. 
     The recording medium  12  is directed through the printing device  10  and, via infeed rollers  28 ,  30  and a plurality of guide rollers  32  through  42 , is thereby directed below and past the print bars  16  through  24  having the print heads  26 , wherein the print heads  26  apply a print image  43  onto the recording medium  12  in the form of dots. In  FIG. 2 , the print image  43  is depicted, by way of example, as two bars printed in parallel across the printable width of the recording medium  12 . 
     In an exemplary embodiment, using the image detector  44 , the printed print image  43  is detected per line or per region over the entire printable width of the recording medium  12 . In an exemplary embodiment, the image detector  44  includes processor circuitry that is configured to perform one or more functions and/or operations of the image detector  44 . 
     With the aid of a takeoff roller  46 , the recording medium  12  is further directed to a drying (not shown) and, if applicable, to a subsequent further printing device in which the back side of the recording medium  12  in particular may be printed to. The recording medium  12  may subsequently or alternatively be supplied to a post-processing in which the recording medium  12  is cut, folded, and/or finally processed in other work steps. In particular, test images printed onto the recording medium  12  in the post-processing may be cut out from said recording medium  12 . 
     Four primary colors are typically used for full-color printing, namely CMYK (cyan, magenta, yellow, and black). Additional primary colors, for example green, orange, or purple, may expand the color space of the printing device  10 . Moreover, additional colors or special inks such as MICR ink (Magnetic Ink Character Recognition=magnetically readable ink) may also be present. Each primary color is printed onto the recording medium  12  with the print heads  26  of a respective print bar  18  through  24 . It is likewise possible that transparent special fluids, such as primer or drying promoter, are likewise digitally applied with the aid of a separate print bar before or after the printing of the print image  43  in order to improve the print quality or the adhesion of the ink to the recording medium  12 . In the exemplary embodiment according to  FIG. 1 , a primer fluid is printed onto the recording medium  12  with the aid of the print bar  16 . 
       FIG. 2  shows a schematic plan view of the printing device  10  according to  FIG. 1 . The print bars  16  through  24  form a print unit  47 . Printing with the full line width is possible with each of the print bars  16  through  24  of the printing device  10 . For this, each print bar  16  through  24  comprises a plurality of print heads  26  that are arranged side by side, with gaps, in two rows. 
     In  FIG. 2 , each print bar  16  through  24  comprises five print heads  26  in order to apply the print image  43  onto the recording medium  12  in a plurality of columns  48 . Each print head  26  comprises a plurality of print nozzles  50  (for simplification, only ten print nozzles are shown in  FIG. 2 ), wherein each print nozzle may apply ink droplets of a variable volume onto the recording medium  12  in the form of dots. In practice, each print head  26  may comprise multiple hundreds to multiple thousands of print nozzles  50  directed toward the recording medium  12 . The print nozzles  50  are arranged in a row transverse to the printing direction T 1 . With the aid of the print nozzles  50  of a print head  26 , a print image  43  may be printed over a portion of a line along the printable width of the recording medium  12 , and in the form of a column  48  along over the length of the recording medium  12  in the printing direction T 1 . A region of the recording medium  12  below the print head  26  is thereby printed to by each print head  26 . 
     The printing onto the recording medium  12  takes place according to a two-dimensional raster matrix in which a print nozzle is associated with each raster point of a line of the raster matrix. A raster point that has been printed to, meaning a dot, along a line across the printable width of the recording medium  12  thus has associated therewith a print nozzle  50  of the print bar  18  through  24 . The print resolution in the line direction x (meaning transverse to the transport direction T 1 ) is indicated in dpi (dots per inch). It is typically in a range from 600 dpi to 1200 dpi. A corresponding print nozzle is associated with each raster point in the line direction x. Given single-line print heads, the print resolution in the transport direction T 1  is determined by the transport velocity of the recording medium  12  and the line timing of the print heads  26  of the print bars  18  through  24  given line-clocked printing. 
     In an exemplary embodiment, using a controller  52 , the individual print nozzles  50  of the print heads  26  of the print bars  18  through  24  are activated, based on print data according to a print raster of raster points, so that the individual ink droplets are applied onto the recording medium  12  at the position in the x-direction and y-direction as defined by the print data, meaning corresponding to the line direction and printing direction T 1 . The ink droplets on the recording medium  12  form the dots that, in their entirety, form the print image  43  on the recording medium  12 . Ink droplets do not need to be applied at each raster point in order to form the print image on the recording medium  12 . As noted, the dots and their position as defined by the print data are arranged in a uniform raster across the printable width of the recording medium  12  and in the printing direction T 1 . It may occur that, due to faulty print nozzles, ink droplets are not printed or ink droplets do not form the provided dot. In an exemplary embodiment, the controller  52  includes processor circuitry that is configured to perform one or more functions and/or operations of the controller  52 . 
       FIG. 3  shows a schematic side view of the image detector  44  and of the recording medium  12  for detecting print images  43  printed onto the recording medium  12 . The image detector  44  has a plurality of light-sensitive dot detection regions  54  arranged in at least one line. The dot detection regions  54  respectively comprise sensor elements for detecting the brightness of the incident light in the colors red, green, and blue (RGB). A separate color channel of the image detector  44  is thereby associated with each color (RGB). An image region  56  having one or more raster points and/or dots of the print image  43  printed on the recording medium  12  is detected with the aid of a dot detection region  54 . 
     For each image region  56 , the image detector  44  thus detects an optical image of the dots at a light-sensitive dot detection region  54 . Each dot detection region  54  thereby has a field of view  58  directed toward the recording medium  12 . With the aid of a plurality of dot detection regions  54  arranged side by side in at least one line, the print image  43  is detected over the entire printable width of the recording medium  12 . In  FIG. 3 , only four dot detection regions  54  are shown for simpler presentation. 
     Typically, depending on the number of dot detection regions  54  of the image detector  44 , a plurality of raster points that contain dots and unprinted raster points of the print image  43  are contained in an image region  56 . Based on the area coverage of the dots in an image region  56 , a brightness value in the color channels RGB of the image detector  44  may be determined for the respective image region  56  of the print image  43  with the aid of the image detector  44 . 
     In an exemplary embodiment, the image detector  44  is executed as a line camera, for example an allPixa Pro camera from the vendor Chromasens that detects the print image  43  line by line. The line camera detects a line of the print image  43  with a plurality of light-sensitive dot detection regions  54  arranged side by side, in particular in the form of a CCD, CMOS, NMOS, or InGaAs sensor. The allPixa Pro line camera has three lines with respectively 4096 dot detection regions  54 , for example. 
     Dot detection regions  54  are also referred to as pixels. Brightness values in a respective color channel of the image detector  44  are detected by each dot detection region  54 . 
     Moreover, in an exemplary embodiment, the controller  52  is configured to compare the image data of the print images  43  with the print data, where the image data being detected in the form of image regions  56  with the aid of the image detector  44 , and produce an association of image regions  56  with raster points and/or with dots. With the aid of the association of image regions with raster points and/or dots, the controller also establishes an association with print nozzles. Furthermore, the controller  52  is designed and configured so that the values, for example brightness values, contrast values, and reference values, may be processed and stored for the image detector  44  and an image pattern processing. With the aid of the image pattern processing, the controller  52  is thereby capable of determining defective and/or faulty print nozzles  50 , as is described further below using  FIG. 7 . 
       FIGS. 4 and 5  show an exemplary embodiment of the method according to the present disclosure. For simplification reasons, the printing device  10  here comprises only 96 print nozzles  50 . A generalization for a greater number of print nozzles (up to multiple thousands) takes place further below. These 96 print nozzles  50  print the print image  43  in the form of a test image  60 ,  62  for a defined primary color. The test image  60 ,  62  comprises four successive line rows L, wherein, in each line row L (see also reference character  66  in  FIG. 5 ), associated print nozzles  50  print a count of 96/4=24 lines  64  at corresponding raster points  76  in the printing direction y. For example, each line  64  comprises ten printed raster points, meaning ten dots  74 . As viewed in the line direction x, the raster points in each line row L are subdivided into groups  59  of four raster points each per print line  78 , meaning that each line row L comprises 96/4=24 groups  59 . In the first line row L, print nozzles  50  print at first raster points associated therewith of each group  59 . The other raster points of each group  59 , meaning the second, third, and fourth raster point, are not printed to. Lines  64  traveling in the printing direction y across multiple print lines  78  are thus created in the first line row L. 
     The second line row L proceeds similarly to the first line row L, but here print nozzles  50  are activated that are respectively associated with the second raster point of each group  59 . The other raster points of the respective group  59  are not printed to. The method proceeds analogously in the third and fourth line rows L. 
     As is apparent, a test image  60  results in which the lines  64  of each line row L have a constant pitch [spacing] of four raster points from one another in the line direction x. The lines  64  of successive lines rows L are respectively displaced counter to one another by one raster point. If the four line rows L were to be printed atop one another, all raster points would thus be printed to in the x-direction by the print nozzles  50 . In the present method example, the test image  60 ,  62  would thus have expanded in the y-direction. 
     The described example with only 96 print nozzles  50  can be generalized. Given a total count j of print nozzles  50  corresponding to a count j of raster points in the x-direction, the print nozzles i is associated with a defined raster point i, with i as a whole-number control variable i of 1, 2, 3, . . . , j. The i print nozzles or i raster points are organized in the line direction x into n successive groups, wherein k print nozzles or k raster points are associated with each group k, wherein n is a control variable of 1, 2, 3, . . . , o, with o equal to the total number of groups, equal to j/k. 
     In an exemplary aspect, k successive line rows L are printed, wherein in the k-th line row the k-th print nozzles of each n-th group are activated in order to print lines of the test image. In the example according to  FIGS. 4A-4B , j=96, k=4, o=96/4=24. 
     In practice, for example for a print width of 600 mm and a print resolution of 1200 dpi, j=27000, k=4, o=27000/4=6750. The value k may be varied from 4 to 16. The dots per line  64  may be varied in a range from 6 to 12 and is preferably 10, meaning that each line  64  is printed across ten print lines  78 . If ten dots in four line rows L are printed per line  64 , the test image  60 ,  62  is in total 0.8 to 0.9 mm long in the printing direction T 1 . The test image printed with a print resolution of 1200 dpi is detected by an image detector at a resolution of 600 dpi, and the faulty print nozzles may be determined from the image data of the detected test image. 
       FIGS. 4A-4B  show the test image  60 ,  62 , respectively, printed with the aid of the printing device  10 . In  FIG. 4A , an error-free printed test image  60  is depicted; by contrast,  FIG. 4B  depicts an incorrectly printed test image  62 . The test image  60  is printed by the print nozzles  50  of a single primary color of the printing device  10 ; preferably, four test images  60 ,  62  are printed in succession in a respective single primary color of cyan, magenta, yellow, or black. The pattern is comprised of individual lines  64  printed by a respective print nozzle  50 , which lines  64  are arranged at a predetermined, uniform pitch  67  relative to one another in a plurality of line rows L across the printable width of the recording medium  12 . The pitch  67  of the lines  64  is thereby chosen so that the image detector  44  respectively detects a line  64  or a portion of a line  64  in each image region  56  upon detection of the test image. The pattern of the test image  60  in the exemplary embodiment results from the method described further below in  FIG. 7 . 
     Given error-free printing of the test image  60 , a uniform pattern of the test image  60  is printed as depicted in  FIG. 4A . In this instance, all print nozzles  60  print without error. In the event that a print nozzle  50  is faulty and does not print, or prints incompletely and/or at an angle on the recording medium  12 , the uniform pattern of the test image  60  is interrupted or disturbed and the respective deviation is apparent. These deviations are in particular optically apparent as light regions on the recording medium  12 . In  FIG. 4B , the test image  62  is depicted with a single faulty print nozzle  50  that is not printing; in this instance, a printed line  64  is missing in the pattern of the test image  62 . This may be associated with a defined group  59  and a defined line row  66 . A unique association of image regions  56  to individual raster points, and therefore print nozzles  50 , is possible due to the pitch between the lines  64 . 
       FIG. 5  shows a schematic detail depiction of a portion of the printed test image  60  of four line rows  66  (corresponding to reference character L in  FIG. 4A ) and four groups  59 . In  FIG. 5 , the individual dots  74  (colored grey) and raster points  76  (white) that yield the test image  60  are arranged in the raster matrix so as to be demarcated from one another and apparent. In most use cases, the recording medium  12  is white paper, so that unprinted raster points appear lighter than printed dots. Each line  64  in  FIG. 5  is printed across ten print lines  78  and thus comprises ten dots  74  in the printing direction T 1 . The pitch  67  between two lines  64  of a line row  66  encompasses three raster points  76  transverse to the printing direction T 1  in a print line  78 . Furthermore, the lines  64  of a line row  66  are arranged offset, respectively by a raster point  76 , from the lines  64  of the following line row  66 . 
       FIGS. 6A-6B  show schematic depictions of the test image  60 ,  62  according to  FIGS. 4A-4B , detected with the aid of the image detection device  44 , after it has been homogenized with the aid of the image pattern processing. The outline of the recording medium  12  is depicted in dashed lines in  FIGS. 6A-6B  for better comprehension; the homogenized test image  68 ,  70  is not located on the recording medium  12 , but rather is stored and processed in the form of image data with the aid of the image pattern processing. An error-free test image  68  is depicted in  FIG. 6A ; a test image  70  having an incorrect region  72  is depicted in  FIG. 6B . Given homogenization, a color property of the image regions  56  of the test image  60 ,  62  is smoothed and/or filtered digitally over the regions of the test image  60 ,  62 . This homogenization corresponds to a low-pass filtering of an image signal, wherein high frequencies are filtered out and thus a softening of the test image  60 ,  62  takes place. In particular, a brightness value of the image regions  56  is used as a color property. 
     Given a test image  60  printed without error, an area with nearly uniform brightness as depicted in the homogenized test image  62  in  FIG. 6A  results from the homogenization. Given an incorrectly printed test image  62 , for example due to a non-printing print nozzle  50 , the incorrect region  72  is apparent as a light region  72  in the homogenized test image  70 , as depicted in  FIG. 6B . For example, due to the homogenization it is easily possible for an operator to achieve a good overview of the quality of the test image  60 ,  62  at a glance. 
     The homogenization is determined as a sliding mean value in the exemplary embodiment. The arithmetic mean of the brightness value of the image region  56  is thereby respectively calculated iteratively over a fixed number of image regions  56  lying next to one another in the line direction. 
     The sliding mean value is the series of mean values of the brightness values of a successive image regions  56 . In the practical example, “a” is equal to five. The arithmetic average of the brightness values is thereby respectively determined iteratively for five successive image regions  56  in the line direction. As a result, a new image data set is determined that forms the homogenized test image  70 ,  72 . 
     Alternatively, the homogenization is implemented by means of a weighted sliding mean value algorithm or an exponential sliding mean value algorithm. For this purpose, linear or exponential weightings are associated with the brightness values before the mean value calculation. 
     Starting from the homogenized test image  70 ,  72 , the determination of the faulty print nozzles  50  then takes place in the image pattern processing. 
       FIG. 7  shows a flowchart of the method for determining faulty print nozzles of a primary color of the printing device  10 . The workflow starts in step S 100 . 
     The test image  60 ,  62  is printed in step S 102 . In the exemplary embodiment according to  FIGS. 4A-4B , 96 print nozzles are activated for this so that, at first, the first print nozzles  50  of each group n respectively print the line  64  of the test image  60 ,  62  in a first line row L, then the second print nozzles  50  of each group n respectively print the line  64  of the test image  60 ,  62  in a second line row L, the third print nozzles  50  of each group n subsequently, respectively print the line  64  of the test image  60 ,  62  in a third line row L, and lastly the fourth print nozzles  50  of each group n respectively print the line  64  of the test image  60 ,  62  in a fourth line row L. The resulting test image  60 ,  62  is depicted in  FIGS. 4A-4B . In the exemplary embodiment, the test image  60 ,  62  is printed on the recording medium  12  in the primary color of black. Alternatively, the test image  60 ,  62  is printed multiple times in a respective primary color of the printing device  10 . 
     In step S 104 , the test image  60 ,  62  printed in step S 102  is detected per dot with the aid of the image detector  44 . 
     In the next step S 106 , the image pattern processing determines, with the aid of the detected test image  60 ,  62 , the color channel of the image detector  44  in which the detected test image  60 ,  62  has the highest contrast. In the further course of the method, the image data of the determined color channel are used in order to analyze the detected test image  60 ,  62 . These image data are brightness values of a color channel of the image detector  44 . 
     In step S 108 , the image data of the test image  60 ,  62  are homogenized by the image pattern processing with the aid of a previously described sliding mean value algorithm. 
     In step S 110 , regions  72  in the homogenized test image  68 ,  70  are determined that are lighter than a preset reference value stored in the controller  52 . A reference value is stored in the controller  52  for each primary color of the printing device  10 . This reference value is also referred to as a threshold. Brightness values of a color channel of the image detector  44  typically range from 0 to 255. In most use cases, the recording medium  12  is white paper. The threshold may then be established between 120 and 150, for example. A region  72  that exceeds the threshold indicates an incorrect and/or unprinted line  64 , and thus a faulty print nozzle. 
     In a further embodiment of the method, in step S 110 , a reference value is determined depending on the average brightness of all image regions of the homogenized test image  68 ,  70 . 
     Based on the print data of the test image  68 ,  70 , in step S 112  the associated n-th group  59  in which the region  72  is located is associated with said determined region  72 . 
     In step S 114 , the k-th line row  66  in which the region  72  is located is associated for each region  72  determined in step S 110 . 
     In step S 116 , the associated print nozzle i is subsequently determined for the region  72  determined in step S 110 , based on the n-th group determined in step S 112  and on the k-th line row determined in step S 114 . The faulty print nozzle is thus uniquely identified. If a plurality of conspicuous regions  72  are present in the homogenized image data of the test image, the associated print nozzle i is thus determined for each region  72 . The workflow ends in step S 118 . 
     The described method is characterized by technologically high efficiency and economy. With the aid of the homogenization of the image data, an analysis of the image data prepared in this way may take place with high speed, and incorrect image regions may be detected and the associated faulty print nozzles may be identified. The digital algorithm or the digital filter that is to be used for homogenization are of simple design and operate at high speed. The resolution in dpi for the image detector  44  may be markedly reduced with respect to the print resolution in the line direction, which enables cost-effective camera systems to be used. 
     To enable those skilled in the art to better understand the solution of the present disclosure, the technical solution in the embodiments of the present disclosure is described clearly and completely below in conjunction with the drawings in the embodiments of the present disclosure. Obviously, the embodiments described are only some, not all, of the embodiments of the present disclosure. All other embodiments obtained by those skilled in the art on the basis of the embodiments in the present disclosure without any creative effort should fall within the scope of protection of the present disclosure. 
     It should be noted that the terms “first”, “second”, etc. in the description, claims and abovementioned drawings of the present disclosure are used to distinguish between similar objects, but not necessarily used to describe a specific order or sequence. It should be understood that data used in this way can be interchanged as appropriate so that the embodiments of the present disclosure described here can be implemented in an order other than those shown or described here. In addition, the terms “comprise” and “have” and any variants thereof are intended to cover non-exclusive inclusion. For example, a process, method, system, product or equipment comprising a series of steps or modules or units is not necessarily limited to those steps or modules or units which are clearly listed, but may comprise other steps or modules or units which are not clearly listed or are intrinsic to such processes, methods, products or equipment. 
     References in the specification to “one embodiment,” “an embodiment,” “an exemplary embodiment,” etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. 
     The exemplary embodiments described herein are provided for illustrative purposes, and are not limiting. Other exemplary embodiments are possible, and modifications may be made to the exemplary embodiments. Therefore, the specification is not meant to limit the disclosure. Rather, the scope of the disclosure is defined only in accordance with the following claims and their equivalents. 
     Embodiments may be implemented in hardware (e.g., circuits), firmware, software, or any combination thereof. Embodiments may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computer). For example, a machine-readable medium may include read only memory (ROM); random access memory (RAM); magnetic disk storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact results from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. Further, any of the implementation variations may be carried out by a general-purpose computer. 
     For the purposes of this discussion, the term “processor circuitry” shall be understood to be circuit(s), processor(s), logic, or a combination thereof. A circuit includes an analog circuit, a digital circuit, state machine logic, data processing circuit, other structural electronic hardware, or a combination thereof. A processor includes a microprocessor, a digital signal processor (DSP), central processor (CPU), application-specific instruction set processor (ASIP), graphics and/or image processor, multi-core processor, or other hardware processor. The processor may be “hard-coded” with instructions to perform corresponding function(s) according to aspects described herein. Alternatively, the processor may access an internal and/or external memory to retrieve instructions stored in the memory, which when executed by the processor, perform the corresponding function(s) associated with the processor, and/or one or more functions and/or operations related to the operation of a component having the processor included therein. 
     In one or more of the exemplary embodiments described herein, the memory is any well-known volatile and/or non-volatile memory, including, for example, read-only memory (ROM), random access memory (RAM), flash memory, a magnetic storage media, an optical disc, erasable programmable read only memory (EPROM), and programmable read only memory (PROM). The memory can be non-removable, removable, or a combination of both. 
     REFERENCE LIST 
     
         
           10  printing device 
           12  recording medium 
           16  to  24  print bars 
           26  print head 
           28 ,  30  take-off roller 
           32  to  42  guide roller 
           43  print image 
           44  image detector 
           46  take-up roller 
           47  printing unit 
           48  columns of the print image on recording medium 
           50  print nozzle 
           52  controller 
           54  dot detection region 
           56  image region 
           58  field of view of the dot detection regions 
           59  groups 
           60  test image 
           62  incorrect test image 
           64  line of the test image 
           66 , L line row 
           67  pitch between two lines  64   
           68  homogenized test image 
           70  incorrect homogenized test image 
           72  incorrect region 
           74  dot 
           76  raster point 
           78  print line 
         T 1  printing direction, transport direction