阅读说明：本技术 借助附加色进行补偿 (Compensating by means of additional colours ) 是由 S·内布 N·R·诺瑞克 A·亨 J·福歇 M·迈尔 于 2019-06-20 设计创作，主要内容包括：本发明涉及一种用于通过计算机(6)对喷墨印刷机(7)中故障印刷喷嘴进行补偿的方法,其中,存在附加印刷头(5a)用于补偿,所述附加印刷头(5a)支承在常规印刷的印刷头(5)后并通过以下方式均衡故障印刷喷嘴的错误部位,即,所述附加印刷头(5a)以呈现灰色调的校正色(12)印刷所述错误部位,该方法的特征在于,所述校正色(12)在CIE-Lab颜色空间中的颜色值这样地优化,使得所述校正色(12)距全部所使用的工艺色(14,15,16)的颜色间隔(11)尽可能最小,黄色(13)除外。(The invention relates to a method for compensating defective printing nozzles in an inkjet printer (7) by means of a computer (6), wherein an additional printing head (5a) is provided for the compensation, said additional printing head (5a) being mounted behind a conventionally printed printing head (5) and balancing the error locations of defective printing nozzles in such a way that the additional printing head (5a) prints the error locations with a correction color (12) that assumes a gray hue, characterized in that the color value of the correction color (12) in the CIE-Lab color space is optimized in such a way that the color spacing (11) of the correction color (12) from all process colors (14,15,16) used is as minimal as possible, with the exception of yellow (13).)

1. A method for compensating for faulty printing nozzles in an ink jet printer (7) by means of a computer (6),

wherein an additional printing head (5a) is provided for compensation, which additional printing head is arranged downstream of the conventionally printed printing head (5) and compensates for an incorrect location of a faulty printing nozzle in that the additional printing head prints the incorrect location with a correction color (12) which assumes a grey tone,

it is characterized in that the preparation method is characterized in that,

the color values of the correction color (12) in the CIE-Lab color space are optimized in such a way that the color spacing (11) of the correction color (12) from all process colors (14,15,16) used is as minimal as possible, with the exception of yellow (13).

2. The method of claim 1, wherein the first and second light sources are selected from the group consisting of,

it is characterized in that the preparation method is characterized in that,

the process colors (13,14,15,16) used are weighted while the color values of the correction color (12) in the CIE-Lab color space are optimized by means of the computer (6).

3. The method of claim 2, wherein the first and second light sources are selected from the group consisting of,

it is characterized in that the preparation method is characterized in that,

when weighting the used process colors (13,14,15,16), the additional process colors, in particular the OGVs, are weighted less.

4. The method according to any one of the preceding claims,

it is characterized in that the preparation method is characterized in that,

using a further print head (5b) with a second correction colour (10),

using the additional print head (5a) with the first correction color (17),

wherein the two calibration colors (10,17) are optimized in the CIE-Lab color space such that they each cover a portion of the CIE-Lab color space.

5. The method of claim 4, wherein the first and second light sources are selected from the group consisting of,

it is characterized in that the preparation method is characterized in that,

the color values of the first correction color (17) are spaced in the CIE-Lab color space from the process colors C and Y (13,14) by a minimum distance,

the color values of the second correction color (10) are spaced in the CIE-Lab color space from the process colors M and K (15,16) by the minimum.

6. The method of claim 5, wherein the first and second light sources are selected from the group consisting of,

it is characterized in that the preparation method is characterized in that,

the two correction colors (10,17) are optimized separately by the computer (6) in such a way that they also take into account the minimum spacing from the additional process color OGV in the CIE-Lab color space,

wherein for each correction color (10,17) additional process colors are considered which are located in a corresponding part of the color space of the correction color (10,17) concerned.

7. The method according to any one of the preceding claims,

it is characterized in that the preparation method is characterized in that,

in order to achieve optimum colour values for the correction colours (12,10,17), the computer (6) modifies the droplet size such that the colour values are shifted on the L axis in the CIE-Lab colour space,

wherein smaller drops correspond to brighter color values and larger drops correspond to darker color values.

Technical Field

The invention relates to a method for compensating faulty printing nozzles in an inkjet printer by adding correction colours.

The technical field to which the invention belongs is digital printing.

Background

In general, in digital (or inkjet) printing, the color to be printed is not usually one of the process colors CMYK in four-color printing, or is not one of the additional or spot colors OVG or other possible additional colors in multi-color printing, but is present as a mixture of a plurality of these base colors. Like the primary/process colors (Prozessfarbe), this mixed color is characterized by a fixed location position in the CIE-Lab color space. The microscopic arrangement of the color points of the base colors furthermore requires a suitable rasterization (Rasterung) of the printed material (Sujet), whereby the impression of a macroscopically generated mixed color is achieved.

In ink jet printers, the printing nozzles may fail or shut down due to specific characteristic values that exceed specified tolerance limits. In both cases, errors in the printed image, so-called White-lines (White-lines), i.e. White lines in full tone printing, can result; this is due to the absence of ink at this location. In order to continue production despite these (statistically unavoidable) errors, a compensation strategy is then used.

In the prior art, defective nozzles are currently compensated for by adapting the ink drop size of those nozzles on the sheet which generate adjacent image dots of the same color in the transverse direction in such a way that a nozzle defect in the printed image is no longer recognizable. These inkjet print heads contain two nozzle arrangements in the transverse direction. The image dots on the sheet are all numbered in ascending order and this numbering is transferred to the inkjet print head, with even numbers in the first nozzle column and odd numbers in the second nozzle column. The nozzles of directly adjacent image dots produced on the sheet are always located in different nozzle rows in the inkjet print head. If a horizontal line is to be produced on a sheet, a plurality of nozzles not located in one nozzle row are controlled at different times.

For this purpose, german patent application DE 102014219965 a1 discloses a method in which, in the event of a nozzle failure, a mixed color is introduced into the region concerned, which is designed in such a way that the spacing from the initial mixed color in the CIE-Lab color space is minimal. In this case, such a mixed color is generated by means of the remaining colors. The process in the machine construction and compensation therefore is as follows: first a printed section with seven colors and then a camera monitoring system that verifies the printed image generated by the seven colors. The disadvantages of this prior art are: for such mixed colors, a local regridding in the region of the nozzles to be compensated must always be carried out.

Another known prior art to this problem is given by the US patent document US 9,475,279B 2. This document discloses a method for compensating for faulty printing nozzles in an inkjet printing system during multicolor printing, wherein faulty printing nozzles are detected and compensated for by an additional achromatic printing color in an additional print head. The disadvantage of this method is primarily that this correction color (korrektorfarbe) is applied directly after the original process color, so that waste pages must first occur before possible errors caused by faulty printing nozzles can be detected and thus also compensated for. In other words, the detected errors detected by the camera monitoring system can be compensated for the first following sheet at the earliest.

In connection with this problem, the more recent german patent application DE 10307136 a1 discloses a similar method, in which faulty printing nozzles of a multicolour printing process are likewise compensated by a correction colour which is achromatic. However, in this method, an image sensor in the form of a camera is installed between the print head for the process color and the print head for the correction color of the achromatic color, and if the nozzle of the current process color is not printed as specified, this is immediately detected by the image sensor, so that the correction color of the achromatic color can be immediately applied by means of the additional print head without generating unnecessary waste pages. A disadvantage of this method is that achromatic gray correction colors are only limitedly suitable for correcting faulty process colors. This choice of grey achromatic correction colour is a compromise, since the colour value of this correction colour lies between the colour values of the process colours used, but cannot perfectly match them. In particular in the case of technical colors having particularly bright color values (for example yellow), compensation by means of a correction color of gray is not optimal.

Disclosure of Invention

The object of the present invention is therefore to provide a method for compensating defective printing nozzles in an inkjet printer, which overcomes the disadvantages of the methods disclosed in the prior art and is improved with regard to the quality and efficiency achieved.

The object is achieved by a method for the computerized compensation of defective printing nozzles in inkjet printers, wherein an additional printing head is provided for the compensation, which additional printing head is mounted downstream of the printing head for regular printing and compensates for the error location (Fehlstellen) of the defective printing nozzle in such a way that it prints the error location with a correction color that assumes a gray shade, characterized in that the color values of the correction color in the CIE-Lab color space are optimized in such a way that the color spacing of all process colors used for the correction color distance (except yellow) is as minimal as possible. Since, in particular, process colors having very bright color values (for example yellow) are usually less important in the event of faults (at least do not cause a clearly visible white line), the color value of the correction color is selected such that it moves only between the color values of the other process colors (for example CMK) and the color spacing from these process colors is as minimal as possible. In this connection, the faulty printing nozzle of the print head to which the process color yellow is applied must not be compensated for, so that a better printing result is obtained than would be the case if the faulty printing nozzle yellow were compensated for by means of a correction color of the achromatic color of gray. Another advantage is that in this way the color values of the correction color are significantly closer to the other process colors (for example CMY), and thus faulty printing nozzles of these process colors are compensated significantly better than in the case of using correction colors which have already been designed for yellow with regard to the color values. In the case of other additional process colors (for example OGV), the correction color of this achromatic gray is of course also adapted to the color values of these additional process colors, as long as no additional process colors having very bright color values (like yellow) are involved.

Advantageous and therefore preferred developments of the invention emerge from the dependent claims and the description with the figures.

In this case, a preferred development of the method according to the invention is to weight the process colors used when the color values of the correction colors in the CIE Lab color space are optimized by means of a computer. In this case, the weighting is to be based on empirical values obtained by the user of the inkjet printer concerned with respect to previous print jobs. It is thus conceivable, for example, that the technical colors which are used less frequently (such as, in particular, the additional technical color OGV) are correspondingly less important when selecting the color values of the correction color of this achromatic gray than other technical colors which are used significantly more frequently. The extreme case of weighting is to ignore a certain process color altogether, as is the case, for example, in very bright process colors (especially yellow).

In this case, a further preferred refinement of the method according to the invention provides that the additional process color (in particular the OGV) is weighted less when the process color used is weighted. Since the additional process colors OGV are used in the conventional case significantly less, they can also be weighted less compared to the standard process color CMK. The weighted intensity is related to the use of the ink jet printer involved and the experience of the user.

In this case, a further preferred development of the method according to the invention is to use a further print head having a second correction color, wherein the two correction colors are then optimized in the CIE-Lab color space in such a way that they each cover a part of the CIE-Lab color space. Another possibility for improving the compensation by means of a correction color consists in using two different correction colors. This is of course accompanied by higher costs due to the use of the second additional print head, so this option should only be used if the correction color of that achromatic gray cannot achieve sufficient compensation quality. In the case of the use of two correction colors, it is of course possible to adapt the correction colors used significantly better in accordance with the process colors to be compensated, since each correction color has to realize a specific part of the CIE-Lab color space covered by these process colors, respectively.

In this case, a further preferred development of the method according to the invention provides that the color values of one of the two correction colors are spaced the smallest from the process colors C and Y in the CIE-Lab color space, and the color values of the other correction color are spaced the smallest from the process colors M and K in the CIE-Lab color space. These two correction colors can be divided into two respective portions of the CIE-Lab color space encompassed by these process colors, with the possibility that: one part should appear as the CIE-Lab color space encompassed by the process colors cyan and yellow, and the other part should appear as the CIE-Lab color space encompassed by the process colors magenta and K (i.e., black). In this case, the color value of the process color yellow is not completely ignored. However, it is also possible and sensible here to bring the color values of such correction colors which should cover cyan and yellow closer to the color values for cyan than to the color values for yellow, since the problem of yellow nozzle failure is less important even in this variant. For the color values of the further correction color, which should cover the color space of magenta and K, the weighting is weaker than in the case of the further process colors for cyan and yellow, although it is also equally feasible and entirely meaningful.

In this case, a further preferred refinement of the method according to the invention provides that the two correction colors are each optimized by the computer in such a way that they also additionally take into account the minimum spacing from the additional process color OGV in the CIE Lab color space, wherein for each correction color the additional process color is respectively taken into account in the respective part of the color space of the correction color concerned. If these additional process colors (e.g. OGV) are used, the two correction colors must of course also be taken into account together accordingly. It is preferred here that for each of the two correction colors, additional process colors located in the respective part of their color space are taken into account. If an additional process color is at the boundary between the two partial color spaces for the two correction colors, it is decided, in particular on the basis of the resulting color spacing, which of the two correction colors together covers the additional process color at the boundary. Here, the possible weighting factors should also be taken into account for the individual process colors and the additional process colors. It is of course also possible in such critical cases for both correction colors to take into account the respective additional process color together.

In order to achieve a corrected color-optimum color value, the computer modifies the drop size in such a way that the color value is shifted on the L axis in the CIE Lab color space, smaller drops corresponding to brighter color values and larger drops corresponding to darker color values. This approach has the advantage that it can be used in addition to other approaches regarding the implementation of correcting color-optimum color values. Thus, for example, the color value of the correction color can first be selected on the basis of the minimum color spacing from the process color used and then shifted accordingly on the L axis in the CIE Lab color space by manipulating the droplet size. This increases the flexibility of the method according to the invention.

Drawings

Such an invention and structurally and/or functionally advantageous refinements of the invention are further described below on the basis of at least one preferred embodiment with reference to the drawings. In the drawings, mutually corresponding elements are denoted by the same reference numerals, respectively.

The figures show:

FIG. 1: examples of the structure of a sheet inkjet printer;

FIG. 2: a schematic example of a "white line" caused by a "missing nozzle";

FIG. 3: an example of a correction color selected based on a minimum color spacing from the CMK;

FIG. 4: examples of two correction colors selected in terms of minimum color spacing from CY and MK.

Detailed Description

The field of application of the preferred embodiment variant is an ink jet printer 7. An example of the basic structure of a machine 7 of this type is shown in fig. 1, which comprises a feeder 1 and even a receiver 3 for supplying a substrate 2 into a printing mechanism 4 where the substrate 2 is printed by a print head 5. Here, a sheet-fed ink-jet printer 7 controlled by a control computer 6 is concerned. As already described, during operation of the printing press 7, it is possible for individual printing nozzles in the printing head 5 of the printing unit 4 to fail. The consequence is then a white line 9, or in the case of multicolor printing, a distorted color value. Fig. 2 shows an example of such a white line 9 in a printed image 8.

The invention describes in its preferred embodiment a method for compensating the white line 9, which method can also be used in addition to other methods. The important differences are: the white line 9 produced is not only compensated within the process colors 13,14,15,16, but also by means of the additional/correction color 12. The additional color 12 is designed in such a way that it is optimized in the CIE-Lab color space, which usually yields grey color values.

The CIE Lab value of the additional color 12 is selected by the computer 6 either automatically or at the direction of the user in such a way that the distance to all process colors 13,14,15,16 is minimal. However, yellow (Y)13 is ignored in this preferred embodiment because it is generally less likely to generate white lines 9 when the printing nozzle fails due to its bright color value, and further, the compensation using the gray correction color 12 is more disadvantageous.

In this case, such error points are replaced in the grid (process) one-to-one, so that no regrinding is necessary. The additional color 12 is applied by an additional print head 5a arranged after the camera monitoring system in the paper travelling direction, in such a way that compensation can be achieved on the first sheet 2. So that there is no waste page. Fig. 3 shows an example of the color values of the grey correction color 12, which are positioned at a minimum spacing 11 from the three process colors CMK14,15,16 described above. Here, yellow 13 is ignored. The optimization with respect to the minimum spacing 11 is carried out by means of the so-called Least-Mean-Square (Least-Mean-Square) method.

Another embodiment introduces weighting of the spacing 11 from the process colors 13,14,15, 16. The smallest weighting is, for example, the exclusion of yellow 13 mentioned. However, Y13 may also be weighted only more weakly than CMK14,15, 16. The other process colors 14,15,16 may also be weighted accordingly. This weighting is dependent on the historical use of the inkjet printer 7 (i.e. the print job so far). In general, the process colors 13,14,15,16 that are used more often (or more importantly) are also weighted more strongly. The corresponding experience of the user can also be taken into account here.

In another embodiment, the optimization of the correction color is also based on the additional process color (e.g., OVG). Furthermore, these additional process colors are weighted less than the standard process colors CMYK13,14,15, 16.

Another embodiment variant involves the use of two correction colors 10, 17. The color optimization can thereby be improved in such a way that each encompasses a part (for example half) of the Lab color space. This also extends generally to N additional colors in the case of M colors. The limit is of course a redundancy of all process colors 13,14,15,16, which is not desirable. For both correction colors 10,17, it is of course also necessary to preset the two printing heads 5a,5b after the monitoring camera, so that the above-described embodiment is only of significance if a very precise compensation with the aid of the correction colors 10,17 very close to the color values of the process colors is required. If additional process colors (such as OGV) are used, the two correction colors 10,17 must of course also be taken into account accordingly. It is preferred here that for each of the two correction colors 10,17 mentioned above, additional process colors located in their respective parts of the color space are taken into account. If an additional process color is at the boundary between the two partial color spaces for the two correction colors 10,17, then it is decided by the computer 6 on the basis of the resulting color interval: the additional process colors at the limit are covered together by which of the two correction colors 10,17 described above. Fig. 4 shows an example of two correction colors 10,17 in the Lab color space, wherein the first correction color 17 covers the left half space for CY and the second correction color 10 implements the right half space for MK.

Another additional embodiment relates to manipulating the ink drop size. This can be used in addition to the embodiment variants described hitherto. The change in the droplet size thus allows the color values of the correction colors 12,10,17 to be shifted in the Lab color space on the L axis after a failure of a specific process color 13,14,15, 16. Here, smaller drops mean brighter color values, and larger drops correspondingly mean darker color values.

List of reference numerals

1 feeder

2 Current substrate/Current sheet

3 material collector

4 ink-jet printing mechanism

5 ink jet print head

5a additional ink jet print head

5b Another additional ink jet print head

6 computer

7 ink jet printer

8 printing image on current printing sheet

9 white line

Second correction color with minimum 10-MK spacing

11 minimum spacing of correction color Grey from Process color CMK

12 corrected color Gray

13 Art color Y (yellow)

14 technical color C (cyan)

15 technical color K (Key color/black)

16 technical color M (magenta)

17 first correction color with minimum spacing from CY