DE102013107942A1 - Method of compensating for streaks in a raster printed printed image on a digital printer - Google Patents

Abstract

In the process, the longitudinal streaks are compensated in a print image printed in raster print. The digital printer has a number of printing elements, wherein each printing element prints printing dots in a printing dot row onto a recording medium in the printing direction. The print elements print a test print image having a predetermined target tone value. The test print image printed on the record carrier (3) is scanned by a scanner and the actual value of the tone value is determined for each print dot row from the scanned test print image. This actual value is compared with the setpoint. If the actual value of the tonal value deviates from its nominal value, the printing data per printing element are corrected as a function of the difference via correction values (12) arranged in a correction table such that for each printing element the printing dots printed by the printing element in the printing dot row match the nominal tonal value for this printing dot row result.

Description

For single or multi-color printing of a recording medium, e.g. a tape-shaped recording medium made of various materials, e.g. Paper, digital printers can be used. A digital printer has a printer controller in which the print image data is stored, and a printing unit with which the print images are printed on the recording medium in accordance with the print image data. Digital printers can be realized as ink printers or electrographic printers.

The construction of ink printers is known, see, for example EP 0 788 882 B1 , Ink printers which operate, for example, according to the drop-on-demand (DoD) principle, have as printing unit a printhead or a plurality of printheads with nozzles comprising ink ducts whose actuators under the control of a printer control excite drops of ink in the direction of the recording medium, which are directed onto the recording medium. to apply pressure points for a printed image there. The actuators can generate ink droplets thermally (bubble jet) or piezoelectrically.

An electrographic printer is off, for example US 5 767 888 A known. Here, with a character generator having an LED comb, charge images of the print images are formed on a photoconductor, which are toned with toner into toner images. The toner images are transferred to the record carrier and fixed there.

According to the offset printing, the print images can be displayed in the print image grid according to known screen printing methods in digital printing. The print image grid corresponds in the printer control a grid of PELs (printed elements), wherein a PEL is the position at which a pressure point on the recording medium can be generated by the printing unit. A pressure point may be greater than the assigned PEL. The gray level or tonal value of such a screened print image is defined by the ratio of the PELs having printing dots to the PELs having no printing dots. The printing dots on the record carrier are thereby generated by printing elements, e.g. a printing element can each be assigned to a number of PELS of the grid. Printing elements are e.g. LEDs on a photographic printing unit or nozzles on an ink printing unit.

In an electrographic printing device, the print dots of a print image raster can be generated via the exposure of the associated PELs on the photoconductor, which can then be developed by toner. This procedure is detailed in US 5 767 888 A it is referred to. In this so-called raster method, the color gradation (halftone gradation) of the print images from paper color to the solid color of a base color can be achieved by adding further printing dots to a printing dot of the same thickness ink. In this case, the dot size of the printing dots can be modulated by the exposure energy of the photoconductor exposure by the character generator. The modulation of the exposure energy at a pressure point is thus used to initially set the size of a pressure point. If a print dot has already been exposed with the highest possible exposure energy and further inking in the print image grid is required, then one PEL or several adjacent PELs can be used for raster formation and their exposure can be changed step by step (cf. US 5 767 888 A ).

Raster printing can also be used with inkjet printers. Here, a row of PELs can each be assigned a nozzle of a print head of an ink printing unit, via which ink droplets are fired onto the recording medium in order to generate the respective pressure point of the associated PEL. As a result, a printed image composed of drops of ink or printed dots is formed on the recording medium, the gray value or tone value of which is determined by the number of printing dots per printed image grid. In this case, a number of pressure points in the printing direction is generated by a respective nozzle of the print head. The printing direction is the direction in which the respective nozzle of the print head successively fires drops of ink onto the recording medium. Such a printing operation is eg off WO 2013/010803 A1 known.

For printing devices that work in halftone printing, the printing dots are thus printed by means of printing elements, e.g. LEDs in photographic printing units or nozzles in ink printing units generated. If the printing elements generate printing dots of different thicknesses on the recording medium, there is a risk that these different functioning of the printing elements can be recognized as stripes in the printed image. These stripes can interfere with the printed image.

Measures to eliminate this streaks in printed images are known. In WO 2013/010803 A1 describes a method that can be eliminated in an inkjet printer, the streakiness in the printed image, which results from the fact that nozzles fail the printhead. It is proposed as a solution to replace the failed nozzles with other functional nozzles, which are arranged adjacent to the failed nozzles. Replacement of failed nozzles functioning nozzles are designed so that the streaks in the printed image is no longer recognizable.

• a) Mit der Druckeinheit wird mindestens ein Druckbild eines vorgegebenen Tonwertes als Testbild (Tonwert-Sollwert) auf den Aufzeichnungsträger gedruckt. Das Testbild wird durch einen Scanner abgetastet und das gescannte Testbild wird digitalisiert.
• b) Aus dem digitalisierten Testbild kann der Intensitätsverlauf (Tonwertverlauf) der mit Hilfe der LEDs erzeugten Druckpunkte festgestellt werden.
• c) Der Druckkopf (LED-Kamm) wird vertikal verschoben und die Schritte a) und b) wiederholt.
• d) Die Differenz der Intensitätsverläufe der Testbilder wird gebildet, um die LEDs zu ermitteln, auf die die mangelhaften Druckpunkte zurückgehen.
• e) Durch Vergleich des Intensitätswerts des durch den mangelhaften LED erzeugten Druckpunkts mit dessen Sollwert wird die Abweichung vom Sollwert ermittelt. Mit Hilfe der Abweichung wird ein Korrekturwert berechnet, mit dem das Ansteuersignal für das LED so geändert wird, dass der erzeugte Druckpunkt seinen Sollwert annimmt. Dazu kann die Belichtungszeit oder die Belichtungsstärke des LEDs verändert werden. Die Korrekturwerte für die LEDs des Druckkopfes können in einem Korrekturspeicher in der Druckersteuerung gespeichert werden.

Out DE 10 2005 009 803 B3 For a photographic printer, a solution to the problem of streaks is described. The printing dots are here created using LEDs which generate dots on a charge image carrier that are developed by toner and then transferred to a recording medium to form the printing dot. In order to prevent the streaks in the printed image due to the LEDs, the following steps are proposed:

• a) With the printing unit at least one printed image of a predetermined tone value is printed as a test image (Tonwert setpoint) on the recording medium. The test image is scanned by a scanner and the scanned test image is digitized.
• b) From the digitized test image, the intensity profile (Tonwertverlauf) of the pressure points generated by the LEDs can be determined.
• c) The print head (LED comb) is moved vertically and steps a) and b) repeated.
• d) The difference of the intensity curves of the test images is formed to determine the LEDs, on which the defective pressure points go back.
• e) By comparing the intensity value of the pressure point generated by the defective LED with its setpoint, the deviation from the setpoint is determined. With the help of the deviation, a correction value is calculated with which the drive signal for the LED is changed such that the pressure point generated assumes its setpoint value. For this, the exposure time or the exposure intensity of the LED can be changed. The correction values for the LEDs of the printhead can be stored in a correction memory in the printer control.

The problem to be solved by the invention is to provide a method for eliminating streaks in a digital printer. The method should be applicable to both inkjet printing devices and photographic printing devices.

This problem is solved by a digital printer according to the features of claim 1.

The method is used in particular to compensate for systematic printhead-specific streaks. First of all, the systematic nonuniformities of the printing properties of the printing unit are determined using one or more print heads. This is done by scanning test images printed by the printing unit at different operating parameters and tonal values. The determined intensity profiles of the tonal values of the test images are examined in order to separate the systematic design-related reproduction properties of the print heads from the properties of the individual print heads used for the analysis.

The information obtained in the analysis is used to modify the print data such that the disturbing effect of the streakiness is at least reduced. If e.g. determined in the analysis that certain nozzles of an ink printing unit achieve a lower coloration than other nozzles, the compensation for these nozzles, a higher tone value is driven in the pressure point row. For this purpose, more ink per printed dot row is printed with these nozzles and compensated in the sequence a previously visible as a stripe lightening.

This process is performed for a plurality of representative tonal values so that compensation can be performed for each level of the tonal range.

Further developments of the invention will become apparent from the dependent claims.

To achieve this goal, the print data can be modified before screening (halftoning) via a look-up table (correction table), the halftone screening then remains unchanged. In this case, an exact assignment of the positions of the lookup table to the printing elements (nozzles) must be ensured.

In a second embodiment, the correction values in the halftone screening can be taken into account. The preparation of the print data up to the halftone screening then remains unchanged.

Advantages of the invention are:
An improvement in print quality is achieved. Systematic longitudinal streaking caused by the construction of the printheads is prevented. It is achieved a simplification of the effort to operate the printer.

An embodiment of the invention will be explained in more detail below with reference to the schematic drawings. Show it:

1 a schematic structure of a printing unit of an ink printer,

2 a printed image printed on a record carrier,

3 Intensity curves for one tonal value for two structurally identical printheads,

4 a representation of the function of the Tonstufenrasterung,

5 a block diagram for illustrating the sequence of screening a printed image with correction of the tonal values,

6 a block diagram illustrating the sequence of the screening of a printed image with correction of the tone values per nozzle in an ink printing unit.

The invention will be further explained with reference to a digital ink printer. The structure of an ink printer can eg WO 2013/010803 A1 be removed.

A schematic example of a part of a printing unit of an ink printer is 1 refer to. The printing unit has a pressure latch 1 with eg five print heads 2 on, with which a record carrier 3 to be printed in the direction y, in the exemplary embodiment, for example, in the transport direction PF of the recording medium 3 , On the record carrier 3 is an example of a printed image 4 shown. The printheads 2 are structurally the same. Every printhead 2 is constructed in a known manner and has, for example, 2558 nozzles with which ink drops to the recording medium 3 can be fired to create pressure points there. Each nozzle is associated with a position x perpendicular to the printing direction y, so that with a nozzle of a printhead 2 a number of pressure points of a print grid in the printing direction y can be printed. In the explanation, the transporting direction of the recording medium will be described 3 with PF, the printing direction, ie the direction in which the respective nozzle generates pressure points, denoted by y.

If the of the nozzles of the printheads 2 For example, given ink drops have different size, this can be in the printed image 4 lead to a streakiness. This is particularly visible in printed images in which larger areas are to be reproduced with nominally the same color.

In order to measure the striations metrologically, test print images can be printed from one or more lines of printing (letters "A" in 1 ) are printed by the printing device, which should lead to a homogeneous coloring of the printed image. Examples are solid areas with maximum coloring or halftone areas, which have, for example, 50% coloring (tone value). Such a printed area is in 2 shown. The printed area 5 is composed of distributed pressure points depending on their tone value, which are arranged according to a grid rule in a print grid. One nozzle each of a printhead 2 can generate a series of pressure points in the y-direction, the pressure points in the x-direction can by adjacent nozzles of the printhead 2 be generated. As discussed above, a print grid consists of PELs (printed elements) at the positions of which the physical pressure points are set, which may be greater than the associated PEL. By the number of pressure points in the print grid, the tonal value of the printed image 5 be determined.

The test print image 5 of the 2 can be scanned in a known manner with a scanner and the scanned values can be digitized to determine the intensity values (tonal values) of the test print image. The intensity values in the x-direction represent the intensity profile transversely to the printing direction y, and the intensity values in the y-direction represent the intensity profile in the printing direction. Strips in the x-direction and y-direction can then be calculated from a print image prepared in this way.

In the following, only the streakiness in the printing direction y is treated, in the following called longitudinal streaking. To calculate the streakiness in one direction, it is common to average the values of the other dimension. Longitudinal streaking generally results from uneven application of colorant over the print width. Longitudinal streaks in the printed image can have many causes. In ink printing, for example, unusual or improperly spraying nozzles are typical causes of longitudinal streaks. However, the resulting longitudinal streaks in obliquely spraying nozzles, for example, by flushing the printhead 2 be eliminated. The longitudinal streaks due to failing nozzles can be correspondingly WO 2013/010803 A1 be compensated. In the following, the longitudinal streaking is considered, the production-related in a printhead 2 is present, for example, where the nozzles do not behave the same. This may, for example, result in slightly different droplet sizes, which then lead to differences in the amount of ink applied in each dot line produced by the respective nozzle; this can lead to a different coloration in the individual rows of pressure points as a result and be visible as longitudinal streaks. In the following discussion, the compensation of this type of printhead-related longitudinal streaking is referred to as systematic long streaking and its elimination is discussed below. Initially, only the longitudinal streaking in a printhead is treated.

• 1. Erfassung der gesamten Längsstreifigkeit;
• 2. Extraktion der systematischen Längsstreifigkeit;
• 3. Kompensation der systematischen Anteile zur Längsstreifigkeit mittels Korrektur der aufgebrachten Farbmittelmenge durch Modifikation der Ansteuerung der Düsen des Druckkopfes 2.

To compensate for systematic longitudinal streaking, the following process steps are carried out in succession:

• 1. detection of total longitudinal streaking;
• 2. extraction of systematic longitudinal streaks;
• 3. Compensation of the systematic components for longitudinal streaking by means of correction of the applied colorant quantity by modifying the control of the nozzles of the printhead 2 ,

To 1) Detection of total longitudinal streaking

The detection of the longitudinal streaking takes place as explained above by printing out a test print image with subsequent scanning and digitizing of the scanned test print image. The result is the intensity curve (tone value curve) over the print width of the print head 2 , ie in the x-direction ( 2 ).

Subsequently, the assignment of the intensities of the tone value to the nozzles of the print head 2 made to determine which nozzles to a lesser or stronger coloring of the recording medium 3 contribute and thus cause the longitudinal streaks.

• – Ist ein Druckkopf 2 Teil eines Druckriegel 1 aus mehreren Druckköpfen 2, werden beim Ausdruck des Test-Druckbildes einzelne Druckköpfe 2 gezielt abgeschaltet. Im Intensitätsverlauf des Tonwertes sind die abgeschalteten Druckköpfe 2 erkennbar.
• – Die Zuordnung von Intensitätsverlauf zu den Düsen eines Druckkopfes 2 lässt sich dadurch herstellen, dass bei der Erzeugung der Testbilder gezielt Düsen, z.B. zwei Düsen, nicht angesteuert werden. Im Intensitätsverlauf sind die abgeschalteten Düsen als Abweichungen in der Helligkeit erkennbar.

To determine the association between the scanned test print image and the nozzles of a printhead 2 For example, the following methods can be used:

• - Is a printhead 2 Part of a pressure bolt 1 from several printheads 2 , printouts appear when printing the test print image 2 specifically switched off. In the intensity curve of the tonal value are the switched-off printheads 2 recognizable.
• - The assignment of intensity curve to the nozzles of a printhead 2 This can be achieved by selectively controlling nozzles, eg two nozzles, during the generation of the test images. In the intensity curve, the disconnected nozzles are recognizable as deviations in the brightness.

To 2) Extraction of systematic longitudinal streaking

The ascertained intensity profiles reproduce the entire streakiness existing at the time of printing the test print image. This streakiness also includes random streaks caused, for example, by obliquely spraying nozzles. With the compensation of the streakiness based on this data, only a short-term improvement would be achieved. For example, after cleaning the respective printhead 2 , in which, for example, the oblique spraying is eliminated, but then a deterioration of the printed image would occur. In addition, the compensation would only the examined printhead 2 a pressure bar 1 affect. For this reason, a variety of intensity gradients are examined to the systematic proportions of the printhead 2 to filter out at the longitudinal streakiness. These test steps are carried out at different operating parameters, for example, different droplet size (if this is adjustable), Tonwertabstufungen, temperature, humidity, etc., since the intensity curves depending on the operating parameters can be different in shape and intensity. By comparing the intensity curves, the systematic components in the longitudinal streak can then be determined and the nozzles causing these proportions can be determined. For example, if structurally identical printheads 2 be examined, it can be seen that their intensity profiles are largely identical. Differences in the intensity curves with different parameters in the printheads 2 therefore do not stir from systematic printhead contingent proportions.

By comparing the intensity curves, the systematic printhead - related portions can be filtered out and assigned to the causing nozzles. 3 shows the intensity curves K1, K2 for two structurally identically constructed print heads 2 , The two curves K1, K2 show a great qualitative agreement in the course. If there are differences here, they are due to causes that are not due to the design of the printheads 2 are conditional.

If the intensity curve, the actual value of the tonal value (coloring) of the test print image on the recording medium 3 is compared with the target value of the tone value of the test printed image, the systematic streaking causing nozzles can be determined and the extent of deviation of the actual tone value from the target tone per nozzle can be determined. The aim of the invention is to compensate for these deviations from the nominal value per nozzle.

According to the 4 , which shows in principle in a block diagram the sequence of the rasterization of a printed image, is intended with a print head 2 a printed image are generated which has a tone value of, for example, 50%. This print image (block 6 ) is eg stored in the printer control of the ink printer with its tone value. In digital printing, first a screening of the printed image (block 7 , Halftone screening) to a screened printed image (block 8th ) supplied to the printing unit and to the print head 2 on the record carrier 3 is printed. A print image is generated in which the pressure points are distributed according to the tone value. Thus, if a print image should have a tone value of 50%, pressure points should be generated on 50% of the print area. The distribution of pressure points on the printing surface (screen area) is done according to known algorithms. If a target Tone of a print image with a printhead 2 For example, because of longitudinal streaking is not achieved, in the print data for the printhead 2 contained tonal values are changed so that the printing elements of the printhead 2 the setpoint of the print image on the record carrier 3 produce. For this purpose correction tables are used, for example, in the printer control, which assign a corrected tonal value to an input tone value in such a way that the print elements print a print image with the desired tone value. This happens eg at DE 10 2005 0009 803 B3 by a corrected control of the LEDs of the printhead, wherein either the exposure intensity or the exposure time can be changed.

Re 3) Compensation of the systematic components for longitudinal streaking

For printheads 2 where sufficient tonal drive to eliminate longitudinal streaking is not possible, the invention provides a way to still minimize longitudinal streaking. Here the tonal values arriving in the print data are also changed before the halftoning with a correction table. This means that every possible input tone value is assigned a corrected tone value in which, after screening, the number of print dots in the print pattern are changed such that the tone value of the print image corresponds to the desired value of the tone value. For example, if the input tone value to be reproduced is 50%, a print image having an actual tone value of 48% on the recording medium 3 is printed, the print image on the recording medium 3 brighter. For compensation, the input tone value can be corrected via the correction table so that, for example, 52% of the possible print positions of the screened print image are activated in order to render the print image darker compared to the print image printed without compensation.

An example of this procedure may follow a block diagram 5 be removed. Here the tonal values (block 9 ) of the print image to be printed (target tonal values of the print image) of a correction table (block 10 ) which changes the target tone values into corrected tone values. With the corrected tonal values, the halftone screening (block 7 ), then the rasterized tonal values (block 11 ) for controlling the printing elements of the print head 2 emits.

This method is used according to the invention to eliminate the longitudinal streaks in the printed images. Here is this procedure for each printing element of the printhead 2 , For example, every nozzle of the ink jet print head, used. Each print element of the printhead 2 will be a correction table 12 assigned according to the step 2 ) has detected correction values for this pressure element. Thus, for each printing element, an individual correction of the tonal values to be reproduced and thus a compensation of the coloring of printing dot rows are possible, for which deviations of the actual values from the nominal values in the tonal value in the printed image have been determined in the analysis. Thus, even on these printing elements going back longitudinal strips are compensated.

6 shows a block diagram illustrating the process of the method. Relative to an inkjet printing device, each nozzle is a printhead 2 a correction table 12 in which the correction values for the target nozzle values for the assigned nozzle determined in the analysis are stored (block 12 ). When printing, the tonal values of a printed image to be scanned become the correction tables 12 supplied per nozzle, the tone values per nozzle are corrected and the corrected tone values after the halftone screening (block 13 ) used for printing. After halftoning, the tonal values per nozzle are thus output, which are used to control the nozzles of the inkjet print head 2 be used. This will be through the nozzles of the inkjet print head 2 caused unwanted influences on the printed image prevented.

As with an ink printer with a pressure bar 1 usually several printheads 2 structurally the same design must be used, the effort for the structure of the correction tables 12 be provided only once; These can then be used for all printheads 2 same type can be used.

The invention has been described for an ink printer. However, the described method can also be used in electrographic printers, e.g. if the change in the control of the LEDs is not sufficient to completely avoid unwanted longitudinal strips.

In the exemplary embodiment, the correction tables per printing element are arranged before the halftone screening (screening) of the printed image. However, it is also possible to include the correction tables in the rule for halftoning, so that due to the longitudinal strip compensation no performance reduction occurs in the printing operation.

LIST OF REFERENCE NUMBERS

K

intensity curve

PF

Transport direction of the recording medium

Y

 Direction in which the printing elements generate printing dots = printing direction

X

Direction in which the printing elements are arranged in the print head

1

pressure lock

2

printhead

3

record carrier

4

print image

5

homogeneously colored print image

6

Block representation: controlled tonal value of the printed image

7

Block diagram: Halftone screening

8th

Block representation: screened printed image

9

Block representation: controlled tonal value of the printed image

10

correction table

11

Block diagram: Halftone screening

12

correction table

13

Block diagram: Halftone screening

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• EP 0788882 B1 [0002]
• US 5767888 A [0003, 0005, 0005]
• WO 2013/010803 A1 [0006, 0008, 0026, 0031]
• DE 102005009803 B3 [0009]
• DE 1020050009803 B3 [0039]

Claims (9)

A method for compensating for streaks in a raster printed printed image in a digital printer, the digital printer comprising a printing unit having a series of adjacent printing elements, each printing element printing dots in a printing dot row on a recording medium in printing direction in response to printing data supplied to the printing unit. 3 ), in which the printing unit to test print images are supplied having different target tone values, wherein for each printing element in response to the desired tone values, the distribution of printing dots for printing elements to be printed by the printing element are varied, in which the on the recording medium ( 3 ) are detected by a scanner in which the actual values for the tone values are determined from the scanned test print images per dot line and these actual values are compared with the nominal values of the tonal values of the test print images in which the actual values deviate Tone values from the respective nominal values of the tonal values, the pressure data per pressure element via correction values arranged in a correction table. 12 ) are corrected such that, for each printing element, the printing dots printed by the printing element in the printing dot series yield the respective desired tone values for this printing dot series. Method according to Claim 1, in which the correction of the tonal values of the printed image supplied to the printing unit is carried out for each printing element of the printing unit before the halftone screening of the printed image. Method according to Claim 1, in which the correction values of the tonal values of the printed image in the halftone screening are taken into account in the raster algorithm. Method according to one of Claims 1 to 3, in which each printing element of the printing unit has a printing element-specific correction table ( 12 ). Method according to one of claims 1 to 4, in which the progression of the tone value intensity profile transverse to the printing direction is determined from the scanned test printed image and the intensity value per printing element is determined therefrom, in which the ascertained intensity value is compared with the pressure data in order to calculate deviations of the ascertained intensity value from the desired value, in which correction values per printing element are determined in accordance with the deviations with which the printing data associated with the printing elements per printing element are corrected. Method according to claim 5, wherein a plurality of test print images are printed at different operating parameters of the digital printer, in which the changes in the tonal value of the print image caused by the construction of the print heads are determined from the scanned print images and assigned to the causing print elements. Method according to Claim 6, in which the digital printer has a plurality of identically constructed print heads ( 2 ), in which from the printheads ( 2 ) Test print images on the record carrier ( 3 ) are printed and these printed images are scanned to determine their intensity curves (K1, K2), are filtered out of the intensity curves (K1, K2) of the printed images, the components that occur in all printheads, in the correction tables only these components for Determining the correction values can be used. Method according to one of the preceding claims, wherein an ink printer is used as the digital printer, which has a pressure bar ( 1 ) with several print heads ( 2 ) as a printing unit, each print head ( 2 ) provides a series of nozzles as printing elements. Method according to one of Claims 1 to 7, in which a digital printer is an electrographic printer which has LEDs arranged in a row as printing elements.

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