DE102009060708B4 - Method for calibrating a printer - Google Patents

Abstract

A printer can basically display different gray values of a color. Different gray levels (64, 66, 68, 70) are assigned different coverage densities of grid cells (80). The occupancy densities are representative of which output pixels within a raster cell (80) are printed with colorant. The number of printing dots thus generated per raster cell (80) conveys a certain color impression, which is referred to as a gray value. To ensure that the same gray value is always achieved when using a gray level (64, 66, 68, 70), the printer is regularly calibrated during productive printing. For this purpose, one of the gray levels (64, 66, 68, 70) can be assigned different occupancy densities, so that the actual gray value, which is obtained by using the corresponding gray level (64, 66, 68, 70), can be varied until it corresponds to a predetermined target gray value (SP).

Description

The invention relates to a method for calibrating a printer. In this case, a colorant is applied to a recording medium according to a plurality of gray levels with the aid of the printer. Each gray level is assigned an occupancy density of a screen cell of the printer. The coverage density is representative of how many pressure points per screen cell are applied to the record carrier.

A printer is suitable for printing digital documents, for example texts or images, on a recording medium. As a recording medium, for example, substrates such as fabric panels, paper webs, individual pieces or single sheets of paper or single sheets of paper are used. The printer may be formed as a standalone device and may be printed in accordance with a digitally stored template or integrated with a copier that regularly includes a scanner for digitally capturing a document in addition to the printer.

When printing with the help of the printer, it is often important that regardless of the printer used, the final product on the record carrier gives the same color impression as the original. The color impression can be achieved for example by different intensities of the primary colors used in color printing. The intensities of the primary colors are also called tonal values or gray values. The different gray values can be mediated by printing on the recording medium with different numbers of printing dots, consisting of output pixels, whose size and shape are determined by the printer. Depending on the gray level, different numbers of print dots are applied per grid cell. The gray value detected by the eye or a sensor and thus the gray value mediated by the pressure points is consequently determined by a coverage density of the screen cell.

Since print results of different printers, identically constructed printers and even one and the same printer can differ from one another at different times with respect to the color impression, it is known to calibrate the printers at regular intervals so that at least approximately the same regardless of the printer used or the time same color impression is mediated. In known calibration methods, a productive printing operation is interrupted for calibrating. This means that a print job to be processed is interrupted and that only for calibration a calibration print job is started in which colorant is printed on the record carrier in accordance with given gray levels or gray scale gradients, whereby the calibrated printed record carrier is no longer useful after the calibration. Due to the interruption of the print job to be processed results in a loss of use of the printer, which leads to high costs.

Furthermore, it is known alternatively or in addition to the calibration to initiate a control method that can be executed during the productive printing operation, but in which, after passing through several control loops, different gray levels achieve the same gray levels and thus individual gray values can no longer be differentiated.

From the US 2001/0004284 A1 a method for calibrating a printer is known in which colorant is applied in different levels of gray on a recording medium. On the basis of the applied colorant gray values are determined, which conveys the colorant at the different shades of gray. The determined gray values are compared with reference gray values. Depending on the comparison, voltage values for heating printheads to apply the colorant are corrected.

From the JP 2009 302 962 A For example, a method for calibrating a printer is known in which colorant is applied to a recording medium according to different gray levels. Subsequently, the spectral characteristic of the colorant is recorded on the recording medium and compared with a standard characteristic. Depending on the comparison, correction colors are determined.

It is an object of the present invention to provide a method for calibrating a printer, which contributes in a simple manner to the fact that the color impression of a template is reproduced the same or at least approximately equal regardless of the printer and regardless of the printing time.

The object is solved by the features of the independent claims. Advantageous embodiments are specified in the subclaims.

The invention is characterized in that during a productive printing operation of the printer by means of pressure points on the recording medium mediated actual gray values are detected. Subsequently, it is checked whether the detected actual gray values correspond to the desired gray values assigned to the corresponding gray levels or at least approximately correspond. If the actual gray values for one or more of the gray levels do not correspond to the corresponding desired gray values, or at least not approximately the corresponding gray level is assigned a different population density.

Since the productive printing operation is not interrupted, the calibration can be performed more frequently and / or at shorter intervals or even continuously, as opposed to a calibration for which the productive printing operation and thus a processing of a print job must be interrupted. This contributes to the color impression of the original and / or the desired printed image as closely as possible regardless of the printer used and regardless of the printing time on the printed record carrier. The assignment of the occupation densities to the gray levels can be done, for example, with the aid of a dither matrix. The different gray values are then mediated by occupying different numbers of cells of the dither matrix and thus a printing of different numbers of print dots within a given grid cell. This is particularly advantageous if the number of possible occupancy densities is significantly greater than the number of gray levels, because then each grayscale can be assigned many different occupation densities, without two different gray levels being assigned the same occupation density. As a result, very small gradations of the gray values and a harmonic gray value curve can be set.

In an advantageous embodiment, the assignment of the occupation density to the gray level is carried out by first assigning a corrected gray level to a controlled gray level in a first calibration step and then assigning the occupancy density assigned to the corrected gray level to the selected gray level in a second calibration step. In this case, the second calibration step can be carried out during a recording pause within the productive printing operation. For example, if the colorant is transferred to the record carrier via one, two, or more transfer ribbons, the record break may arise when collecting the colorant on the transfer ribbon, which collection is part of the productive printing operation. In contrast, the printing of the recording medium only for calibration purposes is not part of the productive printing operation.

In a further advantageous embodiment, the colorant is applied to the recording medium outside a printing area of the recording medium, for example in the form of a measuring strip, for calibration. This has the effect that the pressure range is not affected despite continuous calibration.

The invention is explained in more detail below with reference to schematic drawings.

Show it:

1 a print station of a printer,

2 several gray levels and the corresponding gray values,

3 a grid cell with pressure points,

4 a dither matrix,

5 a record carrier with a reproduction of different gray values outside a print area of the record carrier,

6 a flow diagram of a first program for calibrating the printer,

7 a flow diagram of a second program for calibrating the printer,

8th a first assignment rule in optimally calibrated condition,

9 the first assignment rule after a calibration pass,

10 a flow chart of a third program for calibrating the printer.

Elements of the same construction or function are identified across the figures with the same reference numerals.

1 shows a printing station 20 a printer. The printer may be in addition to the print station 20 one, two or more printing stations 20 exhibit. The printer is a standalone device. Alternatively, the printer may be an assembly of a copier that includes a scanner in addition to the printer. With the help of the printer, especially the printing station 20 , a colorant of a color is applied to a recording medium according to a digital or real original. The recording medium may comprise different substrates, for example paper or textile materials. At an alternative printing station 20 It is also possible to apply colorants of different colors to the recording medium. The colorant includes, for example, liquid developer, dry toner or ink.

The printing station 20 is explained by way of example with reference to an electrophotographic printing station. The printing station 20 includes a photoconductor drum 18 , a loading corotron 22 , an LED character generator 24 , a potential sensor 26 , one control device 28 , a developer station 30 , a transfer station 32 with a transfer corotron 34 and a transfer belt 36 , a firing corotron 38 , a cleaning unit 40 and a discharge lamp 42 ,

The surface of the photoconductor drum 18 is coated with a photo semiconductor material and rotates at a constant speed. The rotating photoconductor drum 18 is done with the help of the charging corotron 22 charged. The LED character generator 24 Illuminates the photosemiconductor material by means of a sharply focused light beam according to a template to be printed. The template is regularly stored digitally. The potential sensor 26 serves to create the latent image on the photoconductor drum 18 to check. A measuring signal of the potential sensor 26 becomes a regulatory device 28 fed, the scheme for a precise representation of the image on the photoconductor drum 18 contributes.

The latent image of charges on the photoconductor drum 18 is using the developer station 30 dyed by the use of electrostatic attraction with the colorant, which is a fine, electrostatically charged powder, in particular a dry toner in this embodiment. At the illustrated printing station 20 The areas which were discharged with the help of the light beam are colored. Alternatively, the charged areas can be inked at another print station.

With the help of the transfer station 32 During a transfer process, the colorant is in the area of the Umdruckcorotrons 34 from the photoconductor drum 18 on the transfer belt 36 transfer. Subsequently, during an in 10 with REP recording process, the colorant from the transfer ribbon 36 applied to the recording medium, in particular on a paper web, not shown. At a printing station 18 with a developer station containing several different colors 30 can during the transfer process with several revolutions of the photoconductor drum 18 and the transfer ribbon 36 several colors on the transfer ribbon 36 be transferred, the transfer belt 36 collecting the different colors and then transferring a colored image to the record carrier during the recording process REP. A recording pause of the printer is characterized in that the printer is in productive printing operation, but currently does not take the recording process REP.

The extinguishing corotron 38 serves after toner transfer from the photoconductor drum 18 on the transfer belt 36 in addition, the residual toner on the photoconductor drum 18 to unload. The uncharged toner layer may then be in the cleaning unit 40 be removed. The discharge lamp 42 removes remaining charges.

2 shows a representation of different gray levels 72 . 74 . 76 . 78 depending on different shades of gray 64 . 66 . 68 . 70 are printed on the recording medium. In particular, the printer generates a first gray level upon request 64 a first gray value 72 when requesting a second gray level 66 a second gray value 74 when requesting a third gray level 68 a third gray value 76 and when requesting a fourth gray level 70 a fourth gray value 78 , The first to fourth grayscale 64 . 66 . 68 . 70 are numeric values given to the printer to generate the first to fourth gray levels 72 . 74 . 76 . 78 be specified. The first to fourth gray values 72 . 74 . 76 . 78 represent different color impressions of a color, and in particular convey different brightnesses of the color. With the help of the printing station 20 256 different gray levels can be realized.

3 shows a grid cell 80 on the record carrier. The grid cell 80 on the record carrier corresponds to a grid cell 80 the printer, in particular a raster cell 80 on the photoconductor drum 18 , The grid cell 80 is magnified so much that individual pressure points on each output pixels of the grid cell 80 are recognizable. Between the individual pressure points 82 If the record carrier is not printed. The different gray values 72 . 74 . 76 . 78 are due to different occupancy densities of the grid cell 80 scored, each grayscale 64 . 66 . 68 . 70 each assigned a density. The higher the occupation density, the darker the mediated gray value 72 . 74 . 76 . 78 , The gray values 72 . 74 . 76 . 78 For example, they vary between 100% coloring and 0% coloring. For example, in a greyscale, not shown 120 the grid cell should be 50% colored.

The occupation density of the grid cell 80 For example, by an in 4 shown dither matrix can be achieved. The dither matrix is an n × n matrix with n = 4. Thus, the first dither matrix comprises 84 sixteen cells. In the cells, the numbers from 0 to 15 are evenly distributed. The first dither matrix 84 is half occupied.

This means that half of the possible printable pixels are printed on the record carrier and that the occupation density is 50%. For this purpose, an occupancy value P _{x is set} to P _x = 7. This causes all output pixels in the range of 0 to 7 with pressure points 82 be printed. Thus, with the help of the first dither matrix 80 sixteen different gray levels 72 . 74 . 76 . 78 be achieved, which then with the help of sixteen shades of gray 64 . 66 . 68 . 70 are requestable.

The printer 18 however, allows you to display 256 shades of gray 64 . 66 . 68 . 70 , Thus, the printer needs 18 at least one 16x16 dither matrix. A fine tuning of the gray values is achieved by using a much larger dither matrix. The dither matrix used then has far more than 256 fields, so that correspondingly more different gray values 72 . 74 . 76 . 78 can be represented. With only 256 shades of gray 64 . 66 . 68 . 70 Then there are several options, one of the grayscale 64 . 66 . 68 . 70 a density and thus different gray values 72 . 74 . 76 . 78 to associate without two different shades of gray 64 . 66 . 68 . 70 cause the same gray value.

5 shows a piece of paper 86 , which serves as a record carrier and that a print area 88 having. Within the print area 88 the printing is done according to the template to be printed. Outside the print area 88 is a gauge 90 printed with different gray values. Because the measuring strip 90 is outside the printing range, this does not distort the reproduction of the original and can be easily separated at the completion of the printed product.

For the sake of representability in the representation of the subsequent programs, the requested gray levels 64 . 66 . 68 . 70 represented by the general controlled gray levels K _x .

6 shows a flowchart of a first program for calibrating the printer. The first program is used to calibrate the printer during productive printing so that one by specifying a gray level 64 . 66 . 68 . 70 requested and transmitted by the colorant on the recording medium actual gray value AV a predetermined desired gray value SP of the corresponding gray level 64 . 66 . 68 . 70 equivalent.

The first program is started in a step S2 in which, if necessary, variables are initiated.

In a step S3, a gray level K _{x is} activated. This causes after processing a step S4 printing multiple grid cells 80 on the sheet of paper 86 outside the print area 88 according to the controlled gray level K _x .

In step S4, an occupancy value P _Y is assigned which is assigned to the controlled gray level K _x and which is representative of a coverage density of the screen cell 80 is. For example, the controlled gray level K _x is 200 and the occupancy value P _Y is 300 , which causes when displaying the gray level 200 to the first 300 output pixels of the screen cell 80 pressure points 82 be applied.

In a step S5, the printed by the grid cells 80 mediated actual gray value AV detected.

In a step S6 it is checked whether the actual gray value AV of the controlled gray level K _x corresponds to the desired gray value SP of the controlled gray level K _x or at least approximately corresponds. For example, a difference between the actual gray value AV and the target gray value SP is compared with a predetermined threshold value, and the actual gray value AV is judged not to correspond to the target gray value SP if the difference is greater than the threshold value. If the condition of step S6 is not satisfied, the processing is continued in a step S7. If the condition of step S6 is fulfilled, then the actual gray value AV corresponds to the desired gray value SP and the processing is preferably continued in step S3, in which case another gray level K _{x is} activated, to which another occupancy value P _{Y is} assigned.

In step S7, the controlled gray level K _{x is assigned} a new occupancy value P _{Y + 1} . In continuation of the above example, for example, the gray level 200 the occupancy value 301 assigned, so that subsequently when driving the gray level 200 on the first 301 cells of the grid cell 80 pressure points 82 be applied.

In a step S8, the first program can be ended. However, the first program is preferably processed on the basis of the step S3 different gray levels K _x driving, for example, for 20 different gray levels 64 . 66 . 68 . 70 , In this case, the same gray levels K can be controlled and, if necessary, corrected in several passes in succession, whereby a control is realized. As a correction of the remaining grayscale 64 . 66 . 68 . 70 can easily switch between the corrected grayscale 64 . 66 . 68 . 70 be interpolated, especially if far more different occupation densities are possible, as there are controllable gray levels K _x .

The first program is shown for clarity only for one controlled gray level K _x per pass. Preferably, however, a selection of a plurality of different gray levels K _x are controlled in each individual passage and the grid cells 80 printed accordingly, for example according to 5 ,

The processing of the first program and in particular the assignment of the occupancy _densities characterized by the occupancy values P _Y to the controlled gray levels K _x requires a high computing power of a control unit of the printer since it must be stored exactly for each occupation density and then retrieved which pressure points 82 exactly inside the grid cell 80 must be printed. In particular, when using a particularly large dither matrix with, for example, 10,000 different possible occupation densities, 10,000 different arrangement variations of the print dots to be printed must be stored and retrieved upon request. To calibrate the printer during productive printing operation, a sufficiently powerful control unit must be provided, or a print speed must be adapted to the performance of the control unit. Alternatively, the calibration may be performed in a first and a second calibration phase.

7 shows a flowchart of a second program for calibrating the printer. The second program represents the first calibration phase and is used to calibrate the printer with low consumption of system resources during the recording process, and in particular the assignment rule specified in the 8th and 9 illustrated diagrams 100 . 110 is represented, adapt.

The second program is started in a step S10 in which variables are initialized if necessary.

In a step S12, the measuring strip 90 outside the print area 88 on the sheet of paper 86 printed. In particular, at least one gray level K _{x is} activated.

In a step S14, which is executed during execution of the step S12, the driven gray level is K _{x x} associated with a corrected gray level L via a stored table. Ideally calibrated printer 18 is a mapping graph 102 in the first diagram 100 linear, so that the controlled gray level _x of the corrected gray level corresponding to L _x K. To be displayed 256 shades of gray 64 . 66 . 68 . 70 are the corresponding occupancy densities and the exact arrangements of those in the grid cells 80 to be printed pressure points 82 stored in a special memory block. As a result, only these 256 devices need to be stored and retrieved, using only a small portion of the system resources.

In a step S16, the actual gray value AV of one of the controlled gray levels K _{x is} detected.

In a step S18 it is checked whether the detected actual gray value AV corresponds to the desired gray value SP of the controlled gray level K _x . If the condition of step S18 is not satisfied, the processing is continued in step S20. If the condition of step S18 is satisfied, the processing is continued in step S12, in which case the same gray level K _{x can be} controlled and reexamined similarly to the first program, or another gray level K _{x is activated} during the first pass and can be examined.

In step S20, the controlled gray level K _{x is assigned} the corrected gray level L _{x + 1} . This is for example for the triggered gray level K ₁₂₀ in the second diagram 110 which is associated with the corrected gray level K ₁₂₁ , thereby creating a level 112 in the mapping graph 102 arises. Thus, in the following, the pressure according to the corrected gray level L _{x + 1} always ensues when the gray level K _{x is} actuated. On the one hand, this leads to the fact that the actual gray-scale value AV is closer to the corresponding desired gray-scale value SP, but this also means that subsequently a representation of the gray levels K _x and K _{x + 1 has} the same gray value 72 . 74 . 76 . 78 mediated on the record carrier.

In one step 522 the second program can be ended. Preferably, however, the second program is executed regularly during the recording process, it being possible to control the same or different gray levels K _x each time the second program is processed again. For example, the second program can be run until all in the gauge 90 illustrated grayscale 64 . 66 . 68 . 70 have been studied.

The two diagrams 100 . 110 can also be referred to as assignment rules or look-up tables. The diagrams 100 . 110 are used to assign the controlled gray levels K _{x to} the corrected gray levels L _x .

The second calibration phase is characterized in principle by the first program and is performed when sufficient system resources are available, for example during the transfer process within the productive printing operation. For optimal calibration then both programs are implemented in a third program as subroutines.

13 shows a flowchart of the third program for calibrating the printer. The The third program serves to process the first program P1 and the second program P2 in a single module, so that calibration takes place both during the recording process REP and in the transfer process, with the aid of the third program also having a synergy effect of the first and second programs Program, in which, as explained below, the calibration results of the second program are used for faster execution of the first program.

The third program is started in a step S32 in which variables are initialized if necessary.

In a step S34, the first program P1 is executed, for example in a first execution of the transfer process before the first execution of the recording process REP.

In a step S36, it is checked if the recording process REP is taking place. If the condition of step S36 is not satisfied, then step S34 is executed again. If the condition of step S36 is satisfied, the processing is continued in step S38.

In step S38, the second program P2 is executed.

In a step S40, it is checked again in step S36 whether the printing operation REP exists. If the condition of step S40 is satisfied, the processing is continued again in step S38. If the condition of step S40 is not met, the processing is continued in step S42.

In step S42, the commanded gray level K _{x is} assigned an exposure value P and thus an occupancy density that was previously assigned to the corrected gray level LX + 1.

In other words, the calibration result of the second program P2, for example, the assignment of the gray level K ₁₂₁ to the gray level K ₁₂₀ in the first diagram 100 used in accordance with the first program P1 of the controlled gray level K _x directly assign the corresponding pixel occupancy P.

Subsequently, in a step S44, the allocation graph 102 linearized.

In a step S46, the third program can be ended. However, the third program is preferably processed continuously during operation of the printer by triggering a selection of activated gray levels K _x several times in succession. The processed controlled gray levels K _x then serve as support points for an interpolation, so that only the selection of the gray levels K _x must be controlled and examined, and yet each of the 256 gray levels can be assigned a new occupation density, whereby an optimal gray value profile can be achieved.

The invention is not limited to the specified embodiment. For example, the printer may also be a copier. Furthermore, arbitrarily large dither matrices can be used, which have more cells than gray levels, which can be controlled with the corresponding printer. Furthermore, the printer can also display more or less gray levels. Furthermore, dither matrices can be used in which an intensity variation takes place within individual pressure points, so-called multilevel rasters.

LIST OF REFERENCE NUMBERS

18

Photoconductor drum

20

printing station

22

corotron

24

LED character generator

26

Potential Sensor

28

control device

30

developer station

32

transfer station

34

Umdruckcorotron

36

transfer tape

38

elimination corotron

40

cleaning unit

42

discharge lamp

64

first gray level

66

second grayscale

68

third grayscale

70

fourth grayscale

72

first gray value

74

second gray value

76

third gray value

78

fourth gray value

80

grid cell

82

pressure point

84

dither

86

sheet of paper

88

pressure range

90

gauges

100

first diagram

102

allocation graph

110

second diagram

112

step

K _x

controlled gray level

L _x

corrected grayscale

BEGIN

program start

P _Y

occupancy value

S2-S30

Steps 2 to 30

AV

Is gray value

SP

Target gray value

n

Condition not fulfilled

j

conditions met

REP

recording process

Claims (7)

Method for calibrating a printer, in which a colorant according to several shades of gray ( 64 . 66 . 68 . 70 ) is applied to a record carrier, each gray level ( 64 . 66 . 68 . 70 ) a coverage density of a grid cell ( 80 ) is assigned to the printer, the occupation density being representative of how many pressure points ( 82 ) per grid cell ( 80 ) and in which actual gray values (AV) of the colorant printed on the recording medium during a productive printing operation are detected, it is checked whether the detected actual gray values (AV) correspond to the corresponding gray levels ( 64 . 66 . 68 . 70 ) and in which, if the actual gray values (AV) correspond to one or more of the gray levels ( 64 . 66 . 68 . 70 ) do not correspond to the corresponding desired gray values (SP) or at least do not correspond approximately, to the corresponding gray level ( 64 . 66 . 68 . 70 ) is assigned a different occupation density. Method according to Claim 1, in which the reassignment of the occupation density to the gray scale (s) ( 64 . 66 . 68 . 70 ) is carried out by first assigning a corrected gray level to a triggered gray level in a first calibration step, and then by assigning the occupation density assigned to the corrected gray level to the selected gray level in a second calibration step. The method of claim 2, wherein the second calibration step is performed during a recording pause within the productive printing operation. Process according to Claim 3, in which the colorant is conveyed via one, two or more transfer belts ( 36 ) is transferred to the recording medium and in which the recording pause due to the application of the colorant to the transfer ribbon or bands ( 36 ) arises. Method according to one of claims 1 to 4, wherein the colorant for calibration according to the gray levels ( 64 . 66 . 68 . 70 ) outside a printing area ( 88 ) of the recording medium is applied to the recording medium. Method according to one of claims 1 to 5, wherein the application of the colorant for calibrating and then assigning the gray scale ( 64 . 66 . 68 . 70 ) to one another during productive printing operation ( 64 . 66 . 68 . 70 ) is carried out a number of times, whereby in the case of a new pass the colorant is treated according to other gray levels ( 64 . 66 . 68 . 70 ) is applied as in the previous pass. Method according to one of claims 1 to 5, wherein the application of the colorant for calibrating and then assigning the gray scale ( 64 . 66 . 68 . 70 ) to one another during productive printing operation ( 64 . 66 . 68 . 70 ) is carried out a number of times, whereby in the case of a new pass the colorant is processed according to the same gray scale ( 64 . 66 . 68 . 70 ) is applied as in the previous passage, whereby a control of the gray values is realized.

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