EP0228347B2 - Process for controlling the application of colours in a printing machine, printing device equipped therewith and measuring device for such a printing device - Google Patents

Abstract

1. Process for controlling the application of ink by a printing machine, wherein a printed sheet printed by the machine is measured photoelectrically in a plurality of test areas, measured data obtained in this manner are processed into control data in combination with set values, and the inking process of the printing machine is controlled automatically using said control data, characterised in that the test areas are measured colorimetrically, and their colour positions relative to a selected colour coordinate system are determined, that for the test areas measured the colour deviations between associated set colour positions related to the same colour coordinate system are determined, and that the formation of control data and control of the inking process take place on the basis of said colour deviations.

Description

Process for ink application control in a printing press, suitably equipped printing system and measuring device for such a printing system.

The invention relates to a method for regulating the ink application of a printing press according to the preamble of independent claim 1, a printing system suitable for carrying out the method according to the preamble of claim 15 and a measuring device intended for generating control data for such a printing system according to the preamble of the independent claim 18th

In the ongoing printing process, the control of the color guide is the most important way to influence the image impression. It takes place after visual assessment or on the basis of densitometric analyzes of color measuring fields that are also printed. An example of the latter control is described in DE-A-2 728 738.

In practice, it has been shown that the control of the color control is often insufficient due to densitometric density measurements alone. So it often happens, among other things, that, when regulating to the same solid ink densities, significant color differences occur between the proof or proof set and the production run. These color differences (image impressions) must then be corrected manually by interactive adjustment of the color guide. The reasons for these color differences lie in the generally different manufacturing processes for proofing / proofing and production and in the color differences of the materials used. Likewise, with a constant color density, in particular solid color density, the constancy of the color impression due to changes in the tonal value due to rubber blanket contamination or other influences is not ensured.

From DD 227 094 a method for ink application control of a printing press is known, in which a printed sheet is measured in areas by colorimetry, the standard color values being determined according to CIE for each measuring area. The surface coverage in the printing inks involved is determined from the standard color values using the Neugebauer equations. These are compared with target values of a test print, and on the basis of the comparison result, zone screws and ink ducts of the printing press are automatically adjusted by means of a control circuit.

In order to determine the quantities included in the Neugebauer equations, this known method requires a special test print with full-tone fields of all possible ink combinations produced under identical conditions and an additional colorimetric measurement of the print sheet after the first printing unit. It therefore requires a double measurement effort and, above all, cannot guarantee a perfect print result with regard to the color impression achieved, if the printing conditions change e.g. B. unnoticed by rubber blanket contamination or by wear-related changes to the printing plates or by other influences. Colorimetry is only used indirectly to determine the area coverage, but the essence of colorimetry is not relevant.

The present invention is now intended to improve the control of the ink flow in printing machines to the effect that a higher degree of agreement in the image impression between the proof or proof set and the production run is achieved and the production run remains stable in the color impression or color changes are recognized.

This goal is achieved by the method according to the invention described in claim 1, the correspondingly equipped pressure system according to claim 15 and the measuring device according to the invention according to claim 18.

The essence of the present invention thus lies in the fact that the principle of densitometric color density measurement is present and is replaced by spectral color measurement, the spectral remissions of the measured test areas being determined and the control of the color guidance (at least during the set-up phase of the printing press) based on these spectral remissions or the one from it derived colorimetric parameters and not based on density measurements. In this way, the image impressions of sensitive, image-important points in the production run can be optimally matched to those of the proof or replacement, whereby to a certain extent color deviations due to different tonal value increases and other material and process influences are compensated for. The color measurement itself can be carried out on printed color test fields or also at suitably selected points (test areas) in the image.

Fig. 1

ein stark vereinfachtes Blockschema einer erfindungsgemässen Druckanlage

Fig. 2

ein Blockschema des Messwerterfassungs-Teiles der Anlage nach Fig. 1 und

Fig. 3

eine schematische Skizze eines Details aus Fig. 2.

The invention is explained in more detail below with reference to the drawing. Show it:

Fig. 1

a greatly simplified block diagram of a printing system according to the invention

Fig. 2

a block diagram of the measured value acquisition part of the system of FIG. 1 and

Fig. 3

3 shows a schematic sketch of a detail from FIG. 2.

The printing system shown in FIG. 1 essentially corresponds to the known systems of this type, for example the system according to DE-A-2 728 738, which is still to be explained, in accordance with the invention in the measured value acquisition part. a control console 20 and a printing press 30 equipped with a remotely controllable ink guide.

With the measured value detection device 10, printing sheets 40 generated by the printing press 30 are photoelectrically measured in a number of test areas, for example in selected locations of the printed image or in the area of color measurement fields 41 that are also printed, and control data 11 are determined from the measurement data obtained in this way, which are the color deviations of the Corresponding printing inks in the individual printing zones and printing units correspond and are supplied as input variables to the control console 20. The control console 20 generates control signals 21 from the control data 11, which adjust the ink guide elements of the printing press 30 in such a way that the color deviations are minimal.

2 shows the basic structure of the measured value acquisition device 10. It largely corresponds to that of the device described in US-A-4,505,589. so that the following description essentially focuses on the differences according to the invention compared to this known device.

The device 10 comprises a measuring head 101 which, for. B. is movable by means of a stepper motor 102 relative to the printed sheet to be measured. In addition, a handheld measuring head 103 is also provided, which can be positioned manually on the desired test area of the printed sheet. The two measuring heads 101 and 103 contain a measurement arrangement, not shown, which the test area to be measured z. B. illuminated according to the usual standard at 45 ° and the light reflected from the test area at 90 ° and couples it into an optical fiber 104, which feeds it to a spectrometer 105. (Of course, the remitted light can also be supplied to the spectrometer by other means.) There the remitted light is spectrally broken down and measured. The measurement data obtained in this way are fed to a computer 106, which determines the control data 11 for the control console 20 from them in a manner to be explained. In addition, the computer 106 operates or controls a driver electronics 107 for the stepper motor 102 and the supply of the light sources in the measuring heads 101 and 103 and a data display device 108, a printer 109 and a keyboard 110. All essentially as in the known one Device.

The primarily relevant difference for the invention of the measured value acquisition device 10 shown compared to the known device mentioned is that it is set up for spectral color analysis of the test areas to be specifically measured and thus for colorimetric analysis, while the known device is only able to measure densitometric color densities , so no color measurement / colorimetry allowed. The second essential difference is the evaluation of the photoelectric measurement data with regard to the control of the color guidance.

3 shows the basic structure (known per se) of spectrometer 105. The measuring light supplied via one of the measuring heads 101 and 103 via the fiber-optic light guide 104 (or directly) acts on a holographic grating 151 via an entry slit and is spatially split by the latter according to wavelengths. The spectrally decomposed light falls on a line-shaped arrangement of z. B. 35 photodiodes 152 that each photodiode is exposed to light of an individual, relatively narrow wavelength range. The measurement signals generated by the 35 photodiodes thus correspond to the spectral intensity distribution of the measurement light at 35 discrete support points (wavelength ranges). To query the photodiodes 152, an interface (interface) 153 is provided which amplifies and digitizes the measurement signals and thus brings them into a form which the computer 106 can understand. The interface could of course also be spatially arranged in the computer.

The measured value acquisition device 10, the control console 20 and the actual printing press 30 form a closed control loop. In the systems of this type known hitherto, the ink guidance is regulated on the basis of densitometric density measurements of the printing inks involved. There are deviations from the corresponding target density values. so these are from the control console by appropriate adjustment of the color guide elements corrected, ie made zero or brought into the permissible tolerance range. The color guide control is therefore color density controlled.

For the reasons mentioned in the introduction, this known type of color guide control is not completely satisfactory in all cases.

In accordance with the basic concept of the invention, the principle of color density-controlled color guidance is abandoned and is substantially supplemented by a control based on spectral color measurement and colorimetry. In other words, for each test area (e.g. color measuring field), the spectral remissions are determined by spectral measurement and, if necessary, the color values of a selected color coordinate system are determined by conversion and compared with corresponding target remissions or target color values. The color guidance is then controlled on the basis of the deviations in the spectral remissions or the color values from the target values (“color distances”) and no longer on the basis of the deviations in the densitometric color densities. The control is preferably carried out with the proviso that the total distance of a printing zone resulting from the sum of the color distances of different test areas should be minimal, wherein each test area and, accordingly, its color distance can, if desired, be taken into account with an individual weight.

The control according to color coordinates is described below. In principle, the same applies to the control according to spectral remissions.

The color coordinate system on which the color measurement is based is in itself arbitrary. However, the L * a * b * system or the L * u * v * system of the CIE (Commission Internationale de l'Eclairage) is preferably used. In the following, the color locus is understood to mean the coordinate triple (L *, a *, b *) or (L *, u *, v *), the color distance corresponds to the vector ΔE _Lab or ΔE _Luv or the individual vectors (ΔL * , Δa *, Δb *) or (ΔL *, Δu *, Δv *). The target values of the color coordinates (target color locations) for the individual test areas can be used for the measured value acquisition device 10 e.g. B. can be entered manually via the keyboard 110. However, it is much easier and more expedient to measure the pressure or pressure substitute, or whatever else should serve as a reference, with the device itself and to save the measured values or the data calculated therefrom as corresponding target values. The same also applies to the color density setpoints required in connection with the overlaid density-dependent control to be described.

For reasons of easier comprehensibility on the one hand and compatibility with existing devices on the other hand, the entire control system is, as shown, divided into the two components of the measured value recording device 10 and control console 20 and are those of Measured value acquisition device 10 generates control signals 11 of exactly the same type as in the known color density measuring systems, so that the measured value acquisition device 10 according to the invention can therefore be connected directly with the known control console 20 mentioned and only the measured value acquisition device has to be replaced in order to convert a corresponding pressure system to the method according to the invention. Of course, it is also possible without further ado to generate the control signals required to correct the color deviations directly from the color distances calculated by the data acquisition device without going through the compatible control signals and to summarize the electrical circuits required for this differently or to integrate them into a single device. The division in two shown is therefore to be understood purely, for example, although it is also very practical.

As already explained, the computer 110 forms the color distance vector ΔE _n for each test area. Each of these vectors ΔE _n is now weighted with a weight factor g _n , so that each test area can be considered individually. Typical test areas will be given greater weight, less important ones a lesser one.

Of course, it is also possible to forego weighting and to treat all test areas equally, or to use only certain test areas for control from the outset. The weighting factors can be done via the keyboard e.g. B. can also be entered interactively or preprogrammed.

The weighted or, if necessary, also unweighted color distance vectors of individual measuring fields are mathematically each with a z. B. multiplied empirically determined transformation matrix, and if certain quality criteria are observed, this results in a color density change vector, the components of which are the density changes or the layer thickness changes of the printing inks involved in the printing, and thus the control data for the printing zone in question and such changes in the setting of the color guide elements causes the total color difference - determined as the sum of the amounts or the sum of the color difference squares of the individual color differences - to be minimal. This total color difference can also serve as a quality measure for the print.

The elements of the transformation matrices essentially contain the partial derivatives of the color coordinates according to the color densities of the printing inks involved. They can be determined empirically by measurements on appropriate test prints or synthetically by modeling.

For three-color printing, the density change vector has three components, and the calculation from the color distance vectors, which also have three components, is therefore relatively free of complications. With more than three inks However, the contributions of the individual test areas must be logically assigned to the individual components of the density change vector in a suitable manner in such a way that a correspondingly multidimensional vector results.

As already mentioned, the control signals for the color guide elements can also be determined directly from the color differences. Again, the criterion that the total color difference must be minimized is expediently used as a basis. The different weighting of the individual test areas can also be used here.

The printing process usually runs in three phases. First there is the more or less rough presetting of the printing press, for. B. on the basis of measured values of the printing plates, then the so-called set-up phase (coordination. Registering), in which the ink guide is fine-tuned in one way or the other until the printed product is satisfied, and finally the production run, where the regulation focuses on maintaining the result achieved at the facility as constant as possible. Usually you do not use the proof or the like as a reference. but rather the print sheet found to be good, the so-called "O.K. sheet", and regulates in the production process to constant densitometric color densities.

The phase of density control in continuous printing can also be implemented very easily with the printing system according to the invention. All that is required is to convert the spectral reflectances into filter color densities (according to densitometry) and to compare them with the target color density values determined by an O.K. sheet. The differences in color densities then directly represent the control data 11 for the control console 20.

According to an advantageous embodiment of the method according to the invention, the setting up of the printing press can be carried out in a color-gap-controlled manner as described further above and the production run can then be stabilized in a conventional manner in a color-density-controlled manner. Another particular advantage is that the color density determination can be based on any filter characteristics, which achieves a high degree of flexibility in such a system.

According to a further advantageous variant, the two control principles can also be superimposed on one another. This means that the total color distance is also determined and monitored during the color density-controlled production stabilization. The total color difference should be for some reason, e.g. B. due to changes in the printing process such as rubber blanket pollution, etc., exceed a predetermined limit, it can be reacted in a suitable manner. For example, a new color distance controlled Correction of the printing press can be initiated, in which case the color density setpoints would then be adapted (updated) for further production stabilization, or a corresponding error message can only or additionally be output.

The total color difference can be viewed as a quality measure and, if desired, displayed or printed out.

An important element for standardized pressure monitoring is the color measuring strip. The screen tones should appear in different color and tonal value combinations or particularly critical tones. It is also possible to include critical tones from the subject in the measuring strips.

According to experience, subjects can be divided into groups, e.g. B. Furniture catalogs - shades of brown determining quality -, cosmetic brochures and portraits - skin tones dominant. There are also groups in which e.g. B. Gray or green tones prevail. Accordingly, special color-oriented color measuring strips can be built up and used in a targeted manner. The areas determining the image can thus be taken into account in a simple manner.

With the proofing / proofing set, the inking is not always controlled zonally. In this case, it is sufficient to print a measuring field for each measuring field type and to adopt it as the target value for the entire printing sheet width or parts thereof.

Each zone can be monitored individually on the production sheet with zonal ink feed. Measuring fields that are important for color control, such as single-color measuring fields for density-controlled regulation of the color guidance or multicolor raster fields for colorimetric control, must therefore be repeated at the smallest possible distance. Control fields for color acceptance, tonal value increase etc. can be mounted at a slightly greater distance.

In three-color printing, the printable color space is limited by the color loci of paper white, the single-color solid tones and the 2 and 3-color solid tone overlays (white, cyan, magenta, yellow, red, green, blue, black). All grid tones are within this color space. When printing, color deviations cannot be compensated for in all color tones at the same time, but the mean color difference can be optimized. It is therefore expedient to use, in addition to the fields for the color density-controlled regulation for the color distance-controlled color guidance, also suitable 2 and / or 3 color grid fields such as gray balance fields or sensitive tones depending on the subject.

In four-color printing, the blackening is created by 3 chromatic colors and / or black. Grid fields with black and 2 or 3 chromatic colors can therefore also be of interest as measuring fields for color location-controlled control. The color tones are advantageously chosen from critical areas of the color space. When using 4-color grid fields, a color must be predetermined as a free parameter or additionally measured on a separate color measuring field.

Depending on the subject, suitable color measuring fields can be determined for special colors according to analogous criteria.

Claims (23)

1. A process for controlling the application of ink by a printing machine in which a printed sheet, printed by the printing machine, is measured colorimetrically in a number of test areas with reference to a selected colour coordinate system, the resulting colour location coordinates are processed in conjunction with reference values to produce control data for the inking process elements of the printing machine, and the inking process of the printing machine is controlled automatically using those control data, wherein there are used as reference values reference colour location coordinates based on the same selected colour coordinate system, wherein starting from the colour location coordinates for the measured test areas the colour deviation vectors to those reference colour location coordinates are determined, wherein the colour deviation vectors are converted into the control data necessary for controlling the inking process of the printing machine, for example into changes in layer thickness, and wherein the inking process of the printing machine is controlled on the basis of the control data converted from the colour deviation vectors.
2. A process according to claim 1, wherein the inking process is so controlled that individual colour deviations of given test areas are minimal.
3. A process according to claim 1, wherein the inking process is so controlled that the total colour deviation resulting from the individual colour deviations is minimal.
4. A process according to any one of claims 1 to 3, wherein the individual colour deviations are given different weights when calculating the total colour deviation.
5. A process according to any one of claims 1 to 4, wherein the control data are calculated for printing zones from the colour deviations of the test areas belonging to the printing zones involved.
6. A process according to any one of claims 1 to 5, wherein the zonal control data are calculated from the colour deviations of zone-overlapping test areas.
7. A process according to any one of claims 1 to 6, wherein the weighting of the individual colour deviations varies by area over the print width.
8. A process according to any one of claims 1 to 7, wherein the control data are calculated from the standard colour values obtained by digital filtering (weighting) of the spectral reflections using CIE-standard spectral value curves and from their conversion into a selected colour coordinate system suitable for colour deviation evaluation, especially the CIELAB or CIELUV system.
9. A process according to any one of claims 1 to 7, wherein in addition the control data are calculated from the colour densities obtained by digital filtering (weighting) of the spectral reflections using selected colour filter curves.
10. A process according to any one of claims 1 to 9, wherein the setting-up of the printing machine, that is to say the matching of the print to the master, is effected with colour deviation control and subsequently the printing run is carried out based on colour densities in such a manner that those colour densities are maintained essentially at constant reference values.
11. A process according to any one of claims 1 to 10, wherein there are used as test areas simultaneously printed colour measuring fields and, in particular, also multi-colour halftone fields are provided as colour measuring fields.
12. A process according to claim 10, wherein the colour-density-controlled printing run stabilisation is superposed on the colour deviation-controlled regulation of the inking process so that a new colour deviation-controlled correction is made, with simultaneous updating of the reference values for the colour densities, if the colorimetric colour deviations exceed a threshold value.
13. A process according to claim 10, wherein the total colour deviation or deviations is or are also formed and monitored during the printing run and a warning is issued, or a new colour deviation-controlled correction of the printing machine is carried out, if the colour deviation tolerance is exceeded.
14. A process according to claim 11, wherein colour measuring fields are used having colour tones taken from selected critical image areas of the printed sheet.
15. A printing installation for carrying out the process according to any one of the preceding claims, comprising a printing machine, a data acquisition device for colorimetric measurement of a printed sheet by area with reference to a selected colour coordinate system, and a control means for processing the colour location coordinates produced by the data acquisition device and for producing adjustment signals for the inking process elements of the printing machine from those colour location coordinates and reference values, wherein the control means is designed to determine colour deviation vectors from the colour location coordinates produced by the data acquisition device by comparison with reference colour location coordinates and to produce the adjustment signals on the basis of those colour deviation vectors.
16. A printing installation according to claim 15, wherein the data acquisition device is arranged for spectral photometric measurement of the printed sheet at a number of different wavelengths and for the production of corresponding spectral photometric measured data.
17. A printing installation according to claim 16, wherein the control means is designed to convert the spectral photometric measured data produced by the data acquisition device also into colour densities and to produce the adjustment signals for the inking process elements also from the result of a comparison of those colour densities with corresponding reference colour densities.
18. A measuring device for producing control data for the inking process elements of a printing machine comprising a data acquisition device for colorimetric measurement of a printed sheet by area with reference to a selected colour coordinate system, and a processing means that from the colour location coordinates produced by the data acquisition device produces the control data that represent the deviation of colour of the measured areas of the printed sheet from corresponding reference values, wherein the processing means is designed to determine from the colour location coordinates produced by the data acquisition device, by comparison with reference colour location coordinates, colour deviation vectors thereto, and to produce the control data for the inking process elements of the printing machine from those colour deviation vectors.
19. A measuring device according to claim 18, wherein the data acquisition device is arranged for spectral photometric measurement of the printed sheet at a number of different wavelengths and for production of corresponding spectral photometric measured data.
20. A measuring device according to claim 19, wherein the processing means is additionally arranged to convert the spectral photometric measured data produced by the data acquisition device into colour densities, compare them with reference colour densities, and produce the control data for the printing machine from the result of that comparison.
21. A measuring device according to any one of claims 18 to 20, wherein it is arranged to perform the process steps according to one or more of claims 1 to 15.
22. A measuring device according to claim 18, wherein, in addition to photoelectric colour measurement with a controlledly movable measuring head, a freely movable measuring head is connected with which colour measurement may be effected at any location and on any samples.
23. A measuring device according to claim 22, wherein the freely movable measuring head acts on the same spectrometer module as the controlledly movable measuring head.

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