EP0196431B1 - Method, control device and auxiliary means for obtaining uniform printing results from a multicolour half-tone offset printing machine - Google Patents

Description

The invention relates to a method and a device for achieving a uniform printing result on a multi-color printing machine that works in an autotypical manner according to the preambles of claims 1 and 19.

In multi-color offset printing presses, the printing inks are printed on the substrate in rapid succession, with a separate printing unit being provided for each printing ink. If, for example, ten levels of brightness are provided for each printing ink, three printing inks can be used to obtain a thousand different color shades.The reproduction of a color shade essentially depends on two factors, namely on the one hand on the layer thickness of the printing inks on the printing material and on the other hand on the area coverage of the halftone dots. In order to influence these factors, the inking units of the printing units of the multicolour offset printing press are each provided with an ink fountain, which extends across the width of the printing substrate, and a large number of actuators in the form of so-called zone screws, by means of which the ink supply to adjacent ink zones (or longitudinal disputes) of the printing plates or the Printing material can be adjusted individually. As a rule, an increase in the ink supply is associated both with a vertically directed increase in the ink layer thickness and with a horizontally directed widening or increase in the area coverage of the halftone dots, while a decrease in the ink supply leads to a corresponding reduction in the ink layer thickness and the area coverage of the halftone dots .

One of the biggest problems when working with such multi-color printing presses is to achieve a uniform printing result during the entire duration of the production or production run, ie the color or gray balance is essentially constant to keep.

In a research report relating to reproduction technology, the opinion is represented that the reproduction of gray without color casts is of central importance in all color reproduction processes, so the basic requirement is to pay attention to gray reproduction and that a balanced color reproduction can only be achieved if the gray balance is observed (Otschik, Renzer , Schirmer "The Gray Condition in Printing and Reproduction", FOGRA Research Report 1.012, Munich 1976). It also shows that the blackening of the raster positives for yellow, magenta and cyan should be in a certain approximately the same ratio, except in the area of the light tonal values (e.g. cyan: magenta: yellow = 100: 70: 75). Which gray is to be regarded as "correct" gray is made dependent on the character of the picture. With regard to the practical implementation of the gray condition in reproduction and printing, no concrete suggestions can be found in the report.

A similar view is held in another research report (Otschik "Studies on changing the color set structure by changing color separations for the partial color black", FOGRA research report 1.203, Munich 1981). This examines in particular the influence of the printing ink black and, with regard to the gray balance, points out that when using the so-called achromatic composition, i.e. when building up gray or black image areas only with the help of the color black, larger tolerances can be observed which facilitates the color guidance during printing.

USA standardization recommendations include that the tonal value increases that occur in relation to the proof are set at 22% ± 4% and should not be more than 3% between the individual colors (Graphics Communication Association GCA, Spectrum News, December 1984, p. 3). However, these recommendations do not show how such a standardization recommendation could be adhered to or targeted.

These and other research reports and recommendations are likely to be the reason why until today only those methods and devices have become known for the practical implementation of the gray balance in production or production printing which are based on the principle that the sizes of the individual printing inks relevant for printing are constant to regulate, because such a regulation should automatically always include compliance with the gray balance. Practice has shown, however, that such a regulation does not lead to the desired result.

Numerous methods and devices for controlling printing processes have become known.

One of these devices consists, for example, of the well-known manually operated or automatically operating densitometers, which enable optical density measurements on preselected measuring fields in the form of grid fields and / or full fields, i.e. areas completely covered with printing ink. The raster and full fields can be parts of the printed image itself or a so-called print control strip (US-A-4,310,248). The densiometric evaluation of a full field leads to a value hereinafter referred to as full tone density, on a grid field, on the other hand, to a value hereinafter referred to as screen density. These density values enable statements about the changes in the ink layer thicknesses or the area coverage of the halftone dots of the individual printing inks and thus also statements about those changes that have to be made on the printing press in order to attribute the density values to predetermined target values. In addition, it is already known (GB-A-1 044 903) to determine the spectral color components of the measuring fields and to calculate the density values from these.

In this context, it is also known to check compliance with the gray balance not only on individual color fields, but also on special multicolor fields, in particular gray fields, which are produced by overprinting of the printing inks involved. This check can be carried out purely visually by comparison (US Pat. No. 4,310,248). or also by scanning the gray fields with measuring devices and calculating the density values of the individual printing inks from the measurement values obtained (GB-A-1 044 903; Kunishi "Estimation of Values of Primary Inks in Color Prints", Graphic Arts Japan Vol 26, 1984-85).

Another known device consists of a special control element which is made up of halftone dots of different sizes and microelements of different sizes, which disappear or are retained during printing and thereby enable an immediate quantitative statement about the change in the halftone dots or their area coverage (DE-A-24 01 672).

Finally, semi-automatic or fully automatic control devices are known, particularly in connection with multi-color offset printing presses (GB-A-2 000 082, EP-A-0 095 649, EP-A-0 089 016). These control devices are based on the principle that the screen and / or solid tone densities of printed screen and / or solid fields are used with the aid of the densitometer or other measuring devices, to compare the density values obtained with specified target values or tolerance ranges and, in the event of deviations of the determined density values from the target values or tolerance ranges, to actuate the actuators of the inking units so that the measured density values again reach their target values or fall within the tolerance ranges

For this reason, the printer has so far only had two options when it comes to producing a typical multi-color print. He can either try to get the print result as close as possible to the original, but only on the basis of subjective criteria, by constantly visually comparing the print result with the original and at the same time making careful manual adjustments to the actuators of the inking units, or to eliminate subjective impressions of the above Use processes and devices to only intervene to help if it is no longer possible to match the original with the printed result.

All known methods and devices have so far not led to the desired results. A main reason for this is seen in the fact that, for practical reasons, only the actuators of the multicolor printing machine that influence the layer thickness or the dot size are available for a corrective intervention in the printing process. Therefore, the color layer thicknesses and area coverage of the halftone dots can only be changed together, but not independently of each other, because a change, e.g. the position of a zone screw or the like, in addition to a change in the ink layer thickness, always also results in a change in the area coverage of the halftone dots in the respective ink zones. The result of this is that both the measured values for the solid ink densities and the measured values for the screen densities change when the printing process is corrected.

This would not be a disadvantage if there was always a clear and constant relationship between changes in the ink layer thickness and changes in the area coverage. In practice, however, this is not the case. Rather, the correlations between changes in screen densities and changes in solid densities change constantly in the course of a printing process. It should be noted that changes in the ink layer thickness have a strong influence on the brightness levels within a given printing ink and have a slight influence on the color nuances formed by the interaction of several printing inks and thus on the color balance, while the reverse applies to changes in the area coverage of the halftone dots. A reasonably fixed relationship or correlation between these changes has so far only been found for periods to be measured after minutes, ie for the short-term range. As a rule, an increase (decrease) in the ink supply is associated both with a vertically directed increase (decrease) in the ink layer thickness and with a horizontally directed increase (decrease) in the halftone dots. In contrast, for the long-term area that is measured after hours and is particularly important for the print run Considerable changes in the correlations between changes in solid and screen densities are observed. The reason for this is to be seen in the rheology of the printing inks and thus in their tendency to form halftone dots of different sizes under the influence of temperature and fountain solution supply. Oxidation processes and other phenomena also affect the correlations. This can go so far that in a border area of a long-term printing process, for example due to very strong changes in the supply of printing ink, combined with a large change in the ink layer thickness, only comparatively small changes in the area coverage of the halftone dots can be brought about while in another border area of the same Long-term printing with small changes in the ink supply or the ink layer thickness, large changes in the area coverage of the halftone dots can be achieved. In these cases, the most important size to be observed during the printing process, namely the color balance, is changed accordingly. It follows from this that the effect of the above-mentioned methods and devices is actually based on one of two compromises, namely specifying either narrow or comparatively large tolerance ranges for the screen and / or solid densities. If narrow tolerance ranges are defined, the color balance can be kept at a sufficiently constant value in the short-term range. However, the printing process often has to be interrupted because changes in the correlations between the screen and solid densities in the long-term range quickly leave the tolerance ranges or the control device becomes uncontrollable because adjustments to the actuators no longer allow the changes in the area coverage of the screen dots required to maintain the color balance If, on the other hand, large tolerance ranges are specified, there is practically no regulation of the color balance, because the human eye is aware of changes in color nuances due to changes in the area coverage of the Halftone dots react very sensitively and therefore, according to previous knowledge, the grid densities or area coverage should remain as unchanged as possible. Overall, therefore, the achievement of a uniform printing result still has many shortcomings.

The invention has for its object to develop a new strategy with regard to the achievement of uniform printing results and to design the method and the device of the genera described in such a way that they a flexible, but still subject to tolerances in terms of color balance and tight tolerances Enable control of the printing process.

The characterizing features of claims 1 and 19 serve to achieve this object.

The invention is based on the knowledge that the color balance does not only depend on the absolute values of the ink layer thicknesses and the area coverage of the halftone dots, but also on the relationships between the area coverage and / or ink layer thicknesses measured in a color zone for different colors or the resulting halftone areas. and / or solid density depends on each other. In other words, a color shade formed, for example, from cyan and magenta will change only slightly if the halftone dots of both the cyan and magenta are changed in the same direction within the relevant screen step due to changed printing conditions and, for example, from 50% area coverage to 55% area coverage for Cyan and grow from 40% to 45% area coverage for magenta. In such a case, the brightness of the color shade should change, but not the color shade itself. In contrast, the color shade will change primarily if the area coverage or screen density of the screen dots are changed in different directions and, for example, the area coverage for cyan is increased from 50% to 55%, but at the same time the area coverage for magenta is reduced from 40% to 35%. The new strategy for achieving a uniform printing result is therefore that not the screen densities and / or solid densities themselves, but selected relationships between the screen densities and / or solid densities of the screen dots are to be kept in preselected narrow tolerance ranges, in order to thereby cause changes in the same direction tolerating inks involved in the formation of a color zone largely in opposite On the other hand, keeping directional changes within narrow limits. "Selected relationships" are understood to mean quantities which are determined by the fact that ink layer thicknesses and / or halftone dot sizes of different printing inks are related to one another. Since the human eye can only distinguish about 50 different color shades, a change in the brightness of a color shade is less critical than a change in the color shade itself. Apart from this, the new strategy has the significant advantage that the tolerance ranges for the absolute values of the full tone or screen densities can be significantly increased compared to the previous methods. Limiting these absolute values only serves the purpose of maintaining the contrast in the print result. Because if the human eye is less sensitive to fluctuations in brightness than to fluctuations in color, the fluctuations in brightness are nevertheless not completely negligible, since the overall contrast is determined by the full-tone density and the color of the printing material, while the limitation of the absolute values of the screen densities or the size of the Halftone dots are desirable because they determine the color nuances within the print result. Since the halftone dot changes in halftone image printing take place according to largely known laws, it is usually sufficient, however, to measure a single halftone level, for example at 50%, per printing ink and possibly per ink zone and to define a tolerance range for this. Finally, when the selected relationships obtained from measurement values obtained on combination measuring fields are used, there is the surprising advantage that their deviations from the values determined on primary colors are considerably smaller than when the absolute values of the halftone dots or solid densities are considered. Therefore, approximation formulas or comparative studies in connection with the selected relationships lead to corrected measured values, which are excellent as a basis for a control process.

The use of combination measuring fields according to the invention also has the advantage that, in the presence of three colored printing inks per color zone, only a single combination measuring field is required for information to obtain the full-tone densities or screen dot sizes of all printing inks involved by densitometric evaluation. Even if each combination measuring field has a width of approx. 8 mm and a combination measuring field is provided both in the form of a full field and in the form of a grid, there will only be a space with a width of approx. 16 mm within each color zone needed to get all information about the solid densities and dot sizes of all colored printing inks. There is therefore always enough space to accommodate additional measuring fields and control elements in each color zone. Alternatively, it can also be provided that only two single-color measuring fields are printed one above the other to form a combination measuring field, so that a total of four combination measuring fields appear in each color zone, for which purpose a width of approximately 32 mm is required in the example above.

The use of the device according to claim 15 to determine the color balance in the printing result has the advantage that the printer, the gray, brown or other mixed tones of the combination measuring fields of the print control strip can be visually assigned to a corresponding control element of the device. With an orderly structure of the device, it is then easily possible to estimate or read the deviations from a defined zero point that occurred in the course of the printing process and to adjust the inking units of the printing press accordingly to eliminate these deviations.

According to a further feature of the invention, the actuators are actuated as a function of the current correlation between the changes in the screen and / or solid density. This takes into account the fact that these correlations can change in the course of the printing process, ie a given change in the color layer thickness can be associated with different changes in the area coverage. Another major advantage of the strategy according to the invention for maintaining a uniform printing result is thus that the control process is made more flexible and can be kept controllable over long periods of time by constant adaptation to the changing correlations.

Further advantageous features of the invention emerge from the subclaims.

Fig. 1

eine schematische Seitenansicht eines einzelnen Druckwerks einer Offsetdruckmaschine;

Fig. 2

die schematische Seitenansicht einer Vierfarben-Offsetdruckmaschine;

Fig. 3

schematisch eine Draufsicht auf ein Druckwerk einer Offsetdruckmaschine mit einem dieses verlassenden, bedruckten Bogen;

Fig. 4

den schematischen Aufbau eines Densitometers;

Fig. 5

schematisch eine erfindungsgemäße Regelvorrichtung;

Fig. 6

weitere Einzelheiten der Regelvorrichtung nach Fig. 5;

Fig. 7

schematisch die Wirkungsweise der Regelvorrichtung nach Fig. 5;

Fig. 8

ein Ausführungsbeispiel des erfindungsgemäßen Einzelfarbenstreifen-Satzes;

Fig. 9

eine Vorrichtung zur Ermittlung der Farbbalance im Druckresultat einer Offsetdruckmaschine; und

Fig. 10

ein Koordinatensystem für die Vorrichtung nach Fig. 14.

The invention is explained in more detail below in connection with the accompanying drawing using exemplary embodiments. Show it:

Fig. 1

a schematic side view of a single printing unit of an offset printing press;

Fig. 2

the schematic side view of a four-color offset printing machine;

Fig. 3

schematically shows a plan view of a printing unit of an offset printing machine with a printed sheet leaving it;

Fig. 4

the schematic structure of a densitometer;

Fig. 5

schematically a control device according to the invention;

Fig. 6

further details of the control device according to FIG. 5;

Fig. 7

schematically the operation of the control device of FIG. 5;

Fig. 8

an embodiment of the single color strip set according to the invention;

Fig. 9

a device for determining the color balance in the printing result of an offset printing press; and

Fig. 10

a coordinate system for the device of FIG. 14th

1, a conventional multi-color offset printing machine contains several printing units, each with a dampening unit 1, an inking unit 2, a plate cylinder 3, around which a printing form 4 carrying the image to be printed, for example an aluminum printing plate, is stretched, a rubber cylinder 5 and a printing cylinder 6.

The dampening unit 1 serves to first coat the printing formes with a thin, uniform water film, and for this purpose has a reservoir 7, from which water is transported to two application rollers 9 by means of rubber rollers 8 covered with fabric, which are applied with slight pressure to the Apply printing form 4 and keep it constantly moist.

The inking unit 2 has the task of constantly supplying the printing form 4 with the required amount of ink. For this purpose, it has an ink fountain 10 which serves as a memory for a printing ink 11 and to which a large number of actuators 12 are attached in the form of zone screws. These actuators 12 are distributed over the entire width of the ink fountain 10 at regular intervals and control the outflow of the printing ink 11 from the ink fountain 10 in such a way that the amount of ink flowing out can be individually adjusted zone by zone over the entire printing width. The ink 11 flowing out of the ink fountain 10 passes via a ductor 13 and a lifter 14 to a number of distribution rollers 15, which have different diameters and are partially axially movably mounted, in order to split and distribute the ink film several times. The printing ink is finally taken over by application rollers 16 which are in contact with the printing form 4 and cover it with a thin ink film.

The printing form 4 carries the image to be printed, the areas which are to be printed being receptive to the printing ink 11 and at the same time water-repellent (hydrophobic), while the areas which are not to be printed are receptive to water (hydrophilic) and do not accept any printing ink 11. Therefore, only the hydrophobic areas of the printing plate 4 are coated with ink by the inking unit 2, while the hydrophilic areas remain free of ink.

The ink is now transferred from the ink-bearing areas of the printing form 4 to the rubber cylinder 5, which lies against the plate cylinder 3 with slight pressure. From the rubber roller 5 the printing ink 4 is finally transferred to a printing material 17 which passes through the gap between the blanket cylinder 5 and the printing cylinder 6. For this purpose, the printing cylinder 6 has a gripper system (not shown in more detail) which has a multiplicity of grippers 18 which are distributed at short intervals over the entire width of the printing cylinder 6 and hold the individual sheets of the printing material while the printing cylinder 6 is rotating.

2 shows the diagram of a four-color offset printing press with four printing units I to IV, printing unit I being assigned to the color black, for example, while printing units II to IV are printing the colors cyan, magenta and yellow, for example. Each printing unit comprises a dampening unit 21, an inking unit 22, a plate cylinder 23, a rubber cylinder 24 and a printing cylinder 25 according to FIG. 1. A number of transfer cylinders 26 is provided in front of and behind the printing cylinder 25. Furthermore, the offset printing machine has at its entrance a storage container 27 for a stack 28 of individual, unprinted sheets 29 of the printing material and a feed table 30, while a storage container 31 for printed sheets 32 is provided at its outlet.

The operation of such an offset printing machine is as follows:
The unprinted sheets 29 are individually separated from the stack 28 and exactly aligned one after the other on the feed table 30. The sheet 29 located on the system table 30 is then taken over by the first transfer cylinder 26, which, like the impression cylinder 25, is equipped with grippers for this purpose. The sheet 29 is transferred from the first transfer cylinder 26 to the printing cylinder 25, whereupon the actual printing process takes place. During the rotation of the printing cylinder 25, the sheet 29 runs between the printing cylinder 25 and the rubber cylinder 24 and takes up the first, for example black, printing ink. After the printing process, the sheet 29 is fed to the second printing unit II by means of the further transfer cylinders 26. There will the sheet 29 is taken from the corresponding printing cylinder 25 with a precise fit, so that the printed image of the second color, for example cyan, is printed with a precise fit. The printing in printing units III and IV takes place accordingly. After all four color images have been printed on the sheets 29 in four printing units arranged one behind the other, these are fed to the storage container 31 by means of a conveyor belt 32 and stacked therein. With modern offset printing machines of this type, approximately 6000 to 8000 sheets can be printed in four colors per hour.

In the top view according to FIG. 3 on a printing unit of an offset printing machine, only an ink fountain 36 with the actuators 37 also indicated in FIG. 2, a plate cylinder 38 carrying the printing form, a rubber cylinder 39 and a printing cylinder 40 are indicated, all of which are via the cover the entire printing width of the machine. A printed sheet 41 is still partially on the printing cylinder 40. Because of the actuators 37, the sheet 41 is printed in a number of imaginary, parallel and adjacent ink zones 42 corresponding to the number of actuators, which consist of strips extending in the transport direction (arrow v ) of the sheet 41. In order to be able to check how thick the color layer applied to the sheet 41 is, measurement fields in the form of raster fields 43 and full fields 44 are also printed on the upper or lower edge of the sheet, expediently at least one raster and full area 43 for each color zone 42. 44 is provided, although each grid or full surface 43, 44 could also extend across the width of a plurality of color zones 42. The grid fields 43 consist of a plurality of grid points of the same size, which have a certain area coverage per unit area of the grid fields. The raster fields 43 are printed by corresponding sections formed in the printing form, which are attached in preselected raster steps with, for example, 25%, 50% or 75% area coverage. It can therefore be concluded from the enlargement or reduction of the screen dots in the screen fields 43 compared to the corresponding sections in the printing form are how the effect of the amount of ink set with any actuator during printing or what changes result with regard to the area coverage of the halftone dots when changing the setting of the corresponding actuator 37. The full fields 44, on the other hand, consist of fields that are completely covered with printing ink and are created by corresponding sections in the printing form. The full fields 44 therefore provide information in particular as to whether much or little printing ink has been supplied by means of an actuator 37, because in the full fields 44 only the layer thickness of the applied printing ink can change.

The grid and full fields 43, 44 are examined with the aid of known densitometers, preferably incident light densitometers, in order to achieve objective measurement results. These can be manually operated densitometers (e.g. Macbeth RD-918) or automatically operating densitometers (e.g. Macbeth PXD-981) manufactured by Kollmorgen-Macbeth or its subsidiary Process Measurements Inc. in Newburgh, NY ( USA) are manufactured and distributed. When using manual densitometers, a sheet 29 is removed from the stack of printed sheets and checked at preselected intervals. If the values determined on the print result differ from those of the original, the printer can try to adjust the measured values again to those of the original by adjusting the actuators. If an automatic densitometer 45 is used, it is expediently mounted on a slide 47, which can be moved back and forth on the rail 48 in the direction of a double arrow w with the aid of controllable motors, for example stepper motors, across the width of the arc 41. 2, the rail 48 can be arranged at any point in the transport path of the sheet 29 between the storage containers 27, 31.

If only one measuring station is desired, the sections producing the measuring fields 43, 44 are applied to the printing form in such a way that after the sheets 29 have been completely printed, the associated measuring fields of all printing inks are printed one above the other.

In other words, the single-color measuring fields of all the printing inks used for printing are printed on top of one another with the aid of sections which are attached to the printing plates in the same locations everywhere, in such a way that a single combination measuring element of corresponding shape and size is produced, which is not due to the overprinting has only halftone dots or a solid surface of a single printing ink, but halftone dots or superimposed solid surfaces of all printing inks and therefore has a gray tone. Alternatively, it is also possible to combine the individual color measuring fields from fewer than all printing inks used for printing or, for example, only from two printing inks to form a combination measuring field. According to a preferred embodiment, only the colored printing colors (for example magenta, cyan and yellow) are used to form the combination measuring field, while achromatic colors (for example black) are assigned a single-color measuring field, if such is desired at all. The same procedure can be used for prints other than four-color. It is also possible to provide such combination measuring fields not in all color zones, but only in selected color zones, for example in every second, third, etc. color zone. In any case, the use of combination measuring fields has the essential advantage that fewer measuring fields are required within each selected color zone than printing inks, preferably colored printing inks, so that there is sufficient space in a row and within each color zone to accommodate measuring fields which provide all the information required for the regulation. If, for example, three single-color measuring fields with the basic colors magenta, cyan and yellow are combined to form a brown or gray combination measuring field, then only a third of the space that would be required if three single-color measuring fields were printed side by side. Therefore, if in each color zone there are two brown or gray combination measuring fields, which are used as grid or Full fields can be formed, are provided, of the available width of a color zone from 30 mm to 40 mm only about 16 mm are required for two 8 mm wide combination measuring fields, so that a number of further measuring fields or Control elements can be housed for the same or different purposes. Because wherever a combination measuring field is provided, the associated single-color measuring fields can be omitted.

When using combination measuring fields only, the densitometer 45 is arranged, for example, between the printing unit IV and the storage container 31. In this case, the densitometer 45 is either provided with a beam splitter, by means of which the incident light beam is divided into a plurality of light beams, which are evaluated simultaneously through a plurality of filters and separately from one another, or with a number of filters, in particular complementary filters, which are arranged one behind the other through the light beams one after the other. Other arrangements are also conceivable, provided that it is possible to obtain information about all the printing inks from each combination measuring field. Apart from this, if necessary, suitable and known measures for synchronization can be provided in order to precisely specify the times at which the densitometer takes a measurement.

Alternatively, further measuring stations can be provided between the individual printing units I to IV and the measuring fields of the individual printing inks can be arranged such that they lie next to one another after printing and therefore each printing ink is assigned to a separate measuring field and a separate densitometer to increase the accuracy. Otherwise, the densitometer 45 is expediently connected via a trailing cable 49 to an automatic evaluation station, an electronic data processing system 50 or the like.

A corresponding procedure can be used when using a roller offset machine. Alternatively, a single sheet removed from the machine can be scanned manually or by means of a densitometer that is automatically guided over the sheet.

4, the functioning of the densitometer 45 is shown schematically. From a light source 56, light beams are directed onto the sheet 29 by means of optics 57, e.g. on a raster or full field 43, 44 of a certain printing ink of the same. A part of the incident light rays is absorbed, while the rest is reflected and directed onto a color filter 59 by optics 58. This color filter 59 has a color complementary to the measured printing ink (cyan-red, magenta-green, yellow-blue), as a result of which the colored light rays are converted into achromatic or gray light rays. Behind the color filter, the light rays arrive at a receiver 60, which consists of an opto-electronic converter and converts the light rays into electrical signals. These are then forwarded to an evaluation circuit 61 and processed in the latter. The measurement results obtained can be digitally displayed on a screen 62. The color filter 59 can be arranged together with other color filters within a swiveling or rotating device in such a way that a color filter assigned to the printing ink to be observed can optionally be swiveled into the light beams in order to make manual examinations possible in a simple manner.

The densitometer 45 measures the optical density D , ie the decimal logarithm of the reciprocal of the degree of reflection, which is the quotient of the reflected luminous flux and the incident luminous flux. If the optical density is determined on a raster field 43, the raster density D _{R is obtained} , while the density determined on a solid surface 44 is referred to as solid tone density D _V. From D _R and D _V , the so-called optically effective area coverage of the raster points can be calculated in a known manner (Murray-Davies, Jule-Nielson), which is somewhat larger than the so-called mechanical area coverage, which is when examining the raster points is obtained with a microscope or the like. For the purposes of the invention, however, it is important that the screen density, like the optically effective or the mechanical surface coverage, is ultimately only a size that enables a statement about the size of the screen points. The same applies to the concept of halftone dot change, which provides information about the extent to which halftone dots are enlarged or reduced during printing. In the following description and also in the claims, these four terms are therefore summarized under the designation "screen dot size". Otherwise, the grid fields can be provided in different grid levels of, for example, 25%, 50% and 75%, based on their optically effective or mechanical area coverage. The sequence and frequency of the measurements depend primarily on the specific properties of the multicolour offset printing press used and the changes in the printing result that occur in the short or long term. Apart from this, manually operated densitometers are mainly used in the preparation phase in order to obtain the data required for the subsequent production print based on a sample or sample print, while fully automatic densitometers are mainly used for print production.

In addition to a measuring device for actual values in the form of the densitometer 45 (or more densitometers), the device according to the invention (FIG. 2) comprises an actuating device which consists of the sum of all actuators 37. The controlled system is the path of the ink from the ink fountains to the sheets to be printed. The controller of the control device consists of an electronic data processing system 65, to which the measured values measured by the densitometer 45 are fed via a line 66 and which emits the control signals to the actuators 37 via lines 67. In addition, the data processing system 65 can be connected to a screen 68 on which measured values or the like can be made visible. The data processing system can also be programmed with previously determined control programs and then calculate a suggestion for actuating the actuators 37 according to these control programs based on the measured values either first made visible on the screen 68 or the like and then released by the printer at the command of the latter or, in the case of fully automatic operation, immediately sent to the actuators 37.

An exemplary embodiment of the control method according to the invention is explained below, it being assumed that all measuring fields consist of single-color measuring fields.

Grids 43 (FIG. 3), which are assigned to one or more color zones 42, are also printed from each colored printing ink, for example cyan, magenta and yellow, and possibly also from black. At the beginning of a printing process, a grid value in the form of a grid density or a mechanical or optically effective area coverage of the grid points is assigned to each grid field 43, which defines the desired grid point size in the respective grid field 43. If, as usual, the area coverage of the halftone dots in the section of the assigned printing form producing the halftone field 43 is known, the halftone dot size in the halftone fields 43 can also be defined by the halftone dot enlargement or halftone dot reduction with reference to the halftone dot size on the corresponding section of the printing plate. Each grid field 43 is also assigned a lower and / or upper limit for the grid point size, which define a tolerance range for the grid point size. Furthermore, selected relationships between the halftone fields of two or more printing inks can be defined, for example the differences or quotients of the halftone dot sizes to the color pairs cyan / magenta, cyan / yellow and magenta / yellow, whereby usually only the selected relationships for two color pairs are required because the corresponding relationships of the third color pair result automatically. Here, as with the other sizes, the choice depends on whether the screen densities, the mechanical or the optically effective area coverage or the screen point changes are used for the definition of the screen dot size, depending on the properties of the densitometer used or other measuring devices, the data processing system used, the respective control program, the multi-color printing machine or the like used in individual cases. Then, upper and / or lower limit values are also defined for the selected relationships, which define further tolerance ranges. It can be provided that the tolerance ranges for the screen dot size remain constant during the entire printing process. However, it is also possible to use the program to specify the data processing system, to repeatedly recalculate the tolerance ranges during the printing process on the basis of the measured values supplied, for example in dependence on changing correlations between the ink layer thickness and the area coverage of the halftone dots.

In a corresponding manner, full fields 44 (FIG. 3) can be printed from each colored printing ink (or also from black), which are assigned to one or more color zones 42. For these full fields 44, guide values, upper and / or lower tolerance ranges defining limit values and, if necessary, selected relationships with associated tolerance ranges are defined in a corresponding manner or are repeatedly calculated using the control program.

If, in addition to the halftone dot sizes, the solid color densities are also taken into account in the control program, the data processing system is also informed of correlations between the halftone dot sizes and the ink layer thicknesses, the ink layer thicknesses being expediently communicated in the form of the associated full surface densities, since these are representative of the respective ink layer thicknesses. Such a correlation can mean, for example, that an increase or decrease in the solid tone density in the range D _V = 1.20 to D _V = 1.40 by ΔD _V = 0.10 an increase or decrease in the screen density by ΔD _R = 0, 03 corresponds. Here, too, the correlation between ink layer thickness or solid color density on the one hand and halftone dot change or halftone dot density on the other hand can be defined by other variables, for example the solid ink density and the associated halftone dot changes in percent. Different correlations can exist for different printing inks and different areas of solid ink density. In addition, the computer can be informed via the control program that it repeatedly calculates the correlations that can be changed during the printing process from the supplied measured values and that it always uses the instantaneous correlations when calculating its suggestions for actuating the actuators 37.

Finally, the data processing system can be informed of preselected priorities which must be taken into account when calculating the suggestions for actuating the actuators 37. These priorities can, for example, require that 1) the dot sizes and / or solid densities must lie within the tolerance ranges assigned to them, 2) the selected relationships for the dot sizes and / or solid densities of different printing inks with each other must lie within the tolerance ranges assigned to them, and 3) the absolute values the dot sizes and solid densities are as close as possible to the specified guide values. The priorities must be set so that the data processing system can make a clear decision in any case. Alternatively, a priority could also be that the data processing system is informed of certain dominances, e.g. state that the calculation of a rule proposal must start with the color on which the greatest deviations in the course of the printing process have been identified, or which, viewed integrally, is most strongly represented in the relevant color zone.

• a) Cyan:
  
  ${\text{D}}_{\text{R}}\text{(Leitwert) = 0,55, Toleranzbereich ± 0,05;}$
  
  
  ${\text{D}}_{\text{V}}\text{(Leitwert) = 1,30, Toleranzbereich ± 0,10;}$
  
  
  
  
• b) Magenta:
  
  ${\text{D}}_{\text{R}}\text{(Leitwert) = 0,45, Toleranzbereich ± 0,05;}$
  
  
  ${\text{D}}_{\text{V}}\text{(Leitwert) = 1,30, Toleranzbereich ± 0,10;}$
  
  
  
  
• c) Gelb:
  
  ${\text{D}}_{\text{R}}\text{(Leitwert) = 0,45, Toleranzbereich ± 0,05;}$
  
  
  
  ${\text{D}}_{\text{V}}\text{(Leitwert) = 1,30, Toleranzbereich ± 0,10;}$
  
  
  
  
• d) ausgewählte Beziehungen:
  
  ${\text{D}}_{\text{R}}{\text{(Cyan) - D}}_{\text{R}}\text{(Magenta) (Leitwert) = + 0,10, Toleranzbereich + 0,08 - + 0,12;}$
  
  
  ${\text{D}}_{\text{R}}{\text{(Cyan) - D}}_{\text{R}}\text{(Gelb) (Leitwert) = + 0,10, Toleranzbereich + 0,08 - + 0,12;}$
  
  
  ${\text{D}}_{\text{R}}{\text{(Magenta) - D}}_{\text{R}}\text{(Gelb) (Leitwert) = 0,00, Toleranzbereich - 0,02 - + 0,02;}$
  
  
  
  
• e) Korrelation zwischen Volltondichten und Rasterdichten: Eine Änderung der drei Volltondichten im Bereich D_V = 1,20 bis D_V = 1,40 um ΔD_V = 0,10 hat eine gleichgerichtete Änderung der Rasterdichte von ΔD_R = 0,03 zur Folge.
• f) Prioritäten:
  • 1) Die Toleranzbereiche für die Raster- und Volltondichten sollen nicht unter- oder überschritten werden. Ergibt die Messung Werte, die außerhalb der Toleranzbereiche liegen, soll ein Vorschlag für eine solche Betätigung der Stellglieder Nr. 24 errechnet werden, daß die Werte wieder in die Toleranzbereiche zurückgeführt werden und nach Möglichkeit auch die Bedingungen nach den Prioritäten 2) und 3) erfüllt sind.
  • 2) Die Toleranzbereiche für die ausgewählten Beziehungen nach d) sollen nicht verlassen werden. Ergibt die Messung Werte, die außerhalb der Toleranzbereiche liegen, soll ein Vorschlag für eine solche Betätigung der Stellglieder Nr. 24 errechnet werden, daß die Werte wieder in die Toleranzbereiche zurückgeführt werden. Dabei soll die Rechnung mit derjenigen Druckfarbe begonnen werden, welche hinsichtlich der Größe D_R am stärksten von ihrem Leitwert abweicht. Der errechnete Vorschlag muß die Bedingungen nach Priorität 1) erfüllen und soll die Bedingungen nach Priorität 3) erfüllen.
  • 3) Wenn die Bedingungen nach 1) und 2) erfüllt sind, aber die Vorschläge noch Alternativen offen lassen, sollen zuerst die Leitwerte der Rasterdichten und dann die Leitwerte der Volltondichten möglichst gut erreicht werden.
  • 4) Erkennt die Datenverarbeitungsanlage keine Regelmöglichkeit, um die Bedingungen nach 1) und 2) zu erfüllen, so wird ein diesbezügliches Fehlersignal abgegeben.

Below is a calculation example for the control of a four-color offset printing press, the inking units of which each have 32 actuators. The example relates to a single ink zone, for example ink zone No. 24, and to the associated actuators No. 24 of the inking units printing this ink zone. The inks cyan, magenta and yellow are considered. The printing forms for these printing inks have a share of 60% printing places for cyan and in the associated ink zone 50% each of the printing areas for magenta and yellow. In addition, the printing forms in this color zone have at least one grid and one full field 43, 44. The densitometer PX-981 from Macbeth is used for density measurement, which measures the measuring fields while the machine is running. The data processing system is informed of the following values for color zone 24:

• a) cyan:
  
  ${\text{D}}_{\text{R}}\text{(Conductance) = 0.55, tolerance range ± 0.05;}$
  
  
  ${\text{D}}_{\text{V}}\text{(Conductance) = 1.30, tolerance range ± 0.10;}$
  
  
  
  
• b) magenta:
  
  ${\text{D}}_{\text{R}}\text{(Conductance) = 0.45, tolerance range ± 0.05;}$
  
  
  ${\text{D}}_{\text{V}}\text{(Conductance) = 1.30, tolerance range ± 0.10;}$
  
  
  
  
• c) yellow:
  
  ${\text{D}}_{\text{R}}\text{(Conductance) = 0.45, tolerance range ± 0.05;}$
  
  
  
  ${\text{D}}_{\text{V}}\text{(Conductance) = 1.30, tolerance range ± 0.10;}$
  
  
  
  
• d) selected relationships:
  
  ${\text{D}}_{\text{R}}{\text{(Cyan) - D}}_{\text{R}}\text{(Magenta) (conductance) = + 0.10, tolerance range + 0.08 - + 0.12;}$
  
  
  ${\text{D}}_{\text{R}}{\text{(Cyan) - D}}_{\text{R}}\text{(Yellow) (conductance) = + 0.10, tolerance range + 0.08 - + 0.12;}$
  
  
  ${\text{D}}_{\text{R}}{\text{(Magenta) - D}}_{\text{R}}\text{(Yellow) (conductance) = 0.00, tolerance range - 0.02 - + 0.02;}$
  
  
  
  
• e) Correlation between full-tone densities and screen densities: A change in the three full-tone densities in the range D _V = 1.20 to D _V = 1.40 by ΔD _V = 0.10 results in a rectified change in the screen density of ΔD _R = 0.03 .
• f) Priorities:
  • 1) The tolerance ranges for the screen and solid densities should not be exceeded or fallen short of. If the measurement yields values that lie outside the tolerance ranges, a proposal for actuating actuators no. 24 should be calculated so that the values return to the tolerance ranges are reduced and, if possible, the conditions according to priorities 2) and 3) are met.
  • 2) The tolerance ranges for the selected relationships according to d) should not be left. If the measurement yields values that lie outside the tolerance ranges, a proposal for actuating actuators no. 24 should be calculated so that the values are returned to the tolerance ranges. The calculation should start with the printing ink that deviates the most from its conductivity in terms of size D _R. The calculated proposal must meet the requirements of priority 1) and should meet the requirements of priority 3).
  • 3) If the conditions according to 1) and 2) are met, but the suggestions still leave alternatives open, first the guide values of the screen densities and then the guide values of the solid densities should be achieved as well as possible.
  • 4) If the data processing system does not recognize a control option in order to meet the conditions according to 1) and 2), then an error signal is issued in this regard.

${\text{D}}_{\text{R}}{\text{(Cyan) - D}}_{\text{R}}\text{(Gelb) = 0,10, erfüllt;}$

${\text{D}}_{\text{R}}{\text{(Magenta)- D}}_{\text{R}}\text{(Gelb) = 0,03, Toleranzbereich verlassen;}$

Bedingung nach Priorität 2) ist für zwei Differenzen nicht erfüllt Die Druckfarbe mit der größten Abweichung der Rasterdichte vom Leitwert ist Magenta mit ΔD_R = 0,05. Es wird daher zunächst versucht, die Rasterdichte von Magenta auf den Wert 0,45 zurückzubringen. Wegen der Korrelation würde das allerdings bedeuten, daß die Volltondichte um ca. 0,167 auf D_V = 1,133 abnehmen würde, so daß die Bedingung nach Priorität 1) nicht erfüllt wäre. Auch bei Absenkung der Rasterdichte von Magenta auf 0,46 würde mit D_V = 1,167 die Bedingung nach Priorität 1) nicht erfüllt sein. Wird dagegen die Rasterdichte von Magenta von 0,50 nur auf 0,47 reduziert, ist die Bedingung nach Priorität 1) mit D_V = 1,20 für Magenta erfüllt. Entsprechend ließen sich die Bedingungen nach Priorität 1) auch mit Reduzierungen der Rasterdichte von Magenta auf 0,48 und 0,49 erfüllen. Da die Priorität 3) jedoch vorschreibt, daß beim Vorhandensein mehrerer möglicher Alternativen zunächst die Rasterdichte möglichst nahe dem zugehörigen Leitwert angenähert werden soll, ist der Wert D_R = 0,47 der beste Wert, der nach dem obgen Regelprogramm erreicht werden kann. Es ist dann zu erwarten, daß die Differenzen der Rasterdichte im weiteren Verlauf folgende Werte annehmen:

${\text{D}}_{\text{R}}{\text{(Cyan) - D}}_{\text{R}}\text{(Magenta) = + 0,10;}$

${\text{D}}_{\text{R}}{\text{(Cyan) - D}}_{\text{R}}\text{(Gelb) = + 0,10;}$

${\text{D}}_{\text{R}}{\text{(Magenta) - D}}_{\text{R}}\text{(Gelb) = 0,00.}$

Measurements in ink zone no. 24 now result in the following measurements, for example during the production run:
A) Cyan: D _V = 1.32 D _R = 0.57; Magenta: D _V = 1.30 D _R = 0.50; Yellow: D _V = 1.28 D _R = 0.47; Priority 1) condition is fulfilled for all colors.
B)

${\text{D}}_{\text{R}}{\text{(Cyan) - D}}_{\text{R}}\text{(Magenta) = 0.07, leave tolerance range;}$

${\text{D}}_{\text{R}}{\text{(Cyan) - D}}_{\text{R}}\text{(Yellow) = 0.10, fulfilled;}$

${\text{D}}_{\text{R}}{\text{(Magenta) - D}}_{\text{R}}\text{(Yellow) = 0.03, leave tolerance range;}$

Condition according to priority 2) is not fulfilled for two differences. The printing ink with the greatest deviation of the screen density from the conductance is magenta with ΔD _R = 0.05. Therefore, an attempt is first made to bring the screen density of magenta back to 0.45. Because of the correlation, however, this would mean that the solid tone density would decrease by approx. 0.167 to D _V = 1.133, so that the condition according to priority 1) would not be met. Even if the screen density was reduced from magenta to 0.46, the condition according to priority 1) would not be fulfilled with D _V = 1.167. If, on the other hand, the screen density of magenta is reduced from 0.50 to 0.47, the condition according to priority 1) with D _V = 1.20 for magenta is fulfilled. Accordingly, the conditions according to priority 1) could also be met by reducing the screen density from magenta to 0.48 and 0.49. However, since priority 3) prescribes that if there are several possible alternatives, the grid density should first be approximated as close as possible to the associated conductance, the value D _R = 0.47 is the best value that can be achieved according to the above control program. It can then be expected that the differences in the screen density will assume the following values in the further course:

${\text{D}}_{\text{R}}{\text{(Cyan) - D}}_{\text{R}}\text{(Magenta) = + 0.10;}$

${\text{D}}_{\text{R}}{\text{(Cyan) - D}}_{\text{R}}\text{(Yellow) = + 0.10;}$

${\text{D}}_{\text{R}}{\text{(Magenta) - D}}_{\text{R}}\text{(Yellow) = 0.00.}$

The conditions according to priority 2) are all met.

As a result, after calculating all the alternatives, the data processing system suggests reducing the grid density of magenta from 0.50 to 0.47. In the case of off-line operation, this proposal is implemented by the printer using a table in a corresponding change in the actuator 37 for the ink zone no. 24 and the magenta printing inks. The amount to which the actuator has to be adjusted depends on the special printing press, ie it must always be determined beforehand what correlation there is between a change in the position of the actuators and the change in ink layer thickness or solid density achieved thereby. In the case of online operation, the printer only gives his consent by pressing an operating key, whereupon the associated actuator is automatically adjusted by means of a stepping or servo motor or the like.

${\text{D}}_{\text{R}}{\text{(Cyan) - D}}_{\text{R}}\text{(Gelb) = 0,10;}$

${\text{D}}_{\text{R}}{\text{(Magenta) - D}}_{\text{R}}\text{(Gelb) = 0,02.}$

If a full-tone density for magenta of D _V = 1.24 instead of D _V = 1.30 had been measured according to a variant of the above calculation example, reducing the screen density of magenta to values between 0.45 and 0.48 full-tone densities would result, that do not meet the conditions of priority 1). Only when D _{R is} reduced to 0.49 does the full-tone density with D _V = 1.207 lie in the required tolerance range, so that the data processing system would recommend reducing the screen density from magenta to 0.49, which, after the control process is completed, will result in the following differences between the Grid densities can be expected:

${\text{D}}_{\text{R}}{\text{(Cyan) - D}}_{\text{R}}\text{(Magenta) = 0.08;}$

${\text{D}}_{\text{R}}{\text{(Cyan) - D}}_{\text{R}}\text{(Yellow) = 0.10;}$

${\text{D}}_{\text{R}}{\text{(Magenta) - D}}_{\text{R}}\text{(Yellow) = 0.02.}$

These values are all within the tolerance ranges according to priority 2).

The examples above show the superiority of the control strategy according to the invention compared to conventional control methods. In the calculation example, it was assumed that the screen densities important for maintaining the color balance had all changed. The change in magenta was relatively large and should have been outside a narrow tolerance range using conventional control devices. As a result of the falling out of the screen density For magenta from the tolerance range, the data processing system would have suggested changing the grid density of magenta to 0.45 or a closely adjacent value. If only the screen density was used as the control variable, it would not have been noticed that the proposed rule simultaneously causes an intolerable change in the solid color density. The same would result if only the full-tone density is regulated, since an increase in the full-tone density for yellow from 1.28 to the conductance of 1.30 at the same time a change in the associated screen density from 0.47 to 0.53 and thus an unnoticed falling out would have resulted in the associated tolerance range. If, on the other hand, both the full-tone densities and the screen densities are used as control variables, then the data processing system could not have calculated a reasonable rule proposal. When using the control strategy according to the invention, on the other hand, it is possible to a) define relatively large tolerance ranges for the absolute values of the solid and screen densities, nevertheless b) maintain the color balance by means of relatively narrow tolerances for the selected relationships, and c) develop reasonable rule proposals by taking the correlation into account . The "selected relationships" serve the purpose of tolerating such changes to one another in the screen densities and / or the solid densities of the printing inks involved that essentially go in the same direction, while largely eliminating changes running in opposite directions. The arithmetic mean of the screen dot sizes and / or solid ink densities of all the colorful and / or achromatic printing inks involved can only be mentioned as selected relationships, for example. Instead of the differences and quotients, other relationships could also be selected and these extended to the relationships between three or more printing inks to one another. The proposed differences and quotients for color pairs have the advantages, however, that on the one hand they can be easily implemented by electrical circuits and therefore can also be calculated automatically using inexpensive switching elements, while on the other hand, closely tolerated changes in the differences and quotients of the screen densities in the associated color cubes practically only result in changes in close to the room diagonals and therefore mainly changes in the brightness of a printed color, but hardly changes the color shade. In contrast to previous control methods and devices, the correlation not only enables a comparison of the absolute values of the screen and solid densities, but also an estimate of the changes caused by an intervention in the printing process with the actuators 37 both with regard to the solid density and also in terms of screen density can actually be achieved. In the case of constant new calculations in the long-term range, the correlation ultimately serves to automatically adapt the control strategy to the changing properties of the printing press.

Details of the process control system of the control device according to the invention are explained in more detail below with reference to FIGS. 5 to 7. The control device first comprises a densitometer 71, e.g. Macbeth PXD-981, which scans a printed sheet and feeds the measurement data obtained to a measured value concentrator 72, which then forwards the data to a process control system 73. This essentially consists of a setpoint or conductance calculator 74, an actual value or measured value calculator 75 and a manipulated value calculator 76 which is connected via lines 77 to the actuators of ink fountains 78 of a multicolor printing press. The conductance calculator 74 is connected to a number of peripheral devices, e.g. with an operating console 80 having buttons 79, a memory 81 in the form of a magnetic tape, blister, perforated tape memory or the like, a printing unit 82 and a monitor 83, for example in the form of a screen.

The operation console 80 is used to enter commands, in particular those relating to the various guide values, tolerance ranges or the like, into the process control system 73. In the memory 81, for example, all the data which relate to a specific edition are stored and have already led to a good printing result and in particular include all the necessary settings for the ink fountains 78. The printing unit 82 can print out the data appearing on the monitor 83 or a log of the printing process during a print run. The monitor 83 serves to visualize the respective operating states of the multicolour printing press, suggestions for a control process or the like calculated by the process control system 73. The master value computer 74 processes the data and commands received from the operation console 80 and from the memory 81, compares them with the data determined by the actual value computer 75, works out rule proposals and, if necessary, forwards them in the monitor 83 and after approval by the printer to the manipulated variable calculator 76. The latter then converts this data into corresponding electrical signals, by means of which the actuators of the actuating device, which consists of the ink fountains and their zone screws or the actuators controlling them, are controlled in the desired manner. The measured value concentrator 72 is connected to the densitometer (s) 71 by means of trailing cables and takes up all measured values determined by them with a large number of parallel lines 84 in very rapid succession. So that these measured values do not have to be forwarded to the process control system 73, which is usually remote from the multicolor printing press, the measured value concentrator 72 is arranged directly on the multicolor printing press, so that it concentrates the measured data supplied and then serially over a few lines 85 can forward to the process control system 73.

The densitometer 71 is guided over the printed sheet according to a program located in the memory 81, which is fed to it via the conductance calculator 74 and the measured value concentrator 72. The program contains data for the motor, for example, by means of which the densitometer 71 is moved over the printed sheet, as well as data relating to the times at which it is to deliver measurement data and, for this purpose, for example, throws a flash of light on the printed sheet. It can be provided that the densitometer 71 moves gradually from color zone 42 to color zone 42 (FIG. 3) and always after reaching a color zone is then triggered to deliver measured values when a raster or full field 43, 44 or any other measuring field of a printed sheet moves beneath it. In this case, for example, densitometers are used which, when a flash of light is emitted, the reflected light beam by means of a prism, by means of optical filters or the like are immediately broken down into the partial beams assigned to the existing printing inks, so that measured values for all printing inks are obtained per flash of light. According to FIG. 6, all data relating to a printing process can be entered into the conductance calculator 74 with the memory 81 or with the operation console 80. These data are distributed to memory units of a conductance memory 86 assigned to them, which are named, for example, with the terms "solid tone densities", "screen dot sizes", "selected relationships" (meaning their conductance values in each case), "tolerance ranges V, R, B" for the Solid density, the screen density and the selected relationships, "correlations", priorities ", color consumption", "color balance", "print type correction" and "color type correction" are designated.

For the values already explained above, data relating to the color consumption can thus first be entered. This means the total amount of used printing ink determined within a color zone, which can fluctuate between 0% and 100% for each color. The sensitivity or the response speed of the control process can be influenced via the color consumption. With high ink consumption in a color zone, the adjustment of an actuator will affect the print result faster than with low ink consumption. In the presence of a given difference between an actual or measured value and the desired guide or target value, it may therefore be expedient to initially adjust the associated actuator more strongly with a low ink consumption than would be necessary with a high ink consumption, in order thereby to approach it more quickly to get the conductance. Apart from this, an adjustment of the actuators can also be made dependent on whether a printing ink is more or less intensive, ie with greater or smaller color layer thickness is applied. A correction value for the control signal supplied to the relevant actuator can thus be entered via the "color consumption" storage unit.

Further corrections for the control signals may prove necessary if there are extreme differences in color consumption and / or color intensity in two adjacent color zones in order to avoid visible changes in these transitions when adjusting the control elements. Finally, the values "printing type correction" and "color type correction" are to be used to generate correction values for the target signals which are required on account of the properties of the printing materials or printing inks used. In particular, it should be taken into account that printing substrates can take up a lot or little printing ink or that the printing inks are applied more or less strongly to the printing substrate due to their rheology under otherwise identical conditions. The actual value calculator 75 contains an actual value memory 87, in particular with storage units for the screen and solid densities measured by the densitometers 71. In addition, storage units can be provided, into which data relating to the "optically effective area coverage", the "mechanical area coverage", the "halftone dot changes" and the "ink layer thickness" are entered. Finally, storage units can be provided in which information is stored which relates to measurement programs, parameters of the raster areas 43 (e.g. their area coverage in%) or the like. This data is repeatedly determined by the measured value computer 75 from the screen and solid densities.

The manipulated value calculator 76 is used to compare the information calculated and supplied by the actual value calculator 75 at certain time intervals or continuously with the guide values or tolerance ranges specified by the conductance calculator 74, based on the priorities or control strategies communicated by the conductance calculator 74 for actuators 88 to be calculated and, if necessary, displayed on the monitor 83 or directly to the actuators 88, which consist of the zone screws, their servomotors or the like, each inking unit of the multi-color printing press having, for example, 32 such actuators. For this purpose, the manipulated value computer has a manipulated value memory 89 with storage units for the information supplied by the conductance computer 74. This information relates, for example, to the starting conditions of the ink ductors or actuators depending on the ink consumption or previously produced, identical or similar editions, as well as correction factors for the printing and / or ink types, compensation factors (e.g. if an ink zone is influenced by an adjacent ink zone, calculated from the Ink consumption), furthermore characterizations of the ink duct openings or the like with the aid of characteristic curves (for example on the basis of the relation ΔOpening / Δ color mass flow) or finally current control strategies, calculated on the basis of priorities or color dominances.

Finally, details of the process control system are shown schematically in FIG. 7. The actual value calculator 75 then contains a computing unit 91 for each color zone, the inputs 92 of which are supplied with the measured values of the screen densities of the available printing inks. These measured values are converted into suitable signals which correspond to the respective actual values and which appear in lines 93. Corresponding computing units 91 can be provided for the area coverage. The computing units 91 for the “selected relationships” between the grid point sizes additionally have difference, divider or other stages 94 in order to form the differences, quotients or the like from two or more measured values.

For each color zone, the master value calculator 74 contains computing units 95, the inputs 96 of which are supplied with the master values or the limit values of the tolerance ranges for the screen dot size and have the stages 97, which calculate or merely determine the differences from the master and actual values, whether the actual values are within or outside the associated tolerance ranges. The data obtained are fed to a microprocessor 98 constructed from programmable matrices, with which the control strategies for the manipulated value computer 76 are calculated with the aid of the correlations and priorities.

For the full-tone densities, similarly constructed computer units 99 can be provided, the inputs 100 of which, for example, the measured and correspondingly converted actual values and the further inputs 101 of which are supplied with the guide values or the limit values of the tolerance ranges. The computer unit 99 has stages 102 which calculate the deviations between the guide and actual values or merely determine whether the full-tone densities are within or outside the tolerance ranges. The corresponding data are also fed to the microprocessor 98. Finally, the information contained in the "priorities" storage units (FIG. 6) is fed to the microprocessor 98 via a line 103. In the example in FIG. 7, it is provided, for example, that a comparator 104, which is also connected to the line 103, is connected in the connecting line between the arithmetic unit 99 and the microprocessor 98 and, for example, specifies as a priority that the microprocessor 98 first processes the data Data of the printing ink whose solid ink density deviates the most from the associated target or guide value should begin.

In the microprocessor 98, the data determined are processed in accordance with the program described above or some other predetermined program, for example stored in the memory 81 (FIG. 5). A proposal is then made for how the actuators should be operated so that all priorities are met. This suggestion is made visible in monitor 83 if necessary and evaluated by the printer. If necessary, 80 corrections can be made via the operating console. Finally, the data calculated by the microprocessor 98 are either direct (with fully automatic operation) or after approval and, if necessary, correction converted by the printer into actuating signals for the actuators and then sent to non-linear controllers 105, a controller 105 being assigned to each actuator. The regulators 105 effect a specific adjustment of the actuators in dependence on the supplied control signals. In this case, further inputs of the controllers 105, for example via lines 106 and 107, can each be supplied with the correction values for the print type or color type correction stored in the corresponding memories of the setpoint computer 74 (FIG. 6). A further correction stage 108 is connected to the outputs of the controllers 105, which has the data of the memory for the color consumption (FIG. 6) via a line 109 and the data of the memory for the color balance with respect to the two adjacent color zones via lines 110 and 111 are fed. The output lines 112 of the correction stage 108 lead to the actuators. It should be taken into account that the correction stage 108 and the controllers 105 are assigned to one of the 32 available color zones and three printing colors, for example cyan, magenta and yellow, and that corresponding correction levels and controllers must be available for the other color zones.

The described exemplary embodiments can be modified in many ways. This applies in particular to the various circuits of the control device. With regard to the specified tolerance ranges, it should be noted that these should be chosen so closely that if a measured value falls out of the tolerance range assigned to it, the print result is still within the limits tolerated by the printer or the customer, and that there are also minor deteriorations that occur before full effect of the control device could not lead to the fact that the now printed sheets are unusable. In particular, further limit values could be entered into the process control system which lie outside the tolerance ranges mentioned and prescribe to the process control system that a printing process must be finally stopped if these limit values are reached or exceeded.

The number and frequency of the measurements with the densitometers is largely at the discretion of the person skilled in the art. To increase the measuring accuracy in each color zone, it is advisable to first take several measurements with regard to the solid density as well as the screen densities, for example by measuring five successive sheets and forming an average from the measured values obtained in this way. At most, a period of a few seconds is required for this, during which the properties of a multicolor printing machine generally do not change significantly. From the mean values obtained in this way, rule suggestions for the color zone in question are then calculated. After completing these measurements, the densitometer is set to the next color zone, where the same measurements are repeated on the next sheet passing through. By constantly moving the densitometer back and forth step by step or in cycles across the entire printing width, information about the printing process is continuously collected in this way and rule suggestions are calculated if necessary. In this case, a further memory of the setpoint computer can additionally be informed of the time periods, measured e.g. in the number of sheets passing through, a given control command must be implemented in the desired change in the associated control variable. Finally, on the basis of the given control commands and the changes actually made to the actuators or the changes in the ink layer thicknesses or the screen densities caused thereby, the correlations between these existing correlations can be continuously recalculated in order to determine system changes during printing and the rule proposals always the last measured To be able to use correlations.

8 shows an embodiment of a single-color stripe set 118 according to the invention, which consists of three single-color stripes 119, 120 and 121. The set or each individual color strip 119 to 121 preferably contains as many zones 122, 123 and 124 in a row and next to one another as color zones in the multi-color offset printing machine used are provided. The upper single color stripe 119 is assigned the color cyan, the middle single color stripe 120 the color magenta and the lower single color stripe 121 the color yellow. The individual color strips are, for example, positive films which are transferred in a known manner to a designated location of the associated printing form in such a way that they are printed by the individual printing units in succession at the same location on the upper or lower edge of the image and there the so-called print control strip form.

The single color strip 119 contains a raster element 126 and a solid element 127 in the zone 122, so that corresponding measuring fields appear at the corresponding location of the printing material. The number and shape of the grid points appropriately correspond to a preselected grid. The size of the halftone dots, on the other hand, is preselected in accordance with a preselected gray value in such a way that those halftone dots of the individual color strip 119 which lead to areas to be printed on the printing form, e.g. have an area coverage of 50%. The solid element 127 is formed such that a correspondingly large area with a defined solid tone density results.

The individual color strips 120 and 121 each have a raster element 128 or 130 and a full element 129 or 131 within zones 123 and 124. Shape and number of grid points in the Screen elements 128 and 130 again correspond to the selected screen fineness, whereas the size of the screen dots in these screen elements lead, for example, to area coverage of 41% each. The solid elements 129 and 131 are selected such that areas with a defined solid tone density result from them.

The raster elements 126, 128 and 130 are each arranged in a region of the zones 122, 123 and 124 in such a way that the corresponding sections of the printing forms print at the same location of the printing material. This means that instead of a set of three grid fields in one color per color zone, only a single, gray or brown grid field with a gray value appears, which is composed of the grid levels 50% cyan, 41% magenta and 41% yellow. In a corresponding manner, the three full-tone elements 127, 129 and 131 are also printed on top of one another on the printing material, so that there is also a single measuring field in gray or brown.

The zones 122, 123 and 124 shown in the right part of the single color strip set in FIG. 8 are designed accordingly. In addition, only two of, for example, 28 zones are drawn.

As an alternative to the sentence according to FIG. 8, it is possible to to produce measuring fields on the printing material which contain, for example, combined raster information about the colors cyan / magenta or cyan / yellow and combined full-tone information about the colors cyan / magenta or cyan / yellow. In addition, a measuring field can be obtained that only has information about the color black. Additional halftone and full fields, which are not printed on top of each other with any other halftone or full field, and which are expediently arranged over the entire length of the individual color strips, are used to continuously determine measured values from which the correlations between the halftone dot sizes and solid densities are calculated. These measured values are preferably first collected and then statistically evaluated in order to obtain an average. The computer programs for this are generally known.

If the measurements are carried out on combination measuring fields, then the full-tone densities and / or screen dot sizes and / or selected relationships obtained therefrom often deviate from the corresponding values obtained with the aid of single-color measuring fields, which is attributed to various causes can be. Surprisingly, however, it has been shown that the observed deviations are not only significantly smaller if, instead of the absolute full-tone densities and screen dot sizes, only the selected relationships between these, in particular differences, are determined, but are also made negligibly small by simple and schematic corrections to the measured values obtained can. This is particularly true if the deviations in the selected relationships fluctuate only within the relatively small tolerance ranges specified above, for example. It is therefore in principle sufficient to subsequently subject the measurement values obtained by scanning combination measurement fields to a correction. This can be done, for example, with a set of mathematical approximation formulas for correcting the measured values or with a color table or color table which enables the determination of corrected measured values by means of a comparison.

In colorimetry, the sensory, i.e. Assessment of color differences depending on the particular perception of the individual viewer using known formulas according to CIELAB, CIE-USC, Hunter or the like. The color difference is defined as the distance between two color points in the color space. In contrast, with regard to the sensitivity of images, the invention is based on the surprising finding that such assessments of color differences can only be used meaningfully if selected color nuances are compared with neighboring color nuances and no contrasts are effective. This is usually not the case when assessing an image, since images have more or less strong contrasts, which change the perceived assessment of color differences to a very great extent.

Up to now it has not been possible to quantitatively assess the color differences of images in the presence of contrasts. However, in order to better define the tolerance range for the control technique described above or another control technique, it would be very useful to know which color differences are just perceived as acceptable for any image, taking into account the existing contrast. In this respect, the invention proposes the following procedure:
First, a test image is selected that is representative in its contrast for a group of images with the same or similar Contrast ratios. Reproductions and a test print are made from this test image in a known manner. If this sample print is described by an average viewer as having the correct color, ie the color matching the test image, then variants with preselected color distances are produced from this test image. These variants are characterized in that, for example, the area coverage of the halftone dots of each variant deviate from the area coverage of the halftone dots of the sample print designated as color correct in at least one color by a fixed value of, for example, 2%, 4% or the like, these deviations in each case a preselected area coverage, e.g. 50%, can be obtained. The changes for the other stages result from this in a known manner. In order for these variants to be meaningful, great precision must be maintained in their manufacture. For this purpose, for example, the area coverage of the halftone dots of rasterized films are changed photographically in a preselected manner using the contact method, and accuracy in the midtones is preferably maintained at least 0.5%.

The variants obtained with the known color differences are now preferably presented to a plurality of viewers individually with the request to designate each variant which can still be accepted. An average is formed from the answers of the various viewers, which is then typical for an average viewer in the assessment of all images that have similar or identical contrast ratios as the associated test image. Since it is known which color distances are assigned to the individual variants, the desired values for the tolerance ranges can be derived directly from these.

In the case of very low-contrast images, which are predominantly composed of achromatic tones, color differences, which are caused by differences in the halftone dot changes in the three primary colors of the order of 3% to 4%, lead to color differences which the viewer considers to be at the limit of acceptance to be felt lying down. On the other hand, very high-contrast images, which are predominantly made up of pure, complementary, intensive colors, only become at the acceptance limit when color differences caused by differences in the halftone dot changes in the three basic colors are reached perceived lying.

In order to avoid having to produce a large number of variants with preselected color distances for a large number of test images, it is proposed according to the invention to divide a small number of carefully selected, typical test images into a number of image contrast classes, so that in each image contrast class a number of typical images with different subjects, but with the same or similar contrast ratios. Since the experts on the In the field of reproduction and printing technology because of their professional activity in classifying images with similar contrast ratios, they are also able to classify any other image to be reproduced or printed into one of the image contrast classes. The number of variants per test pattern can also be limited to a small number, for example three.

Finally, according to the invention, the individual image contrast classes are assigned tolerance ranges for the control method according to the invention described above. In this way, it is sufficient to divide an image to be reproduced or printed into one of the existing image contrast classes and to use the quantitative tolerance ranges assigned to the respective image contrast class for the control method monitoring the print.

The method described has the essential advantage that the person skilled in the art can use the test images and their variants to show the customer which color fluctuations are possible during printing and what the consequences could be in terms of printing costs.

FIG. 9 shows a device for determining the color balance in the printing result of a multicolor offset printing machine or for displaying the image contrast classes. The device consists of a hexagon 152, which is constructed from a multiplicity of small control elements 153 which are arranged around a central control element 154 which defines a zero point and is delimited by an outline 155. The control elements 153 preferably consist of hexagons of the same size, which adjoin one another with their side edges. A first group of six control elements 155 to 161 surrounds the central control element 154 in an approximately circular manner, this group being delimited on the outside by an outline 162. The first group is surrounded by control elements 153 of a second group which is delimited by an outline 163 and in turn is approximately circularly surrounded by the control elements 153 of a third group delimited by an outline 164.

The central control element 154 is produced by printing three individual color fields of the three printing colors cyan, magenta and yellow on top of one another. A certain combination of surface coverings is chosen, which should form the zero point of the gray balance or the color balance during printing. For example, an area coverage of 50% for the color cyan and an area coverage of 41% for the colors magenta and yellow are provided in the screen film used for the production of the printing form.

The control elements 156 to 161 surrounding the control element 154 and representing selected color nuances, on the other hand, have surface coverings which differ from those of the zero point in different but defined ways. For example, the upper control element 156 is characterized by a halftone dot enlargement in magenta of 2% and halftone dot reductions of 2% in cyan and yellow. The lower control element 159 is characterized by a halftone dot reduction in magenta by 2% and halftone dot enlargements of 2% each in cyan and yellow. The upper left control element 161 has a halftone dot reduction of 2% in yellow and halftone dot enlargements of 2% each in magenta and cyan, while the lower right control element 158 has an halftone dot enlargement of 2% in yellow and dot increases of 2% in magenta and cyan. Finally, the control elements 160 and 157 are distinguished by corresponding screen dot enlargements or reductions of 2% each in cyan and corresponding screen dot reductions or enlargements of 2% each in magenta and yellow. The control elements 156 to 161 of the first group are thus characterized in that the area coverage of the halftone dots in the halftone film differs from that of the central control element 154 by exactly + 2% or - 2%.

bezeichnet, dann ergeben sich daraus die folgenden Zuordnungen: Kontrollelement 156 B1 = - 4 % B2 = 0 % B3 = + 4 %, Kontrollelement 157 B1 = - 4 % B2 = - 4 % B3 = 0 %, und Kontrollelement 158 B1 = 0 % B2 = - 4 % B3 = - 4 %. According to the invention, the proposed control strategy, on the other hand, is based on the idea that a color shade changes only slightly if the screen levels of all the colors involved change in the same direction. This also applies accordingly to the respective zero point and in particular within certain limits. Therefore, the control elements 156 to 161 of the first group are not assigned the absolute values of the screen dot sizes, but rather the selected relationships derived from these, for example the differences which are preferably used, while the screen levels of the control element 154 receive the values "zero", so that instead of C = 50%, M = 41% and Y = 41% results in C = 0%, M = 0% and Y = 0%. If the difference C - M is B1, the difference C - Y is B2 and the difference $\text{M - Y = B3}$

the following assignments result: Control element 156 B1 = - 4% B2 = 0% B3 = + 4%, Control element 157 B1 = - 4% B2 = - 4% B3 = 0%, and control element 158 B1 = 0% B2 = - 4% B3 = - 4%.

The differences B1, B2 and B3 for the control elements 159, 160 and 161 can be calculated accordingly. These assignments thus mean that the differences B1, B2 and B3 within the first group differ by a maximum of + 4% or - 4% from those of the central control element 154, for whose differences, by definition, B1 = B2 = B3 = regardless of their actual value 0 applies.

The first group, which contains the control elements 156 to 161, is now referred to as image contrast class X. At the same time, this means that the image contrast class X includes all images in which the differences B1, B2 and B3 may not change by more than ± 4%, based on the selected zero point, during printing and therefore the tolerance ranges for the selected relationships are set to ± 4%.

Correspondingly, the control elements of the second group, which is delimited by the outline 163, can be assigned to images, whose differences B1, B2 and B3 may not change by more than ± 8% in relation to the selected zero point during printing and therefore the tolerance ranges for the selected relationships (in this case the differences B1, B2, B3) to ± 8% can be set.

In the third group, delimited by the outline 164, the tolerance ranges for B1, B2 and B3 are each ± 12%. This group is called image contrast class Z.

Another advantage of the hexagon 152 is that its control elements, like the combination measuring fields in the pressure control strip (cf. FIG. 8), can also be produced under the same conditions as these. Therefore, the printer can visually assign a combination measuring field of the print control strip to the control element best matching this in the hexagon 152 and from this directly estimate the distance of the combination measuring field from the defined zero point or recognize whether the printed combination measuring field is still within the tolerance range to be observed lies. The coordination system shown in FIG. 10 can serve him as a further aid. In this the lines between the letters M and C mean the values for B1, the lines between the letters C and Y the values for B2 and the lines between the letters Y and M the values for B3. If, for example, a combination measuring field of the print control strip corresponds to a control element 165 of the hexagon in terms of its color shade, then by placing the coordination system according to FIG. 10 on the hexagon 152 it can be immediately read that the color shade has the values B1 = 0, B2 = 8 and B3 = 8 are assigned and must therefore be corrected in the printing process if an image is currently being printed that is assigned to image contrast class X.

If the measured values C = + 4%, M = + 4% and Y = 0% are determined on a combination measuring field of the pressure control strip, the values B1 = 0, B2 = 4 and B3 = 4 result from this. With the help of the coordinate system it follows that the control element 161 is assigned to this combination measuring field. It can be seen from this that the tolerance range has not yet been left when printing, provided that it is an image that is assigned to image contrast class X. If the absolute values of the screen dot size were used instead of the selected relationships, the tolerance range would be erroneously displayed because within the image contrast class X the deviations of the screen dot sizes compared to the zero point are a maximum of ± 2%, but the measured deviations for cyan and magenta are + 4% .

Instead of the selected zero point with the area coverage 50%, 41% and 41%, zero points with any other level can be selected. Such a zero point can also be any control element 153 of the hexagon 152, since in such a case only the numerical values for the special relationships B1, B2 and B3 need to be changed, as can be easily determined by placing the coordinate system according to FIG. 10, if there is one Zero point is placed on any other control element instead of on the central control element 154. Moreover, the coordinate system can be used to assign a unique triplet of numbers for the values B1, B2 and B3 to each individual control element of the hexagon 152. If the hexagon 152 is produced in other gradations, the coordinate system must be changed accordingly. The same can apply if, instead of hexagons, other shapes, for example circles, are provided or a completely different spatial arrangement is chosen instead of the spatial arrangement of the control elements shown in FIG. 9.

In addition, the tolerance ranges assigned to the image contrast classes can be freely selected and adapted to the respective needs. The class division described is only an example. In addition, more or less than three image contrast classes can be selected and the levels between the individual image contrast classes can be selected differently. Furthermore, the Hexagon 152 can be replaced by a device in which the control elements consist of overprinted full fields instead of grid fields. It would also be possible to assign other selected relationships to the individual control elements or to convert the differences in the grid point sizes into other values. Furthermore, it would be conceivable to produce devices of a similar type which are produced by the overprinting of more or less than three individual color fields.

Claims (20)

1. A method of achieving a uniform printing result in an offset multi-colour printing press in which the feed of the printing ink to adjacent coloured zones (42) of a printing substrate (41) can be adjusted by means of regulating elements (37) and in which ink film thicknesses and/or screen dot sizes on measuring fields (43, 44) printed inside the colour zones are repeatedly determined for regulation of the printing process and on falling outside their associated tolerance ranges are taken into account in the printing process by actuation of the regulating elements so as to effect correction, characterized in that magnitudes are used to regulate the desired colour balance, which magnitudes are so determined that ink film thicknesses and/or screen dot sizes of different printing inks are related to one another and in that, on these magnitudes falling outside their associated tolerance ranges, they are taken account of in the printing process so as to effect correction, in order to bring these magnitudes back into the tolerance ranges by operation of the regulating elements (37).
2. A method according to claim 1, characterized in that the regulation of the colour balance is effected in such a way that, alterations in the same sense in the ink film thickness and/or screen dot size of different printing inks are tolerated more strongly that alterations in the opposite sense.
3. A method according to claim 1 or 2, characterized in that measuring fields in the form of screen and/or solid surfaces (43, 44) are printed and as the magnitudes which are so determined that ink film thicknesses and/or screen dot sizes of different printing inks are related to one another there are used the differences of the screen dot sizes and/or ink film thicknesses determined at the screen and/or solid areas of two different printing colours.
4. A method according to any one of claims 1 to 3, characterized in that the regulating elements (37) are actuated having regard to the correlation between changes in the ink film thickness and the screen dot size.
5. A method according to claim 4, characterized in that the correlation is repeatedly determined during the printing process and the actuation of the regulating elements (37) is effected on the basis of the correlation so obtained.
6. A method according to any one of claims 1 to 5, characterized in that the printing process is interrupted if the screen dot sizes and/or the ink film thicknesses and/or the magnitudes which are so determined that ink film thicknesses and/or screen dot sizes of different printing inks are related to one another fall outside associated tolerance ranges for a predetermined length of time.
7. A method according to claim 4, characterized in that the regulating elements (37) are so actuated in dependence on the correlation that the screen dot sizes and/or the ink film thicknesses and/or the magnitudes which are so determined that ink film thicknesses and/or screen dot sizes of different printing inks are related to one another approximate as closely as possible predetermined guide values.
8. A method according to any one of claims 1 to 7, characterized in that, at least for the magnitudes which are so determined that ink film thicknesses and/or screen dot sizes of different printing inks are related to one another there are provided groups of tolerance ranges, where each group comprises tolerance ranges determined for a predetermined quality of the printing results, and in that a selected one of these groups is used for each printing.
9. A method according to claim 8, characterized in that the choice of the group of tolerance ranges is made dependent on the contrast in the image to be printed.
10. A method according to claim 8 or 9, characterized in that a number of image contrast classes with contrasts typical for multi-colour images are provided, each image contrast class being associated with one of the groups of tolerance ranges and being used for printing that group of tolerance ranges which is associated with the image contrast class of the image to be printed.
11. A method according to claim 10, characterized in that there are prepared for each image contrast class a correctly coloured test print of at least one test picture with a contrast typical for the image contrast class and a number of variants of the test print with precisely defined colour distances, in that in the production of the test prints and variants, the screen point sizes and/or ink film thicknesses and/or magnitudes which are so determined that ink film thicknesses and/or screen dot sizes of different printing inks are related to one another are suitably preselected in the individual colour separations, and in that the group of tolerance ranges associated with the image contrast class is determined in dependence on the colour distances.
12. A method according to any one of claims 1 to 11, characterized in that measuring fields in the form of combination measuring fields are formed in selected colour zones, in which single colour measuring fields (126 to 131) of at least two different printing colours are overprinted, in that the combination measuring fields are sensed densometrically and measured values are thereby obtained, in that corrected values for the ink film thicknesses and/or screen dot sizes and/or magnitudes which are so determined that ink film thicknesses and/or screen dot sizes of different printing inks are related to one another are derived from these measured values, in order to correct at least partially the errors resulting from the use of the combination measuring fields, and in that regulating signals for the regulating elements (37) are derived from the corrected values.
13. A method according to any one of claims 1 to 12, characterized in that a colour balance control is used with the half-tone combination measuring field (43, 44) determined by multi-colour printing to obtain information for the adjustment of the regulating elements (37), the measuring field being obtained by overprinting at least two single colour measuring fields with predetermined screen dot sizes and/or ink film thicknesses in two print units (I - IV) of the multi-colour printing press, in which the combination measuring field is sensed by means of measuring device (45), and in that the automatic proposals for the adjustment of the regulating elements (37) are computed by means of a process control system (73) connected to the measuring device (45) and by using the measured values obtained from sensing the combination measuring field in a manner such that the colour balance of the combination measuring field serves as a regulating magnitude, and these proposals are displayed by means of a monitor (83) and/or are converted into adjusting signals for the regulating elements (37).
14. A method according to any one of claims 1 to 13, characterized in that, to determine the colour balance in the printing results of the multi-colour printing press, there is used a device which has a first control element (154) defining a datum point, formed by overprinting single colour fields of selected printing colours , which control element is surrounded by at least a first group of further control elements (156 - 161) formed by overprinting the same printing colours and defining predetermined distances from the datum point, wherein the screen dot sizes and/or in film thicknesses in the control elements (154, 156 - 161) are so selected that magnitudes which are so determined that ink film thicknesses and/or screen dot sizes of different printing inks are related to one another have first selected values in the control element (154) defining the datum point and other preselected values in the further control elements (156 - 161) of the group, these other preselected values differing from the values in the control element (154) defining the datum point by preselected but different amounts in each case and determining the limits of the tolerable changes in the magnitudes within the group during printing.
15. A method according to claim 14, characterized in that the control element (154) and the further control elements (156 - 161) consist of like-sized hexagons.
16. A method according to claim 15, characterized in that the first group of six control elements (156 - 161) is surrounded by a second group of twelve hexagonal control elements (153) of corresponding sizes and all control elements combine to form a hexagon (152).
17. A method according to claim 17, characterized in that all control elements (153, 154, 156 - 161) are formed from single colour elements of the colours cyan, magenta, and yellow and whose screen dot sizes and/or ink film thicknesses as so selected that, along the three principle axes intersecting at the middle point of the first control element (154), extending perpendicular to its side edges, each of the magnitudes which are so determined that ink film thicknesses and/or screen dot sizes of different printing inks are related to one another increase in one direction, decrease however in the opposite direction or stay the same in both directions.
18. Apparatus for achieving a uniform printing result in an offset multi-colour printing press with a plurality of print units which each have printing forme with a section for printing measuring fields for printing a printing medium and each have an inking system (78) associated with one printing ink, which inking system is provided with a plurality of regulating elements for the transfer of the associated printing ink to neighbouring printing zones of the associated printing forme or of the printing medium, with an adjusting device comprising the plurality of regulating elements and with a process control system which comprises an actual value computer (75) for processing the measured values repeatedly determined during a printing operation from the measuring fields in the form of screen dot sizes and/or ink film thicknesses, a guide value computer (74) for input of tolerance ranges associated with the measured values, and a set point computer (76) for repeatedly comparing the derived measured values and the tolerance ranges and for proposing adjusting signals for the adjusting device on the measured values falling outside the tolerance ranges, characterized in that the actual value computer (75) has computing units (91) which convert the measured values into signals corresponding to magnitudes which are so determined that the ink film thicknesses and/or screen dot sizes of different printing colours are related to one another, and in that the guide value computer (74) has a guide value memory (86) for input of tolerance ranges for these magnitudes as well as microprocessors (98) for repeated comparison of the magnitudes and the associated tolerance ranges and for computing proposals for actuation of the regulating elements when the magnitudes fall outside the tolerance ranges, in such a manner that these magnitudes are returned to the tolerance ranges on following the proposals.
19. Apparatus according to claim 18, characterized in that, with on-line operation, the data computed by the microprocessors (98) can be converted into adjusting signals and can be fed directly or after enabling to the adjusting device.
20. Apparatus according to claim 18 or 19, characterized in that it has an operator console (80) for correcting the proposals.

Images