Method of controlling or regulating the ink supply in a printing press.

0001The invention relates to a method for color control or color regulation of a printing press according to the preamble of claim 1.

0002From EP-A 228 347 (corresponding to US patent applications Serial No. 939,966 dated December 10, 1986 and Serial No. 213,000 dated 29. June 1988), a method of the type mentioned is known in which a plurality of reference fields are evaluated in order to optimally adapt the color impression in order to compare the color location of the respectively scanned reference field with a color location specified for this reference field and from the color difference between the actual To determine the color location and the target color location a layer thickness change control vector which adjusts the ink guide elements of the printing press in such a way that the smallest possible color deviation is achieved. Sometimes, however, it is not possible due to predetermined boundary conditions, in particular predetermined minimum and / or maximum layer thicknesses of the printing inks, to shift the actual color location to the predetermined target color location. In such cases, there remains a color difference error, which has a more or less disruptive effect, since the specified target color location is outside the correction color space, the dimensions of which are predetermined by the permissible changes in the layer thicknesses of the solid densities of the printing inks involved.

0003EP-A 124 908 describes a device and a method for determining the required raster area coverage of color separations, which can be displayed in percent, in order to reproduce the color of a given template pattern to be reproduced as accurately as possible. The known device has a measuring head which contains, for example, filters for the colors red, green and blue and allows color information, in particular color densities, of the respectively scanned originals to be measured using these filters. The measuring head is connected to a data processing device, which has a keyboard used when scanning predetermined reference patterns for entering raster area coverage in percent. The data processing device furthermore has a display device for displaying raster area coverage degrees calculated on the basis of the scanning of a template pattern.

0004Before the known device for determining the raster area coverage of a set of color separations can be used, it is necessary to create a conversion table for converting color information into raster area coverage levels, which is stored in a memory of the data processing device. First a color sample card is printed. The colors cyan, magenta, yellow and black are used to print the color sample card, whereby grid area covers between 0% and 100% are used in increments of 10% for all colors. This results in 14,641 combinations for the grid area coverings and the associated color information, for example recorded as color densities.

0005When each sample of the color sample card is scanned, the combination of the screen area covers used is entered via the keyboard and assigned to the color densities detected by the measuring head.

0006When the conversion table is created, the device allows a previous day's pattern to be reprinted to be scanned using the measuring head and, by comparing the color densities recorded with the aid of the various filters with the color densities stored in the conversion table, the row in the conversion table whose color density values ​​match those with be determined measured color densities of the original pattern match or best match. If this line is found in the conversion table, the assigned screen coverage levels for, for example, three or four color separations are displayed on the display device or forwarded to an external device. Since the grid area covers for printing were changed in increments of 10% when creating the color sample card, the conversion table is relatively rough and inaccurate. For this reason, according to an improved method, interpolation of the values ​​in the conversion table determines additional intermediate values ​​for the color information and the associated raster area covers. The interpolation can be carried out in such a way that increments of 1% are provided, which results in a more accurate reproduction of the template pattern to be reprinted.

0007In order to determine the grid area coverage, the color differences between the color information of the template pattern and the color information in the conversion table are calculated in the data processing device. The known method can also be designed in such a way that before the output of values ​​for the screen coverage levels, a query is made as to whether values ​​of 0% or 100% are present. By extrapolating the screen coverage levels and the color densities, an extended color space for screen coverage between - 10% and 110% is determined due to the color density changes in the range between 0 and 10 or 90 and 100%. In this way, the known method allows an indication of the non-reproducibility of a template pattern.

0008Proceeding from this prior art, the object of the invention is to sharpen a method which allows the highest possible print quality to be achieved even if the specified target color location lies outside the correction range limited by the specified boundary conditions. This object is achieved according to the invention by the characterizing features of claim 1.

0009Because the predetermined target color location that cannot be reached is replaced by an accessible target color location according to a control strategy, an optimal position in the color coordinate space can be controlled for the actual color location. In the simplest case, the color location that is defined by the intersection of the color distance vector between the actual color location and the target color location with the surface of the color correction body is selected as the achievable target color location. However, it is more advantageous to select the color location on the surface of the correction color space that has the smallest distance from the specified target color location as the achievable target color location. Depending on the position of the predetermined target color location, the achievable target color location with the smallest distance from the predetermined target color location can be found in that a solder is achieved from the predetermined target color location to the surface of the correction color space by the predetermined target color location. If no solution for this is possible, a solder is built on the next side edge instead of a solder on the surface. If no solution is possible for this either, the closest corner of the color correction room is the closest point.

0010If a color space with a brightness coordinate axis is used as the color space, it is expedient to negotiate a larger brightness error against a smaller color error, since brightness errors have less of an effect on the print quality than color errors. According to this strategy, an achievable target color location is calculated by selecting the intersection point closest to the predetermined target color location of a parallel to the brightness coordinate axis through the predetermined target color location with the surface of the correction color space as the achievable target color location. If such an intersection does not exist, it is expedient to proceed according to the control strategies according to subclaims 7 to 8.

0011Exemplary embodiments of the invention are explained in more detail below with reference to the drawing. Show it:<ul id="ul0001" list-style="none"><li>Fig. 1 is a greatly simplified block diagram of a printing system for implementing the control strategy according to the invention and</li><li>2 shows a summary of the control strategy according to the invention using a correction color space within a color space with a coordinate axis assigned to the brightness and two coordinate axes assigned to the chroma and the color.</li></ul>

00121 shows a closed control system of a printing system which has an electronic device for measured value processing 10 in order to generate control data 11 with which a control console 20 is acted upon, which generates control signals 21 for the ink guide elements of a printing press 30 from the control data 11 which is, for example, a multi-color offset printing machine. (Only the colors cyan, magenta and yellow are relevant for the following). The control loop of the printing system serves to keep the color deviations on the printed sheets 40 printed by the printing press 30 as small as possible compared to predetermined target colors.

0013The detection of the colors on the printed sheet 40 is carried out by measuring color measurement fields 41 with printed color measurement strips, which are preferably colorimetrically and / or densitometrically automatically and continuously optically scanned with the aid of a measuring head 42.

0014The color measuring device supplies densitometric measured values ​​of the single-color full-tone measuring fields and colorimetric measuring values ​​of the single-color or multi-colored measuring fields, from which a computer in measured value processing 10 uses the specified density limit values ​​from the measured solid color densities to correct the color space around the actual color location I in the L measured on the multicolor measuring field^*a^*b^*-Color space (CIE 1976) determined. Although other color spaces can also be used, the invention is based on the L^*a^*b^*-Color space explained, which represents a color system with equally spaced sensations, with the same deviations in the three coordinates (delta L^*, delta a^* or delta b^*) can be recognized equally well. However, these deviations are not equivalent for the print quality assessment, since brightness deviations (in the direction of the L^*-Coordinate) have less of a disturbing effect than deviations of the same size in the coordinates assigned to the color a^* and b^*.

0015If it is determined in the measured value processing 10 that the actual color location of the area scanned by the color measuring device 42, in particular a color measuring field 41, on the printing sheet 40 does not correspond to the desired target color location S, which is caused, for example, by scanning a printing sheet which is found to be good or by direct data input is defined, the measured value processing 10 generates control data 11, which are entered via the control console 20 and cause the actuating signals 21 for the ink guide members of the printing press 30 in order to readjust the layer thicknesses of the printing inks on the printing sheet 40 and thus the solid densities in such a way that when the next printing sheet 40 is measured a collapse or at least an approximation of the actual Color locus I and the target color locus S. In measured value processing 10, the color distance vectors are multiplied by the computer by a sensitivity matrix in order to calculate the layer thickness change control vector or the density change vector which must be taken into account the next time a printing sheet 40 is printed in order to achieve the desired color location shift. The sensitivity matrix, with which the density differences for the color locus shift between the target color locus S and the actual color locus I are calculated, can be determined empirically and by measurement using a test series.

0016Further details on the empirical / metrological determination of the sensitivity matrix can be found in the two above-mentioned US patent applications Serial No. 228 347 corresponding to EP-A. 939,966 and 213,000. The mathematical determination is described in detail in Swiss Patent Application No. 120/88 of January 14, 1988 and 1268/88 of April 6, 1988 (corresponding to US Patent Application Serial No. 1989). The cited documents and patent applications are expressly explained as an integral part of the present description.

0017Fig. 2 shows the L^*a^*b^*-Color space with the color vector i for the actual color location I of an area scanned on the printed sheet 40, in particular a color measuring area 41, which can be a gray field or another grid or solid field specially adapted to the image content on the printed sheet 40 in order to optimally correct the color - and brightness components to be carried out simultaneously.

0018Starting from the actual color location shown in FIG. 2, it is possible to use the colors cyan, magenta and yellow to assign changes in solid density delta D_c, delta D_M and delta D^y2 make changes to the printed color locus in accordance with the directions of the correction vectors c, m and y assigned to the colors cyan, megenta and yellow in FIG. 2 within a correction color space 50, which is shown in FIG. 2 as a cuboid. In multicolor printing, it is known that the color location is roughly determined by the grid area coverage, while fine adjustment is carried out by changing the densities, ie by changing the layer thicknesses of the printing inks. The correction vectors c, m and y are limited in accordance with the full-tone density limit values. The maximum permissible density differences delta Dy are for the correction vector y in FIG. 2_Max and delta dymn. The maximum permissible density differences result from the differences between the actual density D_is and the permissible limit densities D_Max and Dmn for the printing inks involved. The limit values ​​for the solid ink density result, for example, from the requirements for a sufficient relative pressure contrast.

0019The correction vectors c, m and y span the correction color space 50 around the current actual color location I. Although they are usually not at right angles to one another, this is shown in FIG. 2 for the sake of simplicity. In addition, it is assumed that within a sufficiently small correction color space around the actual color location there is a linear approximation of the relationships between the color location coordinates and the densities.

0020Together with the actual color location I measured by colorimetry, target color locations S are shown in FIG. 2 to illustrate the control strategy according to the invention₁ to S₆ are drawn, each of which represents a special case and of course only one of them is the specified target color location S. that should have been achieved instead of the actual color locus I when printing the printed sheet 40.

0021As a first example, the case is discussed in which the target color locus S₁, whose color distance from the actual color location I is illustrated by the color distance vector 51, lies within the correction color space 50, which represents a control body. By changing the color densities of the printing inks involved within the specified limit values, it is therefore possible to determine the target color location S₁ really to be achieved, with the help of the already mentioned sensitivity matrix A the density differences for the color locus shift delta L, delta a and delta b between the actual color locus I and the target color locus S₁ be calculated.

0022Control strategies for the cases in which a target color location S cannot be achieved due to predetermined color density limits or other restrictions are explained below. In these cases, a substitute target color locus, ie achievable target color locus S 'or S ", is to be controlled, which is characterized by a color spacing which is least disturbing to the viewer.

0023If the target color location S lies outside the correction color space 50, it is possible to select the penetration point of the color distance vector through the affected side surface or boundary surface of the correction color space 50 as the achievable target color location S ". FIG. 2 shows how in this way for a SoIl color locus S₂ an attainable target color locus S is obtained. The achievable target color location S lies on the intersection of the color distance vector 52 with the side surface 60 of the correction color space 50. The strategy of choosing the point of penetration of the color distance vector between the actual color location and the target color location has the advantage of a simple calculation and represents an approximation .

0024The recognizable distance in Fig. 2 between the target color locus S₂ and the achievable target color location S represents the uncorrected or uncorrectable color difference. Since the target color locus S₂ lies in a spatial area for the spatial points of which there is a solder on the side surface 60, there is a smaller, non-correctable color difference corresponding to the length of the solder 62 on the side surface 60 if the base point of the solder 62 on the side surface 60 is the target color location S that can be reached is chosen. In Fig. 2, the right angles and the plane 61 are shown, in which the solder 62 and the target color locus S₂ and the attainable target color locus S. In order not to stress the drawing too much, the color difference vector between the actual color location I and the attainable target color location S is not shown. If the achievable target color location S has been determined by means of the computer by analytically determining the minimum distance from the correction color space 50, the necessary density difference vector is calculated for this with the aid of the sensitivity matrix A.

0025The smallest distance between the target color locus S₂ and the closest boundary surface of the correction color space 50, ie the side surface 60, was determined in the exemplary embodiment discussed by determining the base point of a solder 62. Depending on the position of the target color location, however, it is not possible to plumb a solder onto one of the boundary surfaces of the correction color space 50. In such cases, the point with the smallest distance from the target color location S is determined in a different way. If the target color location shifts so far that it comes to lie outside the spatial area, for the points of which there is a solder on the adjacent side surface 60, as is the case for the target color location S, for example₃ applies, the attainable target color location S is determined by determining the solder 73 on the adjacent edge 70 of the correction color space 50 and selecting the intersection of the solder 73 with the edge 70 of the correction color space 70 as the attainable target color location S.

0026The target color locus S₄ in FIG. 2 lies at a point which does not permit the construction of a solder on a side surface or on an edge of the regulating body or correction color space 50. For this reason, the adjacent corner 80 of the correction color space 50 is chosen for the achievable target color location S, since this point has the smallest distance of all points on the surface of the correction color space 50 from the target color location S.₄ Has. The distance between the target color location S that can be achieved in this way and the actual target color location S₄ is illustrated in FIG. 2 by the connecting line 84, with S to illustrate the spatial position of the target color quality₄ a cuboid 85 is shown, the diagonal of which is formed by the connecting line 84.

0027Experience has shown that deviations in brightness are less disruptive than deviations of the same size in the other two coordinates, so that larger deviations in brightness can be accepted in favor of smaller color component errors. A simple way to do this is to linearly compress L^* according to equation L^** = K. L^*, where the compression factor K is between zero and one.

0028In this way, color component errors can be weighted and corrected more than the brightness errors. Under certain conditions, the color component errors can be completely corrected, which is based on the target color locus S₅ is illustrated in Fig. 2. The target color locus S₅ assigned achievable target color location S is obtained in such a way that by S₅ a parallel to the L^*-Axis is constructed which is essentially upward in the direction of the L^*-Axis-facing top surface 90 of the correction color space 50 intersects and thereby defines the achievable target coloring point S. The achievable target color location S is shifted from the target color location Ss to the top surface 90 in relation to the penetration point of a solder (not shown) in such a way that the color coordinates a^* and b^* the achievable target color location S with that of the target color location S.₅ agree, it being accepted that, compared to the choice of the point of penetration of the solder, an additional deviation in the brightness coordinate L^* occurs. The color distance vector 95 between the target color location Ss and the achievable target color location S is longer than the plumb of S₅ on top surface 90, but its components for a^* and b^* are zero. The control strategy according to the invention thus proposes to preferably try to reach the correction color space 50 starting from a target color location by an achievable target color location by shifting the actual target color location parallel to the L^*-Axis is determined.

0029However, it makes sense to set limits for the apparently acceptable brightness errors to avoid color component errors and only a predetermined range for brightness errors between delta L_min and delta L_Max allow and within these areas to search for the point on the surface of the correction color space 50 that is as close as possible in accordance with the strategies already described above. Thus the compression of the L^* Coordinate can be combined with the search for a penetration point, plumb base or corner point.

0030Such a case is shown in FIG. 2 on the basis of the target color locus S.₆ illustrates the location of which is represented spatially with the aid of a cuboid 96. In contrast to the strategy for the target color locus S₄ If a larger brightness error against smaller color component errors is negotiated, the closest corner 97 of the correction color space 50 is not selected as the achievable target color location, but rather the point S on the surface of the correction color space 50 that lies on a plane that is parallel to the a^* and b^* Coordinates at a distance from the target color location S₆ which is defined by the largest permitted brightness error and which is the smallest distance from the parallel to the L^*-Axis through the target color locus S₆ Has. The intersection of this plane with the parallel to the L^*-The axis is provided with the reference symbol 98 in FIG. 2. The attainable target color location S can also be determined in such a way that, starting from the intersection 98, corresponding to that at the target color location S₃ applied strategy the base of the solder on edge 99 is determined. Those skilled in the art will recognize from the above statements that the linear compression of the L^*-Axis not only separately, but also in combination with those based on the target color locations S₂, P₃ and S₄ discussed constructions is possible. The calculations required for this are carried out by the computer of the measured value processing of the printing system. Which strategy is chosen depends on the one hand on the relative position of the target color location S to the correction color space 50 and on the other hand on the type of measuring field and the objectives. It is useful if the operator of the printing system can specify the strategy to be selected in several ways.

0031If the achievable target color location on the surface of the correction color space 50 is determined, this is selected for the control on the minimum color distance, the density difference vector being obtained according to the following equation:<chemistry id="chem0001" num="0001"><img file="EP0321402A1_D0001.tif" /></chemistry>

0032in this equation are AD <_{> c}, AD_m and ADy the components of the solid density change vector. The components of the color distance vector between the actual color location and the achievable target color location are denoted by ΔL, Δa and Ab. The matrix containing the partial derivatives of the solid colors according to the components of the color space is the sensitivity matrix A already mentioned.

0033The control strategies discussed can also be used for measuring fields in which less than three printing inks are printed. The correction color space is reduced to a parailelogram for two-color fields and to a distance in the color space for a single-color field. The registration strategies and calculations described above are applied analogously in such cases. All that needs to be done is to set the correction vectors of the non-existent colors to zero. In the case of two and one-color fields in particular, the target color locations are practically always outside the area or section-shaped correction area. For this reason, the strategies discussed above for finding an achievable target color location are a prerequisite for optimal color control.

0034If the density limit values ​​are exceeded or undershot, the actual color location may not lie within the correction color space. Nevertheless, the next control step is carried out optimally. The only requirement is that the linearization on which the calculations are based is permissible and the sensitivity matrix A is known with sufficient accuracy.

0035Finally, it should be mentioned that with a simultaneous regulation to different color locations, the residual errors can be optimally distributed over the color space. The calculations required for this result readily from the discussions above.