EP0324718A1 - Method and device for ink monitoring in a printing machine - Google Patents

Abstract

During sheet printing, a three-colour offset printing machine produces colour measurement bars with a plurality colour measurement fields. A grey screen field (52) produced by the superimposition of three colours serves as the reference field, the colour location of which is compared in a colour separation computer (68) with the colour location of an associated nominal reference field (51). From the colour separation, a layer thickness modification computer (71) calculates a layer thickness modification control vector with the aid of a sensitivity matrix (74) calculated by a matrix computer (73) on the basis of a linear model. In addition, the matrix computer (73) evaluates a series of adjacent fields which comprise three single- colour screen fields, three single-colour full tone fields, three two- colour full tone fields and one three-colour full tone field. …<IMAGE>…

Description

The invention relates to a method for color control of a printing press with a colorimetric ink guide control, color measuring strips with a plurality of color measuring fields being printed on the printing sheets printed by the printing press, which are optically scanned with the aid of a measuring head for detecting the spectral intensity distributions of the color measuring fields in order to obtain spectral color analysis of the measuring light to determine the spectral reflectances and the color location of a reference field of a scanned color measurement field in a coordinate system and to generate a manipulated variable for adjusting the color guide elements of the printing machine from the predetermined color location by comparing coordinates from the color distance of the scanned color measurement field, so that undesired color deviations in those with the new ink guide setting then printed sheets are minimal, and a device for performing the method according to the preamble of claim 9.

A method of the type mentioned at the outset is known from EP A 228 347, in which a large number of color measuring fields is used to optimally match the color impression are evaluated as reference fields in order to obtain an optimal adjustment of the color impression of sensitive, image-sensitive points of the print when printed, whereby the color distance-controlled color guide can be superimposed on a color density-controlled color guide during production. The spectral color analysis of a large number of color measuring fields and the calculation of a large number of color coordinates for each printed sheet require a relatively high level of effort. This effort is further increased in that, in the known method, distance vectors are weighted for each of the numerous reference fields in order to minimize the overall color distance, which is determined from the sum of the amounts of the individual color distances, as a quality measure for the print. To determine the layer thickness changes associated with the individual color measurement fields, it is necessary to multiply the color distance vectors assigned to the numerous reference fields by empirically determined transformation matrices. The empirical determination and storage of the numerous transformation matrices already represents a very high expenditure. Since the printing ink components involved are corrected more or less independently of one another, there is an unfavorable convergence behavior. For this reason, the known method provides for using separate color measuring fields for particularly critical tones, which in turn increases the overall expenditure.

Starting from this prior art, the invention is based on the object of further improving a method of the type mentioned at the outset. In particular, especially in the case of particularly critical tones, it should be possible to dispense with specially adapted color measurement fields and still obtain color control with a high rate of convergence.

This object is achieved in that the spectral reflectance values of a reference field in the form of a grid are recorded as the main variable for determining an actual color location for determining the color distance, and the spectral remissions of color measurement fields serving as secondary fields are recorded as secondary variables, from which the sensitivity of the color locus shift is determined is calculated on the basis of a change in layer thickness, and that from the distance between the measured actual color location of the reference field and the target color location of the reference field and the sensitivity of the color location shift calculated on the basis of the secondary variables, the layer thickness changes of the printing inks required as a relative correction quantity for the color guidance for compensation the color locus deviation of the actual color locus of the reference field from the target color locus of the reference field can be determined.

In individual cases (i.e. with certain subjects or with colored paper) it can make sense to standardize not on absolute white but on paper white when determining the actual color location.

The change in layer thickness calculated in this way can also be converted into a change in density, for example using a Tollenaar function.
Instead of the layer thickness, the sensitivity of the color locus shift due to a change in density can also be calculated directly. To do this, it makes sense to correct the density values. This can be done, for example, with the Saunderson correction. With a sensitivity matrix determined in this way, a density change control vector is then calculated, which should result in density changes that bring the measured color location as close as possible to the target color location move.
The following exemplary embodiments are based on a sensitivity matrix based on layer thicknesses. However, they can easily be transferred to a sensitivity matrix based on densities. In an expedient exemplary embodiment of the invention for a multicolor, for example four-color offset printing machine, the reference field is a gray field which is produced by printing three rasters with the three colored standard printing inks involved. Black and spot colors on 5 and 6-color machines are treated separately, as will be described in detail later. The control method according to the invention compares the color location of the actual gray field printed on a printing sheet with the stored color location of the gray field on the OK sheet or the numerically entered color location. A color difference vector is determined from the deviation between the actual color location and the target color location and a layer thickness change control vector is calculated, which (theoretically) should result in layer thickness changes that shift the measured color location as close as possible to the target color location. The method according to the invention is therefore on the one hand a relative model, since the measured reflectance or the color location of the corresponding three-color grid is used as the basis and the change in reflectance due to the change in the color guide is calculated relatively to this. Since the requirements and accuracy of such a relative model are lower than those of an absolute model, which carries out a color location determination without reference to an existing intermediate value, a linear substitute function in the measured working point is already sufficient as the simplest type of model formation to achieve a high speed of convergence with a relative model if the natural requirement of a correct sign is met. In a preferred exemplary embodiment, a model is formed to determine the sensitivity of the color locus change due to a change in layer thickness on the basis of partially differentiated Neugebauer equations.

A sensitivity matrix is calculated individually for each color location of the gray field serving as a reference field and, after inverting, is formed as a transformation matrix for generating a layer thickness change control vector from the color distance vector. In order to achieve a high level of accuracy, it is expedient to redetermine all the sizes which are required to form the elements of the sensitivity matrix with each print. The sizes required for this are generated by evaluating the spectral remission values of three halftone patches in the printing inks involved, three solid patches in the printing inks involved, three solid patches each with two printing inks printed one above the other, a solid patch with all printing inks printed one above the other and finally a field for recording the paper remission .

• Fig. 1 ein stark vereinfachtes Blockschema einer Druckanlage mit dem erfindungsgemäßen Regel­verfahren und
• Fig. 2 ein Blockschema zur Veranschaulichung einer möglichen Realisierung der erfindungsgemäßen Einrichtung zur Meßwertverarbeitung.

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

• Fig. 1 is a greatly simplified block diagram of a printing system with the control method and
• 2 shows a block diagram to illustrate a possible implementation of the device according to the invention for processing measured values.

The printing system shown in Fig. 1 has an electronic device for measured value processing 10, which generates control data 11 which correspond to the undesirable color deviations of the printing inks involved in the printing in the individual printing zones and printing units and which are fed as input variables to a control console 20. The control console 20 uses the control data 11 to generate actuating signals 21 for the ink guide members of a printing machine 30 equipped with a remotely controllable ink guide, which printing machine can be, in particular, a three-color offset printing machine, so that the color deviations on printed sheets 40 printed by the printing machine 30 are minimal.

During printing, 30 color measurement fields 41 are also printed as color measurement strips by the printing press, wherein a block of a color measurement strip can extend, for example, over two zones of the print sheet 40 having a plurality of zones.

In order to keep the color deviations of the printing inks involved in the printing small, the color measuring fields 41 are optically scanned by hand or preferably automatically and continuously with the aid of at least one measuring head 42 which is motor-driven in the direction of the arrows 43, 44 along the color measuring fields 41 of the color measuring strips which are also printed is. A second measuring head can be provided for manual scanning.
The measuring head 42 contains a white light source, not shown in the drawing, for illuminating the color measuring fields 41, for example at an angle of 45 degrees, and measuring light optics in order to collect the light remitted by the color measuring fields 41, for example at an angle of 0 degrees, and to lead it via a light guide to the input of a spectrometer 45. The spectrometer 45 is used to spectrally split the portion of the white illuminating light remitted by the color measurement fields 41 printed for pressure monitoring in order to permit spectral color analysis and thus colorimetric analysis. The spectrometer 45 contains, for example, a holographic grating illuminated via an entrance slit for the spatial splitting of the measuring light according to wavelengths, and a line-shaped arrangement of, for example, 35 photodiodes, which are acted upon by the spectrally divided measuring light. The spectrometer 45 thus allows spectral color measurement at, for example, 35 support points to determine the spectral remissions of the manually or automatically scanned color measurement fields 41, in order to allow the measured value processing 10 to derive colorimetric parameters. The measurement data 46 present at the output of the spectrometer 45 arrive via an interface (not shown in the drawing), which among other things also performs a digitization of the measurement data 46, to a computer arrangement contained in the device for the measurement value processing 10.

The computer arrangement of the electronic device for measured value processing 10 has driver electronics for feeding the electrical drive of the measuring head 42 and the measuring head illumination. Furthermore, as with a conventional computer, a data display device including a keyboard and a log printer are provided in order to deal with the spectral data acquisition To be able to display data as required and to be able to enter constants and setpoints manually using the keyboard.

In the electronic device for measured value processing 10, the measured data 46 are converted into spectral remissions based on the paper white of the printed sheet 40 and color location coordinates. By comparing the color location coordinates of a color measuring field 41 serving as a reference field with stored target color location coordinates, the control data 11 are generated on the basis of a color distance determination between the target color location and the actual color location of the reference field actually recorded on the printed sheet, which is preferably a gray field from a three-color grid . The target color location coordinates define a target color location, which can be done either manually via the keyboard or by scanning the reference field of a print sheet that is found to be good, i.e. a so-called "O.K. sheet" has been entered into a memory. In the electronic device for measured value processing 10, the spectrophotometric measurement data, i.e. the spectral remissions, preferably of each printed sheet, converted into color location coordinates and compared with the stored target color location coordinates in order to continuously determine color distances and control data 11 for the control console 20 and the color guide elements for controlling the color application in the manner described in more detail below.

Since the result obtained is scanned by the measuring head 42 when the color guide members are adjusted to correct the coloring, the printing system shown in FIG. 1 has a control circuit for correcting color deviations. The respective rule Softening is determined by the measured value processing 10, which contains the coordinates of the respective target color location as a reference variable in its memory and generates the control data 11 as a manipulated variable. In the printing system shown in FIG. 1, the control data 11 feed the printing machine 30 indirectly via the control console 20 that is usually present. Of course, it is also possible to modify the printing system in such a way that the control data act directly on the ink guide elements, for example the ink fountain keys, of the printing machine 30 .

The mode of operation and the structure of the electronic measured value processing 10 are shown in more detail in FIG. 2. The spectral reflectance values supplied by the spectrometer 45 arrive via an input bus 50 to the reflectance value memories 51 to 63, each assigned to the individual color measuring fields 41. The reflectance value memory 51 serves to store the reflectance values β _{R123 of} a three-color _{grid field} measured at 35 different wavelengths on the OK sheet.

The three-color grid on the OK sheet corresponds to a three-color grid on the newly printed printed sheet 40 in each case. The color appearance of the printed sheet 40 corresponds to the OK sheet if the three-color grid field serving as a reference area on the printed sheet 40 has the same, in particular gray, Color impression. For this reason, the spectral reflectance values of the three-color grid are stored as the actual value in the reflectance value memory 52 and indirectly with the spectral reflectance values present as the target value in the reflectance value memory 51 after the spectral reflectance values have been converted into color location coordinates of a color coordinate system, in particular the CIELAB or CIELUV system. Systems compared.

The standard color values X, Y and Z are calculated from the spectral reflectance values in the reflectance value memory 51, for example assigned to 35 different wavelengths, using the first standard color value calculator 64 in accordance with the formulas defined by the CIE (Commission Internationele de l'Eclairage). Correspondingly, the actual standard color values X, Y and Z are calculated in a second standard color value calculator 65 from the spectral remission values obtained from the reflectance spectrum of the reference field on the printed sheet 40. The standard color value calculators 64, 65 can be combined in terms of hardware and in particular can also be part of the main processor of the printing system and thus, like the reflectance value memories 51 to 63, only exist in software.

Instead of calculating the target standard color values (and, as explained below, the target color locations) from the spectral reflectance values of the reference field of the O.K. sheet read in by the measuring head, the coordinates of the target color location can also be entered manually using the keyboard. This possibility is indicated in the drawing by the input line 69 of the first color locator 66. Theoretically, the corresponding target standard color values or the target reflectance values could of course also be entered manually, but this should not make much sense in practice. Corresponding possibilities are indicated in the drawing by input lines 69 'and 69˝.

The target standard color values calculated or manually entered by the first standard color value calculator 64 and the actual standard color values calculated by the second standard color value calculator 65 according to CIE each serve a first color location calculator 66 and a second color location calculator 67 as input variables. The first color location calculator 66 and the second color location calculator 67 each calculate the color locations with the coordinates L, a and b or L, u and v of a CIE color space from the target standard color values and the actual standard color values according to the CIE formulas. The first color location computer 66 and the second color location computer 67, like all computers of the measured value processing 10, can also be implemented in hardware and / or software with the other computers in the printing system. Although the CIE color space with the color location coordinates L, a and b is discussed below as an exemplary embodiment, it should be pointed out that the invention can also be implemented with other color spaces.

The target color locus vector for the color of the three-color grid of the OK sheet determined by the first color locator 66 is compared in a color distance calculator 68 with the actual value determined by the second color locator 67 for the color locus of the three-color grid on the freshly printed printed sheet 40, which serves as a reference field. in order to determine a color distance vector from the difference between the two color locus vectors, the length and orientation of which in the color space indicate the undesired color deviation between the OK sheet and the newly printed printed sheet 40.

The output of the color distance calculator 68 is connected to the first input of a layer thickness change computer, which calculates a layer thickness change control vector Δ S from the color distance vector Δ F, a transformation function being fed in via a second input 72, which is used for the respective by the actual standard color values X, Y, Z or the actual color point L, a, b defined working point is a linear replacement function of what is extremely complex in practice Relationship between layer thicknesses and color locations for an infinitesimal surrounding area of the working point. The quantities fed into the second input 72 for calculating the layer thickness change control vector, the components of which for the three printing colors, for example cyan, yellow and magenta, form the control data 11, are determined with the aid of a matrix computer 73 which components of a matrix A (i, j ), which is a three-dimensional normal matrix with nine elements in three columns and three rows.

2 shows a matrix memory 74 for the components of the matrix A (i, j).

The matrix A (i, j) is inverted with the aid of a matrix inverter 75, so that the elements of the inverted matrix A der¹ are present at the second input 72 as elements of a transformation function, which are preferably determined anew each time a reference field is measured. If there is a discrepancy between the actual color location of the reference field scanned with the aid of the measuring head 42 and the target color location, the layer thickness change computer 71 is thus used to calculate which layer thickness changes are required for the three printing inks in order to approximate them when printing the next printing sheet 40 the actual color location to reach the target color location. The matrix A (i, j) stored in the matrix memory 74 contains, as information, the sensitivity of the color location change due to the layer thickness changes. Therefore, the matrix A (i, j) is referred to below as the sensitivity matrix. Although their elements can be determined experimentally, different matrix elements already apply to each color locus. Because of the multitude of possible color locations and other influences, there is a considerable storage volume Required if experimentally determined sensitivity matrices are to be saved in order to read out the values required for a particular operating point. For this reason, the elements of the sensitivity matrices in the exemplary embodiment of the invention shown in FIG. 2 are calculated separately for each working point defined by the standard color values X, Y, Z. The elements of the sensitivity matrix A (i, j) are the partial derivatives of the color locus vector, in particular the color locus vector of one of the color spaces mentioned, according to the components of the layer thickness control vector. These components are calculated with the aid of the matrix calculator 73 using computation rules which are based on a relative and linear model in which the color locus shift dL, db, is calculated from the partial derivations of the reflectance changes per change in layer thickness, because db is calculated on the basis of the layer thickness changes of the printing inks.

In order to allow the matrix computer 73 to calculate the sensitivity matrix assigned to a freshly printed reference field, it is necessary to provide additional halftone fields and solid color fields in addition to the field with a three-color grid when printing the color measurement strip with the color measurement fields 41. The color measuring fields 41 on the printing sheet 40 therefore comprise a single-color grid for each of the three printing inks, the film surface coverings corresponding to the three-color grid or reference field for the grid fields. If the film surface coverings do not match those of the three-color grid, the calculated surface coverings must be interpolated. In addition, solid color fields are provided for the three printing inks. The color measurement fields 41 also include three solid tone fields, in which two printing colors have been printed on top of each other. Finally, the color measurement strips of the printed sheet 40, which are also printed, each contain a solid field with all three colors printed one above the other and a white field for determining the paper remission.

To determine the sensitivity matrix of a specific freshly printed sheet, it is therefore necessary for the measuring head 42 to record the spectral reflectance values for a large number of different color measuring fields 41. For this reason, remission value memories 53 to 63 are shown in FIG. 2, which can be implemented in software or hardware. In the case of a hardware implementation, the input bus 50 is in each case connected to that reflectance value memory 51 to 63, the associated color measuring field 41 of which is just being scanned by the measuring head 42. Like the reflectance value memories 51 and 52, each reflectance value memory 53 to 63 stores the spectral remissions assigned to a multiplicity of wavelengths, for example 35 different wavelength ranges.

The matrix computer 73 has an operating point input 77, via which the respectively applicable standard color values are fed. Further inputs of the matrix computer 73 are connected to the three remission value memories 53 to 55, which contain, for example, the spectral remission values of grid fields of the colors yellow, magenta and cyan. The reflectance value memories 56 to 58 each store 35 reflectance values of solid color fields of the colors yellow, magenta and cyan, the layer thicknesses of which change when the color guide elements are adjusted in the same way as the layer thicknesses of the respective printing color in the grid fields.

As can be seen from FIG. 2, three reflectance value memories 59 to 61 for solid tone fields are assigned to the matrix computer 73, each of which is produced by overprinting two printing inks and, in the exemplary embodiment described, stores the spectral remission values of the colors red, green and blue produced by overprinting. Finally, a reflectance value memory 62 is provided for storing the spectral reflectance values of a solid-color field, which was created by printing all three printing colors on top of one another and thus essentially has a black color. Finally, in order to store the spectral reflectance of the paper of the printed sheet 40, the reflectance value memory 63 is provided, so that the matrix computer 73 can process reflectance values relating to paper white which are between 0 and 1.

In order to supply the matrix computer 73 with constants and parameters, a constant and parameter input 76 is provided. The computers and inputs mentioned above can be present physically or in software in the measured value processing 10.

After the closed control loop of the printing system (and possibly via the operator) and the organization of the measured value processing 10 have been discussed, it is subsequently discussed how, after the color measurement areas 41 of a fresh printing sheet have been scanned, the sensitivity matrix A (i, j) is determined, with which Layer thickness change control vectors are generated in order to adjust the ink guiding elements with the greatest possible convergence speed in such a way that during the printing of the printed sheets 40 a color-spaced control takes place.

To determine the sensitivity matrix A (i, j), it is necessary to calculate its components. The elements of the sensitivity matrix are the partial derivatives of the components of the color locus vector according to the components of the layer thickness control vector. If the L * a * b * system of the CIE is used in accordance with the exemplary embodiment described, the partial derivatives of the coordinates L, a and b must therefore be calculated according to the component of the layer thickness vectors. The partial derivatives of the color space coordinates contain the actual standard color values X, Y and Z of the measured reference field and the partial derivatives of these standard color values according to the component of the layer thickness vector.

The partial derivations of the standard color value according to the three components of the layer thickness vector could be determined empirically, the values obtained being stored in a memory. However, this case is hardly useful in practice. Another possibility is to calculate these quantities from time to time, for example at the beginning of a printing process, from many printing sheets 40 from the spectral remission values stored in the remission value memories 53 to 63. Instead of a calculation from time to time, a calculation can also be carried out for each individual printed sheet. However, it is preferable to determine the partial derivatives of the measured actual standard color values according to the three components of the layer thickness vector for each measurement of a reference field in a zone or a block of the printed sheet. The information stored in the remission value memories 53 to 63 represent secondary variables which make it possible to determine which color guide for the main variable stored in the remission value memory 52 tion changes are necessary to ensure that the color location assigned to the measured main variable is closer to the target color location in the color space for the next print and for the next measurement.

The nine partial derivatives of the standard color values according to the components of the layer thickness vector or layer thickness control vector are obtained by integrating a printout over the entire spectral range, which essentially contains the partial derivatives of the reflectance values of a three-color grid, calculated on the basis of the model, according to the three layer thicknesses of the three printing inks. The simplest model is the calculation using the Neugebauer equations, which, in their differential form, indicate the changes in reflectance of a three-color grid depending on the optically effective area coverage and the remissions of the solid fields printed together with the grid.

For this reason, the matrix calculator 73 calculates the sizes contained in the Neugebauer equations in a differentiated form and in particular the partial derivations of the remissions of the full-tone fields according to the layer thicknesses of the respectively assigned colors, as well as the optically effective area coverage from the relationships specified by Murray-Davies and partial derivations of the optically effective surface coverings after the remissions of the single-color solid color fields assigned by the color.

The above explanations show how it is possible to implement a relative and linear model which allows the new color location of a three-color grid, which is expected due to color guide changes not to be calculated absolutely, but with significantly higher accuracy and reliability, based on the measured reflectance or the color location of the actually printed three-color grid, with the addition of the change in reflectance due to the change in color. With a relative model, errors (with the correct sign) mainly affect the speed of convergence, but not the convergence as such. For this reason, a linear model can be used as the simplest type of model formation with a linear substitute function at the operating point for calculating the reflectance change in the three-color grid that occurs when the color guide changes. The operating point depends on the remission actually measured and the color location actually determined. Based on the measured reflectance, the linear substitute function at the operating point allows an approximate theoretical determination of the color location of the new three-color grid that will be printed later when the color is changed. The "slope" or sensitivity at the operating point is used to determine the required changes in layer thickness or changes in color from the color difference between the actually measured color location and the desired color location for the three-color grid.

The formulas and the calculations for the control algorithm with a linear model based on the Neugebauer equations and the relationships of colorimetry known to the person skilled in the art result from the exemplary embodiment discussed below.

The matrix computer 73 shown in FIG. 2 calculates the sensitivity matrices for all zones or blocks A (i, j) so that linear regulation can take place.

To calculate the sensitivity matrix A (i, j), the spectral single-color grid emissions are first interpolated to the corresponding grid values of the three-color grid (square) and stored in the corresponding reflectance value memories 53-55. As a result, only these interpolated values are used.

In a further step, the ten secondary measurement values contained in the reflectance value memories 53 to 62, each with 35 spectral individual values, are weighted with densitometric filter profiles and, based on paper white, the geometric area coverage for the three colors is calculated according to the following formula.
F _RjGeom = F _RjFilm + (F _Dj - F _RjFilm ) / 3.

In this equation, F _{Dj means} the optically effective area coverage (according to Murray-Davies) for the color j, where j = 1 means cyan, j = 2 magenta and j = 3 means, for example, yellow. The film area _coverage F _RjFilm is predetermined by the measurement strip definition and does not need to be measured. The formula for calculating the geometrical area coverage is based on the assumption that the increase for the optically effective bottle coverage is composed of 1/3 mechanical point magnification and 2/3 light trapping.

The calculations mentioned below are performed spectrally for the three colors over the wavelengths between 380 and 730 nanometers, for example in 35 steps. To produce a paper white reference, the coefficients _Vj ß 'ß = _Vj / ß _paper are determined, wherein ß _Vj is the measured for the respective wavelength reflectance value for a solid field V _{j of} the printing ink j.

The matrix calculator 73 calculates the partial derivatives of the spectral full tron emissions according to the layer thicknesses for each of the 35 wavelengths and for all solid colors j according to the following formula:

In this equation, S _{j means} the current layer thickness of the printing ink j. It results from the machine characteristic curve (ie the relationship between a manipulated variable of the machine control and the resulting layer thickness). r _Oj is a constant that indicates the surface reflection of the paper for ink j. In the first approximation it is the same for all printing inks j. In addition, the surface reflection r _Oj can be assumed to be negligible due to the measuring optics (45 °, 0 °) and due to the polarizer used. It is therefore advisable to set this constant to zero in most cases. The constant r _2j expresses the total reflection in the color layer and is also approximately the same for all printing inks j. If the internal reflection r _2j is set to zero, the layer thickness is assumed to be proportional to the density. Reasonable values for r _2j range from 0.4 to 0.6. The larger r _2j , the greater the sensitivity and thus the controlled variable.

Darin ist ß′_Rj ein auf Papierweiß bezogener Wert eines in den Remissionswertspeichern 53 bis 55 gespeicherten Wertes für ein Rasterfeld mit einer einzigen Farbe j
(ß′_Rj = ß_Rj / ß_Papier).The optically effective area coverage F _{Dj is} then calculated in all 35 support points for all three colors according to the Murray-Davies formula. If the related paper white spectral reflectance value _PY 'of the associated Volltenfeldes is greater than 0.95, the light capture is assumed to be zero and replaces the optically effective area coverage by the geometric area coverage in the further calculation to the calculation of the optically effective area coverage division to avoid by zero. Such a division by zero could otherwise occur because the measured values are subject to noise.

Therein, ß ′ _{Rj is} a value related to paper _{white of} a value stored in the remission value memories 53 to 55 for a grid with a single color j
(ß ′ _Rj = ß _Rj / ß _paper ).

Next, the program of the matrix calculator 73 calculates the derivatives of the optically effective area coverings F _Dj for all wavelengths and for all colors according to the reflectance values β ′ _{Vj of} the monochrome solid tone fields according to the following equation:

In this equation, the paper constant P is a constant which contains the paper and printing ink properties and can be entered into the matrix calculator 73, for example via the input 76. The above relationship is based on a light trapping model, where the paper constant P can be set equal to 1. The values of the paper constant P are between 0.1 and 1. The smaller the paper constant P is, the greater the sensitivity and thus the controlled variable.

When all the quantities are available that are needed to calculate the partial derivatives of the spectral remissions of a three-color grid from the layer thicknesses of the printing inks from the differential Neugebauer equations, the following equations are used, in which ß _R123 the reflectance of a three-color grid and ß _V12 , ß _V13 and ß _{V23 are} the remissions of the _{full-tone fields assigned to} the remission _{value memories} 59 to 61 from two different _overprinted colors and ß _{V123 is} the remission of a _{full-tone field} with three colors printed one above the other.

The differential Neugebauer equations above each contain a first addend, which contains the changes in reflectance due to the changes in light trapping, and a second addend, which contains the changes in reflectance due to the changes in layer thickness. The influences of the color acceptance were neglected. The change in the reflectance of a color layer due to the change in the layer thickness is assumed to be independent of whether the color was printed entirely on paper or partly on another color.

When the differential Neugebauer equations have been evaluated for all three colors and all wavelengths, the matrix computer 73 calculates the sensitivity matrix, which is inverted in the matrix inverter 75, which can also be software-integrated in the matrix computer 73.

The following nine relationships result from the CIE definition equations for the standard color values for the partial derivations of the standard color values according to the layer thicknesses:

The above equations for the three printing inks, which are designated by j = 1, j = 2 or j = 3, provide 9 numerical values for further processing after inserting the various sizes and integrating them over the wavelength or adding them up over the 35 reference points in the spectrum. In the above equations, B (λ) means the spectral characteristic of the lighting and x (λ), y (λ) and e.g. (λ) the standardized weight functions according to CIE. The quantities dß _R123 (λ) / dS _j are the quantities calculated using the differential Neugebauer equations, the dependence on the wavelength λ being specified for clarification and dS _j standing for dS₁, dS₂ and dS₃.

mit j = 1, 2, 3. X_N, Y_N und Z_N sind Normfarbwerte der vollkommen weißen Fläche der entsprechenden Lichtart und des entsprechenden Beobachters gemäß CIE.If the nine values of the differential derivatives of the standard color values are available according to the layer thicknesses of the three printing inks, these quantities are used to derive the nine elements of the sensitivity matrix from the following relationships, which have been obtained by differentiating the definition equations for L, a and b according to CIE to determine:

with j = 1, 2, 3. X _N , Y _N and Z _N are standard color values of the completely white surface of the corresponding illuminant and the corresponding observer according to CIE.

After the nine derivatives of the three color space coordinates have been calculated according to the layer thicknesses of the three colors, the sensitivity matrix A is formed and recorded in the matrix memory 74, which can be implemented in software or hardware. The nine elements of the sensitivity matrix listed below, which can be calculated from the equations above, result in the sensitivity matrix A (i, j):

In this matrix, the derivatives according to S 1 mean the derivatives according to the layer thickness of the first printing ink, for example cyan. The derivatives according to S₂ and S₃ relate to the second and third printing inks, especially magenta and yellow.

It follows from the above explanations that only ten or, taking into account the paper white, eleven spectral reflectance measurements with 35 individual values each and some constants are included in the entire calculation, which can either be found in tables or by other separate measurements in a manner known per se be determined once and for all.

In the normal case of a gray reference field discussed above, ie a reference field with a three-color raster, A (i, j) has a matrix with three columns and three rows in the manner described, which can be easily inverted in order to calculate the components of the layer thickness change control vector as control data.

However, it may also be the case that, for example, a pure cyan field or a field which contains only two instead of three colors printed one above the other is to be used as a reference field. This means that you want to control the color impression of a cyan grid or two-color grid. In such cases the 3 x 3 matrix degenerates to a 1 x 3 matrix (one color, vector) or to a 2 x 3 matrix (two colors). This is evident because the colors that are not taken into account, ie the colors that do not appear in the reference field, cannot make a contribution and the corresponding elements of the matrix must therefore disappear. Matrices with empty rows or empty columns cannot be inverted, since an inversion would result in a division by zero. For this reason, such "degenerate cases" must be treated separately. Here, the desired target color location will generally not be in the printable color space, since a color deviation can also run in the direction of the "foreign" colors. The characteristic of the printable color space only indicates the relationship between the layer thicknesses of the colors taken into account and the color locations achieved. On the other hand, this means that the desired target color location can generally not be achieved at all. In such cases, the matrix computer 73 allows a replacement target color location to be determined which lies on the replacement characteristic curve or replacement characteristic surface defined by the degenerated "matrix" A in the color space. This replacement target color location can then of course be reached. The he Set-target color difference is calculated so that the distance between the original target color location and the replacement characteristic or replacement characteristic surface is minimal. In the one-dimensional case the sensitivity matrix is a vector, in the two-dimensional case an area is defined. The replacement target color location is determined as the point of penetration of the solder onto the surface or the vector by the original target color location. Once this has been done, the values for the components of the layer thickness change control vector can simply be determined according to the rules of the vector geometry from the color vector of the measured field (actual value, operating point) and the color vector of the replacement target value.

It may also be the case that you want to use the solid field (e.g. black) as a reference field for individual colors. This means that the sensitivity is calculated without secondary fields. The sensitivity matrix mentioned above is reduced to a vector; there is no need to calculate the light catch and the area coverage. All further steps proceed like a grid of a color as a reference field.

Claims (15)

1. A method for color control of a printing press with a colorimetric ink guide control, whereby color measuring strips with several color measuring fields are also printed on the printing sheets printed by the printing press, which are optically scanned with the aid of a measuring head for detecting the spectral intensity distributions of the color measuring fields, in order to obtain a spectral color analysis of the measuring light to determine the spectral remissions and the color location of a reference field of a scanned color measurement field in a coordinate system and to generate a manipulated variable for adjusting the color guide elements of the printing press from a predetermined target color location by comparing coordinates from the color distance of the scanned color measurement field, so that undesired color deviations in those with the new one Color guide setting then printed printed sheets are minimal, characterized in that the spectral reflectance values of a reference field in the form of a grid field for determining the color distance s as the main variable for determining an actual color location, the spectral remissions of color measurement fields serving as secondary fields are recorded as secondary variables, from which the sensitivity of the color location shift due to a change in layer thickness / density is calculated, and that the distance between the measured actual values Color location of the reference field and the target color location of the reference field as well as the sensitivity of the color location shift calculated on the basis of the secondary variables, which is required as a relative correction quantity for the color guidance Changes in layer thickness / density of the printing inks to compensate for the color locus deviation of the actual color locus of the reference field from the target color locus of the reference field are determined. 2. The method according to claim 1, characterized in that the grid selected as a reference field is a multi-color grid, in particular a three-color grid. 3. The method according to claim 2, characterized in that the reference grid is a gray field. 4. The method according to claim 1, characterized in that the secondary fields include solid tone fields and halftone fields of the printing inks involved. 5. The method according to claims 2 or 3 and 4, characterized in that the secondary fields include solid color fields in the printing inks involved, solid color fields each with two superimposed printing colors, full color fields with all superposed printing colors and halftone fields in the printing colors involved. 6. The method according to any one of claims 1 to 5, characterized in that the sensitivity of the color locus shift is determined on the basis of a linear model. 7. The method according to claim 6, characterized in that the linear model from the Influences of the individual color components and the statistics of the overprinting depending on the area coverage of the individual printing inks describing Neugebauer equations is derived taking into account the light catch. 8. The method according to any one of the preceding claims, characterized in that the sensitivity of the color locus shift is recalculated as a sensitivity matrix for each operating point defined by the standard color values of the scanned reference field. 9. Device for carrying out the method according to one of the preceding claims, with at least one measuring head (42) coupled to a spectrometer (45) for scanning the printed color measurement fields (41), with one of the measurement data (46) of the spectrometer (45) for manipulated variables ( 11, 21) for the measured value processing unit (10) processing the printing press (30), characterized in that the measured value processing unit (10) calculating means (64 to 68) for determining the color distances between an actual reference field (52) and a target reference field (51 ), Computing means (71) for determining the layer thickness change control vector (11) required for a color correction and matrix computing means (73) for determining the sensitivity (74, 75) of the color locus shift. 10. The device according to claim 9, characterized in that the computing means for determining the color distance means (64, 65) for calculating standard color values and means (66, 67) to calculate color locations. 11. The device according to claim 10, characterized in that the standard color value computing means (64, 65) with reflectance value memories (51, 52) for storing the spectral reflectance values of the target reference field and the actual reference field and that the matrix computing means (73) with reflectance value memories (53 to 63) are connected to store the spectral remissions of the secondary fields. 12. Device according to one of claims 9 to 11, characterized in that the matrix computing means (73) have a working point input (77) connected to the standard color value computing means (65) for the actual reference field (52). 13. Device according to one of claims 9 to 12, characterized in that the means for calculating the color difference (64-68) target data can be entered, for example, via the keyboard. 14. The method according to claim 1, characterized in that the grid selected as a reference field is constructed from one or two colors. 15. A method for color control of a printing press with a colorimetric color control, whereby color measuring strips with several color measuring fields are also printed on the printing sheets printed by the printing press, which are optically scanned with the aid of a measuring head for detecting the spectral intensity distributions of the color measuring fields to determine the spectral remissions and the color location of a reference field of a scanned color measurement field in a coordinate system from a spectral color analysis of the measurement light and to generate a manipulated variable for adjusting the color guide elements of the printing press by comparing the color distance of the scanned color measurement field from a predetermined target color location, so that undesired ones Color deviations in the printed sheets subsequently printed with the new ink guide setting are minimal, characterized in that the spectral reflectance values of a reference field are recorded in the form of a full tone field for determining an actual color location in order to determine the color spacing, from which the sensitivity of the color location shift due to a change in layer thickness / density is calculated and that from the distance between the measured actual color location of the reference field and the target color location of the reference field and the calculated sensitivity of the color location diff The changes in the layer thickness / density of the printing inks required as a relative correction quantity for the color guidance to compensate for the color locus deviation of the actual color locus of the reference field from the target color locus of the reference field are determined.

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