EP0408507B1 - Method for the determination of the distances between the color coordinates of two halftone regions printed with a printing machine and method for monitoring or adjusting the color printing of a printing machine - Google Patents

Description

The invention relates to a method for determining the color difference between two screen fields printed with the aid of a printing press according to the preamble of independent claim 1.

In addition, the invention relates to a method for color control or color regulation of the printing of a printing press according to the preamble of independent claim 2.

Methods of this type are known from EP-A - 196 431.

Methods for determining the color difference are generally used for quality assessment of printed products and require the use of color measuring devices or spectrophotometers in order to determine the coordinates assigned to a grid field, in particular a gray balance field, in a color space. The use of such devices is perceived in many cases as expensive and complex because of the high optical and electronic complexity. It is also known to carry out a quality assessment on the basis of measured densitometric quantities. A quality assessment based on a densitometric measurement system or densitometric parameters (cf.EP-A-196 431) has the advantage that cheaper devices, namely densitometers can be used instead of spectrophotometers, but densitometric sizes are not particularly practical for a quality assessment and are not equivalent the sizes that occur in a real color measurement system. In this prior art (EP-A-196 431), the use of densitometers is limited to a densitometric measuring system which is worse for a quality assessment than color mass numbers in a color space that is equally sensitive in terms of sensibility, such as the L * a * b * color space or LUV color space.

From EP-A-228 347 and EP-A 321 402 a method for color control and Color control of a printing press is known, in which measuring fields are scanned with the aid of a color measuring device or a spectrophotometer in order to obtain color coordinates in a colorimetric measuring system and, by means of a coordinate comparison from the color distance of the scanned measuring field from a predetermined target color location, a manipulated variable for adjusting the color guide elements of the printing machine to create. The procedure according to EP-A-321 402 is such that a predetermined target color location lying outside a correction color space is replaced by an attainable target color location on the surface of the correction color space with a color distance from the predetermined target color location, which is for the Print quality essential components are minimal. To implement such a control strategy, an operation in a colorimetric coordinate system, for example the L * a * b * color space, is required. According to EP-A 321 402, this requires the use of a spectrophotometer or, in general, a color measuring device instead of a densitometer.

EP-A-255 924 discloses a method for controlling or regulating the color control of a printing press, in which a color change vector is determined from the target / actual color density deviations of the colored partial colors measured with the aid of a densitometer, from which a color change vector is then determined "Control recommendations" or "Regulation recommendations" for the color control are derived in a manner that is not described in detail. The multiplication factors used in the "vectorial auxiliary construction" are determined by previous test prints with systematic variation of the coloring of the individual colors and subsequent evaluation using a densitometer and color measuring device.

The method disclosed in this document essentially only allows qualitative statements about the color deviations between the measured production print and the corresponding reference, since the "vector auxiliary construction" on which it is based, even with the most accurate density measurement, leads to mean errors of up to 16 ° in the polar angle and 3 delta-E -Units in the color difference leads. Although these errors are described as being sufficiently small to obtain sufficient information about the type and extent of a color cast for control processes, precise and reliable quantitative control of the color guide is not possible with this method.

Quite apart from this, the method according to this document is also relatively cumbersome and complicated insofar as the use of a multicolored overprint field (gray balance field) as a color control field means that the color density differences measured on it before Further processing by means of the "vectorial auxiliary construction" must first be "unmasked", ie the density value differences between the measured actual measured values and the target measured values of a target specimen must be converted using a linear system of equations into such density value differences as would have resulted from single-color prints. An additional series of test prints is required to determine the coefficients of the linear system of equations.

The invention has for its object to improve a method for determining the color difference and a method for color control or color control of the printing of a printing press of the generic types in such a way that a more precise determination of the color numbers in the selected color space and thus a more precise and reliable control or regulation of the Ink guide elements of the printing press is possible.

This object is achieved with respect to the method mentioned at the outset for determining the differences in color number and thus for quality assessment by means of the characterizing features of claim 1.

The solution according to the invention for a method for color control or color regulation is specified in the characterizing part of claim 2.

Appropriate embodiments of the invention are the subject of the dependent claims.

The invention is based on the knowledge that transformation matrices exist within small areas around a given color location in a colorimetric coordinate system, which make it possible to convert changes in color measures into changes in screen densities and in changes in solid densities of printed solid fields. A third relationship consists in a transformation of changes in solid density of solid fields and changes in screen density of printed fields. If two transformations of the three transformations mentioned are known, the third can easily be calculated.

Fig. 1

vier Eichdrucke mit Eichfarbflächen in einer schematischen perspektivischen Ansicht,

Fig. 2

ein Schema zur Veranschaulichung von Transformationen zwischen einem Farbraum, einem Volltondichteraum und einem Rasterdichteraum,

Fig. 3

eine schematische Darstellung des Verfahrens zur Bestimmung der Transformationsmatritzen zwischen den in Fig. 2 veranschaulichten Koordinatenräumen und

Fig. 4

eine schematische Darstellung der Funktionsweise des Verfahrens zur Qualitätsbeurteilung durch Bestimmen von Farbmaßzahl differenzen und zur Farbsteuerung oder Farbregelung des Druckes einer Druckmaschine.

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

Fig. 1

four calibration prints with calibration color areas in a schematic perspective view,

Fig. 2

1 shows a diagram for illustrating transformations between a color space, a solid color density space and a screen density space,

Fig. 3

a schematic representation of the method for determining the transformation matrices between the coordinate spaces illustrated in FIG. 2 and

Fig. 4

a schematic representation of the operation of the method for quality assessment by determining the color measure differences and for color control or color regulation of the printing of a printing press.

In order to implement the method according to the invention for quality assessment and / or influencing the color appearance of a color area constructed from a plurality of colorful partial colors in a printing process, it is first necessary to produce calibration prints which allow empirically the relationships between densitometric quantities and colorimetric quantities for a chosen base or operating point depending on the paper used, the printing ink, the printing press and the particular densitometer device or type of densitometer to be used.

In Fig. 1 four calibration prints or calibration cards with calibration color areas are shown schematically. The calibration plate or calibration card shown below in FIG. 1 and produced under nominal pressure conditions is referred to below as reference calibration pressure 1. The reference calibration print 1 has a color measuring strip or a calibration color area with four fields, of which the first a grid field 2, the second a cyan solid field 3, the third a magenta solid field 4 and the fourth a yellow or yellow solid field 5 is.

The grid 2 consists of three rasters printed one above the other with the colors and layer thicknesses of the solid tone fields 3 to 5, it being particularly expedient to use a gray balance field whose tone value or gray level is sensitive to layer thicknesses Changes and color is similar to a grid, as it occurs in the color measurement strip in the printed product to be produced later. In particular, the raster field 2 of the reference calibration print 1 can be a dark gray gray balance field.

With the help of a densitometer, preferably precisely with the densitometer that will later be used in quality assessment or influencing the color appearance by controlling the layer thickness of the printing press, the screen 2 and the solid fields 3, 4 and 5 of the reference calibration print 1 are measured densitometrically . This results in the cyan full tone density CV für for the cyan solid tone field 3, the magenta full tone density MV₀ for the magenta solid tone field 4 and the yellow solid tone density YV₀ for the yellow solid tone field 5 of the reference calibration print 1.

By measuring the preferably, but not necessarily, dark gray grid 2 with the aid of the densitometer, the cyan grid density CR₀, the magenta grid density MR₀ and the yellow grid density YR₀ are obtained for the grid field 2. The three reference calibration measurement values for the three full tone densities and the three reference calibration measurement values for the screen densities are stored for comparison with other calibration prints in a memory, in particular in a memory of the densitometer or a memory of a connected computer or as a printout on a sheet of paper.

The grid 2 of the reference calibration print 1 is used in addition to the densitometric measurement of the densitometer can also be measured colorimetrically using a spectrophotometer. The colorimetric measurement values recorded by the spectrophotometer are color measures of a color space, the L * a * b * color space (CIE 1976) preferably being used. The L * a * b * color space is a color system with equally spaced sensations, the color dimensions of which are particularly useful for quality assessment in the color space, since they lead to greater flexibility and meaningfulness than solid color densities or screen densities. Another color space system that can be used is the LUV system.

With the help of the spectrophotometer that will later no longer be required for production printing, the colorimetric quantities or color dimension numbers L₀, a₀ and b₀ are thus recorded and also stored for comparison with the values of other calibration prints. This storage takes place electronically in the spectrophotometer itself or in a computer connected to it. It is also possible to save the color measure numbers as a printout, in particular as a printout on the reference calibration print 1 itself.

The densitometric and colorimetric quantities for the reference calibration print 1 can have the following values, for example: CV₀ = 1.58, MV₀ = 1.45, YV₀ = 1.48, CR₀ = 0.80, MR₀ = 0.95, YR₀ = 1 , 12, L₀ = 34.21, a₀ = 3.58 and b₀ = 6.06.

The additionally required calibration prints shown in FIG. 1, which are constructed similarly to the reference calibration pressure 1, are a first additional calibration pressure 6, a second additional calibration pressure 7 and a third additional calibration print 8. The fields of the additional calibration prints 6 to 8, like the fields 2 to 5 of the reference calibration print 1, were each printed in such a way that the layer thicknesses of the differently colored full-tone fields match the layer thicknesses of the individual screen dots of the three printed on top of one another different colored grids of the respective grid are assigned.

The first additional calibration print 6 differs from the reference calibration print 1 in that a greater layer thickness of the ink guide elements of the printing machine has been set when printing the cyan solid-color field 9 and thus the associated cyan screen printed in the screen field 12, so that the solid cyan field 9 gives a larger solid cyan density CV₁ than for the solid cyan field 3. The higher cyan solid density corresponding to the increased layer thickness CV₁ can be expressed as the sum of the solid cyan density CV₀ and the change ΔCV₁ (CV₁ = CV₀ + ΔCV₁).

The magenta solid field 10 of the first additional calibration pressure 6 has a magenta solid density MV₁, which corresponds to the solid density MV₀ within the pressure tolerances. The same applies to the solid density YV₁ of the yellow solid field 11th

The printed screen 12 of the first additional calibration print 6 differs from the screen 2 due to the greater layer thickness for the printing ink cyan in that the screen dots of the cyan screen each have a higher layer thickness. For this reason, the grid density is CR₁ Grid 12 a higher measured value when measuring with the densitometer than was the case when measuring grid 2. The magenta screen density MR₁ of the screen 12 of the first additional calibration print 6 corresponds essentially to the magenta screen density MR₀ of the reference calibration print 1. The same applies to the yellow screen density YR₁ of the first additional calibration print 6.

$$\text{ΔMV₁ = MV₁ - MV₀}$$

$$\text{ΔYV₁ = YV₁ - YV₀}$$

$$\text{ΔCR₁ = CR₁ - CR₀}$$

$$\text{ΔMR₁ = MR₁ - MR₀}$$

$$\text{ΔYR₁ = YR₁ - YR₀}$$

The three measured values for the solid densities of the solid fields 9, 10 and 11 as well as the measured values of the halftone densities of the halftone field 12 of the first additional calibration print 6 are stored and used to compare the deviations of these six measured density values with respect to the corresponding measured six density values of the reference calibration print 1 to determine. These six measured deviations or changes are values for: $$\text{ΔCV₁ = CV₁ - CV₀}$$

$$\text{ΔMV₁ = MV₁ - MV₀}$$

$$\text{ΔYV₁ = YV₁ - YV₀}$$

$$\text{ΔCR₁ = CR₁ - CR₀}$$

$$\text{ΔMR₁ = MR₁ - MR₀}$$

$$\text{ΔYR₁ = YR₁ - YR₀}$$

$$\text{Δa₁ = a₁ - a₀}$$

$$\text{Δb₁ = b₁ - b₀.}$$

After the densitometric measurement of the solid areas 9 to 11 and the screen 12 of the first additional calibration print 6, the screen 12 is measured with the aid of the spectrophotometer already mentioned in order to determine the color deviation of the screen 12 with respect to the screen 2. If the three color measures of the grid 12 are denoted by L₁ a₁ and b₁, the result for the changes in Color measure numbers between the reference calibration print 1 and the first additional calibration print 6 following three additional values. $$\text{ΔL₁ = L₁ - L₀}$$

$$\text{Δa₁ = a₁ - a₀}$$

$$\text{Δb₁ = b₁ - b₀.}$$

By colorimetric and densitometric measurement and comparison of the reference calibration print 1 and the first additional calibration print 6, nine deviations or nine difference values are thus obtained, which are three full-tone density differences, three screen density differences and three color measure number differences . The full-tone density differences .DELTA.MV₁ and .DELTA.YV.sub.1 in the additional calibration print 6, like the associated screen density differences .DELTA. For example, the following values result: ΔCV₁ = 0.19, ΔMV₁ = -0.01, ΔYV₁ = -0.02, ΔCR₁ = 0.09, ΔMR₁ = 0.04, ΔYR₁ = 0.01, ΔL₁ = -1.85 , Δa₁ = -2.87 and Δb₁ = -2.44.

These sample values depend not only on the type of densitometer device or densitometer device used, but also on the printing inks, the printing press and the paper used.

In order to empirically determine a general relationship valid for a working point in the color space, which shows a relationship between the changes in solid ink densities or layer thicknesses and the changes in screen densities and the changes in the colorimetric values of a print, two are required create and measure additional additional calibration prints.

$$\text{Δa₂ = a₂ - a₀}$$

$$\text{Δb₂ = b₂ - b₀}$$

$$\text{ΔCV₂ = CV₂ - CV₀}$$

$$\text{ΔMV₂ = MV₂ - MV₀}$$

$$\text{ΔYV₂ = YV₂ - YV₀}$$

$$\text{ΔCR₂ = CR₂ - CR₀}$$

$$\text{ΔMR₂ = MR₂ - MR₀}$$

$$\text{ΔYR₂ = YR₂ - YR₀}$$

In Fig. 1, a second additional calibration print 7 is shown together with the associated solid fields 13, 14 and 15 and the screen 16 also printed. When printing the second additional calibration print 7, the same environmental conditions, in particular the same paper, printing ink and printing press, were used as when printing the reference calibration print 1 and the first additional calibration print 6. In contrast to the reference calibration print 1, however, the second additional calibration print 7 has a much greater layer thickness for the magenta printing ink and thus a full tone density of the solid tone field 14 which is larger by ΔMV₂ than the solid tone density MV₀ of the solid tone field 4 of the reference calibration print 1. The change in solid tone density ΔMV₂ can be 0.26, for example. When printing the second additional calibration print 7 changes in the solid densities CV₂ and YV₂ for the solid fields 13 and 15 are largely avoided. While the solid tone fields 13, 14 and 15 are again only measured densitometrically, the screen 16 of the second additional calibration print 7 is again recorded both densitometrically and colorimetrically. This results in deviations from the values recorded in the densitometric and colorimetric measurement of the reference calibration pressure 1, which are determined and stored in the manner mentioned above. These are the following values: $$\text{ΔL₂ = L₂ - L₀}$$

$$\text{Δa₂ = a₂ - a₀}$$

$$\text{Δb₂ = b₂ - b₀}$$

$$\text{ΔCV₂ = CV₂ - CV₀}$$

$$\text{ΔMV₂ = MV₂ - MV₀}$$

$$\text{ΔYV₂ = YV₂ - YV₀}$$

$$\text{ΔCR₂ = CR₂ - CR₀}$$

$$\text{ΔMR₂ = MR₂ - MR₀}$$

$$\text{ΔYR₂ = YR₂ - YR₀}$$

$$\text{Δa₃ = a₃ - a₀}$$

$$\text{Δb₃ = b₃ - b₀}$$

$$\text{ΔCV₃ = CV₃ - CV₀}$$

$$\text{ΔMV₃ = MV₃ - MV₀}$$

$$\text{ΔYV₃ = YV₃ - YV₀}$$

$$\text{ΔCR₃ = CR₃ - CR₀}$$

$$\text{ΔMR₃ = MR₃ - MR₀}$$

$$\text{ΔYR₃ = YR₃ - YR₀}$$

Analogously to the additional calibration prints 6 and 7, a third additional calibration print 8 is finally produced, the layer thickness for the printing ink yellow or yellow being increased considerably in the solid field 19 shown at the top right in FIG. 1. The resulting increase in solid density ΔYV₃ can be, for example, 0.16. By densitometric scanning of the solid tone fields 17 to 19 and colorimetric scanning of the printed grid 20 of the third additional calibration print 8, nine further measured values are finally obtained in a manner corresponding to the first and second additional calibration prints 6 and 7, namely: $$\text{ΔL₃ = L₃ - L₀}$$

$$\text{Δa₃ = a₃ - a₀}$$

$$\text{Δb₃ = b₃ - b₀}$$

$$\text{ΔCV₃ = CV₃ - CV₀}$$

$$\text{ΔMV₃ = MV₃ - MV₀}$$

$$\text{ΔYV₃ = YV₃ - YV₀}$$

$$\text{ΔCR₃ = CR₃ - CR₀}$$

$$\text{ΔMR₃ = MR₃ - MR₀}$$

$$\text{ΔYR₃ = YR₃ - YR₀}$$

It can thus be seen that the additional calibration prints 6, 7 and 8 differ from the reference calibration print 1 in that a full-tone density differs comparatively strongly by changing a layer thickness Lich has been made, while the other two colors have largely been left unchanged in their layer thicknesses. Corresponding to the changes in the solid ink density, there are changes in the grid field that is also printed, which, in contrast to the solid tone fields, is measured not only densitometrically but also colorimetrically as part of the calibration process.

2 illustrates the concept on which the method according to the invention is based. In Fig. 2 on the left you can see three solid tone fields 21, 22, 23 of a color measuring strip or calibration print with the changes in the associated solid density densities ΔCV, ΔMV and ΔYV measured with the aid of a specific densitometer, which can be understood as the components of a three-dimensional solid density change vector [ΔV]. These changes also cause changes in the grid field 24, 24 ', which is drawn twice on the right in FIG. 2, which can in particular be a gray balance field for monitoring the color balance of cyan, magenta and yellow in the overprint, the changes which can be detected by densitometry Screen densities are ΔCR, ΔMR and ΔYR. If the grid 24, 24 'is measured colorimetrically as a grid 24' with the aid of a spectrophotometer, the changes in the colorimetric values ΔL, Δa and Δb in L * a * b * caused by the changes in the solid density in the solid fields 21 to 23 are recorded. Color space.

oder abgekürzt $$\text{[ΔR] = [X] · [ΔV]}$$

In FIG. 2, an arrow 25 illustrates an assignment between the solid fields 21, 22, 23 and the grid field 24. The connection of the solid fields 21 to 23 assigned full-tone density changes in a full-tone density space with the assigned raster density changes of the raster field 24 in a raster density space means a transformation of a three-dimensional vector, which can be represented by a full-tone density-raster density transformation matrix, which is hereinafter referred to briefly as transformation matrix [X]. The transformation matrix [X] has nine matrix elements and assigns the three solid density changes ΔCV, ΔMV and ΔYV to the three grid density changes ΔCR, ΔMR and ΔYR. The transformation matrix [X] thus transforms the full-tone density change vector [ΔV] formed from three full-tone density changes ΔCV, ΔMV and ΔYV into a screen density change vector [ΔR] with the components Δ CR, ΔMR and ΔYR. In matrix notation, this can be represented as follows:

or abbreviated $$\text{[ΔR] = [X] · [ΔV]}$$

The transformation matrix [X] for three-dimensional vectors contains nine elements X₁₁ to X₃₃, which are the partial derivatives of the components of the grid density vector according to the components of the solid tone density vector. The following applies to the transformation matrix [X]:

2, an arrow 26 between the grid field 24 'and the solid fields 21 to 23 illustrates an assignment between changes in the color location assigned to the color of the grid field 24' in the L * a * b * color space or the assigned changes in the colorimetric values or Color measures on the one hand and the changes in solid density of the solid fields 21 to 23 printed on the other. This corresponds to a transformation of a three-dimensional color change vector [ΔF], the components of which are formed by the changes in color measure number ΔL, Δa and Δb, in the L * a * b * color space into the assigned three-dimensional full-tone density change vector ΔV in the full-tone density space. The color metric full-tone density transformation matrix assigned to the transformation thus illustrated by the arrow 26 is designated in FIG. 2 with [Z], the following in abbreviated form: $$\text{[ΔV] = [Z] · [ΔF]}$$

The nine components of the matrix [Z] are formed in an analogous manner to the matrix [X] by the partial derivatives of the components of the solid density vector according to the components of the color vector.

2 finally shows an arrow 27 between the grid field 24 'and the grid field 24. The arrow 27 illustrates an association between changes ΔL, Δa, Δb of the colorimetric values L, a, b in L * a * b * - Color space of the grid field 24 'and the associated densitometrically detectable grid density changes ΔCR, ΔMR and Δ YR of the grid field 24 which is physically identical to the grid field 24'. The assignment between three changes in color measures and three changes in grid densities illustrated by arrow 27 can be determined by a color measure - Describe the grid density transformation matrix. The matrix, briefly referred to as the transformation matrix [W], allows the transformation of the three-dimensional color change vector [ΔF] in the L * a * b * color space into a grid density change vector [ΔR] in the grid density space. The transformation matrix [W] has nine elements because it transforms a three-dimensional vector into another three-dimensional vector. The elements W₁₁ to W₃₃ are formed by the partial derivatives of the components of the vector [ΔR] after the components of the vector [ΔF]. The following therefore applies to the transformation matrix [W]:

oder abgekürzt $$\text{[ΔR] = [W] · [ΔF]}$$

The transformation between the changes in color measure and the changes in the grid densities can thus be represented as follows:

or abbreviated $$\text{[ΔR] = [W] · [ΔF]}$$

From Fig. 2 and the above explanations it follows that the transformation matrices [X], [W] and [Z] can be assigned inverse transformation matrices [X⁻¹], [W⁻¹] and [Z⁻¹], respectively are illustrated by arrows 28, 29 and 30 in FIG. 2 and can each be used in a transformation in the opposite direction to the transformations illustrated by arrows 25, 27 and 26. It can be seen from FIG. 2 and the above explanations that it is sufficient to know two transformation matrices which are not inverse to one another in order to calculate any conversions between changes in the full-tone density space, screen density space and L * a * b * color space. The transformation matrices [X], [W] and [Z] discussed above only apply to the operating point for which they were determined, because in the above considerations linear relationships have been assumed, which are however always correct when the considered changes take place in a relatively small volume of the entire three-dimensional (color) space. The The working point is the point in the respective space around which the changes take place.

$$\text{[ΔF] = [W]⁻¹ · [ΔR]}$$

$$\text{[ΔF] = [Z]⁻¹ · [ΔV]}$$

$$\text{[ΔV] = [X]⁻¹ · [W] · [ΔF]}$$

$$\text{und [ΔF] = [W]⁻¹ · [X] · [ΔV]}$$

If, in addition to the above-mentioned transformation matrices, the inverse transformation matrices, which are easy to calculate, are taken into account, the following further relationships apply in abbreviated form, which can also be seen in FIG. 2: $$\text{[ΔV] = [X] ⁻¹ · [ΔR]}$$

$$\text{[ΔF] = [W] ⁻¹ · [ΔR]}$$

$$\text{[ΔF] = [Z] ⁻¹ · [ΔV]}$$

$$\text{[ΔV] = [X] ⁻¹ · [W] · [ΔF]}$$

$$\text{and [ΔF] = [W] ⁻¹ · [X] · [ΔV]}$$

With knowledge of the nine elements of two transformation matrices, it is thus possible to make any calculations between the solid densities of the solid fields, the halftone densities of the halftone fields and the color dimensions of the halftone fields of printed calibration color areas or color measurement strips. The calibration color areas are used initially to determine the matrix elements, which will later be available for conversions when monitoring color measuring strips.

FIG. 3 illustrates how, in accordance with the method according to the invention, by measuring the calibration prints described in connection with FIG. 1, the transformation matrices [X], [W] and [Z] are determined for a working point predetermined, for example, by a gray balance field. In Fig. 3, a grid R _i with i = 0, 1, 2 or 3 can be seen at the top left, whereby it depends on the index i is the grid 2, 12, 16 or 20 from FIG. 1.

In Fig. 3 top right you can see a trio of solid fields V _i , the index i running from 0 to 3. If the index i = 0, the trio of solid fields V₀ consists of solid fields 3, 4 and 5 according to FIG. 1. The solid fields 9, 10 and 11 known from FIG. 1 correspond to the trio of solid fields V₁, the solid fields 13, 14 and 15 the trio of solid fields V₂ and the solid fields 17, 18 and 19 the trio of solid fields V₃.

At the beginning of the calibration measurements for the determination of the transformation matrices [X], [W] and [Z], the reference calibration print 1 is measured with the raster field R₀ and the trio of solid fields V₀. In Fig. 3 you can see a spectrophotometer 30, which allows the grid fields R₀, R₁, R₂ and R₃ to be measured colorimetrically. In addition to a colorimetric measurement of the grid fields R₀ to R₃, which, as mentioned, correspond to the grid fields 2, 12, 16 and 20), a densitometric measurement of the grid fields R₀ to R₃ takes place with the aid of the densitometer 31 shown schematically in FIG. 3.

The spectrophotometer 30 delivers color dimensions L₀, a₀, b₀ for the grid R₀ of the reference calibration print 1, L₁, a₁, b₁ for the grid R₁ of the first additional calibration print 6, L₂, a₂, b₂ for the grid R₂ of the second additional Calibration print 7 and L₃, a₃, b₃ for the grid R₃ of the third additional calibration print 8. From the output 32 of the spectrophotometer, the triples of the color measures L _i , a _i and b _i either go directly electrically or with the interposition of a display and manual keyboard input into a computer 33 assigned to the spectrophotometer 30 and the densitometer 31.

The computer 33 has a difference calculator 34 for the color measure numbers detected by the spectrophotometer 30 and forms the differences between the color measure numbers L _i , a _i , b _i with i = 1, 2, 3 of the grid fields R 1, R 2 and R 3 on the one hand and the other Color measures L₀, a₀, b₀ of the grid R₀ on the other hand. Then the difference calculator 34 stores the calculated difference values for the color measures, ie the numerical values for ΔL₁, Δa₁, Δb₁, ΔL₂, Δa₂, Δb₂, ΔL₃, Δa₃ and Δb₃. The three color measure number differences for the first additional calibration print 6 can be used as components of a three-dimensional vector [Δ F] ₁, those for the second additional calibration print 7 as components of a vector [ΔF] ₂ and those of the third additional calibration print 8 as components of one three-dimensional vector [ΔF] ₃ can be understood. 3, these three three-dimensional vectors are represented as [ΔF] _i with i = 1, 2, 3.

The grid fields R₀, R₁, R₂ and R₃ are additionally measured with the aid of the densitometer 31 in order to determine the grid densities for each of the colors cyan, magenta and yellow, so that the differences between the grid densities of the grid R₁ are then determined in a difference calculator 35 for grid densities , R₂ and R₃ on the one hand and the grid density of the grid R₀ on the other hand can be calculated. After saving the Grid density differences are available at the output of the difference calculator 35 for grid densities the following nine values: ΔCR₁, ΔMR₁, ΔYR₁, ΔCR₂, ΔMR₂, ΔYR₂, ΔCR₃, ΔMR₃ and ΔYR₃. These screen density differences can be written in short form as three-dimensional screen density change vectors [ΔR] _i with i = 1, 2, 3.

The densitometer 31 is also used during the calibration measurements on the calibration prints also for the densitometric measurement of the full tone fields V₀ of the reference calibration pressure 1, the full tone fields V₁ of the first additional calibration pressure 6, the full tone fields V₂ of the second additional calibration pressure 7 and the full tone fields V₃ of the third additional calibration pressure 8. These solid tone fields have the reference numerals 3, 4, 5, 9, 10, 11, 13, 14, 15, 17, 18 and 19 in FIG. 1.

As can be seen in FIG. 3, the densitometer 31 is also directly electrically connected to a difference computer 36 for solid densities in the computer 33 or with the interposition of a densitometer display and a keyboard. The difference calculator 36 for full tone densities calculates the difference between the full tone density of an additional calibration print 6, 7 or 8 and the same color of the reference calibration print 1 from the full tone densities recorded by the densitometer 31 for each of the three printing inks. These values are then used for further processing in the difference calculator 36 for Solid densities saved. These are the following nine full-tone density differences: ΔCV₁, Δ MV₁, ΔYV₁, ΔCV₂, ΔMV₂, ΔYV₂, ΔCV₃, ΔMV₃ and ΔYV₃. These triple numbers assigned to an additional calibration pressure can be summarized in the short summarize notation into a three-dimensional vector [ΔV] _i with i = 1, 2, 3.

erhält man die folgenden neun Gleichungen für die neun Unbekannten der Transformationsmatrix [X]: $$\text{ΔCR₁ = X₁₁ · ΔCV₁ + X₁₂ · ΔMV₁ + X₁₃ · ΔYV₁}$$

$$\text{ΔCR₂ = X₁₁ · ΔCV₂ + X₁₂ · ΔMV₂ + X₁₃ · ΔYV₂}$$

$$\text{ΔCR₃ = X₁₁ · ΔCV₃ + X₁₂ · ΔMV₃ + X₁₃ · ΔYV₃}$$

$$\text{ΔMR₁ = X₂₁ · ΔCV₁ + X₂₂ · ΔMV₁ + X₂₃ · ΔYV₁}$$

$$\text{ΔMR₂ = X₂₁ · ΔCV₂ + X₂₂ · ΔMV₂ + X₂₃ · ΔYV₂}$$

$$\text{ΔMR₃ = X₂₁ · ΔCV₃ + X₂₂ · ΔMV₃ + X₂₃ · ΔYV₃}$$

$$\text{ΔYR₂ = X₃₁ · ΔCV₁ + X₃₂ · ΔMV₁ + X₃₃ · ΔYV₁}$$

$$\text{ΔYR₂ = X₃₁ · ΔCV₂ + X₃₂ · ΔMV₂ + X₃₃ · ΔYV₂}$$

$$\text{ΔYR₃ = X₃₁ · ΔCV₃ + X₃₂ · ΔMV₃ + X₃₃ · ΔYV₃}$$

The difference calculator 35 for screen densities and the difference calculator 36 for solid tones feed, as can be seen from the block diagram in FIG. 3, a first matrix calculator 37. The matrix calculator 37 serves to determine the nine elements of the transformation matrix [X]. For this purpose, he receives from the difference computer 35 for the screen densities the nine numerical values for screen density differences mentioned above and from the difference computer 36 for full tone densities the above-mentioned nine measured values for full tone density differences. By inserting these numerical values into the three matrix equations $${\text{[ΔR]}}_{\text{i}}{\text{= [X] · [ΔV]}}_{\text{i}}\text{with i = 1, 2 and 3}$$

the following nine equations are obtained for the nine unknowns of the transformation matrix [X]: $$\text{ΔCR₁ = X₁₁ · ΔCV₁ + X₁₂ · ΔMV₁ + X₁₃ · ΔYV₁}$$

$$\text{ΔCR₂ = X₁₁ · ΔCV₂ + X₁₂ · ΔMV₂ + X₁₃ · ΔYV₂}$$

$$\text{ΔCR₃ = X₁₁ · ΔCV₃ + X₁₂ · ΔMV₃ + X₁₃ · ΔYV₃}$$

$$\text{ΔMR₁ = X₂₁ · ΔCV₁ + X₂₂ · ΔMV₁ + X₂₃ · ΔYV₁}$$

$$\text{ΔMR₂ = X₂₁ · ΔCV₂ + X₂₂ · ΔMV₂ + X₂₃ · ΔYV₂}$$

$$\text{ΔMR₃ = X₂₁ · ΔCV₃ + X₂₂ · ΔMV₃ + X₂₃ · ΔYV₃}$$

$$\text{ΔYR₂ = X₃₁ · ΔCV₁ + X₃₂ · ΔMV₁ + X₃₃ · ΔYV₁}$$

$$\text{ΔYR₂ = X₃₁ · ΔCV₂ + X₃₂ · ΔMV₂ + X₃₃ · ΔYV₂}$$

$$\text{ΔYR₃ = X₃₁ · ΔCV₃ + X₃₂ · ΔMV₃ + X₃₃ · ΔYV₃}$$

After inserting the special 18 difference number values supplied by difference calculators 35 and 36 into the above system of equations with nine Equations, the first matrix calculator 37 calculates the numerical values for the nine unknowns X₁₁, X₁₂, X₁₃, X₂₁, X₂₂, X₂₃, X₃₁, X₃₂ and X₃₃. These numerical values are output by the first matrix computer 37 at the output 38 as the nine elements of the transformation matrix [X].

$$\text{ΔCR₂ = W₁₁ · ΔL₂ + W₁₂ · Δa₂ + W₁₃ · Δb₂}$$

$$\text{ΔCR₃ = W₁₁ · ΔL₃ + W₁₂ · Δa₃ + W₁₃ · Δb₃}$$

$$\text{ΔMR₁ = W₂₁ · ΔL₁ + W₂₂ · Δa₁ + W₂₃ · Δb₁}$$

$$\text{ΔMR₂ = W₂₁ · ΔL₂ + W₂₂ · Δa₂ + W₂₃ · Δb₂}$$

$$\text{ΔMR₃ = W₂₁ · ΔL₃ + W₂₂ · Δa₃ + W₂₃ · Δb₃}$$

$$\text{ΔYR₁ = W₃₁ · ΔL₁ + W₃₂ · Δa₁ + W₃₃ · Δb₁}$$

$$\text{ΔYR₂ = W₃₁ · ΔL₂ + W₃₂ · Δa₂ + W₃₃ · Δb₂}$$

$$\text{ΔYR₃ = W₃₁ · ΔL₃ + W₃₂ · Δa₃ + W₃₃ · Δb₃}$$

As can also be seen in FIG. 3, the computer 33 contains a second matrix computer 39 for calculating the transformation matrix [W]. The second matrix calculator 39 uses the difference values determined and temporarily stored by the difference calculators 34 and 35 in the matrix equation [ΔR] _i = [W] · [ΔF] _i . This results in the following nine equations for the nine unknowns of the elements of the transformation matrix [W]: $$\text{ΔCR₁ = W₁₁ · ΔL₁ + W₁₂ · Δa₁ + W₁₃ · Δb₁}$$

$$\text{ΔCR₂ = W₁₁ · ΔL₂ + W₁₂ · Δa₂ + W₁₃ · Δb₂}$$

$$\text{ΔCR₃ = W₁₁ · ΔL₃ + W₁₂ · Δa₃ + W₁₃ · Δb₃}$$

$$\text{ΔMR₁ = W₂₁ · ΔL₁ + W₂₂ · Δa₁ + W₂₃ · Δb₁}$$

$$\text{ΔMR₂ = W₂₁ · ΔL₂ + W₂₂ · Δa₂ + W₂₃ · Δb₂}$$

$$\text{ΔMR₃ = W₂₁ · ΔL₃ + W₂₂ · Δa₃ + W₂₃ · Δb₃}$$

$$\text{ΔYR₁ = W₃₁ · ΔL₁ + W₃₂ · Δa₁ + W₃₃ · Δb₁}$$

$$\text{ΔYR₂ = W₃₁ · ΔL₂ + W₃₂ · Δa₂ + W₃₃ · Δb₂}$$

$$\text{ΔYR₃ = W₃₁ · ΔL₃ + W₃₂ · Δa₃ + W₃₃ · Δb₃}$$

After evaluating this system of equations, the second matrix computer 39 supplies the nine elements of the transformation matrix [W] at its output 40.

The outputs 38 and 40 of the first matrix computer 37 and the second matrix computer 39 feed the two inputs of a third matrix computer 41, which it allows the transformation matrix [X] to be inverted and multiplied by the transformation matrix [W] in order to calculate the nine elements of the transformation matrix [Z] described in connection with FIG. 2.

As soon as the elements of the transformation matrices [X], [W] and [Z] are present in the computer 33, the spectrophotometer 30 is no longer required in order to carry out a quality control and quality assessment in the L * a * b * color space with the densitometer 31.

Following the calibration described above, the system consisting of the densitometer 31 and the computer 33 can now be placed by placing the densitometer 31 on a grid 43 similar to the grid of the calibration prints, in particular a gray balance grid of a sample sheet or OK sheet 44 (FIG. 4) determine the differences between the color measures of the grid 43 of the OK sheet 44 and the color measures of the grid 2 of the reference calibration print 1. After knowledge of these differences or deviations, it is possible, taking into account the color dimensions of the screen 2 of the reference calibration print 1 known from the measurement with the spectrophotometer 30, also to determine the absolute color numbers of the screen 43 of the OK sheet 44 without the OK -Bow 44 has been scanned with a spectrophotometer. For high accuracy, it is assumed that the nominal conditions as used for producing the OK sheet 44 were used when producing the reference calibration pressure 1 and that the densitometer 31 used for scanning the OK sheet 44 was the same or at least the same Densitometer type is how it was used when scanning the calibration prints.

FIG. 4 shows the system consisting of the computer 33 and the densitometer 31 with a printing machine 42 which can be controlled with the aid of this system, as well as the OK sheet 44 and a production sheet 45 in a schematic representation.

The sample sheet or OK sheet 44 is drawn in FIG. 4, top left with its grid 43 which serves as a reference color area and which is similar in color to the grid 2 of the reference character print 1. The color appearance of the raster field 43 becomes continuous with a raster field 46, in particular a corresponding gray balance field in the color measuring strip, when printing the production sheets 45, one of which is shown schematically with a color measuring strip in FIG. 4, top right compared.

The arrangement shown in FIG. 4 with the densitometer 31 and the computer 33 is used to check the continuous printing sheets 45 printed continuously with the printing machine 42 with regard to their color correspondence with the OK sheet 44 and to readjust the ink guiding elements of the printing machine 42 in the event of deviations . For this purpose, with the aid of the computer 33, a control variable is output at its output 47, which is fed as an input variable via the input 48 to the layer thickness control of the printing press 42.

The control signals present at input 48 are a layer thickness change control vector, the components of which are shown in FIG. 4 are. The component ΔCV of the layer thickness change control vector specifies the amount by which the layer thickness for the printing ink cyan has to be changed in order to correct the color appearance of the grid field 46 if there is a deviation from the color appearance of the grid field 43 on the OK sheet 44. Accordingly, the components ΔMV and ΔYV of the layer thickness change control vector [ΔV] are assigned to the required layer thickness changes for the printing inks magenta and yellow.

As illustrated in FIG. 4, the densitometer 31 first serves to determine and to store the target value for the screen density vector [R] _SOLL which can be obtained by measuring the screen field 43 of the OK sheet 44. These are the components CR _SOLL , MR _SOLL and YR _{SOLL of} the screen density vector [R] _SOLL .

Correspondingly, the actual value of the screen density vector [R] _{IST is} measured with the aid of the densitometer 31 by measuring the grid field 46 in the color measurement strip of the printing sheet 45.

The computer 33 has a plurality of computing units designed in terms of hardware or software, which represent an evaluation computer which makes it possible, on the one hand, to determine a quality measure for the respectively printed production sheet 45 from the comparison of the vector [R] _ACTUAL with the vector [R] _SHOULD , which is available at the output 49 of the computer 33, and on the other hand to generate an input variable for the layer thickness control, which is provided at the output 47 of the computer 33.

The part of the computer 33 designated as the evaluation computer in FIG. 4 not only receives the measured values of the densitometer 31 as input variables, but also the matrix elements of the transformation matrices [X], [W] and [Z] determined beforehand on the basis of the calibration prints. These values reach the part of the computer 33 referred to as an evaluation computer via the inputs 50, 51 and 52.

As can be seen in FIG. 4, the computer 33 has a screen density difference calculator 53 which calculates the deviations between the actual screen densities measured on the respective printing sheet 45 and the target screen density recorded on the OK sheet 44. The output 56 of the grid density difference calculator 53 is connected to a first input 57 of a quality measurement calculator 54, the second input 58 of which is supplied with the values of the nine matrix elements of the transformation matrix [W]. In accordance with the relationship illustrated in FIG. 2, the transformation matrix [W] is first inverted in the quality measurement computer 54 and then multiplied by the grid density difference vector [ΔR]. At the output 59 of the quality measure computer 54, the calculation results are available in the form of color measure difference ΔL, Δa and Δb, which can be viewed as the components of a three-dimensional color difference vector [ΔF].

Thanks to the calculations of the quality measurement computer 54 and the transformation matrix [W], there are 49 color measurement values or their differences at the output 49, although the computer 33 does not use the data of a color measurement device, but has been fed with those of the densitometer 31. The color measure number changes at the output 49 of the computer 33 permit a quality assessment in the color space, which results in a much simpler and more meaningful quality control than in a quality assessment based on density values. The quality measurement computer 54 has converted the deviations from density values into deviations from color coordinates of a color space that is equally graduated in terms of sensation. Based on the known deviations and the color dimensions for the calibration print or OK sheet, it is then also possible to determine the absolute color coordinates.

The grid density difference calculator 53 also feeds the first input 60 of a first layer thickness control computer 55. The first layer thickness control computer 55 receives at its second input 61 the values of the elements of the transformation matrix [X] fed in via the input 50 of the computer 33. After inverting the transformation matrix [X], the first layer thickness control computer 55 calculates the components ΔCV, ΔMV and ΔYV of the layer thickness change control vector [ΔV], the values of which output from the output 62 of the first, from the product of the inverted transformation matrix [X] ⁻¹ and the grid density difference vector [ΔR] Layer thickness control computer 55 to the output 47 of the computer 33 and from there to the input 48 of the layer thickness control for the ink guide members of the printing press 42.

In addition to the determination of the layer thickness change control vector [ΔV] described above, in FIG. 4 Two further possibilities for the determination of the layer thickness change control vector are shown, whereby the interruptions 97, 98 and 99 in the drawn lines are intended to illustrate that an interruption 97, 98, 99 is bridged depending on the selected option.

berechnet.With the first additional possibility, the first layer thickness control computer 55 can be dispensed with. Then, with the aid of the color difference vector [ΔF] at the output 59 of the quality measurement computer 54 using the transformation matrix [Z] in an alternatively provided second layer thickness control computer 55 ′, the layer thickness change control vector [ΔV] according to the equation $$\text{[ΔV] = [ΔF] · [Z]}$$

calculated.

It can be seen that the determination of the layer thickness change control vector [ΔV] takes place via the L * a * b * color space in such a procedure. This opens up a further possibility shown in FIG. 4, in which the color dimension difference ΔL, Δa and Δb provided by the output 59 does not directly, but under the influence of a correction using a control strategy, which is shown in FIG. 4 as block 63, the input variable of a third layer thickness control computer 55 ″.

berechnet werden kann, wobei [ΔF]' den Vektor aus den Ersatz-Farbmaßzahldifferenzen ΔL', Δb' und Δa' bedeutet. Der am Ausgang 68 des dritten Schichtdickensteuerungsrechners 55'' anliegende und bei geschlossener Unterbrechung 99 über den Ausgang 47 zum Eingang 48 der Druckmaschine 42 eingespeiste Schichtdickenänderungssteuervektor ist entsprechend der im Regelstrategieblock 63 festgelegten Strategie gegenüber einem Schichtdickenänderungssteuervektor verändert oder verbessert, wie er am Ausgang des Schichtdickensteuerungsrechners 55' zur Verfügung steht.The control strategy block 63 generates substitute color measure number differences ΔL ', Δa' and Δb 'from the color measure number differences fed in at the color measure number input 64, which are output via the output 65 and feed the first input 66 of the third layer thickness control computer 55 ″. The second input 67 of the third layer thickness control computer 55 ″ is used to feed the matrix elements of the transformation matrix [Z]) so that the layer thickness change control vector [ΔV] corresponds to the equation $$\text{[ΔV] = [ΔF] '· [Z]}$$

can be calculated, where [ΔF] 'means the vector from the substitute color measure differences ΔL', Δb 'and Δa'. The layer thickness change control vector which is present at the output 68 of the third layer thickness control computer 55 ″ and is fed in when the interruption 99 is closed via the output 47 to the input 48 of the printing press 42 is changed or improved in accordance with the strategy defined in the control strategy block 63 in relation to a layer thickness change control vector as it is at the output of the layer thickness control computer 55 'is available.

A wide variety of strategies can be used as the control strategy for the control strategy block 63, in which color measure number differences are to be replaced by more useful color measure number differences. In particular, a control strategy can be implemented in the control strategy block 63, which allows the highest possible print quality to be achieved even if the specified target color location in the color space lies outside a correction range which is limited as a result of maximum and minimum full-tone layer thicknesses.

In the exemplary embodiment described, the control strategy block 63 therefore has a limit value input 69 via which the boundary conditions, i.e. the minimum and maximum layer thicknesses that are permitted are entered. In order to record the actually existing layer thickness of the three printing inks, when using the control strategy described here, it is necessary to measure 45 additional solid tone fields 70, 71 and 72 densitometrically on the production sheet. The solid field 70 is a solid field with the solid density CV for the color cyan. The solid field 71 is a solid field with the solid density MV for the magenta printing ink and the solid field 72 is a solid field with the solid density YV for the color yellow or yellow.

With the aid of the densitometer 31, when using the control strategy block 63, in addition to the densitometric measurement of the grid field 46, a densitometric detection of the full tone fields 70 to 72 is carried out in order to be able to determine within the control strategy whether an adjustment of the layer thicknesses would lead to the change in a layer thickness leads to an area that is no longer permitted. When using the control strategies, the densitometer 31 is therefore additionally connected to an actual full-tone input of the control strategy block 63.

The control strategy implemented in control strategy block 63 is described in detail in EP-A 321 402.

The color measure numbers of the are in the control strategy block 43 via an input, not shown in FIG Reference character print 1 has been entered so that, based on these color measure numbers and the color measure number differences at the color measure number input 64, the color location of the color of the grid 46 is available for carrying out the control strategy.

The color location of the raster field 43 of the OK sheet 44 results simply from the fact that, with the aid of the arrangement shown in FIG. 4, the reference character print 1 and the OK sheet 44 are measured densitometrically in succession. The color locus of the grid field 43 of the OK sheet 44 results from the known color measure numbers of the reference character print 1 and the color measure number differences for the grid field 43 calculated with the aid of the evaluation computer of the computer 33. By comparing the grid field 43 of the OK sheet 44 with the grid field 46 of the production sheet 45 finally results in the differences in the color measure between the grid 46 and the grid 43, so that ultimately not only the differences in color measures but also the absolute color measures or color coordinates in the L * a * b * color space are known for the grid 46.

The color coordinates in the color space determined in this way indicate an actual color location by which the control strategy block determines a correction color space for the colors that can be achieved on the basis of the predetermined limit densities for the solid fields 70 to 72 and the solid densities of the solid fields actually measured with the aid of the densitometer.

If by comparing the actual color location of the grid 46 with the target color location of the grid 43 shows that the target color location lies outside of the correction space, then, in accordance with the control strategy implemented in control strategy block 63, the predetermined target color location is replaced by an achievable target color location on the boundary surface of the correction color space with a color distance from the predetermined target color location, whose for the print quality essential components are minimal.

In particular, in the implemented control strategy, the color location on the surface of the correction color space that has the smallest color distance from the specified target color location is selected as the achievable target color location. Depending on the position of the target color locus in the L * a * b * color space outside the correction color space spanned by the actual color locus in the L * a * b * color space, there are various options for determining an optimal replacement target color locus according to Control strategy. One possibility is that a solder is dropped from the predetermined target color location onto the adjacent side surface of the correction color space and the intersection point of the solder with the side surface is used as an attainable target color location.

If such a solution does not exist, it is possible according to the control strategy to proceed in such a way that a solder is dropped from the specified target color location onto the adjacent side edge of the correction color space and the intersection of the solder with the side edge is used as an achievable target color location.

If no solution is also possible for this, it is provided that the specified target color location adjacent corner of the correction color space is used as an attainable target color location.

It is known that chrominance errors are more critical than pure brightness errors (luminance errors). Therefore, according to an alternative of the control strategy, it is provided that the intersection of a parallel to the brightness coordinate axis closest to the predetermined target color location through the predetermined target color location with the surface of the correction color space is selected as the achievable target color location or replacement target color location. According to a special modification of this method, the closest points on the surface of the correction color space as achievable target points for the points lying on a parallel to the brightness coordinate axis through the given target color location within a given brightness error range with a maximum and a minimum brightness. Color locations can be determined. It is possible that the closest point on the surface of the correction color space is determined for the point on the parallel) which is assigned to the largest acceptable brightness error.

The control strategy can also provide that the intersection of the color distance vector between the actual color location of the grid field 46 and the predetermined target color location of the grid field 43 with the surface of the color correction space is selected as the achievable replacement target color location.

From the options for the control strategy mentioned above, for example, it can be seen that a rule Strategy takes place in the L * a * b * color space, although the grid 46 of the printed sheet 45 has not been scanned with a colorimeter, but only with the densitometer 31. The use of the control strategy block 63 and the third layer thickness control computer 55 ″ thus allows an inaccessible color location specified on the OK sheet 44 to be replaced by an achievable target color location according to a control strategy, so that for the actual color location of the grid field 46 of the printing sheet 45 an optimal position in the color coordinate space can be controlled, although the color coordinates of the raster field 46 have not been recorded with the aid of a color measuring device or spectrophotometer.

In the control strategy described in the above-mentioned EP-A 321 402, measurement value processing is provided in which the color distance vectors between the target color location and the actual color location are multiplied by a sensitivity matrix in order to calculate the layer thickness change control vector which is the next time a production run is printed -Bogens 45 must be taken into account to achieve the desired color location shift. The sensitivity matrix, with which density differences for the color locus shift between the target color locus and the actual color locus are calculated, can be determined empirically and in terms of measurement technology using a test series in the control strategy mentioned.

According to FIG. 4, however, the control strategy can also be implemented in such a way that the control strategy block 63 does not have a layer thickness change control vector at the output 65, but only substitute color dimension difference 55 '' can be converted into a layer thickness control vector using the transformation matrix [Z].

Another particularly simple possibility for a control strategy, in which the boundary conditions of the solid densities are taken into account, can be realized in such a way that the output of the second layer thickness control computer 55 'feeds a further input of the control strategy block 63 in a manner not shown in FIG. 4 in order to Avoid that 63 color measure number differences occur at the output of the control strategy block, which, after conversion in the third layer thickness control computer 55 ″, would lead to an overriding of the ink guide elements beyond the layer thickness limit values.

Claims (6)

1. A process for the determination of colorimetric measure differences between two half-tone fields, especially two grey balance fields, printed by a printing machine, by optical scanning by means of a densitometer of the half-tone fields to be compared, there being determined for each printing colour involved the difference of the associated half-tone densities and those differences being transformed, by a transformation that includes a multiplication, into colorimetric measure differences in a colour space that is equidistantly graduated in accordance with perception, the multiplication factors being determined by a previous printing test with systematic variation of the coloration of the individual colours and subsequent evaluation by means of a densitometer and a colorimeter, wherein a half-tone density difference vector having as its components the half-tone density differences of the printing colours involved is transformed by multiplication with an inverted colorimetric measure half-tone density transform matrix into a colour variation vector, having the colorimetric measure differences as its components, in the colour space which is equidistantly graduated in accordance with perception, the colorimetric measure half-tone density transform matrix being determined by printing a reference calibration print and a plurality of additional calibration prints under nominal conditions by means of a printing machine, each of which prints has a plurality of full-tone fields and a co-printed half-tone field, especially a grey balance field, similar in colour to the half-tone fields to be compared, each additional calibration print having for at least one full-tone field a full-tone density that differs from the corresponding full-tone field of the same colour of the reference calibration print, measuring the half-tone density differences between the half-tone densities of the half-tone field of the reference calibration print, on the one hand, and those of the half-tone fields of the additional calibration prints, on the other hand, by means of the densitometer, and measuring the colorimetric measure differences between the colorimetric measures of the half-tone field of the reference calibration print, on the one hand, and those of the half-tone fields of the additional calibration prints, on the other hand, by means of a spectrophotometer, and, by substituting the values of the half-tone density differences and colorimetric measure differences determined in that manner in the equations $$\text{[ΔR]i =[W] ·[ΔF]i ,}$$

   

   determining the elements of the colorimetric measure half-tone density transform matrix [W], where [ΔR]i is the half-tone density difference vector associated with the i-th additional calibration print and having the components formed by the half-tone density differences for each of the printing colours, and [ΔF]i is the colorimetric measure difference vector associated with the i-th additional calibration print and having the components formed by the colorimetric measure differences.
2. A process for the colour control or colour regulation of the print of a printing machine, wherein measuring fields on the production sheets printed by the printing machine are optically detected in order to determine the colour deviation of the measuring field detected from a given desired colour location and from this to produce an adjusting value for adjusting the colour control elements of the printing machine so that undesirable colour variations in the production sheets subsequently printed with the new colour control setting become minimal, wherein a measuring field in the form of a half-tone field, especially a grey balance field, composed of several printing colours is further provided on the production sheets, to which measuring field a corresponding half-tone defining the desired colour location is assigned on an OK sheet, wherein the determination of the colour deviation is further carried out by comparing the half-tone densities obtained in the scanning of the two half-tone fields by means of a densitometer, in a manner such that for each of the printing colours involved the difference of the associated half-tone densities is determined and those differences are transformed, by a transformation that includes a multiplication, into a colour variation vector in a colour space that is equidistantly graduated in accordance with perception, the multiplication factors being determined by a previous printing test with systematic variation of the coloration of the individual colours and subsequent evaluation by means of a densitometer and a colorimeter, and wherein control or regulating recommendations for adjusting the colour control elements of the printing machine are produced from the colour variation vector obtained in that manner, wherein a half-tone density difference vector having as its components the half-tone density differences of the printing colours involved is transformed by multiplication with an inverted colorimetric measure half-tone density transform matrix into a colour variation vector, having the colorimetric measure differences as its components, in the colour space which is equidistantly graduated in accordance with perception, the colorimetric measure half-tone density transform matrix being determined by printing a reference calibration print and a plurality of additional calibration prints under nominal conditions by means of a printing machine, each of which prints has a plurality of full-tone fields and a co-printed half-tone field, especially a grey balance field, similar in colour to the half-tone fields to be compared, each additional calibration print having for at least one full-tone field a full-tone density that differs from the corresponding full-tone field of the same colour of the reference calibration print, measuring the half-tone density differences between the half-tone densities of the half-tone field of the reference calibration print, on the one hand, and those of the half-tone fields of the additional calibration prints, on the other hand, by means of the densitometer and measuring the colorimetric measure differences between the colorimetric measures of the half-tone field of the reference calibration print, on the one hand, and those of the half-tone fields of the additional calibration prints, on the other hand, by means of a spectrophotometer, and, by substituting the values of the half-tone density differences and colorimetric measure differences determined in that manner in the equations $$\text{[ΔR]i =[W] ·[ΔF]i ,}$$

   

   determining the elements of the colorimetric measure half-tone density transform matrix [W], where [ΔR]i is the half-tone density difference vector associated with the i-th additional calibration print and having the components formed by the half-tone density differences for each of the printing colours, and [ΔF]i is the colorimetric measure difference vector associated with the i-th additional calibration print and having the components formed by the colorimetric measure differences, and from the colour variation vector obtained in that manner a layer thickness variation control vector is produced for adjusting the colour control elements of the printing machine.
3. A process according to claim 2, wherein, using the given boundary densities and the measured full-tone densities of the full-tone fields printed together with the half-tone field on the production sheets, a correction colour space around the actual colour location measured on the half-tone field is determined and a given desired colour location lying outside the correction colour space is replaced by an attainable desired colour location on the boundary surface of the correction colour space with a colour deviation from the given desired colour location, the components of which deviation that are essential for printing quality are minimal.
4. A process according to claim 2 or 3, wherein the colour variation vector or a substitute colour variation vector calculated in accordance with a regulation strategy in the colour space taking into consideration the boundary values for the attainable full-tone densities is multiplied by a colorimetric measure full-tone density transform matrix in order to obtain the layer thickness variation control vector.
5. A process according to claim 4, wherein the colorimetric measure full-tone density transform matrix is determined by measuring for each colour the full-tone density difference between the full-tone densities of the full-tone field of the reference calibration print, on the one hand, and those of the full-tone fields of the additional calibration prints, on the other hand, by means of the densitometer and measuring the colorimetric measure differences between the colorimetric measures of the half-tone field of the reference calibration print, on the one hand, and those of the half-tone fields of the additional calibration prints, on the other hand, by means of the spectrophotometer, and, by substituting the values of the full-tone density differences and colorimetric measure differences obtained in that manner in the equations $$\text{[ΔV]i =[Z] ·[ΔF]i ,}$$

   

   determining the elements of the colorimetric measure full-tone density transform matrix [Z], where [ΔV]i is the full-tone density difference vector associated with the i-th additional calibration print and having the components formed by the full-tone density differences for each of the printing colours, and [ΔF]i is the colorimetric measure difference vector associated with the i-th additional calibration print and having the components formed by the colorimetric measure differences.
6. A process according to claim 4, wherein the colorimetric measure full-tone density transform matrix is determined by determining the colorimetric measure half-tone density transform matrix [W] and multiplying it by the inverted full-tone density / half-tone density transform matrix [X]⁻¹ which is in turn determined by measuring, by means of a densitometer, the half-tone density differences between the half-tone densities of the half-tone field of the reference calibration print, on the one hand, and those of the half-tone fields of the additional calibration prints, on the other hand, and, for every colour, the full-tone density difference between the full-tone densities of the full-tone field of the reference calibration print, on the one hand, and those of the full-tone fields of the additional calibration prints, on the other hand, and, by substituting the values of the half-tone density differences and full-tone density differences obtained in that manner in the equations $$\text{[ΔR]i =[X] ·[ΔV]i ,}$$

   

   determining the elements of the full-tone density / half tone density transform matrix X are determined, where [ΔR]i is the half-tone density difference vector associated with the i-th additional calibration print and having the components formed by the half-tone density differences for each of the printing colours, and [ΔV]i is the full-tone density difference vector associated with the i-th additional calibration print and having the components formed by the full-tone differences.

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