EP0649743B1 - Method for controlling the ink supply in a halftone printing machine - Google Patents

Abstract

A method for controlling the ink supply in a halftone printing machine, in particular an offset printing machine, is described. Both in the case of a reference master and on the printed sheet, the standard colour values of the four-colour combined halftone print are determined at test regions located within the subject. For the printing colour black, the infrared colour density is determined within the close infrared. These standard colour values of the four-colour combined print are converted by linear transformation into standard colour values which correspond to the combined printing of only the three chromatic colours. In this case, the coefficients of this transformation depend on the infrared colour density of the printing colour black, for which the corresponding correlations are determined empirically. The effective colour area for the printing colour black depends on the infrared colour density which is likewise determined empirically. The effective colour areas for the chromatic colours are calculated using a modified approach according to Neugebauer using the transformed standard colour values. <IMAGE>

Description

The invention relates to a method for controlling the color guidance of an autotypical working printing press according to the preamble of claim 1.

The visual color impression of offset printing products is created in a known manner through the interplay of subtractive and additive color mixing. The reason for this is that the individual halftone dots of each printing ink are printed in different sizes both side by side and overlapping one another to a greater or lesser extent. The printing inks used are glazed, i.e. the effect corresponds to a filter lying on the white substrate. The color direction of the combined printing is determined both by the layer thickness of the applied printing ink and by the size of the halftone dots (geometric area coverage). By changing the setting of the ink guide elements in the individual printing units, the color location of a print image location can thus be changed. As a rule, three chromatic colors cyan, magenta, yellow and a fourth printing color black (increase in contrast) are printed for color printing.

It has been known for a long time to photoelectrically detect the ink application on a printed product on measuring elements which have also been printed and to derive a measure for the amount of ink applied. This is usually done using densitometers, because between the color density value and the layer thickness of the color and thus also the position of the For example, there is a relatively simple connection between the color guide members designed as a slide valve. The disadvantage here is that a densitometric detection of the color guide does not allow any numerical statements regarding the visual perception of color. Checking the ink flow on measuring elements that are also printed in addition has the disadvantage that space on the printing material is used for these measuring fields and furthermore only the target ink application of these measuring fields is controlled. Accordingly, the color impression of the actual subject is only changed indirectly.

From EP 0 228 347 B1 a method for ink application control in a printing press is known, in which test areas on the printed sheet are measured colorimetrically, color locations relating to a selected color coordinate system are determined, and color distances are formed between actual and target values (template) and the control data are to be determined from precisely these color differences. This method uses an empirically determined dependency of the color location coordinates as a function of a change in the layer thickness of the printed color.

This method works colorimetrically, i.e. the determined color locations and color distances also allow conclusions regarding the visual color impression. Because of the described way of calculating positioning commands for the color guide via the partial derivatives of the color coordinates according to the color densities of the printing inks involved, this principle seems to only work with measuring fields that are also printed. This font alone does not identify a way in which setting commands for the color guidance can be obtained by measurements directly in the printed image, the subject.

A method for colorimetric evaluation of printed products is known from DD 227 094 A1. Measuring devices in the machine determine the color coordinates of certain test areas and, based on the relationship between Neugebauer, the area coverage printed colors. If the color black is printed in addition to the chromatic colors, a second colorimetric measuring device for this color is necessary, which is to be regarded as disadvantageous.

EP 0 143 744 A1 discloses a method for assessing the print quality and for regulating the color guidance. The reflectances in four spectral ranges are measured on image elements in the subject using one or more measuring heads. According to the specifications in this document, the color densities for the chromatic colors and spectral reflectance in the infrared range for the printing ink black are determined. Again, area coverings are determined (unmasked) using the Neugebauer equations. This is done on the same image locations of the copies printed on the machine as well as on a target template. Control commands for the color control of the printing press can be derived from the target / actual comparison of the surface coverings.

The disadvantage here is that density values have to be determined for the chromatic colors, which, as already mentioned, do not represent any relation to the visual color impression. Although this document teaches that color locations can also be determined from the detected remissions, this only leads to inaccurate results, since color density values form the starting variable.

It is therefore known from the prior art to determine the geometric surface coverages of test areas either via color density values or color values and to determine control variables for the color guidance therefrom. As is well known, the Neugebauer relationship, which has been expanded to four colors, describes the theoretical relationship between the color locus of a four-color overprint and the geometric area coverage of the individual colors and their overlays. The standard color values used for the individual colors, the combinations of the combined prints and the paper white are determined on samples printed over the entire surface. However, the geometric surface coverings determined in this way and the change in color guide determined from a target-actual comparison can only deliver inaccurate results because light scattering or light trapping are not taken into account. As is known, the optically effective surface covering or colored surface is decisive in a raster structure and not the surface that is only geometrically covered by color.

The object of the present invention is to extend a method according to the preamble of claim 1 such that the control of the color guidance is possible with great accuracy.

This object is achieved by the characterizing features of claim 1. Further developments of the invention result from the subclaims.

Furthermore, the invention is explained in more detail using an exemplary embodiment. The control of the color control of a sheet-fed offset printing machine is described. A so-called template or OK sheet is specified. The color of the sheets to be printed should differ as little as possible from this template sheet.

A number of test areas are specified on the template sheet, which are either particularly important for the image, show particularly typical or difficult color nuances or are otherwise typical of the overall image structure. In this example it should be assumed that these test areas were all created by printing together the three chromatic colors and the black color in the halftone area.

Furthermore, the method according to the invention for a test area of the template sheet and the corresponding test area of the printed sheet is described. The remaining test areas are processed accordingly.

The spectral reflectance of the test area on the template sheet and the spectral reflectance of the corresponding test area on the printed sheet are recorded by means of a spectrophotometer. The X, Y, Z components of the color values are determined from these spectral remissions, using the standardized sensitivity curves of a CIE normal observer. Using the known transformation equations, color locations of the CIE-LUV color space can be determined. For this test area of the template, you get a target color location. The color location of the corresponding test area in the printed sheet thus represents the actual color location.

According to this process example, a spectrophotometer is used, which also delivers spectral intensities in the near infrared at wavelengths between 0.85µ and 1.0µ. A narrow-band color density for a wavelength L (IR) is now determined in this specified wavelength range of the near infrared. This narrowband color density is also referred to as DIR.

Instead of a spectrophotometer selected in this way, a color measuring device (spectral; triple range), which is designed in a known manner and to which an infrared color density measuring device is connected (beam splitter) can also be used in the method according to the invention. The color location and the infrared color density are then recorded by two measuring systems.

If a large number of test areas are measured on template and printed sheets and compared with one another, it is advantageous if the measuring device, which is to be designated generally as a photoelectric scanning device, is attached to an automatically controlled device which can be moved in one plane. With such a known device, a large number of stored test areas can be approached and automatically measured. For this purpose, the template is first placed on the surface of this device and then measured. The same applies to the printed sheet method. Of course, the position of the individual test areas with regard to the so-called color metering zones is taken into account when determining control commands for the color guide.

Before the further method steps are explained in more detail, the procedure according to the invention is briefly reproduced. The invention makes use of the knowledge that a color location can be determined from the infrared color density values DIR of the test area in the template and printed sheet described above, which results when only the chromatic colors cyan, magenta and yellow are printed will. X (CMYK), Y (CMYK), Z (CMYK) denote the standard color values of the test area in the template or printed sheet. X (CMY), Y (CMY), Z (CMY) are understood to mean the corresponding standard color values of the color location, which results when only the chromatic colors cyan, magenta, yellow are printed.

The following relationship is used to convert the standard color values of four-color overprinting into the alternative standard color values of hypothetical three-color printing: $$\begin{array}{c}\begin{array}{c}\text{X (CMY) = ax (1) X (CMYK) + ax (2),}\\ \text{Y (CMY) = ay (1) * Y (CMYK) + ay (2) and}\\ \text{Z (CMY) = az (1) * Z (CMYK) + az (2).}\end{array}\end{array}$$

As shown above, the standard color values of the four-color overprint in the test area are converted linearly into another color location. The conversion coefficients ax (1), ax (2); ay (1), ..., az (2) are not constant parameters, but, as has been found, in each case a function of the measured infrared color density DIR. The relationship between these parameters and the infrared color density DIR is determined empirically.

The determination of the coefficients ax (1), ax (2), ....., az (1), az (2) will now be explained below with reference to the drawings. Described the empirical determination of the coefficients ax (1), ax (2) for the linear transformation of the standard color value X (CMY) from the standard color value for the four-color overprint X (CMYK). The procedure for determining the coefficients ay (1), ay (2), az (1), az (2) for the transformation of the standard color values Y (CMY), Z (CMY) from the standard color values Y (CMYK), Z ( CMYK) is analog.

A test print is produced on which a multitude of three-color or four-color overprints lie side by side. FIG. 1 shows a section of an exemplary arrangement of the measuring fields. In the individual measuring field pairs, the proportion of printing area is varied both for the three chromatic colors and for the printing color black in the overprinting of the four colors. The gradation of the individual measuring fields can be such that the proportions of printing areas are changed by 10%, for example.

Fig. 1 shows that pairs of measuring fields are arranged on the sample sheet, the three chromatic colors C, M, Y with a predetermined screen tone in a measuring field CMY and the three chromatic colors C, M, Y with the same in the adjacent measuring field CMYK Halftone (portion of the printing area) and additionally the color black K can be printed with the specified halftone value. On the test sheet, the pairs of measuring fields can be arranged in a matrix, that is, in rows 1, 2, 3, ... and columns A, B, C, .... In Fig. 1, a pair of measuring fields is highlighted in dashed lines.

According to the matrix-like arrangement of the measuring field pairs CMY, CMYK shown in FIG. 1, it can be provided that in columns A, B, C,... The screen tone value of the color black K in the metering area CMYK of the four-color overprinting is constant and the screen tone values of the three bright colors varies. This means that in a column A, B, C, ... in different rows 1, 2, 3, ... the three chromatic colors produce different color fields due to the variation of the halftone values. So if the color values X, Y, Z in one Column detected on the measuring field pairs CMY / CMYK and additionally determined the infrared color density DIR at the measuring fields CMYK with the overprinting of the four colors, so it can be seen that along the column the color values X, Y, Z due to the different halftone values of the Change chromatic colors in a wide range. The infrared color density DIR, on the other hand, remains almost constant, since the color black has always been printed along the column in the measuring fields CMYK with the same proportion of the printing area.

Now different-colored measuring fields CMY of the overprinting of the three chromatic colors C, M, Y and the associated measuring fields CMYK of the superimposing of the three chromatic colors C, M, Y and the black color K are measured on a sample sheet described above, the proportion of the printing area (screen value) for the color black always remains the same. The color values X, Y, Z and the infrared color density DIR along one of the columns A, B, C, ... are thus recorded. Furthermore, only the procedure for the color value X is described. If the results of a series of measurements are now plotted in a diagram according to FIG. 2, in which the standard color value X (CMY) of the measuring field CMY with the overprinting of only the three chromatic colors as a function of the associated standard color value X (CMYK), determined on the measuring field CMYK with the additional printing of the color black K is shown, the measuring points (X (CMYK); X (CMY)) of this series of measurements lie almost on a straight line.

2 shows the standard color value X (CMYK) as the abscissa, that is to say the standard color value X, which results in each case at the measuring field CMYK with the overprinting of the four printing inks C, M, Y, K. Correspondingly, the ordinate represents those standard color values X (CMY) that result in the measuring fields CMY, in which only the three chromatic colors C, M, Y are involved. The bisector between the ordinate and the abscissa represents the straight line that results if the color black is not printed.

Various straight lines are plotted in FIG. 2, the increase of which increases with the proportion of the printing surface of the printing ink black K in the measuring field CMYK, which is indicated by the arrow. Each line is a best-fit line through a large number of individual measuring points in a series of measurements. Each straight line of a measurement series is also assigned a (averaged) value of the infrared color density DIR.

As shown in FIG. 2, these straight lines intersect both the bisector (area coverage of the printing ink black = 0) and the ordinate. Because of the different slopes of the straight line, they intersect the ordinate at different points, i.e. there are different ordinate intersections.

und $\text{ax(2) = fktx2(DIR)}$

. Entsprechend werden durch Auswerten der Y(CMY)/Y(CMYK) und Z(CMY)/Z(CMYK) - Korrelationen ebenfalls Interpolationsfunktionen $\text{ay(1) = fkty1(DIR)}$

, $\text{ay(2) = fkty2(DIR)}$

, $\text{az(1) = fktz1(DIR)}$

, $\text{az(2) = fktz2(DIR)}$

ermittelt.Now that the correlation of the standard color value X (CMY) / X (CMYK) has been determined for different portions of the printing area of the color black, it is now possible to calculate the coefficients ax (1), ax ( 2) to take both the ordinate intersection from the diagram according to FIG. 2 as a corresponding slope (solving a straight line equation). Since each straight line corresponds to a series of measurements with known infrared color density DIR, these infrared color densities DIR can now be assigned to the corresponding coefficients ax (1), ax (2). By using interpolation methods, the dependence ax (1) or ax (2) on the infrared color density can now be functionally represented. So you get two functions: $\text{ax (1) = fktx1 (DIR)}$

and $\text{ax (2) = fktx2 (DIR)}$

. Correspondingly, by evaluating the Y (CMY) / Y (CMYK) and Z (CMY) / Z (CMYK) correlations, interpolation functions also become $\text{ay (1) = fkty1 (DIR)}$

, $\text{ay (2) = fkty2 (DIR)}$

, $\text{az (1) = fktz1 (DIR)}$

, $\text{az (2) = fktz2 (DIR)}$

determined.

The conversion of the standard color values X, Y, Z of the four-color overprint to the standard color values of the hypothetically resulting three-color overprint according to the above approach takes into account the fact that the printing color is black does not only represent a pure "filter function", that is to say that the corresponding standard color values X (CMYK), Y (CMYK), Z (CMYK) are not just attenuated by a certain amount. The presence of the printing ink black does not only result in a pure change in the brightness of a four-color overprint compared to the associated overprinting of the three chromatic colors C, M, Y. The presence of the printing ink black also leads to a slight change in the color location itself, which is expressed by the coefficients ax (2), ay (2) and az (2). Since these coefficients depend on the infrared color density DIR, it follows that the "color location correction" brought about by the presence of the printing ink black is also a function of the proportion of the printing area of the printing ink black.

.From the infrared color density DIR of the test area described above, the effective color area for the color black EFF (black) is determined via an empirical relationship between the infrared color density DIR and this effective color area. Pressure tests are carried out to determine these parameters. For this purpose, a series of halftone patches of the color black is also printed on a test sheet, the halftone tone value being varied in stages or continuously and the measured infrared color density DIR being plotted against the effective color area EFF, for example measured by video or planimetry. Again, using an interpolation function gives a functional representation of the relationship $\text{EFF (K) = fkt (DIR)}$

.

It was described above how the standard color values X (CMY), Y (CMYK), Z (CMYK) of the measured test areas are used to determine the standard color values X (CMYK), Y (CMYK), Z (CMYK) of the theoretical chromatic color combination. These standard color values, which would theoretically result if only the three chromatic colors were printed together, now serve as a starting point for calculating the effective color areas EFF (cyan), EFF (magenta), EFF (yellow). These effective color areas EFF for the three chromatic colors are just like the effective color area EFF (black) dimensionless and physically mean the optically - including scattering - effective area share of a unit area. Depending on the calibration of the value of the effective color area EFF - numerical value in% or as a value between 0 and 1 - these values are similar to the area coverage in the film or on the printed image.

A system of modified Neugebauer equations is used to calculate the effective color areas EFF for the three chromatic colors. The modified Neugebauer equations used in the invention are based on the same mathematical approach as the well-known Neugebauer equation. As is known, the standard color values of a three-color overprint can be calculated by the Neugebauer equations by linking the geometric surface coverings of the three chromatic colors in full tone and paper white and also the standard color values of the corresponding full tone overprint.

According to the invention, the effective color areas EFF of the three chromatic colors described are used in the modified Neugebauer equations instead of the geometric area coverings. Furthermore, it is provided that instead of the standard color values for the respective solid color areas (single and together) standard color values are used which take into account the changes caused by scattering effects on printed screen areas. This data is determined on a printed sample board, which contains a certain amount of defined CMY color fields, which essentially consist of screened color areas - individually and also in overprinting - of defined parts.

The system of Neugebauer equations consists of three equations (for one standard color value of the CMY color field). The Neugebauer equation is shown here in a vector representation. $$\left(\begin{array}{c}\text{X (CMY)}\\ \text{Y (CMY)}\\ \text{Z (CMY)}\end{array}\right)\text{= (1-EFF (C)) (1-EFF (M)) (1-EFF (Y))}\left(\begin{array}{c}\text{X (W)}\\ \text{Y (W)}\\ \text{Z (W)}\end{array}\right)\text{+ EFF (Y) (1-EFF (M)) (1-EFF (C))}\left(\begin{array}{c}\text{X (Y)}\\ \text{Y (Y)}\\ \text{Z (Y)}\end{array}\right)\text{+ (1-EFF (Y)) EFF (M) (1-EFF (C))}\left(\begin{array}{c}\text{X (M)}\\ \text{Y (M)}\\ \text{Z (M)}\end{array}\right)\text{+ (1-EFF (Y)) (1-EFF (M)) EFF (C)}\left(\begin{array}{c}\text{X (C)}\\ \text{Y (C)}\\ \text{Z (C)}\end{array}\right)\text{+ EFF (Y) EFF (M) (1-EFF (C))}\left(\begin{array}{c}\text{X (YM)}\\ \text{Y (YM)}\\ \text{Z (YM)}\end{array}\right)\text{+ EFF (Y) (1-EFF (M)) EFF (C)}\left(\begin{array}{c}\text{X (YC)}\\ \text{Y (YC)}\\ \text{Z (YC)}\end{array}\right)\text{+ (1-EFF (Y)) EFF (M) EFF (C)}\left(\begin{array}{c}\text{X (MC)}\\ \text{Y (MC)}\\ \text{Z (MC)}\end{array}\right)\text{+ EFF (Y) EFF (M) EFF (C)}\left(\begin{array}{c}\text{X (YMC)}\\ \text{Y (YMC)}\\ \text{Z (YMC)}\end{array}\right)$$

In the above-mentioned approach, EFF (C), EFF (M), EFF (Y) mean, as already mentioned, the effective color areas for the three chromatic colors C, M, Y. X (W), X (C), X (M), X (Y), Y (W), Y (C), ...., Z (Y) are the color values of the paper white W or a cyan-magenta or yellow-colored grid; X (CM), X (CY), X (MY), X (CMY) the corresponding standard color values for two or three-color halftone fields printed one above the other. These values are determined in test prints (sample board) and saved for later calculation.

Using the three equations indicated above for the standard color values X (CMY), Y (CMY), Z (CMY), the effective color areas EFF (C), EFF (M), EFF (Y) are now calculated for the three chromatic colors. The equations are solved for these quantities.

According to the description above, the effective color areas for both the color black and the three chromatic colors are calculated for each test area of the template sheet and the printing sheet. Then the differences between the effective color areas of a test area of the template sheet and the printed sheet are formed. These differences are then converted into control commands for the ink guide elements via empirical relationships, which take into account in particular the inking unit behavior, the inking unit structure of the printing press and the properties of the printing inks used. If several test areas are evaluated in one ink metering zone, an optimally achievable difference in the effective ink area for the respective printing ink is formed (e.g. mean value). Provision can also be made to carry out a check of the differences in the effective color areas for each color zone determined for all selected test areas - for adjacent color zones, if applicable - and for each inking unit in a logical network for plausibility and compatibility of the values.

After the color guide elements have been changed on the basis of the calculated positioning commands, as described above, new ones become Sheet printed. This is followed by the repetition of the method and, if necessary, the calculation of corrections from stored data for the parameters used in the empirical equations for adaptation to the current printing conditions until the difference between the color locations of the test areas of the template and the printed sheet falls below predetermined tolerances.

Claims (3)

1. Process for controlling the ink feed of a half tone working printing press, particularly an offset printing press, in which in selected test regions of the image of a proof as well as corresponding test regions of the printed product, reflectance values are photoelectrically sensed, from which surface covering values for the printed inks are determined, for which for the printing ink black the reflectance is sensed in the near infra-red, and the control commands for the ink feeding elements of the printing press are determined by a comparison of the corresponding surface covering values of master and printed product characterised in that for the printing ink black the infra-red colour density DIR is determined from which via an empirically determined relationship the optically effective colour surface value EFF(K) for the printing ink black is determined, that the reflectance from the test regions are calculated to standard colour values X(CMYK), Y(CMYK), Z(CMYK), that from the standard colour values X(CMYK), Y(CMYK), Z(CMYK) of the four colour superimposed print by a linear transformation the following standard colour values are calculated: $$\begin{array}{c}\begin{array}{c}\text{X(CMY) = ax(1) · X(CMYK) + ax(2),}\\ \text{Y(CMY) = ay(1) · Y(CMYK) + ay(2) and}\\ \text{Z(CMY) = az(1) · Z(CMYK) + az(2).}\end{array}\end{array}$$

   

   Wherein these standard colour values X(CMY), Y(CMY), Z(CMY) correspond to a colour locus which arises by the combined printing of only the three coloured inks, and wherein the coefficients ax(1), ax(2), ay(1), ay(2), az(1), az(2) are determined empirically as a function of the infra-red ink density DIR and that from the standard colour values X(CMY), Y(CMY), Z(CMY) obtained in this way the optically effective colour surface covering values EFF(C), EFF(M), EFF(Y) of the three coloured inks are determined.
2. Process according to claim 1 characterised in that for determining the values of the effective colour areas EFF(C), EFF(M), EFF(Y) for the three coloured inks C, M, Y the Neugebauer equations are used wherein the theoretically calculated standard colour values X(CMY), Y(CMY), Z(CMY) are used and for the standard colour values to be substituted into the Neugebauer equation the individual colour and also superimposed print combination standard colour values are used which have been determined in print tests on raster areas.
3. Process of according to claim 1 or 2 characterized in that the process is repeated sufficiently until the colour deviations lie within a predetermined tolerance range.

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