EP1033247B1 - Method for controlling colour in a printing machine - Google Patents

Abstract

A decorative pattern in color is available as a reference and this is camera sampled. The color at any point is based upon three color components and these are defined in look up tables that relate to position. A comparison between measured and reference values is used to control the color printing process.

Description

The invention relates to a method for controlling the coloring of a printing press.

In offset printing, the desired color tones of the reproductions are produced on the substrate by overprinting a small number of primary colors. The hue is mainly dependent on the intrinsic color of the substrate, the layer thickness and the degree of screening of the primary colors used in the overprint and the so-called dot gain. The rasterization is set in the prepress stage and represents the desired degree of attenuation of the color impression of the pure base color at each image location in compression. The tone value increase alters this originally desired attenuation due to mechanical and optical action mechanisms, e.g. the grid points are flattened, causing a visually higher tone value. However, this can be observed and compensated for in the pre-press, so that the desired tonal value is created again during printing. Problematic are the trend or random variations of dot gain occurring in printing.

Trendy deviations due to technical properties can only be compensated by changes in the layer thicknesses of the primary colors according to a selected quality criterion. There is no remedy for short-term random deviations, they are the equivalent of noise in electronic amplifiers and must be taken into account when specifying tolerances.

The layer thicknesses of the primary colors serve as a constant parameter in the screening of the individual colors in the prepress stage, so they normally have to remain in precisely defined tolerance ranges. For the reason explained above, a constant deviation of the solid density may be necessary during later printing in order to achieve on average acceptable deviations in the subject.
The measurement of the layer thicknesses of the pure primary colors takes place on so-called full-tone control surfaces by densitometers which determine the ratio of irradiated to reflected light in wavelength ranges defined by a DIN standard. Interesting are only the channels for CYAN, MAGENTA and YELLOW, whose English initials "C", "M", "Y", serve as a name for the measurement channels. The density value for SCHWARZ (English "Key", short name "K") is usually calculated mathematically from the three independent density values. However, even in the case of a measurement with its own black channel, it does not constitute useful information because there is no specific spectral range for "black" other than the near infrared, and the black filter is basically a broadband filter is. The CMY densities therefore represent a triple or even vector of density values waiving a value for black.

Color measurement devices with 3 filter channels (tristimulus) on the basis of the eye sensitivity curves are used to measure the hues, especially in combination printing. Modern measuring devices also use spectrophotometers with more than 30 channels, which simulate eye sensitivity by mathematical treatment of the measured values in the channels.

From the patent DD 227 094 a method is known which determines the absolute proportions of these colors on the target and actual sheets on the basis of test prints of all occurring primary and mixed colors and makes use of the comparison of these proportions to control the coloring.

A disadvantage of this method is u.a. the effort to produce the proofs and the use of the absolute proportions to control the coloring, since in this way no colors can be compared based on different printing methods.

From the DE 43 11 132 A1 a method is known in which complete spectra at the actual and nominal measuring points are represented as a linear combination of the spectra of the individual colors involved in the printing. The difference of the coefficients of the linear combination of the Istspektrums and the coefficients of the linear combination of the desired spectrum is used to control the coloring. This saves the pressure of all possible combinations of colors, but has the disadvantage that again only prints of the same production technology, but for example, no offset prints can be compared with set values of a proofing device.

From the DE 4343 905 A1 a method is known in which the spectral differences are optimally decomposed into weighted fractions of the existing fundamental spectra. This method also permits the processing of spectral deviations between actual spectra and target spectra, also produced with another printing method, but has the disadvantage that the entire spectrum must always be obtained. Even with newer spectrometers take measurements much longer than with a simple densitometer, so that can not be measured with the required speed in a control strip or in a continuous image area.

• mindestens 2 Grundfarben haben dasselbe Spektrum bzw. eines der beteiligten Spektren kann aus einer Linearkombination der übrigen Spektren dargestellt werden
• bildwichtige Messstellen sind so kontrastreich im Bereich der geometrischen Auflösung des Sensors, so dass kleinste Lageänderungen des Messortes große Messwertschwankungen zur Folge haben

All of the above methods have the disadvantage that they fail, even if only one of the following conditions is met:

• at least 2 primary colors have the same spectrum or one of the spectra involved can be represented from a linear combination of the remaining spectra
• Image-critical measuring points are so rich in contrast in the range of the geometric resolution of the sensor, so that the smallest changes in position of the measuring location have large fluctuations in the measurement result

The object of the invention is to provide a method for accurate and rapid statement about the size and direction of the layer thickness changes of the participating primary colors.

This object is achieved by a method according to the features of claim 1.

The invention will be described on an exemplary embodiment decorative printing of wood wallpaper. The decor print consists of the combination of three primary colors, two of which are very similar; Measuring strips for zonal measurement and control of the layer thickness do not exist.

Color One takes over the pressure of the basic color tone. Color Two contains the same pigments as Color One, but at a higher concentration, and is responsible for most of the grains. The spectrum of color two is therefore derivable by a factor from the spectrum of color one. Color Three can not be deduced from the spectra of color one and color two. This color takes over the printing of other picture elements like leaves, grass etc.

Because of the similarity of color one and color two none of the methods mentioned in the prior art can give a correct statement as to whether the clearly visible color differences between the customer signed proof and the current printing result from the inking unit one, two or three or one Combination come from.

For this reason, quantitative information and location information are included in the determination of the control information. Therefore, an image is taken with an RGB camera from each of the single color separations of the current print. Alternatively, these extracts can also be supplied by the pre-press, which saves the expense of printing technology production of the color separation. Picture 1 contains the pixel-wise RGB-values of excerpt 1, picture 2 the of excerpt 2 etc. The picture-pixels contain the required quantitative information "RGB" and the position information about their arrangement in the picture. This includes the image contrasts, edges or grains in the printed subject.

Now, a formula-based or look-up-table-based transformation of the pixel-by-pixel RGB values into a logarithmized value representation that allows a linear combination. In the This transformation can be omitted by recording pixel-wise density spectra or density values, since these values can already be combined linearly.
Then there is a linear decomposition of the pixel-by-pixel difference between the actual image and the target image, eg during the tuning process, in decomposition factors. The decomposition factors are the solution of the following equation, taking into account each of the N pixels. There are therefore N equations with 3 unknowns. The solutions for K1, K2 and K3 represent the weights of the decompositions. In anilox or gravure printing, the pixels of the entire arc can be included in the decomposition. In the zone inking unit in offset printing, the pixels from each of the respective zones can be taken into account in a system of equations. The control information from the decomposition factors is calculated individually for each zone.

The decomposition factors can also be represented as solid density changes.
To do this, calibrate the ink master masters in density values by assigning the full-tone density to a basic color master (= color separation) by measurement or calculation.

The control information "full tone density changes" is then the product of colorwise decomposition factor and the assigned solid density.
The decomposition into decomposition factors does not take place or not exclusively by the solution of equations but by simulation of the combined pressure.
For example, 100,000 variants are calculated by calculation. The variant with the best quality (depending on the criterion) is chosen. Although the computing time limits the possible solutions, there is the advantage that any models (including Fuzzy models) can be used with any boundary conditions.
The measurement is preferably carried out with an automatic measuring system at all desired image locations. As described, an RGB camera or a combined measuring head is used to measure spectra with more than 30 channels

The image data obtained during the measurement are collected in a processing unit. Thereafter, the difference of the data to the measured or calculated target data takes place. When a quality criterion formed from the differences is exceeded, the differences are divided into the individual colors involved in the printing at the measuring point, eg according to the principle of the smallest deviation squares. The differences are represented component by component as sums of products from coefficients with values of the individual inks, with area coverage values and empirical weighting factors in the form of a system of equations. The coefficients result after the solution of the system of equations by methods of calculation known per se. The coefficients are then converted into curves by means of characteristic curves Volltondichteänderungen and then converted into slider changes the zone inking unit. In this case, infinitely many different solutions of the equation system are possible in particular when one or more printing inks have high density values in all spectral ranges. It is then selected that division, which leaves the optimum layer thickness of the offset printing least according to an extended quality criterion.

When distributing the density differences, current or previously known information about the trapping behavior or spectral nonlinearities can be included. It is e.g. It is known that the reprinting colors with their main and secondary densities can not simply be added to the density of the first printing color. The reason lies in the non-ideal remission behavior of the real colors. This is very clear when printing opaque white in the last inking unit. Opaque white outshines all pre-printed colors. This type of blooming effects can be empirically described by compensation factors for the behavior around the working point.

The observation of boundary conditions (in particular solid density or layer thickness) allows compliance with the offsetting typical conditions. For this purpose, in addition to obtaining the target and actual templates, the current full-tone density value is necessary. This value comes from a special measuring field eg in a measuring strip or by evaluation (profile scan) of a single grid point.
The consideration of boundary conditions allows the limitation of the control movements to layer thickness ranges, in which the offset printing still works. (0.6-1.3 μm).
The acquisition of the boundary conditions can be done by video image analysis of individual halftone dots and evaluation of the precursor information at this image location. Leaving the offsetting typical image area is absolutely clear in the 1000-fold magnification of halftone dots.

Claims (29)

1. Method of controlling coloration at a printing machine, wherein

   - quantitative image data inclusive of the associated positional data (x; y) are determined from a target comparison copy,

   - quantitative image data inclusive of the associated positional data (x; y) are determined from an actual copy,

   - quantitative image data inclusive of the associated positional data (x; y) are determined from respective printing mechanism basic copies,

   characterised in that

   - quantitative image data are determined in each instance from N pixels per printing image copy,

   - N differences are formed from the quantitative image data of the target comparison copy and the actual copy,

   - resolution factors K1, K2, K3 are determined by computer from the differences with the assistance of the quantitative image data of the printing mechanism basic copies by way of an equation system which in the case of three basic colours participating in the printing assumes the form: $$\begin{array}{l}{\left(\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{u}\mathrm{a}\mathrm{l}\mathrm{c}\mathrm{o}\mathrm{p}\mathrm{y}-\mathrm{t}\arg \mathrm{e}\mathrm{t}\mathrm{c}\mathrm{o}\mathrm{m}\mathrm{p}\mathrm{a}\mathrm{r}\mathrm{i}\mathrm{s}\mathrm{o}\mathrm{n}\mathrm{c}\mathrm{o}\mathrm{p}\mathrm{y}\right)}_{\left(\mathrm{1...}\mathrm{N}\right)}=(\mathrm{K}1\mathrm{x}\mathrm{q}\mathrm{u}\mathrm{a}\mathrm{n}\mathrm{t}\mathrm{i}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{e}\mathrm{i}\mathrm{m}\mathrm{a}\mathrm{g}\mathrm{e}\mathrm{i}\mathrm{n}\mathrm{f}\mathrm{o}\mathrm{r}\mathrm{m}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{o}\mathrm{f}\mathrm{p}\mathrm{r}\mathrm{int}\mathrm{i}\mathrm{n}\mathrm{g}\mathrm{m}\mathrm{e}\mathrm{c}\mathrm{h}\mathrm{a}\mathrm{n}\mathrm{i}\mathrm{s}\mathrm{m}\mathrm{b}\mathrm{a}\mathrm{s}\mathrm{i}\mathrm{c}\mathrm{c}\mathrm{o}\mathrm{p}\mathrm{y}1\\ +\mathrm{K}2\mathrm{x}\mathrm{q}\mathrm{u}\mathrm{a}\mathrm{n}\mathrm{t}\mathrm{i}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{e}\mathrm{i}\mathrm{m}\mathrm{a}\mathrm{g}\mathrm{e}\mathrm{i}\mathrm{n}\mathrm{f}\mathrm{o}\mathrm{r}\mathrm{m}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{o}\mathrm{f}\mathrm{p}\mathrm{r}\mathrm{int}\mathrm{i}\mathrm{n}\mathrm{g}\mathrm{m}\mathrm{e}\mathrm{c}\mathrm{h}\mathrm{a}\mathrm{n}\mathrm{i}\mathrm{s}\mathrm{m}\mathrm{b}\mathrm{a}\mathrm{s}\mathrm{i}\mathrm{c}\mathrm{c}\mathrm{o}\mathrm{p}\mathrm{y}2\\ {+\mathrm{K}3\mathrm{x}\mathrm{q}\mathrm{u}\mathrm{a}\mathrm{n}\mathrm{t}\mathrm{i}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{e}\mathrm{i}\mathrm{m}\mathrm{a}\mathrm{g}\mathrm{e}\mathrm{i}\mathrm{n}\mathrm{f}\mathrm{o}\mathrm{r}\mathrm{m}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{o}\mathrm{f}\mathrm{p}\mathrm{r}\mathrm{int}\mathrm{i}\mathrm{n}\mathrm{g}\mathrm{m}\mathrm{e}\mathrm{c}\mathrm{h}\mathrm{a}\mathrm{n}\mathrm{i}\mathrm{s}\mathrm{m}\mathrm{b}\mathrm{a}\mathrm{s}\mathrm{i}\mathrm{c}\mathrm{c}\mathrm{o}\mathrm{p}\mathrm{y}3)}_{\left(\mathrm{1...}\mathrm{N}\right),}\end{array}$$

   

   wherein N ≥ 3, and

   - are converted into control data for the printing mechanism setting elements.
2. Method of controlling coloration at a printing machine according to claim 1, characterised in that boundary conditions are included in the determination of the resolution factors.
3. Method of controlling coloration at a printing machine according to claim 2, characterised in that the boundary condition - full-tone density value or layer thickness - is obtained from a special measuring field on a measuring strip.
4. Method of controlling coloration at a printing machine according to claim 3, characterised in that the boundary condition - full-tone density value or layer thickness - is obtained from an individual dot.
5. Method of controlling coloration at a printing machine according to claim 4, characterised in that the dot is analysed by means of video image analysis.
6. Method of controlling coloration at a printing machine according to claim 1, characterised in that the full-tone density is included in the determination of the resolution factors and the control data are a product of the resolution factors and the full-tone density.
7. Method of controlling coloration at a printing machine according to claim 1, characterised in that the determination of the resolution factors is carried out by simulation of the compression.
8. Method of controlling coloration at a printing machine according to claim 1, characterised in that the determination of the resolution factors is carried out partly by simulation of the compression.
9. Method of controlling coloration at a printing machine according to claim 1, characterised in that the image data are brightness values, density values, spectral values, RYB values or LAB values
10. Method of controlling coloration at a printing a printing machine according to claim 1, characterised in that the comparison copy or printing mechanism basic copy is a print pattern, a print pattern copy, a proof information or a digital copy.
11. Method of controlling coloration at a printing a printing machine according to claim 1, characterised in that the positional data are obtained from selectable positions (x; y).
12. Method of controlling coloration at a printing a printing machine according to claim 1, characterised in that the positional information is obtained in the manner of an array.
13. Method of controlling coloration at a printing a printing machine according to claim 1, characterised in that the determination of the image data and positional information is carried out by measurements.
14. Method of controlling coloration at a printing a printing machine according to claim 1, characterised in that the determination of the image data is carried by selection of values.
15. Method of controlling coloration at a printing a printing machine according to claim 1, characterised in that the printing mechanism basic copy is quasi a colour extract.
16. Method of controlling coloration at a printing a printing machine according to claim 1, characterised in that the resolution by computer takes place according to the method of smallest squares.
17. Method of controlling coloration at a printing a printing machine according to claim 1, characterised in that the image data inclusive of the associated positional data are obtained from an array representing the entire printing or arrays representing sub-regions of the printing.
18. Method of controlling coloration at a printing a printing machine according to claim 1, characterised in that the arrays representing sub-regions are colour zones.
19. Method of controlling coloration at a printing a printing machine according to claim 1, characterised in that for determination of the image data use is made of intensity sensors for regions of any spectral width and characteristic (UV, visible or infrared), special density sensors (CMYK), special colour sensors (LAB), spectrally resolving sensors and video cameras (RYB and/or B/W).
20. Method of controlling coloration at a printing a printing machine according to claim 12, characterised in that the local spacings of the positional data in the array lie in the range of microns and the total printing width.
21. Method of controlling coloration at a printing a printing machine according to claim 1, characterised in that the determination of the control data is carried out by resolving equations.
22. Method of controlling coloration at a printing a printing machine according to claim 1, characterised in that the determination of the control data is carried out by computerised simulation.
23. Method of controlling coloration at a printing a printing machine according to claim 12, characterised in that the image data are obtained from a measuring spot in the array, the size of which lies in the range of microns and the total printing width.
24. Method of controlling coloration at a printing a printing machine according to claim 1, characterised in that sub-regions with specific weightings are taken into consideration for the resolution in that the resolution factors contain K weighting factors.
25. Method of controlling coloration at a printing a printing machine according to claim 24, characterised in that the weighting is the image weighting.
26. Method of controlling coloration at a printing a printing machine according to claim 19, characterised in that the sensors are arranged off-line or in-line in the printing machine.
27. Method of controlling coloration at a printing a printing machine according to claim 1, characterised in that the control data are evaluated automatically or manually by an operator.
28. Method of controlling coloration at a printing a printing machine according to claim 1, characterised in that the resolution factors are recalculated into full-tone changes.
29. Method of controlling coloration at a printing a printing machine according to claim 28, characterised in that full-tone changes are converted into control data.