EP2727730A1 - Method for controlling the inking of an offset printing press - Google Patents

Abstract

The method involves calibrating printing control elements of a printing substrate, where the elements are printed outside a virtual subject on the substrate by a camera e.g. charge coupled device camera. Determined color density actual value is compared with predetermined color density reference value to generate a control signal for color adjusting elements including duct roller, jack roller or film roller, depending on comparison between the actual and the reference values. Color register marks (11-14) are calibrated as the elements for determining the color density actual value.

Description

The invention relates to a method for controlling the coloring in an offset printing press according to the preamble of claim 1.

When printing a substrate is successively moved through several printing units of a printing press, wherein in each printing unit usually a printing ink is applied to the substrate. In the case of autotypical se-tenant printing, the four scale colors black, magenta, cyan and yellow and, if appropriate, spot colors are generally printed, with a separate printing unit and therefore inking unit being present for each of these printing inks. The inking unit of each printing unit has a Farbdosiereinrichtung, wherein the Farbdosiereinrichtung comprises a number of ink zones corresponding number of Farbzonenstellelementen. The Farbzonenstellelemente are also referred to as a color slider or color meter. Depending on the position of the ink zone setting elements, ink is applied to an ink fountain roller, which is also referred to as a ductor roller. The amount of ink applied to the ductor roller per ink zone as a function of the ink zone setting elements is transferred from a lifting roller or from a film roller to a downstream inking unit roller of the inking unit. The ink is moved over several inking rollers in the direction of a plate cylinder or plate cylinder of each printing unit. On the forme cylinder of a printing unit rolls at least one inking roller serving as the inking roller of the respective inking unit. Accordingly, the printing ink reaches at least one printing plate positioned on the forme cylinder via the or each inking roller. With the forme cylinder acts a so-called transfer cylinder or blanket cylinder, which transfers the ink from the forme cylinder to the substrate.

The color design of a printed product to be printed is set in a so-called pre-press. In this case, for example, so-called area coverage values are defined for each ink zone to be printed and thus for all inking units of the printing machine involved in the printing for each color zone. As a function of these area coverage values, the ink zone setting elements are adjusted and thus the zonal coloration is determined.

It is already known from the state of the art to measure the coloring which takes place on the printing substrate during printing and, depending thereon, regulates the zonal coloring of the inking units involved in the printing. For this purpose, measuring ranges printed on the printing material, so-called pressure control elements, are usually measured outside a subject. For each color zone, there are typically more than one so-called full tone fields, grids or gray balance fields executed pressure control elements each having a size of e.g. 2x2mm or 3x6mm or 5x5mm. The Druckkotrollelemente can be part of continuous pressure control strip. Actual values of the zonal colorimulation determined here are transmitted to a control device of the printing press, in which the actual values are compared with setpoint values, depending on the comparison between the actual values and the setpoint values and thus dependent on the control deviation between the measured actual values and the predetermined setpoint values to create the color zone actuators. Already an automatic control of the zonal coloring is possible.

Particularly in newspaper printing, in which no trimming of printed copies takes place, print control elements printed on the printing material outside the subject are perceived as disturbing. Therefore, it is also already known to measure measuring ranges within a subject and, depending thereon, to regulate the zonal coloring. The measurement of measurement areas within the subject is, however, more error-prone and therefore less accurate than the measurement of print control elements printed on the printing substrate outside of the subject.

From the US 2004/0163562 A1 a method according to the preamble of claim 1 is known.

Further prior art is disclosed in DE 10 2004 003612 A1 and in the EP 1 551 635 A1 as well as in the US 2003/169345 A1 , in the DE 195 38 811 A1 and in the EP 1 470 918 A2 ,

On this basis, the present invention is based on the problem to provide a novel method for controlling the color in an offset printing machine.

This problem is solved by a method according to claim 1.

Hereinafter, color density actual values determined for the compensation of scattered light effects during the measurement of the measuring ranges are corrected using image data of the measuring ranges and using data on the optics of the camera.

Preferably, color density actual values for controlling the zonal coloration are determined by measuring color register marks, which are usually measured to determine actual values for a color register control, by means of a camera. It is then dispensed with separate pressure control elements for determining color density actual values. Rather, the color density actual values are then detected by measuring the color register marks. The color register marks are typically relatively small, dot-shaped or rectangular print control elements having a size between 0.5 mm and 2 mm. Since color register marks are relatively small print control elements, it is important to compensate for scattered light effects when determining color density actual values by measuring color register marks.

Likewise, however, it is also possible to use the method according to the present invention when sections or measuring ranges of the actual subjects of the printing material are measured to determine color density actual values. Also in this case, the determination of color density actual values can be made more accurate by the compensation of scattered light effects.

Fig. 1:

eine schematisierte Darstellung eines flächig ausgeleuchteten Blickfelds einer Kamera mit Messbereichen zur Ermittlung von Farbdichte-Istwerten;

Fig. 2:

das Blickfeld der Fig. 1, wobei zur messtechnischen Kompensation von Streulichteffekten nur solche Abschnitte des Blickfelds ausgeleuchtet sind, innerhalb derer die Messbereiche zur Erfassung der Farbdichte-Istwerte liegen; und

Fig. 3 und 4:

schematisierte Darstellungen eines Blickfelds einer Kamera zur Ermittlung von Farbdichte-Istwerten unter Verwendung einer rechnerischen Kompensation von Streulichteffekten.

Preferred embodiments of the invention will become apparent from the dependent claims and the description below. An embodiment of the invention will be described, without being limited thereto, with reference to the drawing. Showing:

Fig. 1:

a schematic representation of a flat illuminated field of view of a camera with measuring ranges for determining color density actual values;

Fig. 2:

the field of vision of Fig. 1 , wherein for metrological compensation of scattered light effects only those portions of the field of view are illuminated, within which the measuring ranges for detecting the color density actual values are; and

3 and 4:

schematized representations of a field of view of a camera for determining color density actual values using a computational compensation of scattered light effects.

The present invention relates to a method for controlling the color in at least one inking unit of an offset printing machine, namely for controlling the adjustable by Farbzonenstellelemente the respective inking zonal coloring. For this purpose, measuring ranges of a printed printing material are measured with the aid of a camera and color density actual values determined are compared with predetermined color density nominal values in order to generate actuating signals for the ink zone setting elements of the respective inking unit, depending on the comparison between the actual color density values and the color density setpoints.

Preferably, it is assumed that pressure control elements of the printing material which are printed on the printing material outside of the actual subject are measured as measuring ranges for determining color density actual values.

These are preferably color register marks. Color density actual values are then recorded by measuring color register marks by means of a camera.

Accordingly, the color register marks serve not only to provide actual values for color register control but, according to the invention, also to provide actual values for controlling the zonal coloration.

Preferably, the color register marks continue to serve to provide actual values for cut-off register control. In order to obtain additional information for the control, the color register marks can be supplemented by rastered fields of the same size.

Preferably, color register marks printed on the printing substrate outside the subject are detected in a metrological manner such that they supply actual values for color register control and for cutting register control and for controlling the zonal coloration.

In this way, the number of print control elements that are printed on a substrate outside of the actual subject can be reduced to an absolute minimum, so that even in newspaper printing, in which no trimming of printed copies occurs, the pressure control elements are perceived as not disturbing.

Since color register marks are typically not present in each color zone of a printed substrate, the control of the zonal color scheme is such that for those color zones within which a color register mark lies, the color density actual value for color control determined in the color register mark is measured in the respective color registration mark Color zone is used.

For those color zones within which there is no color register mark, the corresponding color density actual value is determined using an interpolation between the color density actual values determined during measurement of the register marks or by derivation from zonal presetting values of these color zones in combination with color density actual values. If appropriate, empirically obtained correction values can be taken into account from previous printing processes. However, such color register marks may also be present in each color zone.

As already mentioned, the measurement of the color register marks for the metrological detection of color density actual values takes place with the aid of a camera, wherein the camera is preferably a CCD camera or CMOS camera. In this case, a color camera with white illumination of the substrate or a black and white camera with colored illumination of the substrate can be used. The field of view of the camera used captures at least one to be measured color register mark completely.

The color register marks are relatively small, dot-shaped or rectangular print control elements, which typically have a diameter or size between 0.5 mm and 2 mm. When determining color density actual values at such relatively small, punctiform pressure control elements, there is the problem that the measured color density actual values recorded can be falsified by scattered light effects. Therefore, according to the present invention, in the measurement of the register marks for determining the color density actual values, a compensation of scattered light effects is carried out.

One possibility for compensation of scattered light effects in the metrological detection of color density actual values at relatively small color register marks is described below with reference to FIG Fig. 1 and 2 described.

Fig. 1 and 2 each show a field of view 10 of a camera schematized, wherein in the field of view 10 four punctiform Farbregistermarken 11, 12, 13 and 14 are. In a first step (see Fig. 1 ), the entire field of view 10 of the camera is illuminated areally. Here, the position of the color register marks 11, 12, 13 and 14 is determined, and then in a second step (see Fig. 2 ) are illuminated depending on the position of the Farbregistermarken 11, 12, 13 and 14, only those portions of the field of view 10 of the camera, within which the Farbregistermarken 11, 12, 13 and 14 are located.

Accordingly, the first step primarily serves to determine the position of the color register marks, furthermore, measured values of the color register marks determined in the first step can be used as actual values for color register control and / or cut register control. In the second step, measured values of the color register marks are used as actual values for controlling the zonal coloration. The determination of the position of the color register marks in the first step can be carried out using known image processing techniques, e.g. By segmentation and feature extraction, or using cross-correlation techniques.

For flat illumination of the entire field of view of the camera in the first step and for partial illumination of the field of view of the camera in the second step, one and the same source of illumination can be used, which then has to be designed to be switchable over a wide area. In the beam path of such a surface switchable illumination source is at least one switchable element, which either transmits or blocks radiation depending on the switching state or refelcted or reflected at different angles.

In the first step, in which the field of view 10 of the camera is illuminated flat, the or each switchable element of the camera is permeable or in employment, in the second step, however, in which the field of view of the camera is illuminated exclusively in those sections in which the color register marks, however, at least one of the switchable elements is impermeable or in exhibition. Alternatively, the full-surface illumination of the field of view of the camera in the first step and the selective illumination of the field of view in the second step can also be realized by using different illumination sources.

According to the invention, the compensation of scattered light effects, in particular during the measurement of the color register marks to determine the color density actual values, additionally or alternatively via a mathematical correction, which is dependent on image data to be measured color register marks and data on the optics of the camera. In the mathematical correction of metrologically detected color density actual values for compensation of scattered light effects, an angle-independent correction and an angle-dependent correction are preferably performed, by means of which the color density actual values determined during the measurement of the color register marks are corrected.

The correction of metrologically recorded color density actual values using an angle-independent correction factor and an angle-dependent correction factor is based on the finding that in a camera a measurement location 16 and a background 15 are imaged and scattered by an optical system 17 of the camera.

In the angle-independent case of scattering (see Fig. 3 ) is a light beam repeatedly to z. B. a tube 18 of the camera scattered and reaches as a constant offset 19 the location on a camera chip of the camera.

In the angle-dependent case (see Fig. 4 ), a light beam is scattered at the optics 17 and gives an offset 20 decreasing in intensity with the scattering angle which is greatest at an edge of a color register mark to be measured. For round color register marks, an angle-dependent correction factor that depends on the size of the color register mark can be determined.

The angle-independent correction factor and the angle-dependent correction factor are then used to correct color register marks for metrologically recorded color density actual values.

wobei I die Intensität, I₀ die Intensität aus Weißmessung, R die Remission und D die Farbdichte ist.In order to determine the angle-independent correction factor and the angle-dependent correction factor for the mathematical correction of the color register marks metrologically determined color density actual values is proceeded in detail so that a preferably black full-tone field is positioned in the field of view of the camera and covered with a white element in half, on the one hand a Density distribution of the full black solid field and on the other hand, a density distribution of half-covered with the white element solid field is determined. The distribution of the color density can then be determined according to the following formula: $$D=-\log \left(\frac{I}{I_0}\right)=-\log \left(R\right)$$

where I is the intensity, I ₀ the intensity of white measurement, R the remission and D the color density.

$$D_{\mathit{korr}}=-\log \left(\frac{I-A_R{I}_S}{I_0-I_S}\right)$$

wobei D_korr der winkelunabhängig korrigierte Farbdichte-Istwert ist, β das Verhältnis der Remissionen (gleich dem Verhältnis der Intensitäten bei gleichem I₀) ist und weit weg von der Kante gebildet wird, wobei I_S die Streuintensität ist, wobei I_R die Referenzintensität des Volltons des unabgedeckten Feldes ist und A_R der Anteil der weißen Fläche ist.A ratio of the remissions is then formed far away from the edge formed by covering the solid field with the white element, and an angle-independently corrected color density is calculated from this ratio of reflections by the reference intensity of the solid of the uncovered field and by the proportion of the white area can: $$I_S=2\left(1-\beta \right)I_R$$

$$D_{\mathit{corr}}=-\log \left(\frac{I-A_R{I}_S}{I_0-I_S}\right)$$

wherein D _corr of the angle-independent corrected color density value is the ratio of the remissions β (equal to the ratio of the intensities at the same I _0), and is formed away from the edge, where I _S is the scattering intensity, where I _{R is} the reference intensity of the is full tone of the unmasked field and A _R is the percentage of white surface.

$$D_{\mathit{korr}2}=D_{\mathit{korr}}\left(1+2*f\left(x\right)-1\right)$$

wobei D_korr2 der winkelabhängig korrigierte Farbdichte-Istwert, f(x) der normierte Korrekturfaktor und D_∞ die Dichte weit weg von der Kante ist. In der korrigierten Dichte ist für x hier der Radius des Druckkontrollelements einzusetzen.For the angle-dependent correction of the metrologically recorded color density actual values, the edge produced by covering the full-tone field with a white element is examined in more detail, with a curve fit being carried out by means of a least square fit for the color-independent actual values corrected for angle. From this, a normalized correction factor f (x) can be determined and the angle-dependent correction of the metrologically recorded color density actual values can be carried out according to the following formula: $$f\left(x\right)=D_{\infty }/D\left(x\right)$$

$$D_{\mathit{corr}2}=D_{\mathit{corr}}\left(1+2*f\left(x\right)-1\right)$$

where D _{korr2 is} the angle-dependent corrected color density actual value, f (x) is the normalized correction factor and D _{∞ is} the density far away from the edge. In the corrected density, the radius of the pressure control element is to be used here for x.

The above-described correction of scattered light effects can also be used if sections of the actual subject of the printed printing material are measured as measured objects. In particular, in this case the spatially resolved lighting according to Fig. 1 and 2 Use, in which case the subject can be illuminated with white light and metrologically recorded RGB values can be converted into LAB values using a Bradford matrix. Alternatively, it is also possible to record several images sequentially when the subject is illuminated with different wavelengths, wherein the illumination with different wavelengths can be achieved by either assigning different filters to a white light illumination source or by using different colored light sources to illuminate the subject ,

The method according to the invention preferably takes place within the printing machine inline for printing on the printing substrate. Alternatively, however, it is also possible for the method to be used offline outside the printing press for printing on the printing substrate.

LIST OF REFERENCE NUMBERS

10

field of vision

11

Color register mark

12

Color register mark

13

Color register mark

14

Color register mark

15

background

16

Measuring location

17

optics

18

tube

19

offset

20

offset

Claims (14)

Method for controlling the color scheme in at least one inking unit of an offset printing machine, namely for controlling the zonal coloration which can be set by ink-zone setting elements and preferably a fountain roller or film roller of the respective inking unit, wherein measuring ranges of a printed substrate are measured with the aid of a camera and color density actual values determined thereby be compared with predetermined color density setpoints to produce depending on the comparison between the actual values and the setpoints control signals for the ink zone actuators and optionally the ductor roller and vibrating roller or film roll, characterized in that to compensate for scattered light effects in the measurement of the measuring ranges determined color density Actual values are corrected using image data of the measuring ranges and using data on the optics of the camera. A method according to claim 1, characterized in that are measured as measuring ranges pressure control elements of a printed substrate, which are printed outside the actual subject on the substrate. Method according to Claim 1, characterized in that sections of the actual subject are measured as measuring ranges. Method according to one of claims 1 to 3, characterized in that in this case an angle-independent correction and an angle-dependent correction can be determined, the measurements from the measurement of the color register marks, color density actual values are corrected using those. A method according to any one of claims 1 to 4, characterized in that to compensate for stray light effects in a first step, the field of view of the camera is illuminated surface, and that only those portions of the field of view of the camera are illuminated in a second step, within which measuring ranges for detecting the color density actual values are. A method according to claim 5, characterized in that are measured as measuring ranges pressure control elements of a printed substrate, which are printed outside the actual subject on the substrate. Method according to Claim 5, characterized in that sections of the actual subject are measured as measuring ranges. Method according to one of claims 5 to 7, characterized in that for flat and partial illumination of the field of view of the camera, a single, surface switchable illumination source is used. Method according to one of claims 5 to 8, characterized in that for the areal and partial illumination of the field of view of the camera different sources of illumination are used. Method according to one of claims 1 to 9, characterized in that color register marks are measured as pressure control elements for determining the color density actual values. A method according to claim 10, characterized in that the Farbregistermarken be measured for color register control and for controlling the zonal color scheme, and that the Farbregistermarken preferably continue to be measured for the cut register control. A method according to claim 10 or 11, characterized in that for such color zones, within which a color register mark lies, the color density actual values determined upon measurement of the color register mark are used for controlling the zonal coloration of the respective color zone. A method according to claim 12, characterized in that for such color zones within which there is no color register mark, the color density actual values is determined depending on an interpolation between color density actual values determined when measuring the color registration mark or by derivation from zone presets in combination with actual color density values. Method according to one of Claims 10 to 13, characterized in that, in addition to the color register marks, equally sized raster fields of the same size are measured and measured values are used to control and optionally compensate for scattered light effects.

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