Multi-stage regulation and measurement of opaque white

Abstract

Method for color control in printing machines with a computer (41) by detecting color areas (22) on a surface (20) to be printed with a color measuring device (30), characterized in that the surface to be printed is a substrate (20), that the substrate (20) is coated with opaque white, that the color measuring device (30) detects several color measurements of opaque white, and that the computer (41) compares the detected color measurements of the opaque white with each other or against a reference color value of the opaque white and stores the deviations determined in the comparison in the computer (41), that the substrate (20) coated with opaque white is overprinted with color measurement fields (26).that the overprinted color measurement fields (26) are detected by the color measuring device (30) and that the computer (41) takes into account the influence of the stored determined deviations when comparing the color measurement values ​​of the color measurement fields (26) underlaid with opaque white to the target color values ​​of the print template for controlling the coloring of the color measurement values ​​of the color measurement fields (26).

Description

[0001] The invention relates to a method for color control in printing machines with a computer by detecting color areas on a surface to be printed with a color measuring device.

[0002] For measuring the color of printing inks applied to a substrate, spectral measurement using colorimetric strips is a common and proven method. This requires that, firstly, the substrate is lighter than the ink being printed, and secondly, that the ink appears darker with increasing layer thickness. The latter is the case with offset printing inks. Experts are familiar with, for example, the regulations developed by Heidelberger Druckmaschinen AG under the name Image Control. ® and / or Axis Control ® and / or Inpress Control ® known for distributing printing presses, especially offset printing presses.

[0003] The layer thickness is determined in spectral measurement based on a measured difference in brightness between the substrate and the applied ink. Typically, the substrate is a white sheet of paper. In this case, the described spectral measurement works without problems, as any ink applied even in a thin layer appears darker than the white of the sheet. However, this is not always possible with darker substrates, as the ink may appear lighter than the substrate. The same problem arises with spectral color measurement on metals and transparent films. Metals have reflective properties. Therefore, they appear black to a spectral measuring device under standard conditions (0° or 45° angle measurement). When measuring spectrally on transparent films, the surface on which the transparent film rests is always visible through the spectral line.For measurements taken outside the machine, this is the dark printing table, and for inline measurements, it is a metal cylinder.

[0004] This problem can be circumvented in practice by applying a layer of opaque white to the colorimetric strip. This enables color measurement and control within the strip itself. However, to ensure accurate and correct measurement and control of the printing ink, the opaque white must be applied evenly. Applying the opaque white is done manually, for example, in a printing unit preceding the printing units with the colored inks, without automatic color measurement and control. This inevitably leads to deviations both between the measuring strips and between the color zones of a single measuring strip. These deviations result in measurement differences that are incorrectly interpreted as variations in the ink layer thickness, as a uniform background is assumed.

[0005] Another practical workaround for the described problem involves using a halftone patch, for example, 70%, within a color measurement strip containing opaque white. During the measurement process, the opaque white and the substrate are reversed. The halftone patch is treated as if the opaque white were the background and the substrate the printing ink. This means that the tonal value of a black halftone patch, here for example, 30%, is measured on a white substrate. While this method allows for the relative adjustment of multiple color zones on a zonal printing press and the detection of fluctuations during operation, the opaque white itself must be measured using an external measuring device and not during operation. Furthermore, this method does not work with conventional color control systems because it operates in the wrong direction.This is because with colored printing inks, the color becomes darker with increasing layer thickness, while with opaque white on a dark background, the opaque white becomes lighter with increasing layer thickness.

[0006] With transparent substrates, it is still possible to place a white strip of paper, plastic, or similar material behind the substrate. However, this option is not available for opaque substrates.

[0007] German patent application DE 10 2007 005 018 A1 discloses a method for color control of reproduction copies from a printing press, in which a substrate that appears dark in a standard-compliant color measurement is printed with at least one printing ink that is lighter than the substrate, and at least one color value of the printing ink is determined from a measured quantity. The method is characterized by the fact that the color value of the at least one printing ink is controlled using a defined reference value of a color point in the color space that is lighter than the printing ink.

[0008] German patent application DE 197 24 171 A1 discloses a method for controlling color in printing presses, in which the inhomogeneities of the unprinted substrate are taken into account when measuring the colors after printing. For this purpose, the unprinted substrate is first measured at several points to identify inhomogeneities such as contamination. Then the printed substrate is measured. The color measurements obtained on the printed substrate are then corrected using the measurements of the unprinted substrate, thus preventing the distorting influence of contamination on the color control.

[0009] The object of the present invention is to enable a color control system for printing machines which can also reliably measure and adjust printing inks on surfaces that are darker than the printed colors or transparent.

[0010] This problem is solved according to the invention by a method for color control with the features according to claims 1 and 2. Advantageous embodiments of the invention are characterized in the dependent claims.

[0011] In the inventive method for color control, several color measurements are first recorded on a surface to be printed, coated with opaque white, using a colorimeter. These color measurements of the opaque white area are compared with each other or against a reference color value of the opaque white in the computer. The deviations determined during the comparison are stored in the computer. Subsequently, the substrate coated with opaque white is overprinted with color measurement patches. These color measurement patches are again recorded by the colorimeter. When comparing the color of the opaque white-coated color measurement patches with the target values ​​of the print template to control the color, the stored deviations are taken into account by the computer. With this embodiment, printing inks can also be controlled on highly reflective and very dark surfaces.

[0012] An alternative or additional embodiment of the invention provides that several color measurements of the opaque white are recorded on the substrate using a colorimeter. The averaged color measurements of the opaque white are then compared with each other or against a reference color value of opaque white by a computer. The deviations of the color measurements determined during the comparison, either between themselves or against the reference value of the opaque white, are then corrected by the computer and thus brought to a uniform value. This method can be combined with the aforementioned correction method by, for example, first correcting the opaque white and then, after the correction step, taking into account the remaining deviations from the ideal opaque white as correction values ​​when controlling the printing inks, i.e., the color inks and the black ink. Thus, both a parallel and a sequential combination of the two methods is possible.Furthermore, opaque white and printing inks can be controlled in parallel or sequentially. With parallel control, the effects of expected changes in the opaque white on the printing inks during the opaque white control step must be considered for simultaneous control of the printing inks. Simultaneous control of printing inks and opaque white has the advantage of saving material and time.

[0013] In a further advantageous embodiment of the invention, after the opaque white has been adjusted, the color measurement fields underlaid with opaque white are detected by the colorimeter. Another practical form of the invention involves detecting the opaque white in or alongside a color measurement strip containing color measurement fields. The color measurement strip required for color measurement is thus also used for opaque white control by adding an opaque white field.

[0014] In another advantageous embodiment of the invention, the reference color value is either a stored predetermined value, a stored print job value, or the brightest value of the opaque white across the entire width of the substrate.

[0015] A further advantageous embodiment of the invention comprises that the substrate is printed in color zones and that the color measuring device detects an opaque white color measurement value and the color measurement fields underlaid with opaque white in each or every second color zone on the substrate.

[0016] Furthermore, in an advantageous embodiment of the invention, it is provided that the reference color value for opaque white in the opaque state is stored in a database and retrieved by the computer.

[0017] In a particularly advantageous embodiment of the invention, the computer displays the measured values ​​for opaque white on a display device and suggests the brightest measured value for opaque white as a reference color value on the display device for confirmation by the operator.

[0018] In a further particularly advantageous embodiment of the invention, the color control process first involves homogenizing the opaque white on the substrate. Only then are the colors adjusted in the color measurement fields of the color measurement strip backed with opaque white.

[0019] Another highly advantageous embodiment of the invention provides that the effects of deviations in the opaque white on the color measurement values ​​of the color measurement fields backed with opaque white are taken into account by considering color changes of the opaque white background in the form of spectra in the computer and are considered in the color control of the color measurement values ​​of the color measurement fields backed with opaque white in the computer. Alternatively, consideration in the form of color density values ​​or color coordinates is also possible.

[0020] One possible embodiment is described with reference to the following figure. It shows in detail: Fig. 1 a schematic representation of an embodiment of an arrangement according to the invention for color measurement.

[0021] The Fig.Figure 1 is a schematic representation of an embodiment of an arrangement according to the invention for color measurement. A specimen of the substrate 20, printed in an offset printing press, has a color measuring strip 22. The color measuring strip 22 is backed with opaque white 24. The color measuring strip 22 has gaps for measuring the opaque white and includes measuring fields 26 for the other printing inks, in particular the standard printing inks, in each color zone 28. Instead of gaps, it is also possible to provide measuring fields printed with opaque white in the color measuring strip 22. Opaque white and other printing inks can thus be measured and controlled in a single measurement process. A measuring device 30 performs a relative movement 32 with respect to the substrate 20, so that the measuring fields 26 and the opaque white 24 in the color measuring strip 22 in the individual color zones 28 can be detected by a control device 34.

[0022] First, a line of opaque white 24 is printed. The color measurements of the opaque white 24 from the various color zones 28, recorded by the colorimeter 30, are then compared in the computer 41 with a reference color value of opaque white. Subsequently, deviations of the color measurements of the opaque white 24 from the reference color value of opaque white are identified by the computer 41 and corrected by the control unit 34. Next, the colored measurement fields 26 are applied to the color measurement strip 22, which previously consisted only of opaque white 24, while the printed image outside the color measurement strip 22 is printed simultaneously. These colored measurement fields 26 are then also recorded by the colorimeter 30 and compared in the computer 41 with target values ​​stored in the computer 41. Deviations of the colored measurement fields 26 from the respective target color value are identified by the computer 41 and corrected by the control unit 34.

[0023] The reference color value for the opaque white control is either a stored predefined value, a stored print job value, or the lightest measured value of opaque white across the entire width of the substrate 20. The target value or reference value is supplied as a specification 38 to a storage unit 36 ​​of the control device 34. A signal output 40 is then sent to the actuators of the offset printing press for color control of opaque white 24 and the colors in the color measurement fields 26 of the measuring strip 22, together with the printed image. Reference symbol list 1 measuring table 2 Measuring table surface 20 Printing material 22 colorimetric strips 24 white surface area 26 measuring fields in the colorimetric strip 28 color zone 30 Colorimeter 32 Relative motion 34 Control device 36 storage units 38 Specification of the reference value 40 Signal output for actuators 41 control computers

Claims

[1] Method for color control in printing machines with a computer (41) by detecting color areas (22) on a surface to be printed (20) with a color measuring device (30), characterized by, that the surface to be printed is a substrate (20), that the substrate (20) is coated with opaque white, that the colorimeter (30) records several color measurements of opaque white, and that the computer (41) compares the recorded color measurements of the opaque white with each other or against a reference color value of the opaque white and stores the deviations determined in the computer (41) during the comparison, that the substrate (20) coated with opaque white is overprinted with color measurement fields (26), that the overprinted color measurement fields (26) are recorded by the colorimeter (30), and that the computer (41) takes into account the influence of the stored determined deviations when comparing the color measurements of the color measurement fields (26) underlaid with opaque white to control the coloring of the color measurements with the target color values ​​of the printing template. [2] Method for color control in printing machines with a computer (41) by detecting color areas (22) on a surface to be printed (20) with a color measuring device (30), wherein the surface to be printed is a substrate (20), wherein the substrate (20) is coated with opaque white, wherein the color measuring device (30) records several color measurements of opaque white, wherein the computer (41) compares the recorded color measurements of the opaque white with each other or against a reference color value of the opaque white, and wherein the computer (41) compensates for deviations of the color measurements of the opaque white with each other or against the reference color value of the opaque white. characterized by , that after the opaque white has been adjusted, the color measurement fields (26) underlaid with opaque white are detected by the color measuring device (30). [3] Method according to claim 1, characterized by, that additionally, sequentially or in parallel, the computer (41) corrects deviations of the color measurement values ​​of the opaque white among themselves or against the reference color value of the opaque white. [4] Method according to claim 3, characterized by , that after the opaque white has been adjusted, the color measurement fields (26) underlaid with opaque white are detected by the color measuring device (30). [5] Method according to any one of the preceding claims, characterized by , that the opaque white is detected in a color measurement strip (22) with color measurement fields (26) or next to the color measurement strip (22) by the color measuring device (30). [6] Method according to any one of the preceding claims, characterized by , that the reference colour value is a stored predefined value, a stored print job value or the brightest measured value of the opaque white across the entire width of the substrate (20). [7] Method according to any one of the preceding claims, characterized by, that the substrate (20) is printed in color zones (28) and that the color measuring device (30) records an opaque white color measurement value and the color measurement fields (26) underlaid with opaque white in each or every second color zone (28) on the substrate (20). [8] Method according to any one of the preceding claims, characterized by , that the reference colour value for opaque white in the opaque state is stored in a database and is retrieved by the computer (41). [9] Method according to any one of the preceding claims 6 to 8, characterized by , that the computer (41) displays the measured values ​​for opaque white on a display device and suggests the brightest measured value for opaque white as the reference colour value on the display device for confirmation by the operator. [10] Method according to claim 2 or 3, characterized by, that first the color control process homogenizes the color of the opaque white on the substrate (20) and only then regulates the colors in the color measurement fields (26) of the color measurement strip (22) backed with opaque white. [11] Method according to claim 1 or 3, characterized by , that the effects of the deviations of the opaque white on the color measurement values ​​of the color measurement fields (26) backed with opaque white are taken into account by considering color changes of the opaque white substrate in the form of spectra in the computer (41) and are taken into account in the color control of the color measurement values ​​of the color measurement fields (26) backed with opaque white in the computer (41).

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