Color shift associated with a substrate

Abstract

The invention relates to a method for color control of a printing press (3) for a printing job on a non-white printing substrate by means of a computer (4), wherein, for carrying out the printing job, the printing press (3) first undercoats the printing substrate with an opaque white color and subsequently applies the actual printing color to the undercoat, wherein the color control with regard to the application of the opaque white color and the application of the printing color is calibrated by means of a computer-supported analysis of at least one test sample (8, 9) which is printed, characterized in that, within the calibration scope of the color control, the computer (4) optimizes the application of the opaque white color and the application of the printing color in accordance with the color values (7) of the non-white printing substrate.

Description

Technical Field

[0001] This invention proposes a method for color control of a printing press, which takes into account the color value of the substrate to be printed.

[0002] This invention is based on the technical field of color management for printing presses. Background Technology

[0003] To print on colored, non-white substrates and paper, a white primer is often applied to the areas to be printed in an opaque manner to achieve not only high color saturation and brightness but also a uniform print designed specifically for the white substrate. This results in unnecessarily high consumption of colorants (such as inks or toners), as a white primer is first applied to the black substrate, for example, in the case of printing from black to a black substrate. On the other hand, there is usually a total area coverage limit determined by the printing process used in the application, which means that less colorant can be applied to that area, resulting in a smaller color gamut. By achieving a color structure (Farbaufbau) in dark colors without using white, a smaller brightness can be achieved in the form of a larger density range, which overall contributes to a larger color gamut.

[0004] While various methods for directional adjustment of opaque white (or primer) are known in the prior art, these methods typically involve applying an opaque white primer in a way that optimizes the application of the substrate color to achieve the most uniform white output color value possible for printing the actual image; similar to optimized paper white. There is no known solution in the prior art that intentionally makes full use of the substrate's color value even when applying opaque white. Summary of the Invention

[0005] Therefore, the object of this invention is to provide a method for color control of a printing press that covers a large color space with minimal material application.

[0006] This objective is achieved through a method for color control of a printing press for printing tasks on non-white substrates using a computer. In this method, to perform the printing task, the printing press first applies a primer to the substrate with zinc white / opaque white (Deckweiβ), and then applies the original / actual printing color onto the primer. Color control regarding the application of opaque white and the application of printing color is calibrated by analyzing at least one test sample in a computer-supported manner. The method is characterized in that, within the scope of color control calibration, the computer optimizes the application of opaque white and the application of printing color based on the color values ​​of the non-white substrate. The core of the method according to the invention is that, instead of completely covering the substrate with opaque white and then reconstructing a printed image with its target color value on the opaque white as in the prior art, the method according to the invention fully utilizes the color values ​​of the substrate to achieve the desired target color values ​​of the printed image. Thus, the color values ​​of the substrate contribute in part to the printed image. Therefore, it is necessary to adjust the amount of opaque white accordingly. This reduces the amount of material used for opaque white (and possibly printing color). Furthermore, the substrate size facilitates the creation of a wider color space and gamut, potentially enabling the achievement of target color values ​​that cannot be achieved with printing color alone.

[0007] Advantageous and therefore preferred improvements to the method according to the invention arise from the preferred embodiments and from the description with the accompanying drawings.

[0008] A preferred improvement to the method according to the invention lies in that a computer creates a color transformation table (particularly in the form of an ICC color profile) by means of calibration, which maps the application of opaque white and printing colors to optimized color control according to the printing task. To achieve color control for a printing task, a color space transformation is typically performed, which generates a color transformation table (e.g., an ICC color profile or ICC color profile table). These color transformation tables show which combinations of process colors (i.e., the printing colors of the applied printing press) result in which color values ​​in a neutral, machine-independent color space (e.g., Lab). For the method according to the invention, these color transformation tables are adapted such that they take into account not only the color values ​​of opaque white and printing colors but also the color values ​​of the substrate.

[0009] Another preferred improvement of the method according to the invention is that, in order to optimize the application of opaque white and printing colors, the color transformation table considers the substrate color to achieve a total surface coverage for multiple target color coordinates. Minimize and apply opaque white only where necessary to achieve the target color coordinates. Ideally, some areas of the printed image should not require any opaque white at all, thus ensuring maximum material savings.

[0010] Another preferred improvement to the method according to the invention lies in that, in order to create a color transformation table, the computer generates a process model from measurements of a testform. This process model presents a mapping of color shares (including opaque white) in a device-dependent color space to a device-independent color space. To generate the process model, measured values ​​from the color regions of the testform are applied, and the computer uses these values ​​to generate the process model via extrapolation or interpolation, or alternatively via a color prediction model. This represents a preferred, however not unique, approach for implementing the method according to the invention that considers substrate color values. The preferred device-dependent color space is the Lab color space.

[0011] Another preferred improvement to the method according to the invention is that a test sample is applied in a full-surface manner. And / or in a rasterized manner, it includes color combinations with opaque white, color combinations without opaque white, and color combinations with opaque white as the sole printing color. Using the analysis of this test sample, a color transformation table can then be created to perform the method.

[0012] Another preferred improvement to the method according to the invention is that the color value of the non-white printing substrate is pre-given by the user of the printing press or determined by measurement using a colorimeter. If the printing press includes an automated inline image detection system with a colorimeter for monitoring color values, then such a colorimeter should preferably be used. However, an external colorimeter can also be used. If the color value of the substrate is already known accurately enough, then the color value can also be pre-given directly by the user, for example, by obtaining the color value from a substrate catalog (or database).

[0013] Another preferred improvement to the method according to the invention is that the test samples for each printing task are selected by choosing from available test samples, or created by a computer. The latter occurs more frequently. However, the test sample can also be applied if a printing task with the same substrate and the same or at least similar printing colors and opaque white has already been performed. This reduces costs because the step of creating test samples can be eliminated.

[0014] Another preferred improvement to the method according to the invention is that, in order to create test samples for a specific printing task or for a specific paper grade, a large number of printing color combinations, including opaque white, are first printed. However, a computer then determines relevante color combinations based on these printing color combinations to create the actual test samples. This requires additional method steps, which increases cost but provides relatively accurate results.

[0015] Another preferred improvement of the method according to the invention is that the computer determines the color combinations relevant to the test sample by applying a color prediction model to create an overprint of each printing color, including opaque white. The prediction. As an alternative for printing a wide range of color combinations, including opaque white, additional steps for printing these color combinations can be eliminated by applying a color prediction model via computer. Therefore, test samples may not be as accurately aligned with the requirements of the printing task as when performing such printing.

[0016] Another preferred improvement to the method according to the invention is the application of a toner printing press. This method can naturally also be used with other printing presses (e.g., inkjet or offset printing presses that operate in opaque white). However, the substrate must then have a color value different from the target opaque white. In the case of a transparent substrate, this method is only meaningful when a colored substrate is present (on which the transparent substrate is applied; for example, applying a film to a colored bottle). Attached Figure Description

[0017] The invention itself, as well as its advantageous structural and / or functional improvements, will now be further described with reference to the accompanying drawings and preferred embodiments. In the drawings, corresponding elements are given their respective identical reference numerals. The drawings show:

[0018] Figure 1 Schematic diagram of a printing press system;

[0019] Figure 2 The schematic flow of the method according to the present invention; and

[0020] Figure 3 Examples of various opaque white values ​​for the test sample. Detailed Implementation

[0021] This invention proposes a method that takes into account the chromaticity of paper in its color structure, thereby minimizing the applied color dosage. Furthermore, this method may also potentially contribute to color gamut expansion.

[0022] The method according to the invention is preferably used in printing press system 2. This printing press system 2... Figure 1 The diagram is schematically shown. In addition to the printing press 3 itself (which is preferably a digital toner printer, and which prints opaque white as the first printing color), the printing press system 2 also includes a control computer 4 for the printing press 3, on which the ICC color profile 6 is stored in a database 5. Besides the control computer 4, another computer can also be used, which the user 1 uses to access the color management system of the printing process. It is necessary that the printing press 3 can also output white printing color in a rasterized manner. Other printing colors include at least the process colors commonly used in printing (e.g., CMYK), but may also include other colors. Besides operating the printing press 3, the computer 4 is particularly needed to select or generate suitable test samples 8 and to create color separations. Furthermore, a color measuring instrument 14 is needed to measure the printed color areas of these test samples 8, 9.

[0023] Figure 2 The flowchart of a preferred embodiment of this method is schematically illustrated, in which a color structure is generated that maps a device-independent color space (e.g., Lab) to a device-specific process space (e.g., CMYK plus white). This mapping can (and typically) be saved as a color transformation table 6 in the form of an ICC color profile file 6. To create this color transformation table 6, test samples 8 must be printed, wherein the printed test samples 9 then contain a series of combinations of printed colors. The printed test samples are then measured using a colorimeter 14, and the measured values ​​10 of the test samples 9 are correspondingly analyzed by a computer 4. To perform the method presented herein, the test samples 8 and 9 must not only contain color combinations in which opaque white is included in test area 11 and test area 13 respectively in a full-area manner and in a rasterized manner; and the test samples 8 and 9 must also contain color combinations in which test area 12 does not contain white. Furthermore, there are color combinations where these test areas 11 and 12 contain opaque white as the sole printing color.

[0024] When generating color transformations, for multiple target color values, the total area coverage is minimized by considering the base color's color value (7). This is naturally less suitable for very bright target color values, as opaque white is typically required to achieve these very bright target color values. Generally, white is only used for positions necessary to achieve the color coordinates.

[0025] The method according to the present invention includes the following steps:

[0026] 1. Calibrate the printing press 4 on the colored substrate 7, especially for printing white;

[0027] 2. The operator inputs the color value 7 of the substrate, or the color of the substrate 7 is measured by the color measuring instrument 14, wherein such measurement may also have already occurred during the calibration process;

[0028] 3. Select a suitable test sample 8, or generate test sample 8;

[0029] 4. Print the selected or generated test samples 8;

[0030] 5. Measure color areas 11, 12, and 13 located on the test sample;

[0031] 6. This color transformation is generated and stored in database 5 as ICC color profile file 6.

[0032] If test sample 8 is to be generated, there are several possible approaches to doing so. One possible approach is that, for a given printing process or a given paper grade, printing press 3 prints a large number of process color combinations at once. These process color combinations are then used to determine color combinations relevant to the color structure. These determined color combinations then generate the actual test sample 8.

[0033] One possible solution is to use computer 4 to apply a color prediction model and thereby generate corresponding process color combinations for overprinting. Thus, computer 4 can determine the relevant color combinations. The color prediction model is preferably available through database 5.

[0034] After the test samples 9 are generated in the printing press, these test areas are measured. This is typically done using a spectrophotometer, which serves as the color measuring instrument 14. To generate a color transformation, a process model is generated by computer 4 from the measurements 10 of the test samples 9. This process model maps the color proportions of the device-dependent process space, including opaque white, to a device-independent color space (e.g., CIELab). To generate this process model, the measurements 10 from the color areas are applied. This process model is generated from these measurements 10 using extrapolation, interpolation, or alternatively, a color prediction model. This process model can then predict the corresponding and expected device-independent color values ​​with sufficient accuracy for any combination of process values ​​(including opaque white).

[0035] Therefore, using the process model, computer 4 can determine the color transformation from the device-independent color space to the process space of the printing press 3, thereby minimizing the total color application. Since the process space typically has at least the process colors CMYK and opaque white, while the device-independent color space is only three-dimensional, various possible color transformation schemes exist, each contributing to different process color combinations for the device-independent color values. However, this color transformation must fit the process model, as the process model is the inverse mapping of the color transformation. When determining the color transformation, we define a transformation that minimizes the total color application, considering additional boundary conditions or optimization criteria if necessary.

[0036] The main purpose is to minimize the application of opaque white.

[0037] The resulting and optimized color transformation table 6 is typically stored in a database (similar to the transformation scheme described by the process model). This transformation table 6 comprises regular gratings, where each grating point represents a point in the machine-dependent process color space, and at each grating point, it contains the color transformation result in a machine-independent space as a value. The transformation table 6 thus represents discrete sampling of the color transformation. The storage of the color transformation table 6 in the database is preferably implemented in the form of an ICC color profile file 6.

[0038] The method according to the invention can also be used on white printing substrates, in which paper white has a different color value than the white produced by printing opaque white.

[0039] Furthermore, this method can also be used in other specific embodiments if printed onto a transparent film (foie) and that film is subsequently applied to a colored article (e.g., a bottle). In this case, the color of the article serves as the color of the substrate.

[0040] Therefore, the advantages of the method described here are that it results in less consumption of the applied printing color material and a potentially larger color gamut.

[0041] List of reference numerals

[0042] 1 user

[0043] 2 Printing press system

[0044] 3 printing press

[0045] 4. Control computer

[0046] 5 databases

[0047] 6 Color transformation tables / ICC color profile files created

[0048] 7. Substrate color / Substrate color value

[0049] 8 Test samples with opaque white and printing colors

[0050] 9 printed test samples

[0051] 10. Measurement values ​​of the test samples

[0052] 11. Solid white test sample area

[0053] 12. No white test sample area.

[0054] 13 rasterized white test sample areas

[0055] 14 Color Meter

Claims

1. A method for color control of a printing press (3) for printing tasks on non-white substrates using a computer (4), in, In order to perform the printing task, the printing press (3) first applies a primer to the substrate with an opaque white color, and then applies the actual printing color onto the primer. Color control regarding the application of opaque white and printed colors is calibrated by analyzing at least one test sample (8, 9) using a computer-supported method after printing. Its features are, Within the calibration scope of color control, the computer (4) optimizes the application of opaque white and printing color based on the color value (7) of the non-white printing substrate. The following test samples (8, 9) are applied: the test sample contains color combinations with opaque white in full-area mode (11) and raster mode (13), the test sample contains color combinations without opaque white (12), and the test sample contains color combinations with opaque white as the only printing color, and The computer (4) uses the analysis and processing of the test samples to create a color transformation table (6), which maps optimized color control for the application of opaque white and printing color according to the printing task. To optimize the application of opaque white and printing color, the color transformation table (6) minimizes the total surface coverage by considering the substrate color for multiple target color coordinates, and applies opaque white only at the locations necessary to achieve the corresponding target color coordinates. In order to create the color transformation table (6), the computer (4) generates a process model from the measurements (10) of the test samples (8, 9), which gives a mapping of the color share, including opaque white, in the equipment-related color space to the equipment-independent color space. In order to generate the process model, the computer (4) uses the values ​​measured from the color regions of the test samples (8, 9) to generate the process model by extrapolation or interpolation or alternatively by a color prediction model.

2. The method according to claim 1, Its features are, The color transformation table (6) is created in the form of an ICC color profile file.

3. The method according to claim 1 or 2, Its features are, The color value (7) of the non-white printing substrate is pre-given by the user (1) of the printing press (3) or determined by measurement by a color measuring instrument (14).

4. The method according to claim 1 or 2, Its features are, Test samples (8, 9) for the corresponding printing tasks are selected by choosing available test samples (8, 9) or created by computer (4).

5. The method according to claim 4, Its features are, In order to create test samples (8, 9) for a specific printing task or for a specific paper grade, multiple printing color combinations containing opaque white are first printed, and then the computer (4) determines the color combinations related to the color structure based on these printing color combinations in order to create the actual test samples (8, 9).

6. The method according to claim 4, Its features are, The computer (4) determines the color combinations relevant to the test sample by applying a color prediction model to create predictions about the overprinting of each printing color, including opaque white.

7. The method according to claim 1 or 2, Its features are, As a printing press (3), toner printing press is used.

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