Methods for color measurement in a printing press

Abstract

A method for direct color measurement in the printed image of at least two colors used in a printing process in a printing press is described using an optical image acquisition system such as an inline camera, in which a computer defines image areas for color measurements in the digital printed image such that a maximum number of measuring points are located in the same horizontal plane with respect to the direction of travel of the substrate through the printing press.

Description

The present disclosure relates to a method for measuring the color of at least two colors used in a printing process in a printing machine and to a printing machine equipped for carrying out this method. Printing presses can print a variety of products, such as commercial or packaging printing. Modern printing processes often use more than one color, for example, cyan, magenta, yellow, and black, abbreviated CMYK, from which numerous color gradations can be created through color mixing. In addition, special colors, such as metallic colors, can be used, which cannot be produced on the substrate through color mixing. Furthermore, colorless varnishes are used in standard printing processes, which also require a separate application of material and therefore usually a separate printing unit or varnishing unit. Experts are familiar with various printing processes, which are carried out on different types of printing presses. Common printing processes include offset printing, flexographic printing, screen printing, and inkjet printing. A key requirement for many printing jobs carried out on printing presses is maintaining color accuracy and uniformity on the substrate. The finished printed product should have the color specified in the print job and must not deviate from the specified color values, or only within narrow tolerance limits. Printed products that deviate too much from the specified color values ​​are generally unsaleable and must be discarded as waste paper. To ensure that color values ​​remain within tolerance, color values ​​are typically continuously monitored during the execution of a print job on a printing press using color measurement methods on the substrate. For this purpose, printed samples can be removed from the printing process and examined outside the printing press using colorimeters, often handheld devices. Alternatively, one or more colorimeters are permanently installed within the printing press, which regularly perform color measurements on the substrates to ensure compliance with the color values. Another option is to perform color measurements using cameras integrated within the printing press, which are frequently part of an inline image inspection system.This enables continuous monitoring of the moving web or sheets of printed material, allowing for the early detection of errors such as creases and contamination during the printing process. Simultaneously, it allows for monitoring of color values. If the measured color values ​​deviate from the specifications, the printing press is typically adjusted to correct at least one of the deviations. In an offset printing press, for example, this might involve instructing the cyan inking unit to introduce a larger quantity of ink into the corresponding printing unit. For color measurement purposes, colorimetric strips are typically printed on the substrate. These strips are located outside the actual printed image at the edge of the image and are later cut off during post-press processing, thus no longer appearing on the finished printed product. As the substrate passes through the printing press, the colorimeters or cameras detect the colorimetric strip, which usually contains all the monitored colors, and can then determine whether all color values ​​are within the tolerance range. Another method works without color measurement strips by taking the color measurement directly into the actual printed image. The advantage is that no separate color measurement strip is needed, which takes up space, requires printing ink, and later has to be removed as waste. An increasingly desirable approach is color measurement directly within the printed image itself, for which the cameras of the inline image inspection system are used. This combines the advantages of not having to print a color measurement strip with the elimination of a comparatively expensive and, in some cases, slow inline colorimeter. WO 95 / 00335 A1 describes a method for image inspection and color measurement of at least one printed product in which a camera is used and image areas within the printed image are used for color measurement. However, it turns out that color measurement with inline cameras can be problematic. Despite prior calibration of the inline cameras, reliable and rapid color measurement in the printed image is still not possible in many cases. A major problem is ink falloff, the phenomenon where the same color exhibits different color values ​​at different locations on the substrate due to uneven ink application. This leads to variations in ink density in the printing direction on the substrate, because in rotary printing presses, ink removal in the direction of travel of the printing cylinders is not homogeneous, meaning that the same color has different ink densities at different heights on the substrate. As a result, color measurements taken at any point on the substrate are generally not comparable. Therefore, there is a need for further methods for color measurement using camera systems that at least partially overcome the disadvantages of state-of-the-art methods. It has now been found that fast and reliable color measurement with inline cameras is possible by having a computer analyze the digital print image beforehand, select image areas within the digital print image for subsequent color measurements, and define as many of these image areas as possible in the same horizontal plane relative to the direction in which the substrate passes through the printing press. This ensures comparability of the color measurements with regard to color falloff, because a larger portion or all of the colors in the printing process are measured in a single sequence.In the event that, due to the specific appearance of the digital print image, not all colors of the printing process can be defined in a horizontal row to the direction in which the substrate passes through the printing press in the form of image areas to be measured, an auxiliary color measurement field, which includes the missing colors, can be printed outside the print image onto the substrate and used for supplementary color measurements. The method according to the invention offers several advantages. Printing a colorimetric strip onto the substrate is unnecessary, or only required in exceptional cases where not all colors in a row can be measured. A separate inline colorimeter in the printing press is not required because cameras of the inline image inspection system can be used. Color measurement can be performed at high speed due to the use of cameras and the pre-selection of image areas by a computer before the start of the printing job. No or less waste is generated because the colorimetric strip is omitted. Post-processing of the printed product can be simplified because the colorimetric strip does not need to be cut away.The results of the color measurements obtained according to the invention allow not only statements to be made about color fidelity, i.e., any deviation of the measured color value from the specified color value, but also about the homogeneity of the measured color area, and in particular about the uniformity of the amount of ink applied to the substrate. It is therefore possible to assess color values ​​and homogeneity of the color gradients simultaneously, for which, in the prior art, two separate measuring devices are sometimes required. Accordingly, a first aspect of the invention relates to a method for measuring the color of at least two colors used in a printing process in a printing press, in which the digital print image of a print job, comprising the at least two colors, is analyzed by means of a computer in order to define at least two image areas for color measurements in the digital print image, wherein each of the at least two image areas is assigned to a different one of the at least two colors, the digital print image is printed onto a substrate using the printing press, color measurements are taken in the at least two image areas on the substrate by means of at least one optical image acquisition system, and the color values ​​of the at least two image areas are determined from the color measurements, which is characterized in that the computer defines the at least two image areas for color measurements in the digital print image in such a way thatthat a maximum number of the at least two image areas lie in the same horizontal plane in relation to the direction in which the substrate passes through the printing press. The digital print image is the digital version of the image that will later be printed on the substrate. The print order is also in digital form and includes the digital print image. However, in addition to the digital print image, the print order typically includes further information for the printing press, such as print run, image position, ink quantities, and other machine settings. The computer in question can be a prepress hardware computer, which may be integrated into the control panel of the printing press. The image areas defined by the computer within the digital print image can preferably be created in the form of a digital map, on which the image areas are distributed across the digital print image. According to the invention, the computer defines image areas for subsequent color measurements, which can also be referred to as measuring points, in the digital print image such that a maximum number of the at least two image areas lie in the same horizontal plane with respect to the direction in which the substrate passes through the printing press. The at least two image areas defined by the computer thus lie in a row that is perpendicular to the direction in which the substrate passes through the printing press. By definition, the direction in which the substrate passes through the printing press is the direction that the sheet-shaped or web-shaped substrate takes during transport and printing through the printing press. According to the invention, color measurement is carried out in a printing process with at least two colors. In a preferred embodiment, the printing process is selected from offset printing, flexographic printing, screen printing, inkjet printing, and combinations thereof. In offset printing, flexographic printing, and screen printing, each color is preferably printed in a separate printing unit; in inkjet printing, this is preferably done with separate inkjet printheads. In a particularly preferred embodiment, the inventive process is selected from rotary printing, preferably from offset printing, flexographic printing, screen printing, and combinations thereof. The image areas defined according to the invention can all have the same surface area or differ in the size of their areas. When defining the image areas, the computer can take into account whether the color in question is present in the digital print image as a solid color or only in halftone form, i.e., not as a solid color. In a preferred embodiment, the computer adds at least one auxiliary color measurement field to the print job if not all of the at least two image areas can be defined in the same horizontal plane with respect to the direction in which the substrate passes through the printing press. In a particularly preferred embodiment, if the at least one auxiliary color measurement field is actually at least two auxiliary color measurement fields, these at least two auxiliary color measurement fields are added to the print job in the same horizontal plane with respect to the direction in which the substrate passes through the printing press, preferably in the same horizontal plane in which the at least two image areas defined by the computer lie, or in a plane arranged parallel to it.In a particularly preferred embodiment, at least one auxiliary color measuring field is arranged outside the digital printed image on the substrate, for example at the edge of a substrate sheet. Preferably, the ink is applied—in rotary printing presses by printing cylinders, in inkjet printing presses by inkjet printheads—also in a horizontal plane relative to the direction in which the substrate passes through the printing press. Thus, the plane of ink application via the printing cylinders and / or inkjet printheads and the plane of the image areas defined for subsequent color measurements are preferably parallel or nearly parallel to each other. Therefore, in a preferred embodiment of the inventive method, the ink is applied during the printing of the digital image onto the substrate by means of one or more printing cylinders and / or one or more inkjet printheads perpendicular to the direction in which the substrate passes through the printing press.Accordingly, the rotation axes of the printing cylinders are preferably arranged parallel to the horizontal plane of the defined image areas; the same preferably applies to the spatial arrangement of any inkjet printheads that may be present. Within the at least one auxiliary color measuring field, the colors present therein, if there are two or more colors, are preferably arranged in the same horizontal plane in relation to the printing direction of the substrate through the printing machine. In a preferred embodiment, the color measurements in the at least two image areas provide information about the color fidelity and / or color homogeneity of the colors measured in the at least two image areas. If color measurements of image areas and / or auxiliary color measurement fields are performed in different, for example, two, horizontal planes relative to the direction in which the substrate passes through the printing press, it is also possible to convert the color values ​​between the different horizontal planes to ensure comparability. For example, the at least two image areas defined by the computer can lie in one horizontal plane, and the at least two auxiliary color measurement fields, or even just a single auxiliary color measurement field, can lie in another horizontal plane relative to the direction in which the substrate passes through the printing press. Advantageously, this conversion can be performed by the computer based on predefined conversion parameters, which may have been determined beforehand, for example, through calibrated comparative measurements. In another preferred embodiment, each of the at least two defined image areas is assigned a different color for color measurements, either in solid or halftone form. The halftone form is generally a regular pattern. In a further preferred embodiment, all colors used in the printing process in the printing press are included, along with all image areas and all auxiliary color measurement fields. In another preferred embodiment, image acquisition is performed line by line by the at least one optical image acquisition system. In a further preferred embodiment, the at least one optical image acquisition system is at least one digital camera, preferably at least one digital RGB camera. In the method according to the invention, one or more digital RGB line scan cameras can preferably be used. In a further preferred embodiment, image acquisition in the inventive method is carried out by at least one digital RGB camera, which is arranged within the printing press in the form of one or more inline cameras and captures the color values ​​in the form of RGB color values. From the captured RGB color values, the corresponding Lab color values ​​can be precisely calculated after prior color calibration of the at least one digital RGB camera, preferably using a highly accurate colorimeter. This calculation is preferably performed in the computer used to define the at least two image areas to be measured or directly in the control hardware of the digital RGB cameras.This conversion of color values ​​from the RGB color space to the Lab color space makes it possible to compare the measured color values ​​with the specified color values ​​of the print job by the computer in a particularly simple and advantageous way. In a further preferred embodiment, the computer uses the determined color values ​​of the at least two image areas to readjust the color control of the printing press in order to correct the deviation of the determined color values ​​from the color values ​​specified in the print job. In the case of an offset printing press, this could, for example, involve adjusting the amount of ink supplied by the inking unit to the corresponding offset printing unit.Since the inventive method allows for statements about the color fidelity and / or color homogeneity of the colors measured in at least two image areas using color measurements in those at least two image areas, it is also possible for the computer to adjust the color control of the printing press based on the determined color values ​​of the at least two image areas in order to correct any deviation of the determined color homogeneity from the color homogeneity specified in the print job. Thus, deviations with respect to both color values ​​and color homogeneity can be corrected. If a correction of these color parameters is not possible, for example, because the deviations are too large, a corresponding acoustic and / or visual warning can be issued to the operator of the printing press. Another aspect of the invention relates to a printing press configured for carrying out the method according to the invention. In a preferred embodiment, the printing press is selected from offset printing presses, flexographic printing presses, screen printing presses, inkjet printing presses, and combinations thereof, particularly preferably from offset printing presses, flexographic printing presses, screen printing presses, and combinations thereof. The printing press according to the invention comprises at least one optical image acquisition system, preferably at least one digital camera. Fig. 1 schematically shows a preferred embodiment of the method according to the invention. This preferred embodiment comprises the following steps: providing the print job, analyzing the digital print image in the computer, defining image areas for color measurements, sending the print job to the printing press, executing the print job in the printing press, performing color measurements in defined image areas, and determining color value deviations from the color measurements. Fig. 2 schematically shows the sequence of a preferred embodiment of the method according to the invention. A print job (3) is provided and sent to the computer (4), which is connected to the printing press (1) via a communication link. The computer (4) identifies image areas (5) in the digital print image (2) in which the color measurements are to be taken later. The image areas (5) are defined by the computer (4) such that as many as possible lie in the same horizontal plane (8), indicated by the dashed line, with respect to the direction in which the substrate (6) passes through the printing press (1). Typically, the horizontal plane (8) is arranged at a right angle or an almost right angle with respect to the direction in which the substrate (6) passes through the printing press (1).The horizontal plane (8) is typically positioned at a right angle or nearly a right angle to the leading edge of the substrate (6), which may be in the form of a sheet. The digital print image (2) is then transmitted to the printing press (1) as a print job (3) containing the image areas (5), and the print job (3) is executed in the printing press (1). For this purpose, a substrate (6) is printed with the digital print image (2) within the printing press (1), for example, using at least one offset printing unit. Color measurement is then performed using the optical image acquisition system (7), which may be, for example, an inline camera. The color measurement data is sent back to the computer (4), which then corrects any color deviations for subsequent substrates (6) by adjusting the settings of the at least one offset printing unit.The printed substrates (6) are finally removed from the printing machine (1). Fig. 3 schematically shows a sheet of substrate (6) inside the printing press (1) as it is transported through the printing press (1) during the method according to the invention. The arrow (9) indicates the direction in which the substrate passes through the printing press (1). The image areas (5) are arranged on the substrate (6) in a horizontal plane (8), indicated by the dashed line, with respect to the direction of travel (9). The ink is typically applied—by printing cylinders in rotary printing presses and by inkjet printheads in inkjet printing presses—also in a horizontal plane (8) with respect to the direction in which the substrate (6) passes through the printing press (1). Thus, all image areas (5) lie in the same plane, which is at a right angle or an almost right angle with respect to the direction of travel (9). Reference symbol list 1 Printing press 2 Digital print image 3 Print job 4 Computer 5 Image area 6 Substrate 7 Optical image capture system 8 Horizontal plane 9 Direction of substrate flow through the printing press

Claims

A method for measuring the color of at least two colors used in a printing process in a printing press (1), in which the digital print image (2) of a print job (3), comprising the at least two colors, is analyzed by means of a computer (4) in order to define at least two image areas (5) for color measurements in the digital print image (2), wherein each of the at least two image areas (5) is assigned to a different one of the at least two colors, the digital print image (2) is printed onto a substrate (6) using the printing press (1), color measurements are taken in the at least two image areas (5) on the substrate (6) by means of at least one optical image acquisition system (7), and the color values ​​of the at least two image areas (5) are determined from the color measurements, characterized in that the computer (4) defines the at least two image areas (5) for color measurements in the digital print image (2) in such a way thatthat a maximum number of the at least two image areas (5) lie in the same horizontal plane (8) with respect to the direction of travel (9) of the substrate (6) through the printing machine (1). Method according to claim 1, wherein the computer (4) adds at least one auxiliary color measurement field to the print job (3) if not all of the at least two image areas (5) can be determined in the same horizontal plane (8) with respect to the direction of travel (9) of the substrate (6) through the printing machine (1). Method according to claim 2, wherein in the case that the at least one auxiliary color measuring field is at least two auxiliary color measuring fields, the at least two auxiliary color measuring fields are added to the printing job (3) in the same horizontal plane (8) with respect to the direction of passage (9) of the substrate (6) through the printing machine (1). Method according to one of claims 2 or 3, wherein the at least one auxiliary color measurement field is arranged outside the digital print image (2) on the substrate (6). Method according to one of the preceding claims, wherein, in the color measurements in the at least two image areas (5), statements are made about the color fidelity and / or color homogeneity of the colors measured in the at least two image areas (5). Method according to one of the preceding claims, wherein each of the at least two defined image areas (5) is assigned a different color in solid tone or in rasterized form for color measurements. Method according to one of the preceding claims, wherein all color fields (5) and all color measurement fields include all colors used in the printing process in the printing machine (1). Method according to one of the preceding claims, wherein the ink application during the printing of the digital print image (2) with the printing machine (1) onto the substrate (6) is carried out by means of one or more printing cylinders and / or by means of one or more inkjet printheads perpendicular to the direction of travel (9) of the substrate (6) through the printing machine (1). Method according to one of the preceding claims, wherein the image acquisition is performed line by line by the at least one optical image acquisition system (7). Method according to one of the preceding claims, wherein the at least one optical image acquisition system (7) is at least one digital camera. Method according to one of the preceding claims, wherein the computer (4) performs a readjustment of the color control of the printing machine (1) on the basis of the determined color values ​​of the at least two image areas (5) in order to correct the deviation of the determined color values ​​from the color values ​​specified in the print job (3). Printing press (1), equipped for carrying out the method according to any one of claims 1 to 11.

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