Angle-dependent color correction

Abstract

Methods for recording color measurement values ​​( α F αα , 0F 00 ) on printing materials (2) by means of a color sensor (1), wherein color measurement values ​​( α F αα , 0F 00 ) on the printing materials (2) under different viewing angles (α) of the color sensor (1), wherein the angular dependence of the color measurements taken on the substrates (2) under different observation angles (α) α F αα , 0F 00 ) is corrected in a computer (3), characterized by that the correction function is determined using a color model which, based on a known spectrum at a known color layer thickness, calculates the spectrum expected at a different color layer thickness.

Description

[0001] The present invention relates to a method for detecting color measurement values ​​on printing materials using a color sensor, wherein color measurement values ​​on the printing materials are detected under different viewing angles of the color sensor.

[0002] For quality control of printing materials produced on printing presses, their color is measured using a colorimeter, and the measurements are compared with the corresponding values ​​of a printing template. If there is too great a deviation between the measurements on the printing materials and the values ​​of the printing template, the printing materials are considered waste. This waste must be avoided as much as possible, as it incurs unnecessary costs and cannot be sold. While in the past, printing materials were measured outside the printing press, for which a sample sheet was removed from the press at specific intervals, so-called inline color measurement systems are now becoming increasingly common. These systems are integrated into a printing unit of the printing press and measure the printing materials during the printing process, preferably in the last printing unit. In this way, it is possible to perform color measurements on every printing material.Due to high printing speeds, substrates are currently not measured across their entire surface in the printing press, but only within the area of ​​a print control strip, which contains corresponding color measurement fields. Cameras are increasingly being used as color measuring devices, as they are more cost-effective than spectrometers. Cameras naturally have a lens with a wider field of view, and it must be ensured that identical colors on the substrate are measured regardless of the viewing angle, i.e., the position of the color area to be measured on the object line. This means that the recorded color values ​​must be independent of the position of the measured color area. However, this is not the case here for various reasons.Firstly, the lenses of such cameras are not fully color-corrected, or there are color filters applied to the pixels of the camera's CCD matrix, which absorb the captured spectra differently when illuminated from different angles.

[0003] If the aforementioned camera is used as a color measurement and color control system, color values ​​measured at different angles are not comparable, even with identical coloring of the original. Therefore, reliable quality control of the coloring on printed materials is not possible with existing camera systems.

[0004] The German patent application EP 1 694 048 A1 relates to a colorimeter with one color measurement channel and a control unit that generates color measurements from the measurement signals of the color measurement channel and stored white calibration data. Correction mechanisms are also provided to calculate or correct color measurements depending on different distances or angles to the object being measured. In this way, the colorimeter can correct for fluctuating distances and angles relative to the object being measured.

[0005] German patent application DE 10 2008 010 562 A1 discloses a method for instrument calibration of multi-angle colorimeters, in which a test device is profiled with a reference device. A first data set of initial remission data measured by the reference device and a second data set of subsequent remission data measured by the test device are recorded. A mathematical relationship between the two data sets is established using a statistical analysis method. This allows for the correction of remission data subsequently measured by the test device. The data sets recorded by the reference device and the test device are acquired as a function of different measurement angles and corrected using angle-dependent data.

[0006] German patent application DE 10 2004 046 461 A1 discloses a method for determining the paint quality of a painted surface on vehicle body parts. In a first step, a colorimeter is used to determine a plurality of angle-dependent brightness values ​​for a surface coated with a test paint under defined illumination. In a second step, a computer is used to calculate a characteristic value that characterizes the test paint with respect to its paint quality. In a third step, this characteristic value is compared to at least one reference value or range stored in a memory device, which is defined according to Flop effect parameters.

[0007] German patent application DE 10 2004 021 599 A1 relates to inline measurement of printed sheets during the printing process. Due to the inherent instability of the sheets, measurement deviations occur during printing compared to external reference measurements. To obtain color measurement values ​​compatible with external reference measurements, these measurement errors, caused by ink splitting and drying effects of the sheets, are corrected by a combination of metrological and computational correction measures. The measurement correction is based on a model that describes the measurement errors as a combination of three error contributions caused by surface effects, layer thickness modulation, and light trapping.

[0008] It is therefore an object of the present invention to provide a method for detecting color measurements on printed materials using a color sensor, in which color measurements on printed materials can be detected under different viewing angles of the color sensor, which are nevertheless comparable with each other.

[0009] The problem is solved according to the invention by claim 1. Further embodiments of the invention can be found in the dependent claims and the drawings.

[0010] The method according to the invention is particularly suitable for measuring the color of printing materials using RGB cameras in the printing press. To ensure the accuracy of color measurements taken from different viewing angles of the RGB camera, the RGB camera is connected to a computer, which computationally corrects the angular dependence of the color measurements taken from the printing materials at different viewing angles. This computational correction makes it possible to compare color measurements taken from different viewing angles and thus to check the print quality of the printing materials. This is necessary because the camera performs many color measurements on the printing material in different areas of the material and therefore at different viewing angles.

[0011] In a first embodiment of the invention, the correction of the angular dependence is performed by the computer using a correction function. A color value is generated by convolving a color, a spectrum, with an angle-dependent spectral value function. This angle-dependent function is then corrected in the computer by a correction function so that the color measurements are ultimately angle-independent. The correction function is therefore a mathematical formula that eliminates the angular dependence of the acquired color measurements. Such a correction function can be determined by storing the spectrum of a color to be measured for at least one angle in the computer. Preferably, this is the spectrum at angle α = 0. This spectrum is then converted, using the correction function, into a spectrum adapted to the respective camera angle.In a further embodiment of the invention, so-called angle-dependent spectral value functions are used for this purpose. These angle-dependent spectral value functions can, in turn, be generated in several ways. To eliminate the angle dependency, the angle-dependent spectral value functions must be converted into angle-independent spectral value functions, with the conversion depending on the current observation angle. The angle dependency can be eliminated by a one-time measurement of the camera in the laboratory, and the recorded measurements can be stored as angle-dependent correction values ​​in the computer.

[0012] According to the invention, the correction function is determined using a color model that calculates the expected spectrum for a different color layer thickness based on a known spectrum for a known color layer thickness. In this way, it is possible to approximate the spectral reflectance (spectrum) not measured at a different angle α using the spectral reflectance determined and stored once at angle α = 0, via the color model on the computer. This color model calculates the expected spectrum for a different color layer thickness based on a known spectrum for a known color layer thickness.

[0013] In a further embodiment of the invention, the computer is connected to a printing press, which controls the coloring in the printing press's inking units. The color measurement values ​​of the camera in the color measurement system, corrected by the inventive method, can then be compared with the corresponding values ​​of the printing template. In case of deviations, the computer can intervene accordingly in the control of the printing press's inking units and compensate for deviations between the measured actual values ​​and the target values ​​of the printing template by adjusting the color control accordingly.

[0014] As mentioned earlier, an RGB camera can be used as a color sensor. This sensor can be implemented as an inline measuring device within the printing press, as an offline measuring device outside the printing press with a separate measuring table, or even as a handheld device. For calibrating the color sensor, especially when using an RGB camera, a spectral measuring head can also be used. This head captures at least some of the measurement points recorded by the RGB camera, and its readings are used to calibrate the RGB camera. Using the calibrated RGB camera, a large area of ​​the printed material can then be measured for color within the printed image without further assistance from the spectral measuring head.

[0015] Furthermore, the color sensor is designed to detect several identical color patches on a substrate from different viewing angles. In both sheet-fed substrates and printed webs in offset printing presses, it is common practice to have so-called color measurement strips in the edge area, which comprise periodically applied color measurement patches in several colors and shades of gray. The color sensor is thus enabled to detect several color measurement patches of the same color simultaneously. However, since these color measurement patches are located at different positions within the color measurement strip, they are consequently detected by the color sensor from different viewing angles. Due to the angle correction according to the invention, these detected color measurements can nevertheless be corrected according to their different viewing angles and recorded as reliable actual values ​​for comparison with the print template in the computer.

[0016] The invention is described and explained in more detail below with reference to a figure. It shows: Figure shows an offset sheet-fed printing machine with a connected computer and an connected RGB camera, which takes color measurements on a substrate from different viewing angles.

[0017] The figure schematically depicts a sheet-fed offset printing press 4. The sheet-fed offset printing press 4 consists, by way of example, of two printing units, a feeder, and a delivery unit. Furthermore, the printing press 4 is connected to a computer 3, which also serves as the control computer for the printing press 4. An RGB camera 1 is connected to the computer 3 as a color sensor, which in this case captures sheet-shaped substrates 2. In the figure shown, the substrates 2 are measured for color outside the printing press 4. However, it is also possible for the camera 1 to be arranged in one of the printing units, preferably in the last printing unit of the printing press 4, and to capture the substrates 2 directly within the printing press. The figure shows that two color areas 5 of the same color are captured from different viewing angles α.The right colored area 5 is captured at an observation angle α=0, with a corresponding color value 0F. 00 is captured by camera 1. The left colored area 5 is captured at the observation angle α≠0, so that a different color value is obtained. α F α0 The color is captured. Due to the different viewing angle α, the two color values ​​differ from each other.

[0018] For comparison with the print template, it is important that the two color values ​​are the same, as the differences result solely from the different viewing angle and are not reflected in the actual color. Therefore, the angle-dependent color value must be... α F α0 They must be corrected accordingly. In both cases, the spectral remission β0 is the same. While the spectral remission β0 is present in the right case, a different spectral remission β is present in the left case. αHowever, computer 3 only knows the spectral reflectance β0, which is by definition angle-independent. Therefore, the angular dependence of β must be determined. α This can be replaced by a correction function that incorporates the only known spectral reflectance β0. The spectral reflectance β0, in turn, can be determined by a single spectrophotometric measurement of the corresponding ink area during a print job at an observation angle α=0. This procedure will be explained in more detail below. Color measurement: Angle dependence of the color value

[0019] An identical remission of a color β0 is not converted into identical color values ​​F by the camera 1 depending on the observation angle α.

[0020] The following applies to the nomenclature: α F πσ : Color value at position α generated by spectral value function and spectrum β σwith F ≡ {R, G, B} as the trivalent color value. The reason is the angular dependence of the camera's spectral function.

[0021] Generally speaking: βα≠β0

[0022] Angle independence, on the other hand, means: identical spectral value functions for all angles. must go through The position α=0 is defined as angle-independent, since it represents the reference and the common datum. Color measurement: Angle correction of the color value

[0023] The problem is that in general β α is unknown. However, the following are known: α F αα and 0F 00 by current measurement of the two colors with camera 1 by a one-time measurement of camera 1 in the manufacturer's laboratory β0 by spectrophotometric measurement of the substrate 2 during the printing process

[0024] Ultimately, the goal is to find the color value F of the color β. α measured at angle α, but evaluated using the angle-independent spectral value function :

[0025] Since β αwhich is generally unknown, β must be α represented by β0: β α ← β0(s) with s as relative layer thickness according to a color model

[0026] Determine s such that |αFαα−αFα0(s)|→min

[0027] Replace after that

[0028] The quality measure of the procedure "representation of the unknown remission β" α The following criterion serves as the basis: dRGB=|αFαα−αFα0(s)|→min

[0029] As a measure of the quality of the "replacement" procedure α F αα through α F 00 (s)“ with the remission β0 measured (for this purpose) serves as the criterion: dRGB=|αF0α−αF00(s)|→min

[0030] The central idea of ​​this embodiment is to measure the unmeasured remission β αThe measured remission β0 is approximated using a color model. This color model is state of the art and functions to calculate the expected spectrum at a different layer thickness based on a known spectrum and a known layer thickness. This allows for the computational reproduction of similar colors as a spectrum to a practical extent, using the free parameter of a relative layer thickness. This substitution is controlled by the quality measure "representation of the unknown remission β". α “ , which determines the similarity of the measured to the calculated color value at position α. ​​The color value resulting from the remission at position α and from the spectral value function 7 at the same position is replaced by the color value of the approximated spectrum and by the spectral value function 7 to be used at all positions. Reference symbol list 1 camera 2. Printing material 3 computers 4 printing press 5 color area β0 spectral remission α F αα Color value, measured at angle α 0F 00 Color value, measured at an angle = 0 α observation angle 7 Spectral value function

Claims

[1] Methods for recording color measurement values ​​( α F αα , 0F 00 ) on printing materials (2) by means of a color sensor (1), wherein color measurement values ​​( α F αα , 0F 00 ) on the printing materials (2) under different viewing angles (α) of the color sensor (1), wherein the angular dependence of the color measurements taken on the substrates (2) under different observation angles (α) α F αα , 0F 00 ) is corrected in a computer (3), characterized by , that the correction function is determined using a color model which, based on a known spectrum at a known color layer thickness, calculates the spectrum expected at a different color layer thickness. [2] Method according to claim 1, characterized by , that the correction of the angular dependence is carried out by the computer (3) using a correction function. [3] Method according to claim 2, characterized by , that the spectrum of the measured color (β0) is known to the computer (3) for at least one angle. [4] Method according to any of the preceding claims, characterized by , that the correction of the angle dependence in the computer (3) by taking into account angle-dependent spectral value functions ( ). [5] Method according to claim 4, characterized by , that the angle-dependent spectral value functions ( ) created by a one-time measurement of the color sensor (1) and stored in the computer (3). [6] Method according to any of the preceding claims characterized by , that the color sensor (1) is an RGB camera. [7] Method according to any of the preceding claims, characterized by , that the computer (3) is connected to a printing press (4) and controls the coloring in the printing press's inking units (4). [8] Method according to claim 6 or 7, characterized by, that the color sensor (1) is arranged in the printing machine (4). [9] Method according to any of the preceding claims, characterized by , that the color sensor (1) detects several identical color fields (5) on a substrate (2) under different viewing angles.

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