Design and preparation method and system of special color code for camera color correction for detecting spot color of laser printing product

Abstract

The invention provides a color code design and preparation method and system special for camera color correction for detecting the spot color of a laser printing product, and relates to the technical field of printing, and the method comprises the steps: setting a stepping value according to a basic spot color and a ground white ink dot value, and generating a dot combination value; generating corresponding digital color bars and arranging the digital color bars into a page image; printing and outputting a color bar printed product; measuring a chromatic value and calculating a chromatic aberration, a lightness difference, a saturation difference and a hue angle difference with the target spot color; color bars with the color difference smaller than or equal to 6.3 and uniform color difference size and parameter positive and negative distribution are screened and fixed to the black rough surface base material to form the special color code. The color detection device is matched with a laser paper base material and is used for spot color printing online detection of cigarette packets, cosmetics and the like, and the color detection precision and the detection efficiency are improved.

Description

Technical Field

[0001] The present invention relates to the field of printing technology, and in particular to a method and system for designing and preparing a special color target for camera color calibration for detecting spot colors of laser-printed products. Background Art

[0002] The traditional printing industry is developing towards automation and intelligence. In terms of printing color control, the online control of the printed product color is not only an important aspect, but also a technical direction that is not yet mature but urgently needed by users.

[0003] The online control of the printed product color includes a series of processes such as chromaticity detection of the product color, processing and negative feedback of chromaticity information, and automatic adjustment of corresponding signals of the equipment. Among them, the accurate detection of the chromaticity information of the product color is the key technical problem that needs to be solved first.

[0004] In the field of cigarette package printing with laser paper as the base material, there are often large areas of uniform colors on the packaging printed products, which are printed with 1 or 2 - 3 kinds of spot color inks pre-mixed into specific colors and white base ink, and can obtain more vivid colors that cannot be included in the normal CMYK four-color printing gamut (the range of printable colors), so as to improve the visual effect and overall quality of the product. Since it cannot be printed by normal CMYK four-color printing, the large-area uniform color on the packaging printed product printed with 1 or 2 - 3 kinds of spot color inks is called "product spot color" here, and the 1 or 2 - 3 kinds of spot colors pre-mixed into specific colors that make it up are called "basic spot colors".

[0005] During the printing process, due to fluctuations in the control of process parameters such as printing pressure, ink volume, and water volume, fluctuations in the color of printed products are almost inevitable. When there is a large deviation in the product color, experienced operators often manually adjust the ink volume of the corresponding color on the machine control panel. Further, samples are taken at intervals at the product outlet, the color of the extracted samples is measured with an instrument, and then the ink volume is finely adjusted based on this; the fine adjustment is repeated several times to achieve the consistency between the product spot color and the target color (usually required to be no greater than 1 ΔE * ab color difference). This is the commonly used method for color control in the printing process in the current industry. Obviously, the sampling detection method will inevitably cause untimely detection results and an increase in unqualified products, thus having an adverse impact on the improvement of product qualification rate and production efficiency.

[0006] Analyzing the above color control process, it is not difficult to find that large color deviations are easy to adjust, while correcting small color deviations is the key to solving the problem. Therefore, how to obtain the accurate chromaticity of the product spot color in a timely manner is the primary task in the online color control during the cigarette package spot color printing process.

[0007] Online color control first requires rapid color detection, which cannot be satisfied by traditional spectrophotometers. Therefore, realizing the online detection of printed matter color by means of the imaging and image information processing technology of digital cameras has become a most likely detection method. In fact, the online detection of printing defects based on machine vision, that is, digital camera imaging, has been applied in the industry, indicating the applicability of digital camera imaging and image processing technology to online quality detection. At present, the online detection of printed matter color quality has not been broken through, mainly due to insufficient color detection accuracy, rather than problems with the camera imaging and image processing system itself.

[0008] For the digital camera to take an image of the printed matter, the directly obtained image information is the RGB data formed by the camera's responses to the red, green, and blue components contained in the reflected light of the printed matter. Affected by the photoelectric performance of the camera itself, the RGB data does not correspond to the objective chromaticity value of the printed matter color. If the RGB value of the image is to reflect the chromaticity value of the photographed color, the technical methods that can be adopted are: 1) Calibrate the color of the digital camera, that is, the camera takes an image of a color target (a reflected color sample composed of several colors), and establishes the conversion relationship between the RGB value of the color target color imaging and the CIE chromaticity value of the color target color, which is called the color calibration relationship. This process is also simply called color calibration. 2) Take an image of a color with unknown chromaticity, obtain its imaging RGB value, and further calculate the CIE chromaticity value of this color from the color calibration relationship of the camera. This technical idea is also the method of the mature ICC color management technology. If this technical method is used to realize the color calculation of the printed matter, it can be used in the online color detection of the printed matter. However, when implementing it in the color detection of laser cigarette packages, there is still a restrictive bottleneck problem, that is, there is no color target for digital camera color calibration with laser paper as the substrate and suitable for the high-precision requirements of product spot color calibration (ΔE * ab color difference not greater than 1). The existing standard color targets are not based on laser paper substrates, and the color calibration accuracy during application cannot guarantee ΔE * ab color difference not greater than 1). Therefore, designing and preparing a special color target for digital camera color calibration that meets the application requirements has become the primary task for the online color control of spot color printing of cigarette package products. Summary of the Invention

[0009] In order to overcome the deficiencies of the prior art, the purpose of the present invention is to provide a method and system for designing and preparing a special color target for digital camera color calibration for detecting spot colors of laser printing products, which can obtain a color target for product spot color detection with high color calibration accuracy. It can not only meet the high-precision requirements for the color detection of large-area spot colors in laser printing, but also use a simple color calibration algorithm, greatly improving the color detection speed and being suitable for online detection requirements.

[0010] To achieve the above object, the present invention provides the following solutions:

[0011] A method for designing and preparing a special color target for camera color calibration for detecting spot colors of laser-printed products, comprising:

[0012] Setting a step value according to the initial dot values of the base spot color and the under white ink of the laser-printed product spot color, and respectively increasing and decreasing the step value within the range of available values to generate dot combination values of all base spot colors and under white ink;

[0013] Generating a corresponding digital color bar for each of the dot combination values, and arranging all the digital color bars into a page image;

[0014] Preparing a printing plate based on the page image, and printing and outputting on a laser paper substrate using the corresponding base spot color ink and under white ink to form a color bar printed product;

[0015] Measuring the chromaticity values of each of the digital color bars in the color bar printed product, and calculating the color difference ΔE * ab between the chromaticity value and the corresponding product spot color, the lightness difference ΔL * the saturation difference ΔC * ab and the hue angle difference Δh * ab ;

[0016] Among all the digital color bars, screening out the digital color bars with a color difference ΔE * ab ≤6.3 and the size of ΔE * ab and the positive and negative distributions of ΔL * ΔC * ab Δh * ab are all uniform, arranging them in an orderly manner and fixing them on a black matte substrate to form a special color target for camera color calibration.

[0017] Preferably, the range of the initial dot value is (0%, 100%]; the step value is not greater than 10%.

[0018] Preferably, the range of increasing and decreasing the step value includes at least 1 times the step value and 2 times the step value.

[0019] Preferably, when the initial dot value is 100%, only decrease it according to the step value, and when the initial dot value is between (0%, 100%), increase and decrease it respectively according to the step value.

[0020] Preferably, the length of the digital color bar > 5 cm; the width of the digital color bar > 1 cm; the length of the digital color bar covers a complete cycle in the transverse direction of the laser paper light column, and the width of the digital color bar meets the requirements of the measuring aperture of a preset spectrophotometer.

[0021] Preferably, the ink volume conditions for the printing output include normal printing ink volume and an increased ink volume of 5% - 10%; the increased ink volume is used to generate digital color bars with a brightness lower than the product spot color.

[0022] Preferably, the chromaticity value is measured using an integrating sphere spectrophotometer, and at least 7 measurement points are evenly selected along the length direction of the digital color bar, and the sampling interval × the number of measurement points ≥ one complete cycle in the transverse direction of one laser paper light column;

[0023] Preferably, the color difference ΔE * ab is calculated by the formula:

[0024]

[0025] wherein, ΔL * , Δa * , Δb * are respectively the brightness difference, the difference on the red - green chromaticity axis, and the difference on the yellow - blue chromaticity axis between the digital color bar and the product spot color.

[0026] Preferably, the number of selected digital color bars is not less than 30.

[0027] A special color target design and preparation system for camera color calibration of laser - printed product spot colors, comprising:

[0028] A dot combination value generation unit, configured to set a step value according to the initial dot values of the base spot color and the under - white ink of the laser - printed product spot color, and increase and decrease the step value respectively within the available value range to generate dot combination values of all the base spot colors and the under - white ink;

[0029] An image arrangement unit, configured to generate a corresponding digital color bar for each of the dot combination values and arrange all the digital color bars into a page image;

[0030] A color bar printing unit, configured to prepare a printing plate based on the page image, and print and output on a laser paper substrate using the corresponding base spot color ink and under - white ink to form a color bar printed product;

[0031] An index calculation unit, configured to measure the chromaticity values of each digital color bar in the color bar printed product, and calculate the color difference ΔE * ab , brightness difference ΔL * , saturation difference ΔC *ab and hue angle difference Δh * ab ;

[0032] A color scale preparation unit for screening out color difference ΔE among all the digital color bars * ab ≤6.3 and ΔE * ab size and ΔL * 、ΔC * ab 、Δh * ab Digital color bars with uniform positive and negative distributions of size and ΔL, ΔC, and Δh are arranged in an orderly manner and fixed on a black matte base material to form a special color scale for camera color calibration.

[0033] According to the specific embodiments provided by the present invention, the following technical effects are disclosed:

[0034] The present invention is mainly aimed at the packaging and printing industry with laser paper as the printing substrate, and designs and prepares special color scales for on-line chromaticity detection of spot colors in the spot color printing of the outer packaging of cigarettes, wines, cosmetics, and electronic products, which can provide a relevant technical tool for on-line color detection of spot color printing of laser products based on camera imaging technology, and can improve the color detection accuracy and detection efficiency of spot color printing. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required to be used in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0036] Figure 1 It is a flowchart of the method provided by the embodiment of the present invention;

[0037] Figure 2 It is a page image of 48 basic spot color combination color bars in the embodiment of the present invention.

[0038] Figure 3 It is a comparison schematic diagram of ΔE color difference between 192 printed color bars and product spot colors in the embodiment of the present invention. * ab It is a comparison schematic diagram of ΔL lightness difference between 192 printed color bars and product spot colors in the embodiment of the present invention.

[0039] Figure 4 It is a comparison schematic diagram of ΔL lightness difference between 192 printed color bars and product spot colors in the embodiment of the present invention. * It is a comparison schematic diagram of ΔL lightness difference between 192 printed color bars and product spot colors in the embodiment of the present invention.

[0040] Figure 5ΔC between the 192 color bars in the printing output and the spot color of the product in the embodiments of the present invention * ab Schematic diagram of the comparison of saturation differences

[0041] Figure 6 Δh between the 192 color bars in the printing output and the spot color of the product in the embodiments of the present invention * ab Schematic diagram of the comparison of hue angle differences

[0042] Figure 7 ΔE between the 49 color bars selected in the embodiments of the present invention and the spot color of the product * ab Schematic diagram of the comparison of color differences

[0043] Figure 8 ΔL between the 49 color bars selected in the embodiments of the present invention and the spot color of the product * Schematic diagram of the comparison of lightness differences

[0044] Figure 9 ΔC between the 49 color bars selected in the embodiments of the present invention and the spot color of the product * ab Schematic diagram of the comparison of saturation differences

[0045] Figure 10 Δh between the 49 color bars selected in the embodiments of the present invention and the spot color of the product * ab Schematic diagram of the comparison of hue angle differences

[0046] Figure 11 Color target image captured by the camera in the embodiments of the present invention

[0047] Figure 12 Schematic diagram of the RGB value extraction area of the color target in the embodiments of the present invention

[0048] Figure 13 Conversion color difference ΔE of the color of the color target in the embodiments of the present invention * ab Distribution schematic diagram Detailed implementation manners

[0049] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0050] The object of the present invention is to provide a method and system for designing and preparing a special color target for camera color calibration for detecting spot colors of laser-printed products, so as to obtain a special color target for product spot color detection with high color calibration accuracy. It can not only meet the high-precision requirements for detecting large-area spot colors in laser printing, but also use a simple color calibration algorithm, greatly improving the color detection speed and being suitable for on-line detection requirements.

[0051] To make the above objects, features, and advantages of the present invention more obvious and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0052] Figure 1 For the method flowchart provided by the embodiment of the present invention, as Figure 1 shown, the present invention provides a method for designing and preparing a special color target for camera color calibration for detecting spot colors of laser-printed products, including:

[0053] Step 100: Set a step value according to the initial dot values of the base spot color and the bottom white ink of the laser-printed product spot color, and increase and decrease the step value within the available value range respectively to generate the dot combination values of all base spot colors and bottom white inks;

[0054] Step 200: Generate corresponding digital color bars for each dot combination value, and arrange all digital color bars into a page image;

[0055] Step 300: Prepare a printing plate based on the page image, and print and output on a laser paper substrate using the corresponding base spot color ink and bottom white ink to form a color bar printed product;

[0056] Step 400: Measure the chromaticity values of each digital color bar in the color bar printed product, and calculate the color difference ΔE * ab between the chromaticity value and the corresponding product spot color, the lightness difference ΔL * the saturation difference ΔC * ab and the hue angle difference Δh * ab ;

[0057] Step 500: Among all digital color bars, select the digital color bars with a color difference ΔE * ab ≤6.3 and the size of ΔE * ab and the positive and negative distributions of ΔL * ΔC * ab Δh * ab are all uniform, arrange them in an orderly manner and fix them on a black matte substrate to form a special color target for camera color calibration.

[0058] As an alternative implementation, the technical route of this embodiment includes the following steps:

[0059] (1) Design a certain-sized step value based on the printing dot values of 1 to 3 base spot colors (inks) and undercoat white (ink) set for the spot colors of laser-printed products (the large-area spot colors appearing in laser-printed products). Based on this dot value, within the available value range, increase and decrease one to several step values respectively and combine them with each other to form all the combined dot values of the base spot colors and undercoat white within a certain range;

[0060] (2) For each combined dot value of the base spot color and undercoat white, form a digital "color" bar (subsequently written as a digital color bar) with a length greater than one laser change cycle and a width not less than the measurement aperture of the color measuring instrument. The digital color bars corresponding to all the combined values are arranged into a page image;

[0061] (3) For each base spot color image information in the base spot color page image, prepare a separate printing plate respectively, and print and output it on a laser paper substrate with an appropriate ink amount output condition using the corresponding base spot color ink to form a color bar printed product of the combined base spot color and undercoat white ink;

[0062] (4) For each color bar on the color bar printed product of the combined base spot color and undercoat white ink, measure its CIEL * a * b * values under the condition of D65 / 10° (D65 standard light source and CIE1964 standard observer function) within one laser cycle, and take the average value as the chromaticity value of this color bar. Further, calculate the color difference ΔE * ab between the chromaticity values of each color bar and the corresponding product spot color, the lightness difference ΔL * the saturation difference ΔC * ab and the hue angle difference Δh * ab ;

[0063] (5) For all the measured color bars, select the lightness difference ΔL * the saturation difference ΔC * ab and the hue angle difference Δh * ab (all three of these differences need to be positive and negative), and the maximum color difference ΔE * ab is not greater than 6.3, and the magnitude of ΔE * ab and ΔL * ΔC * ab Δh * abColor bars with relatively uniform positive and negative values; the selected color bars are arranged and pasted on the black matte substrate in an orderly manner to form a special color target for camera color calibration for the spot color detection of this product.

[0064] Preferably, in step (1), the range of values that the initial dot value can take is (0% to 100%]; the step of change of the basic spot color and the bottom white dot value is set to be no greater than 10%. The number of steps for increasing and decreasing respectively includes at least 1 times and 2 times the step value. When the initial dot value is 100%, it can only be decreased by the step value. When the initial dot value is between 0% and 100%, it needs to be increased and decreased by the step value. All the combined values of the basic spot colors and the bottom white dots are the mutual combinations of all the dot values of all the basic spot colors and the bottom white inks, that is, the combined values in mathematics.

[0065] Preferably, in step (2), the size of the digital color bar should be greater than 5 cm × 1 cm; among them, the length of 5 cm can cover the minimum linear dimension of 1 complete cycle in the transverse direction of the laser paper light column (perpendicular to the light column direction); the width of 1 cm can meet the requirements of the test aperture diameter of common spectrophotometers.

[0066] Preferably, in step (3), the appropriate ink output conditions during the printing output process are the ink amount conditions of each basic spot color and the bottom white during normal printing, or when there is a 100% situation in the combined value of the basic spot colors; in order to obtain a lightness lower than that of the product spot color, increase the ink amount of the corresponding basic spot color, which is equivalent to increasing by more than 5% to 10% of the normal ink amount.

[0067] Preferably, in step (4), the color measurement of each color bar of the printed matter should use the average value of measurements of no less than 7 measurement points within 1 complete cycle in the transverse direction of the light column by an integrating sphere spectrophotometer. And the sampling interval × the data of the acquisition points should ≥ 1 complete light column cycle. The ΔE * ab Color difference and the corresponding lightness difference ΔL*, saturation difference ΔC * ab And hue angle difference Δh * ab The relevant calculation formulas are:

[0068] ΔL * =L * 1–L * 2 (1)

[0069] Δa * =a * 1–a * 2 (2)

[0070] Δb * =b * 1–b *2 (3)

[0071]

[0072] ΔC * ab =C * 1,ab -C * 2,ab (5)

[0073] h * ab =arctan(b * / a * ) (6)

[0074] △h * ab =h* 1,ab –h * 2,ab (7)

[0075]

[0076] Among them, are the lightness of the product spot color and the measurement color bar respectively, C * ab is the saturation of each color, is the saturation difference, h * ab is the hue angle of each color, Δh * ab is the hue angle difference, is the chromaticity index of the product spot color, are the red-green chromaticity and yellow-blue chromaticity of the measurement color bar respectively, Δa * 、Δb * are the differences in red-green chromaticity and yellow-blue chromaticity between the product spot color and the measurement color bar respectively. The subscript 1 represents the chromaticity value of the product spot color, and the subscript 2 represents the chromaticity value of the measurement color bar.

[0077] Preferably, in step (5), the maximum ΔE * ab color difference is not greater than 6.3ΔE * ab color difference, corresponding to 2.5 times the human eye's visually acceptable color difference threshold (ΔE * ab color difference is 2.5), can quite sufficiently characterize the color differences that are easily distinguishable visually. Colors with a color difference smaller than this are left for the camera to detect and process, while larger color differences can be avoided by the printing personnel through rough adjustment on the machine. The number of color bars selected to form the special color target should be no less than 30.

[0078] To better illustrate the design and preparation of a special color target for camera color calibration for detecting spot colors of laser-printed products, this embodiment will explain the design and preparation process of a special color target for digital camera color detection for a certain spot color situation of a rainbow light column laser paper substrate.

[0079] The D65 / 10℃ IEL*a*b* chromaticity values required for the spot color of this product are [45.66 47.34 26.71]. The basic spot colors already used are Red 1 - 100% (dot value), Red 2 - 100%, and Base White - 60%.

[0080] Step (1), obtaining the combined values of basic spot colors and base white dots

[0081] According to the composition of the basic spot colors of this product's spot color, that is, the basic spot colors are Red 1 - 100%, Red 2 - 100%, and the dot value of Base White 60%, a combined design of the basic spot color and base white dot values is carried out.

[0082] First, according to the characteristics of their composition values, the values of the two basic spot colors, Red 1 and Red 2, which play a major role, are increased by two values of 90% and 80% on the basis of 100%. Considering that there is a need for colors with a smaller color difference from the finished product spot color, a dot value of 95% with a slight deviation is added; in addition, the base white color is a neutral color, which mainly affects the lightness and saturation of the finished product spot color. Designing a 10% step, two dot values of 50% and 70% are added. In this way, the designed values of the three components are shown in Table 1.

[0083] Table 1 Basic spot color and base white dot values

[0084]

[0085] The values taken in Table 1 can be combined into a total of 2 4 × 3 = 48 arrays.

[0086] Step (2), generating the digital color bar page image

[0087] Record the values of Red 1, Red 2, and Base White respectively using the three information channels of C (cyan), M (magenta), and Y (yellow). For each array formed in step (1), a CMYK digital color bar image is formed (the information of the K channel is 0); among them, the length of the color bar is 5.6 cm and the width is 1.2 cm. The design of the 5.6 cm length of the color bar is because the color of the light column laser paper and its printed color changes within one light column cycle, and the color of a complete light column cycle needs to be used to characterize it, while the linear dimension of a complete cycle of the used light column laser paper is 5 cm; the design of the 1.2 cm width of the color bar is considered that the aperture diameter of the color measurement instrument is within 1.2 cm, which is sufficient to meet the use requirements of the color measurement instrument. In this way, these 48 color bar images are arranged in a printing page to form a page image, asFigure 2 As shown. It should be noted that Figure 1 In the CMYK mode image, cyan, magenta, and yellow do not represent the actual printed colors, but only indicate the occupation of the information channels.

[0088] Step (3), printing output of the digital color bar page image

[0089] For the CMYK page image formed in step (2), color separation and plate making are carried out, and the information of the C, M, and Y plates respectively corresponds to the dot values of Red 1, Red 2, and bottom white, and the information of the K plate is 0; then, under the conditions of standard printing process, the page is printed and output, and printing outputs are added for three conditions of respectively adding C and M ink channels (corresponding to Red 1 and Red 2 information respectively) and simultaneously adding C and M ink channels on the console for the page shown, each equivalent to about 10% dot ink amount; a total of four pages with 192 color bars are output. Figure 1 As shown, printing outputs are added for three conditions of respectively adding C and M ink channels (corresponding to Red 1 and Red 2 information respectively) and simultaneously adding C and M ink channels on the console for the page shown, each equivalent to about 10% dot ink amount; a total of four pages with 192 color bars are output.

[0090] Step (4), color measurement and color difference calculation of the printed matter color bar

[0091] For each color bar on the color bar printed matter in step (3), a uniform 7-point measurement is carried out in the length (light column cross-section) direction of each printed color bar using an integrating sphere spectrophotometer Ci64, and it is ensured that the sampling interval × the data of the sampling points should be ≥ 1 complete light column period. The CIEL * a * b * average value of each color bar is used as the chromaticity value of the color bar. Further, the chromaticity values of each color bar are used to calculate the color difference ΔE * a * b * =[45.66 47.34 26.71]) with the spot color chromaticity value (L * ab value, lightness difference ΔL * saturation difference ΔC * ab and hue angle difference Δh * ab . Respectively as Figures 3 - 6 shown. Figure 3 The ΔE * ab values of all color bars are sorted from small to large, Figures 4 - 6 then the change situations of ΔL * ab , ΔC * * and Δh ab * ab corresponding to the color numbers sorted by the ΔE value are respectively.

[0092] Step (5), Color Selection of Special Color Marks

[0093] Among the ΔE * ab , ΔL * , ΔC * ab and Δh * ab obtained from step (4), first select 181 colors with ΔE * ab ≤6.3; then, sort them in ascending order according to the ΔE * ab value. Analyze the sorted ΔE * ab values and find that the colors with ΔE * ab color difference less than 1.3 and greater than 5.5 have relatively less color density compared to the colors with other color differences. Therefore, retain the colors within these two ranges without elimination, and take one color every 5 or every 3 from the remaining colors with color differences, obtaining a total of 49 colors. The median values of ΔE * ab , ΔL * , ΔC * ab and Δh * ab of these 49 colors are as shown in Figures 7 - 10 . As can be seen from Figure 7 , the color differences between these 49 colors and the product special color change relatively evenly from small to large; among them, the smallest color difference of the first one is 0.40, which is the color closest to the product special color, and the largest color difference of the last one is 6.09, which is about 2.44 times the color difference threshold acceptable to the human eye.

[0094] As can be seen from Figures 8 - 10 , these 49 colors and their product special color increase and decrease to varying degrees in terms of the three colorimetric quantities of lightness L * , saturation C * ab and hue angle h * ab , and the number of increasing and decreasing colors is quite the same. Generally speaking, the chromaticity values of these 49 colors can vary appropriately around the chromaticity of their product special color in the three dimensions of L * , C * ab , h * ab to meet the usage requirements for product special color detection based on camera imaging.

[0095] Arrange the selected 49 color bars in 7 rows and 7 columns in descending order of lightness, and adjust the color saturation in each row to be low on the left and high on the right, and then paste them onto the black cardboard regularly. The black cardboard has a low gloss, and coupled with the strong light absorption of black, it can effectively avoid the influence of its reflection on the color of the color bars.

[0096] Step (6), color calibration application of the special color target

[0097] This special color target is used for color calibration of digital cameras, that is, to establish a color conversion relationship (color calibration relationship) from the RGB color values responded by the camera to their corresponding CIE chromaticity values.

[0098] In the lighting and camera imaging system commonly used for printed matter defect detection, achieve white balance under the light source used for camera imaging, adjust the exposure and aperture to have a large light intensity response range, and photograph this special color target. The result is as Figure 11 shown.

[0099] For Figure 11 For each color bar of the image, extract an area with a length of 5 cm and a width of 0.8 cm in the center part of the light column period, and calculate the average values of the R, G, and B values of the pixels used on this area element as the camera response values of the color of this color bar. The schematic diagram of the extraction area is as Figure 12 shown.

[0100] After that, use a second-order six-term polynomial fitting relationship to establish the fitting relationship from RGB to the D65 / 10°L * a * b * chromaticity of the color bar, and the relationship formula is:

[0101]

[0102] Among them, RGB is the average value of the image numerical values of each area element of the color bar in the color target photograph, L * , a * , b * are the chromaticity values of the corresponding color bars on the icon, and a ij (i = 1, 2, 3; j = 1, 2, 3, 4, 5, 6) are the fitting relationship coefficients.

[0103] Through the 49 color values of the color target, the specific values of the coefficients a ij in formula (9) can be obtained by using mathematical methods such as the least squares method, and then the camera color calibration relationship from RGB to chromaticity values is obtained.

[0104] In order to characterize the accuracy of the color calibration relationship shown in formula (9), further, the fitting L * a * b *value, and calculate the corresponding L according to formula (8) * a * b * color difference △E between true values * ab value, and the result is as Figure 13 shown

[0105] From Figure 13 it can be seen that the △E of 49 colors * ab The color differences are all below 1, the maximum value is 0.86, and the average value is 0.37. The ΔE with a value of 1 * ab color difference means that when two samples are placed adjacent to each other, the human eye can just see or not see the difference visually. Therefore, the ΔE with a maximum value of 0.86 here * ab color difference indicates that the relationship of formula (9) has a very high color correction accuracy, which also indicates a relatively high color detection accuracy when this special color standard is used for the spot color of this product

[0106] This embodiment shows that by using the color standard color design and preparation method of the present invention, a color standard for spot color detection of products with high color correction accuracy can be obtained. It can not only meet the high-precision requirements for spot color detection of large-area spot colors in laser printing, but also use a simple color correction algorithm, greatly improving the color detection speed and meeting the requirements for on-line detection. In addition, the substrate and color ink for preparing the color samples by the method of the present invention are exactly the same as the products to be detected, most effectively representing the actual color fluctuations of the product spot color, and effectively avoiding the metamerism phenomenon between the camera color recognition and the human eye

[0107] As another optional embodiment, corresponding to the above method, this embodiment also provides a camera color correction special color standard design and preparation system for detecting the spot color of laser printing products, including:

[0108] Dot combination value generation unit, used to set a step value according to the initial dot values of the base spot color and the underwhite ink of the laser printing product spot color, and increase and decrease the step value respectively within the available value range to generate the dot combination values of all base spot colors and underwhite ink

[0109] Image arrangement unit, used to generate a corresponding digital color bar for each dot combination value and arrange all the digital color bars into a page image

[0110] Color bar printing unit, used to prepare a printing plate based on the page image, and print and output on a laser paper substrate with the corresponding base spot color ink and underwhite ink to form a color bar printed matter

[0111] Index calculation unit, used to measure the chromaticity values of each digital color bar in the color bar printed matter and calculate the color difference ΔE between the chromaticity value and the corresponding product spot color* ab , brightness difference ΔL * , Saturation difference ΔC * ab and hue angle difference Δh * ab ;

[0112] A color standard preparation unit is used to screen the color difference ΔE in all the digital color bars. * ab ≤6.3 and ΔE * ab Size and ΔL * , ΔC * ab , Δh * ab Digital color bars with even positive and negative distributions are arranged in order and fixed on a black matte substrate to form a special color standard for camera color calibration.

[0113] In this specification, each embodiment is described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the embodiments can be referred to each other. For the system disclosed in the embodiment, since it corresponds to the method disclosed in the embodiment, the description is relatively simple, and the relevant parts can be referred to the method part.

[0114] The principles and implementation methods of the present invention are described in this article using specific examples. The description of the above embodiments is only used to help understand the method and core idea of the present invention. At the same time, for those skilled in the art, according to the idea of the present invention, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting the present invention.

Claims

1. A method for designing and preparing a special color target for camera color calibration for detecting spot colors of laser-printed products, characterized in that, Including: Based on the initial dot values of the base spot colors and the under white ink for the spot colors of the laser printing product, set a step value, and respectively increase and decrease the step value within the available value range to generate the dot combination values of all the base spot colors and the under white ink; Generate a corresponding digital color bar for each of the dot combination values, and arrange all the digital color bars into a page image; Prepare a printing plate based on the page image, and print and output on a laser paper substrate using the corresponding base spot color ink and under white ink to form a color bar printed product; Measure the chromaticity values of each of the digital color bars in the color bar printed matter, and calculate the color difference ΔE between the chromaticity value and the corresponding product spot color * ab , lightness difference ΔL * , saturation difference ΔC * ab and hue angle difference Δh * ab ; Among all the digital color bars, select the color bars with a color difference ΔE * ab ≤6.3, and the ΔE * ab is of the same magnitude as ΔL * , ΔC * ab , and Δh * ab and which have a uniform positive and negative distribution. Arrange them in order and fix them on a black matte substrate to form a special color target for camera color calibration.

2. The design and preparation method of a special color target for camera color calibration for detecting spot colors of laser-printed products according to claim 1, wherein The value range of the initial dot value is (0%, 100%]; the step value is not greater than 10%.

3. The design and preparation method of a special color target for camera color calibration for detecting spot colors of laser-printed products according to claim 1, characterized in that, The range for increasing and decreasing the step value includes at least 1 times the step value and 2 times the step value.

4. The method for designing and preparing a special color target for camera color calibration for detecting spot colors of laser-printed products according to claim 2, characterized in that, When the initial dot value is 100%, only decrease it according to the step value; when the initial dot value is between (0%, 100%), increase and decrease it respectively according to the step value.

5. The method for designing and preparing a special color target for camera color calibration for detecting spot colors of laser-printed products according to claim 1, wherein The length of the digital color bar > 5 cm; the width of the digital color bar > 1 cm; the length of the digital color bar covers a complete cycle in the transverse direction of the laser paper light column, and the width of the digital color bar meets the requirements of the measurement aperture of the preset spectrophotometer.

6. The method for designing and preparing a special color target for camera color calibration for detecting spot colors of laser-printed products according to claim 1, characterized in that, The ink amount conditions for the printing output include normal printing ink amount and an increased ink amount of 5% - 10%; the increased ink amount is used to generate digital color bars with a brightness lower than the spot color of the product.

7. The method for designing and preparing a special color target for camera color calibration for detecting spot colors of laser-printed products according to claim 1, wherein The measurement of the chromaticity value uses an integrating sphere spectrophotometer, evenly select at least 7 measurement points along the length direction of the digital color bar, and the sampling interval × the number of measurement points > one complete cycle in the transverse direction of one laser paper light column.

8. The design and preparation method of a special color target for camera color calibration for detecting spot colors of laser-printed products according to claim 1, characterized in that, The color difference ΔE * ab is calculated by the following formula: Among them, ΔL * , Δa * , Δb * are respectively the difference in lightness between the digital color bar and the product spot color, and the differences on the red-green chromaticity axis and the yellow-blue chromaticity axis.

9. The method for designing and preparing a special color target for camera color calibration for detecting spot colors of laser-printed products according to claim 1, characterized in that, The number of selected digital color bars is not less than 30.

10. A special color target design and preparation system for camera color calibration for detecting spot colors of laser-printed products, characterized in that, Including: A dot combination value generation unit, configured to set a step value based on the initial dot values of the base spot colors and the under white ink for the spot colors of the laser printing product, and respectively increase and decrease the step value within the available value range to generate the dot combination values of all the base spot colors and the under white ink; An image arrangement unit, configured to generate a corresponding digital color bar for each of the dot combination values, and arrange all the digital color bars into a page image; A color bar printing unit, configured to prepare a printing plate based on the page image, and print and output on a laser paper substrate using the corresponding base spot color ink and under white ink to form a color bar printed product; An index calculation unit, configured to measure the chromaticity values of the digital color bars in the color bar printed matter, and calculate the color difference ΔE between the chromaticity values and the corresponding product spot color * ab , lightness difference ΔL * , saturation difference ΔC * ab and hue angle difference Δh * ab ; A color target preparation unit, which is used to screen out digital color bars with a color difference ΔE * ab ≤6.3 and ΔE * ab in terms of size and ΔL * and ΔC * ab and Δh * ab Digital color bars with uniform positive and negative distributions are arranged in an orderly manner and fixed on a black matte substrate to form a color target dedicated to camera color calibration.

Images