A pre-press color separation correction method for paper fluorescent brightener

Abstract

The present invention discloses a pre-press color correction method for paper fluorescent whitening agents, which includes two parts: creating a color correction formula and pre-press color correction. By statistical analysis and fitting calculation of measurement data, a color correction formula based on the paper fluorescent whitening agent index is obtained. Before printing, the whitening agent index of the printing paper is measured, and b is calculated by the color correction formula. * The value is adjusted, and then the prepress electronic file is corrected based on the ICC characteristic file, so as to reduce the impact of paper fluorescent brighteners on color management and the deviation of color reproduction caused by paper fluorescent brighteners, so as to make the color reproduction on paper reach the ideal state.

Description

Technical Field

[0001] The invention relates to a printing color correction method, in particular to a pre-press color separation correction method. Background Art

[0002] Paper is a common printing substrate material. In order to achieve an ideal reproduction effect, most papers are added with optical brightening agents (OBA). Optical brightening agents can absorb invisible ultraviolet radiation with a wavelength below 400 nanometers and release blue visible light with a wavelength between 400 and 450nm through electrophysical changes. After this light is released from the paper containing the brightener, it produces a color that is whiter than white under the illumination of a light source containing a large amount of ultraviolet light, increasing the brightness and whiteness of the paper and enhancing the color characteristics of the ink.

[0003] From the principle of color management, we know that when color matching is performed, it is necessary to use various software and hardware tools to measure the color data of color samples and generate the ICC characteristic file of the device to record the color gamut characteristics of the device. However, although the measuring instruments and standard light boxes used in the current printing color reproduction system are as unified as possible and simulate standard light sources, if the printing paper contains fluorescent brighteners, the optical properties of the fluorescent brighteners will also affect the color measurement based on the standard lighting source, thereby affecting the accuracy of color management.

[0004] Before 2013, all color management and color targets were developed without considering fluorescent brighteners (OBA). Previous standards and data, such as ISO12647-2 and GRACoL2006, were also based on substrates without fluorescent brighteners. When fluorescent brighteners are contained in printed substrates, the color will appear obviously yellowish visually.

[0005] In the international standards GRACOL2013, SWOP2013, the updated version of ISO12647-2:2013 and the G7 database series (CGATS21) released in 2013, the data are all measured using the M1 (D50 light source illumination) observation condition. However, since M1 defines that the spectral power distribution of the sample illumination source should match the CIE illumination source D50 spectral power distribution (such as Figure 1 As shown in the figure), it contains ultraviolet light components below 400nm. Therefore, the ICC characteristic file produced using the measurement data of the M1 observation condition is b * The value will shift significantly towards negative values.

[0006] In order to solve the impact of the optical properties of fluorescent whitening agents on color management, the ISO 13655 standard specifies two observation conditions, M1 and M2, to solve the changes in measurement results between instruments caused by the optical properties of fluorescent whitening agents and to standardize the exchange of colorimetric data between instruments.

[0007] M2 in the ISO 13655 standard defines that by appropriately designing the light source or by adding a filter between the source and the sample to filter out ultraviolet light below 400nm, the spectral power distribution of the sample illumination light source only contains the radiation power in the wavelength range above 400nm, so as to eliminate the variation in measurement results between instruments caused by the fluorescence of fluorescent whitening agents.

[0008] Prepress color separation refers to the use of the ICC conversion mechanism in Photoshop to convert electronic files in RGB color mode into files in CMYK color mode. In order to meet the national standard, this process usually uses the data set measured under M1 observation conditions to create ICC characteristic files for color mode conversion. However, the content of paper whitening agents will cause deviations in the color table measurement data under M1 observation conditions, that is, b * Value offset (Lab color mode, b * The value is the value of the b channel, and the b channel covers the blue and yellow channels), which causes CMYK dot data deviation and color cast of the printed product. Therefore, the prepress file needs to be calibrated for the fluorescent brightener content before color separation. Currently, there is no standardized correction method in the industry. Summary of the invention

[0009] In order to overcome the deficiencies of the prior art, the present invention provides a pre-press color separation correction method for paper fluorescent brighteners, which enables the color reproduction on paper to reach an ideal state.

[0010] The technical solution adopted by the present invention to solve its technical problem is:

[0011] A pre-press color separation correction method for paper fluorescent whitening agent includes two parts: creating a color correction formula and pre-press color separation correction.

[0012] The color correction formula is created as follows:

[0013] (1) Printing specimens

[0014] Print a standard color chart on paper with different fluorescent brightener content.

[0015] (2) Measure the L of the color block on the printed specimen * a * b * value

[0016] Use a spectrophotometer to measure the L of each color block under M1 and M2 conditions.* a * b * value.

[0017] (3) Calculate |△b for each color block * ∣ 色

[0018] ∣△b * ∣ 色 = |b * M1 - b * M2 ∣;

[0019] b * M1 is the b of the color block under M1 observation conditions * value;

[0020] b * M2 is the b of the color block under M2 observation conditions * value;

[0021] (4) Calculate the standard color table |△b for each paper * ∣ 色 The average value of |△b * ∣ 均

[0022] ∣△b * ∣ 均 =|△b * ∣ 总 / n;

[0023] ∣△b * ∣ 总 is the |△b of n color blocks * ∣ 色 The sum of

[0024] n is the number of color blocks.

[0025] (5) Calculate the whitening agent index |△b for each paper * ∣ 纸

[0026] Using a spectrophotometer, measure the L of each paper under conditions M1 and M2. * a * b * Value, and calculate the whitening agent index |△b of each paper * ∣ 纸 .

[0027] (6) Calculate the correction parameters k and b

[0028] ∣△b* ∣ 均 and |△b * ∣ 纸 Data, through the linear fitting algorithm, get the parameters k and b in the fitting formula y=kx+b, establish b * Color correction formula for value adjustment value Si;

[0029] Si=(ib) / k, where i is the whitening agent index of the printing paper.

[0030] The steps of prepress color separation correction method are as follows:

[0031] (1) Use a spectrophotometer to measure the L * a * b * Value; Calculate the whitening agent index of the printing paper |△b * ∣;

[0032] ∣△b * ∣= ∣b * M1 - b * M2 ∣;

[0033] b * M1 b is the b of the printed paper under M1 observation conditions * value;

[0034] b * M2 b is the b of the printing paper under M2 observation conditions * value.

[0035] (2) According to the color correction formula, calculate b * Value adjustment value Si;

[0036] Si = (ib) / k;

[0037] b and k are correction parameters;

[0038] The value of i is the whitening agent index of the printing paper, i=∣△b * ∣.

[0039] (3) In Photoshop, first convert the electronic file in RGB color mode to Lab color mode, and then use the "Curves" adjustment function to adjust the b channel. * Value correction, that is, b * The value reduces the Si value.

[0040] (4) Convert the electronic file from Lab color mode to CMYK color mode.

[0041] The present invention obtains generally representative values ​​through statistical analysis and fitting calculation of the measured data, namely k=2.24, b=1.09, and the obtained b * The color correction formula of the value adjustment value Si is: Si=(i-1.09) / 2.24, and the i value is the whitening agent index of the printing paper.

[0042] The beneficial effects of the present invention are as follows: the present invention obtains a color correction formula based on the fluorescent whitening agent index of paper by statistical analysis and fitting calculation of the measured data; before printing, the whitening agent index of the printing paper is measured, and b is calculated by the color correction formula. * The value is adjusted, and then the prepress electronic file is corrected based on the ICC characteristic file, so as to reduce the impact of paper fluorescent brighteners on color management and the deviation of color reproduction caused by paper fluorescent brighteners, so as to make the color reproduction on paper reach the ideal state. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0044] Figure 1 This is the spectral power distribution diagram of the M1 light source D50;

[0045] Figure 2 is a fitting chart of the embodiment. DETAILED DESCRIPTION

[0046] Creating a color correction formula

[0047] 1. Print specimens

[0048] Printing a standard color chart on paper with different fluorescent brightener contents (Color ProfileMaker Pro / Full 1584 Chart). In this embodiment, 9 kinds of paper with different fluorescent brightener contents are used, namely 105g copper paper, 160g offset paper, 170g fine check paper, 80 offset paper, 240g offset paper, 250g copper paper, 270g offset paper, 300g copper paper, 315g offset paper, and a standard color chart is printed on each paper.

[0049] 2. Measure the L of the color block on the printed specimen * a * b * value

[0050] Use a spectrophotometer (X-Rite Exact spectrophotometer) to measure the L of each color block under M1 and M2 conditions. * a * b * value.

[0051] (3) Calculate |△b for each color block * ∣ 色

[0052] ∣△b * ∣ 色 = |b * M1 - b * M2 ∣;

[0053] b * M1 is the b of the color block under M1 observation conditions * value;

[0054] b * M2 is the b of the color block under M2 observation conditions * value;

[0055] The fluorescent brightener index is based on ISO 15397 to determine the fluorescent brightener content in the substrate. This value is measured under M1 observation conditions and M2 observation conditions. * The absolute value of the difference in values ​​directly reflects the b caused by the UV component (light with a wavelength below 400nm) in the D50 standard light source. * The value difference, so the fluorescent whitening agent index can determine the OBA content in the substrate, and the influence of OBA content on color measurement data is related to the gradation value, which is basically a negative linear relationship. The fluorescent whitening agent content has a great influence on the paper white measurement data, and the greater the fluorescent whitening agent content, the greater the difference value, where b * The value has the greatest impact, L * The value has the least impact, a * The value is not large, so we only need to calculate b * The impact of the value.

[0056] (4) Calculate the standard color table |△b for each paper * ∣ 色 The average value of |△b * ∣ 均

[0057] ∣△b * ∣ 均 =|△b * ∣ 总 / n;

[0058] ∣△b * ∣ 总 is the |△b of n color blocks * ∣ 色 The sum of

[0059] n is the number of color blocks. In the standard color table, there are 1584 color blocks.

[0060] 5. Calculate the whitening agent index of each paper |△b * ∣ 纸

[0061] The L of each paper was measured using a spectrophotometer (X-Rite Exact spectrophotometer) under conditions M1 and M2. * a * b * Value, and calculate the whitening agent index |△b of each paper * ∣ 纸 .

[0062] 6. Calculate the correction parameters k and b

[0063] ∣△b * ∣ 均 and |△b * ∣ 纸 The data is used to obtain the parameters k and b in the fitting formula y=kx+b through the linear fitting algorithm, and b is established. * Color correction formula for value adjustment value Si;

[0064] Si=(ib) / k.

[0065] In this embodiment, the data is as follows:

[0066] Paper type <![CDATA[∣△b * ∣ 均 ]]> <![CDATA[M1-b * ]]> <![CDATA[M2-b * ]]> <![CDATA[∣△b * ∣ 纸 ]]> 105g double copper paper 1.5 -3.3 1.0 4.3 160g offset paper 3.9 -15.1 -5.3 9.8 170g fine grain paper 2.5 -5.4 1.2 6.6 80 double offset paper 0.6 -1.5 0.5 2.1 240g offset paper 3.5 -7.5 1.6 9.1 250g double copper paper 1.3 -3.9 0.5 4.4 270g offset paper 1.6 -5.2 0.0 5.2 300g double copper paper 1.5 -5.7 -1.3 4.4 315g double glue 3.6 -6.5 2.4 8.9

[0067] M1-b * is the b of the paper under M1 observation conditions * value;

[0068] M2-b * is the b of the paper under M2 observation conditions * value.

[0069] The calculation of parameters k and b is implemented in Excel. Select |△b * ∣ 均 and |△b * ∣ 纸 Then insert a discrete graph, select "Add trend line", check "Show company" and "Show R square value", and you can get the following Figure 2 The fitting chart shown automatically obtains the fitting formula as y=2.24x+1.09, R 2 = 0.99, that is, k=2.24, b=1.09. Goodness of fit R 2It is 0.99, which is very close to 1, which shows that the regression line fits the observed value very well. The obtained k=2.24 and b=1.09 are generally representative. Based on this, the color correction formula can be established. The color correction formula is: Si=(i-1.09) / 2.24. In the subsequent actual production pre-press color separation correction, this parameter can be directly used.

[0070] Prepress color correction

[0071] 1. Use a spectrophotometer (X-Rite Exact spectrophotometer) to measure the L * a * b * Value, and calculate the whitening agent index |△b of the printing paper * ∣.

[0072] ∣△b * ∣= ∣b * M1 - b * M2 ∣

[0073] b * M1 b is the b of the printed paper under M1 observation conditions * value;

[0074] b * M2 b is the b of the printing paper under M2 observation conditions * value.

[0075] 2. According to the color correction formula, calculate b * Value adjustment value Si;

[0076] Si=(i-1.09) / 2.24.

[0077] The value of i is the whitening agent index of the printing paper, i=∣△b * ∣.

[0078] 3. In Photoshop, first convert the electronic file in RGB color mode to Lab color mode (the electronic file refers to the ICC characteristic file made by the data set measured under M1 observation conditions), and then use the "Curve" adjustment function to adjust the b channel. * Value correction (Image-Adjustment-Curves in Photoshop menu), b * The value reduces the Si value (b channel output value minus Si value).

[0079] 4. Convert electronic files from Lab color mode to CMYK color mode for output for actual printing production.

[0080] The present invention obtains a color correction formula based on the paper fluorescent brightener index by statistical analysis and fitting calculation of the measured data. Before printing, the brightener index of the printing paper is measured and b is calculated by the color correction formula. * The value is adjusted, and then the prepress electronic file is corrected based on the ICC characteristic file, so as to reduce the impact of paper fluorescent brighteners on color management and the deviation of color reproduction caused by paper fluorescent brighteners, so as to make the color reproduction on paper reach the ideal state.

[0081] The above implementation modes cannot limit the protection scope of the present invention. The equivalent modifications and changes made by those skilled in the art without departing from the overall concept of the present invention are still within the scope of the present invention.

Claims

1. A pre-press color separation correction method for paper fluorescent brightener, characterized in that The steps of prepress color separation correction method are as follows: (1) Use a spectrophotometer to measure the L * a * b * Value; Calculate the whitening agent index of the printing paper |△b * ∣; ∣△b * ∣= ∣b * M1 - b * M2 ∣; b * M1 b is the b of the printing paper under M1 observation conditions * value; b * M2 b is the b of the printing paper under M2 observation conditions * value; (2) According to the color correction formula, calculate b * Value adjustment value Si; Si = (ib) / k; b and k are correction parameters; The value of i is the whitening agent index of the printing paper, i=∣△b * ∣; (3) In Photoshop, first convert the electronic file in RGB color mode to Lab color mode, and then use the "Curve" adjustment function to adjust the b channel. * Value correction, that is, b * The value reduces the Si value; (4) Convert the electronic file from Lab color mode to CMYK color mode; The color correction formula is created as follows: (1) Printing specimens Printing standard color charts on papers with different fluorescent brightener contents; (2) Measure the L of the color block on the printed specimen * a * b * value Use a spectrophotometer to measure the L of each color block under M1 and M2 conditions. * a * b * value; (3) Calculate |△b for each color block * ∣ 色 ∣△b * ∣ 色 = ∣b * M1 - b * M2 ∣; b * M1 is the b of the color block under M1 observation conditions * value; b * M2 is the b of the color block under M2 observation conditions * value; (4) Calculate the standard color table |△b for each paper * ∣ 色 The average value of |△b * ∣ 均 ∣△b * ∣ 均 =∣△b * ∣ 总 / n; ∣△b * ∣ 总 is the |△b of n color blocks * ∣ 色 The sum of n is the number of color blocks; (5) Calculate the whitening agent index |△b for each paper * ∣ 纸 Using a spectrophotometer, measure the L of each paper under conditions M1 and M2. * a * b * Value, and calculate the whitening agent index |△b of each paper * ∣ 纸 ; (6) Calculate the correction parameters k and b ∣△b * ∣ 均 and |△b * ∣ 纸 Data, through the linear fitting algorithm, get the parameters k and b in the fitting formula y=kx+b, establish b * Color correction formula for value adjustment value Si; Si=(ib) / k, where i is the whitening agent index of the printing paper.

2. The pre-press color separation correction method for paper fluorescent whitening agent according to claim 1, characterized in that k=2.24, b=1.09, and the obtained b * The color correction formula of the value adjustment value Si is: Si=(i-1.09) / 2.24, where the i value is the whitening agent index of the printing paper.

Images