Color matching method of water-based spot color ink based on natural pigment

Abstract

The invention relates to the technical field of spot color matching, in particular to a color matching method based on natural pigment water-based spot color ink, which comprises the following steps: preparing natural pigment water-based primary color ink according to a predetermined concentration gradient, printing to obtain a gradient printed product, collecting optical data of the gradient printed product and a target sample, and constructing a primary color database; converting the spectral reflectance in the primary color database into an absorption-scattering ratio, fitting the absorption-scattering ratio, establishing a linear relation coefficient between the concentration gradient and the optical characteristics of the natural pigment primary color ink, and calculating the absorption-scattering ratio of the mixed primary color ink; solving an initial ink formula by taking the root-mean-square error of the spectral reflectivity and the CIE1931 chromatic aberration or the CIE1976 chromatic aberration as target minimum values; and comparing the spectral difference and chromatic aberration of the initial printing sample and the target printing sample, and optimizing the formula to obtain a corrected formula. Compared with a traditional ink color matching method, an accurate spot color formula can be output for color matching of the natural pigment water-based spot color ink, and therefore the application range of the natural pigment water-based ink is widened.

Description

Technical Field

[0001] The invention relates to the technical field of spot color matching, and in particular to a color matching method based on natural pigment water-based spot color ink. Background Art

[0002] The potential safety problem of existing water-based inks is that they still use colorants such as inorganic pigments, organic pigments and organic dyes, which have potential pathogenic risks. Natural pigments are edible pigments obtained from natural resources, mainly extracted from animal and plant tissues and microorganisms. Many ingredients of edible natural pigments are the same as those contained in food itself. Their safety has been recognized by my country's national standards and EU standards and can be used as water-based ink colorants.

[0003] At present, there are relatively mature formulas for ink color matching methods based on computer technology. For example, the patent with publication number CN117929301A discloses a high-precision printing ink color matching method with automatic gradient screening, including the following steps: (i) preparing base color ink gradient samples and obtaining the absorption coefficient and scattering coefficient ratio of each gradient sample; (ii) calculating the wavelength with the largest absorption coefficient and scattering coefficient ratio in the base color ink gradient sample, and constructing coordinate points; (iii) cleaning abnormal points in the coordinate points and screening the base color ink gradient samples; (iv) obtaining the absorption coefficient and scattering coefficient ratio of the base color ink based on the screened gradient samples; (v) selecting the remaining base color inks and repeating the above steps to obtain the absorption coefficient and scattering coefficient ratio of various base color inks, and then calculating the formula for color matching.

[0004] However, most spot colors are made by mixing or printing four-color CMYK inks, and there are no natural pigments in nature that have exactly the same colors as CMYK inks. Therefore, the ink system with natural pigments as colorants cannot be used to mix spot colors based on the existing CMYK formula, and there is a lack of color matching methods that can be applied to natural pigment water-based spot color inks, which to some extent hinders the promotion and application of natural pigment water-based inks. Summary of the invention

[0005] To solve the above problems, the present invention provides a color matching method based on natural pigment water-based spot color ink. Compared with the traditional color matching method based on CMYK mixing spot color ink, the present invention can output accurate spot color formula for the color matching of natural pigment water-based spot color ink, thereby expanding the application scope of natural pigment water-based ink.

[0006] To achieve the above-mentioned purpose of the invention, the technical solution of the present invention is to provide a color matching method based on natural pigment water-based spot color ink, comprising the following steps:

[0007] S1, printing a plurality of natural pigment water-based primary color inks according to a predetermined concentration gradient to obtain a gradient print, collecting the spectral reflectance of the gradient print and the target sample, and constructing a primary color database;

[0008] S2. Convert the spectral reflectance in the base color database into an absorption-scattering ratio and perform fitting, establish a linear relationship between the concentration gradient and the absorption-scattering ratio of the natural pigment water-based base color ink, predict the spectral reflectance of the mixed ink formed by mixing multiple natural pigment water-based base color inks based on the linear relationship, and construct a prediction model based on the spectral reflectance of the mixed ink;

[0009] S3. On the basis of the prediction model, constraints are added and an optimizer is used to solve the initial ink formula when different natural pigment water-based base inks are mixed at different concentrations, wherein the constraints include taking the root mean square error between the spectral reflectance of the prediction sample and the target sample in the prediction model, the CIE1931 color difference or the CIE1976 color difference as the target minimum value;

[0010] S4. According to the initial ink formula, a natural pigment water-based spot color ink obtained by matching multiple natural pigment water-based base color inks is obtained by proofing, and the natural pigment water-based spot color ink is recorded as the initial sample. The spectral difference and color difference between the initial sample and the target sample are compared, and a correction compensation factor is introduced to optimize the formula to obtain a corrected formula.

[0011] Preferably, in step S1, the natural pigment water-based base ink includes natural pigment, water-based connecting material, solvent, filler and auxiliary agent.

[0012] Preferably, the natural pigment includes one or more of gardenia yellow, curcumin, gardenia blue, monascus red, gardenia red, erythrosine, chlorophyll, gardenia green, cuttlefish ink, and plant carbon black.

[0013] Optionally, the aqueous vehicle comprises a polyacrylate emulsion.

[0014] Preferably, the filler comprises kaolin.

[0015] Preferably, the auxiliary agent includes one or more of wax emulsion, alkaline silica sol and polyvinyl siloxane.

[0016] Preferably, in step S1, the predetermined concentration gradient is 20%, 40%, 60%, 80%, or 100%.

[0017] Preferably, in step S1, the data range of the spectral reflectance is a wavelength of 400nm-700nm.

[0018] Preferably, in step S2, the process of converting the spectral reflectance in the base color database into the absorption-scattering ratio is given based on the Kubelka-Munk theory:

[0019]

[0020] In the formula: K / S is the absorption-scattering ratio; R is the spectral reflectance; λ is the wavelength. The spectral reflectance of each wavelength in the wavelength range of 400nm-700nm is converted into the absorption-scattering ratio by this formula, aiming to establish a linear fitting relationship between it and the concentration.

[0021] Preferably, in step S2, the fitting is a linear fitting using the least square method. Further preferably, the fitting result of the linear relationship coefficient is as follows, which is used to give the linear relationship between the absorption scattering ratio and the concentration of each primary color ink:

[0022]

[0023] In the formula: K / S is the absorption scattering ratio; a is the slope, b is the intercept; c is the concentration of each primary ink; i represents the type of primary ink; λ is the wavelength; the absorption scattering ratio of each primary ink at each concentration is determined by this formula.

[0024] Preferably, in step S2, the spectral reflectance of the mixed ink is obtained by substituting the absorption-scattering ratio of the mixed ink into the Kubelka-Munk formula. Based on the Duncan summation theory, the absorption-scattering ratio of the mixed ink follows the following formula:

[0025]

[0026] Where: K / S is the absorption scattering ratio; a is the slope, b is the intercept; c is the concentration of each primary color ink; λ is the wavelength.

[0027] Preferably, in step S3, the optimizer comprises a nonlinear minimization optimizer. Further preferably, MatLab provides an fmincon optimizer for solving constrained nonlinear functions, and its usage format is as follows:

[0028] [S,objectiveValue]=fmicon(@fun,x 0 ,A,b,Aeq,Beq,lb,ub)

[0029] The explanation of each parameter is as follows: S in the return value is the concentration of each base ink in the formula; objectiveValue is the predicted minimum value of spectral difference or color difference; @fun is the name of the objective function, and its form depends on the target minimum value adopted. The root mean square error of spectral reflectance, CIE1976 color difference, and CIE1931 color difference are calculated by the following three formulas:

[0030]

[0031] x0 is the initial value of the iteration; A and b are linear inequality constraint parameters, so that the iteration process satisfies A*x0≤b, A=[1,1,…,1,1], b=[1], and the number of columns of A is equal to the number of base inks; Aeq and Beq are linear equality constraint parameters, so that the iteration process satisfies Aeq*x0=Beq. When using multiple base inks, set Aeq=[1,1,…,1,1] and Beq=[1], and the number of columns of Aeq is equal to the number of base inks; lb and ub are divided into the lower and upper bounds of the output value. In this application, they are set to lb=[0,0,…,0,0] and the upper bound ub=[1,1,…,1,1], and the number of columns is equal to the number of base inks.

[0032] Preferably, in step S4, the difference in absorption-scattering ratio between the actual print sample and the target print sample is used as the correction compensation factor, as shown in the following formula:

[0033]

[0034] Afterwards, the fmincon optimizer is used again to calculate and obtain the revised formula.

[0035] The beneficial effects of the present invention are:

[0036] The color matching method based on natural pigment water-based spot color ink proposed by the present invention can avoid the phenomenon of metamerism to a large extent, output an accurate spot color formula for the color matching of natural pigment water-based spot color ink, and further promote the application of natural pigment water-based ink. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 Spectral reflectivity of the concentration gradient print obtained in Example 1 of the present invention.

[0038] Figure 2 This is the linear fitting result of the least squares method obtained in Example 1 of the present invention.

[0039] Figure 3 Spectral reflectance of the initial formula and the modified formula obtained in Example 1 of the present invention.

[0040] Figure 4 Spectral reflectance of the initial formula and the modified formula obtained in Example 2 of the present invention.

[0041] Figure 5 Spectral reflectance of the initial formula and the modified formula obtained in Example 3 of the present invention.

[0042] Figure 6 Spectral reflectance of the initial formulations obtained in Example 3 of the present invention and Comparative Example 1. DETAILED DESCRIPTION

[0043] The present invention is further described below in conjunction with the accompanying drawings and specific embodiments of the specification. Those of ordinary skill in the art will be able to implement the present invention based on these descriptions. In addition, the embodiments of the present invention involved in the following description are generally only embodiments of a part of the present invention, rather than all embodiments. Therefore, based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative work should fall within the scope of protection of the present invention.

[0044] Preparation Example 1

[0045] A gardenia yellow water-based base ink is prepared by the following steps: under the condition of magnetic stirring, 8g of deionized water and 5.2g of water-based connecting material are evenly mixed, 2.4g of gardenia yellow powder, 2g of kaolin, 1g of wax emulsion, 1g of alkaline silica sol, and 0.4g of polyethylene siloxane are added in sequence, and ultrasonic homogenization is performed for 30 minutes to prepare the gardenia yellow water-based base ink.

[0046] Preparation Example 2

[0047] The method is basically the same as Preparation Example 1, except that the natural pigment added is gardenia blue powder, and gardenia blue water-based base color ink is prepared.

[0048] Preparation Example 3

[0049] It is basically the same as Preparation Example 1, the only difference being that the natural pigment added is red yeast rice powder, and red yeast rice water-based base color ink is prepared.

[0050] Preparation Example 4

[0051] The method is basically the same as Preparation Example 1, except that the natural pigment added is chlorophyll powder, and chlorophyll water-based base color ink is prepared.

[0052] Preparation Example 5

[0053] The method is basically the same as Preparation Example 1, except that the natural pigment added is cuttlefish ink powder, and cuttlefish ink water-based base color ink is prepared.

[0054] Example 1

[0055] This embodiment provides a color matching method based on natural pigment water-based spot color ink to match the colors of Preparation Examples 1-5, and the specific steps are as follows:

[0056] S1. Gardenia yellow, gardenia blue, red yeast rice, chlorophyll, and cuttlefish juice water-based primary color inks are diluted at a concentration gradient of 20%, 40%, 60%, 80%, and 100%, respectively, and then gravure printed on tipping paper to obtain printed products. A spectrophotometer is used to collect the spectral reflectance of each printed product at a wavelength of 400-700nm, and a primary color database is constructed. The data of five different natural pigment water-based primary color inks are as follows: Figure 1 As shown;

[0057] S2. Convert the spectral reflectance in the base color database into an absorption-scattering ratio and perform fitting, establish a linear relationship between the concentration gradient and the absorption-scattering ratio of the natural pigment water-based base color ink, predict the spectral reflectance of the mixed ink formed by mixing multiple natural pigment water-based base color inks based on the linear relationship, and construct a prediction model based on the spectral reflectance of the mixed ink;

[0058] S3. On the basis of the prediction model, constraints are added and an optimizer is used to solve the initial ink formula when different natural pigment water-based base inks are mixed at different concentrations, wherein the constraints include taking the root mean square error between the spectral reflectance of the prediction sample and the target sample in the prediction model, the CIE1931 color difference or the CIE1976 color difference as the target minimum value;

[0059] S4. According to the initial ink formula, a natural pigment water-based spot color ink obtained by matching multiple natural pigment water-based base color inks is obtained by proofing, and the natural pigment water-based spot color ink is recorded as the initial sample. The spectral difference and color difference between the initial sample and the target sample are compared, and a correction compensation factor is introduced to optimize the formula to obtain a corrected formula.

[0060] In step S2, the process of converting the spectral reflectance in the base color database into the absorption-scattering ratio is given based on the Kubelka-Munk theory:

[0061]

[0062] In the formula: K / S is the absorption-scattering ratio; R is the spectral reflectance; λ is the wavelength. The spectral reflectance of each wavelength in the wavelength range of 400nm-700nm is converted into the absorption-scattering ratio by this formula, aiming to establish a linear fitting relationship between it and the concentration.

[0063] In step S2, the fitting is a linear fitting using the least square method. Further, the fitting result of the linear relationship coefficient is as follows, which gives the linear relationship between the absorption scattering ratio and the concentration of each base ink:

[0064]

[0065] In the formula: K / S is the absorption scattering ratio; a is the slope, b is the intercept; c is the concentration of each base ink; i represents the base ink type; λ is the wavelength. The absorption scattering ratio of each base ink at each concentration is determined by this formula. The linear fitting results are as follows: Figure 2 The spectral reflectance of the mixed ink is obtained by substituting the absorption and scattering ratio of the mixed ink into the Kubelka-Munk formula. Based on the linear relationship and Duncan summation theory, the absorption and scattering ratio of each primary color ink after mixing follows the following formula:

[0066]

[0067] Where: K / S is the absorption scattering ratio; a is the slope, b is the intercept; c is the concentration of each primary color ink; λ is the wavelength.

[0068] In step S3, the optimizer includes a nonlinear minimization optimizer. Further, MatLab provides an fmincon optimizer for solving constrained nonlinear functions, and its usage format is as follows:

[0069] [S,objectiveValue]=fmicon(@fun,x 0 ,A,b,Aeq,Beq,lb,ub)

[0070] The explanation of each parameter is as follows: S in the return value is the concentration of each base ink in the formula; objectiveValue is the predicted minimum value of spectral difference or color difference; @fun is the name of the objective function, and its form depends on the target minimum value adopted. The root mean square error of the spectral reflectance is calculated by the following formula:

[0071]

[0072] x0 is the initial value of the iteration; A and b are linear inequality constraint parameters, so that the iteration process satisfies A*x0≤b, A=[1,1,…,1,1], b=[1], and the number of columns of A is equal to the number of base inks; Aeq and Beq are linear equality constraint parameters, so that the iteration process satisfies Aeq*x0=Beq. When using 5 base inks, set Aeq=[1,1,…,1,1] and Beq=[1], and the number of columns of Aeq is equal to the number of base inks; lb and ub are divided into the lower and upper bounds of the output value. In this embodiment, they are set to lb=[0,0,…,0,0] and the upper bound ub=[1,1,…,1,1], and the number of columns is equal to the number of base inks.

[0073] In step S4, the absorption and scattering ratio between the actual print sample and the target print sample is used as a correction compensation factor, as shown in the following formula:

[0074]

[0075] After that, the fmincon optimizer is used to calculate the corrected formula. The spectral reflectance of the corrected formula is as follows: Figure 3 shown.

[0076] Example 2

[0077] It is basically the same as Example 1, except that: in step S3, the CIE1931XYZ color difference is taken as the target minimum value, and the CIE1931XYZ color difference is calculated by the following formula:

[0078]

[0079] The spectral reflectance of the corrected formula is obtained as Figure 4 shown.

[0080] Example 3

[0081] It is basically the same as Example 1, except that: in step S3, the CIE1976 L*a*b* color difference is taken as the target minimum value, and the CIE1976 L*a*b* color difference is calculated by the following formula:

[0082]

[0083] The spectral reflectance of the corrected formula is obtained as Figure 5 shown.

[0084] Comparative Example 1

[0085] This comparative example uses a color matching method based on CMYK to match the spot color ink in the prior art to match the colors of Preparation Examples 1-5, and the specific steps are as follows:

[0086] S1. Dilute gardenia yellow, gardenia blue, red yeast rice, chlorophyll and cuttlefish ink water-based ink into concentration gradients of 20%, 40%, 60%, 80% and 100% respectively, and then perform gravure printing on tipping paper to obtain printed products. Use a spectrophotometer to collect the spectral reflectance of each printed product at a wavelength of 400-700nm to construct a base color database.

[0087] S2, based on the single constant Kubelka-Munk theory:

[0088]

[0089] In the formula, K / S is the absorption-scattering ratio; R is the spectral reflectance; λ is the wavelength. The absorption-scattering ratio of each base color ink at unit concentration is directly calculated through this formula, and the special color ink formula is obtained through matrix operation.

[0090] Figure 6The spectral reflectance of the formula obtained in Comparative Example 1 and Example 3 is shown. The spectral difference between Comparative Example 1 and the target color is 9.87, and the spectral difference between Example 3 and the target color is 6.59; the color difference between Comparative Example 1 and the target print is 3.7, and the color difference between Example 3 and the target print is 3.1. This shows that both Comparative Example 1 and Example 3 have a similar color feel to the target print, but the difference between the spectral reflectance obtained by the method given in Example 3 and the target print is smaller, reducing the phenomenon of metamerism, so that the print obtained by this method and the target color can achieve the same color perception under different light source conditions compared with Comparative Example 1. Therefore, the color matching method provided in this application is more suitable for the color matching of natural pigment water-based inks than the existing methods.

Claims

1. A color matching method based on natural pigment water-based spot color ink, characterized in that: The steps include: S1, printing a plurality of natural pigment water-based primary color inks according to a predetermined concentration gradient to obtain a gradient print, collecting the spectral reflectance of the gradient print and the target sample, and constructing a primary color database; S2. Convert the spectral reflectance in the base color database into an absorption-scattering ratio and perform fitting, establish a linear relationship between the concentration gradient and the absorption-scattering ratio of the natural pigment water-based base color ink, predict the spectral reflectance of the mixed ink formed by mixing multiple natural pigment water-based base color inks based on the linear relationship, and construct a prediction model based on the spectral reflectance of the mixed ink; S3. On the basis of the prediction model, constraints are added and an optimizer is used to solve the initial ink formula when different natural pigment water-based base inks are mixed at different concentrations, wherein the constraints include taking the root mean square error between the spectral reflectance of the prediction sample and the target sample in the prediction model, the CIE1931 color difference or the CIE1976 color difference as the target minimum value; S4. According to the initial ink formula, a natural pigment water-based spot color ink obtained by matching multiple natural pigment water-based base color inks is obtained by proofing, and the natural pigment water-based spot color ink is recorded as the initial sample. The spectral difference and color difference between the initial sample and the target sample are compared, and a correction compensation factor is introduced to optimize the formula to obtain a corrected formula.

2. The color matching method based on natural pigment water-based spot color ink according to claim 1, characterized in that: In the step S1, the natural pigment water-based base ink includes a water-based linker, a solvent, a natural pigment, a filler, and an additive.

3. The color matching method based on natural pigment water-based spot color ink according to claim 2, characterized in that: The natural pigments include one or more of gardenia yellow, curcumin, gardenia blue, monascus red, gardenia red, erythrosine, chlorophyll, gardenia green, cuttlefish ink, and plant carbon black.

4. The color matching method based on natural pigment water-based spot color ink according to claim 1, characterized in that: In the step S1, the data range of the spectral reflectance is a wavelength of 400nm-700nm.

5. The color matching method based on natural pigment water-based spot color ink according to claim 1, characterized in that: In step S2, the process of converting the spectral reflectance in the base color database into the absorption-scattering ratio is given based on the Kubelka-Munk theory: Where: K / S is the absorption-scattering ratio; R is the spectral reflectance; λ is the wavelength.

6. The color matching method based on natural pigment water-based spot color ink according to claim 5, characterized in that: The fitting result of the linear relationship coefficient is as follows, which is used to give the linear relationship between the absorption scattering ratio and concentration of each base color ink: Where: K / S is the absorption scattering ratio; a is the slope, b is the intercept; c is the concentration of each base ink; i represents the base ink type; λ is the wavelength.

7. The color matching method based on natural pigment water-based spot color ink according to claim 6, characterized in that: The spectral reflectance of the mixed ink is obtained by substituting the absorption-scattering ratio of the mixed ink into the Kubelk a -Munk formula is solved, based on Duncan summation theory, the absorption and scattering ratio of the mixed ink is obtained by the following formula: Where: K / S is the absorption scattering ratio; a is the slope, b is the intercept; c is the concentration of each primary color ink; λ is the wavelength.

8. The color matching method based on natural pigment water-based spot color ink according to claim 1, characterized in that: In the step S3, the optimizer comprises a nonlinear minimization optimizer.

9. The color matching method based on natural pigment water-based spot color ink according to claim 8, characterized in that: The optimizer is the fmincon optimizer for solving constrained nonlinear functions, and its usage format is as follows: [S,objectiveValue]=fmicon(@fun,x0,A,b,Aeq,Beq,lb,ub) The explanation of each parameter is as follows: S in the return value is the concentration of each base ink in the formula; objectiveValue is the predicted minimum value of spectral difference or color difference; @fun is the name of the objective function, and its form depends on the target minimum value adopted. The root mean square error of spectral reflectance, CIE1976 color difference, and CIE1931 color difference are calculated by the following three formulas: x0 is the initial value of the iteration; A and b are linear inequality constraint parameters, so that the iteration process satisfies A*x0≤b, A=[1,1,…,1,1], b=[1], and the number of columns of A is equal to the number of base inks; Aeq and Beq are linear equality constraint parameters, so that the iteration process satisfies Aeq*x0=Beq. When using multiple base inks, set Aeq=[1,1,…,1,1] and Beq=[1], and the number of columns of Aeq is equal to the number of base inks; lb and ub are divided into the lower and upper bounds of the output value. In this application, they are set to lb=[0,0,…,0,0] and the upper bound ub=[1,1,…,1,1], and the number of columns is equal to the number of base inks.

10. The color matching method based on natural pigment water-based spot color ink according to claim 9, characterized in that: In step S4, the difference in absorption scattering ratio between the initial sample and the target sample is used as the correction compensation factor, as shown in the following formula: Afterwards, the fmincon optimizer is used again to calculate and obtain the revised formula.

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