A method for measuring the color of optically variable ink

Abstract

This invention relates to a color measurement method for optically variable ink, belonging to the field of color measurement technology, and includes the following steps: (1) using a multi-angle measuring instrument and connecting the measuring instrument to a computer; (2) placing the sample to be measured on a measuring platform; (3) setting the measurement conditions: the angle θ between the spectral energy receiving direction and the mirror reflection direction, and the angle β between the incident direction of the illumination source and the spectral energy receiving direction; (4) controlling the multi-angle measuring instrument with software to complete the measurement of all samples one by one; (5) exporting the measurement results using software with software to the measuring instrument. * a * b * The chromaticity value is calculated by determining the color difference between the corresponding measurement conditions for multiple measurement results. The color measurement method for optically variable ink in this invention requires at least two measurement conditions, resulting in a small amount of measurement data while possessing a strong ability to reflect color differences in samples. It improves work efficiency while meeting measurement accuracy requirements and can be used to test large batches of samples.

Description

Technical Field

[0001] This invention relates to a method for measuring the color of optically variable ink, belonging to the field of color measurement technology. Background Technology

[0002] Optically variable inks play a crucial role in anti-counterfeiting, exhibiting color variation depending on the viewing angle, and are widely used in the packaging anti-counterfeiting and decoration industries. Currently, ink manufacturers and printing companies rarely employ quantitative methods to characterize the color chromaticity information of inks at different viewing angles, and rarely evaluate the color consistency of products printed with different optically variable inks from the same batch. Similarly, quantitative methods for comparing the color changes of anti-counterfeiting inks from different manufacturers are also scarce. Currently, comparisons of ink color changes at different viewing angles are based solely on visual inspection. The uncertainty of this visual evaluation introduces significant subjective factors into the authentication, quality control, and evaluation of anti-counterfeiting products.

[0003] The draft GB / T17001.7 recommends using illumination at 45° relative to the sample plane normal, and receiving spectral energy at 60° relative to the normal (opposite side). Similarly, invention patent CN1124484C uses 22.5° incident light source relative to the sample surface normal, receiving spectral energy at 0°, and 45° incident light source relative to the sample surface normal, receiving spectral energy at 67.5° (opposite side) as measurement conditions. The measurement conditions in the GB / T17001.7 draft do not directly correspond to the color changes of photochromic inks typically described by ink manufacturers, such as a change from dark green (color A) to dark brown (color B). Furthermore, the measurement conditions in patent CN1124484C do not fully correspond to the geometric measurement conditions of existing multi-angle spectrophotometers. The American Society for Materials (ASTM) standard E2539-14 recommends four measurement conditions: r45as-15, r45as15, r15as-15, and r15as15. These conditions are primarily defined by the angle between the incident direction of the illumination source and the normal direction of the sample plane, and the angle between the receiving direction and the specular reflection direction. These four measurement conditions correspond to the measurement conditions of existing measuring instruments. Establishing a color measurement method for optically variable inks that can accurately characterize their color changes (from color A to color B) and comprehensively characterize the maximum range of color changes is also a problem that needs to be considered during the measurement process.

[0004] Therefore, a color measurement method for photochromic inks is proposed, specifying its geometric measurement conditions as (θ, β), where θ is the angle between the spectral energy receiving direction and the specular reflection direction, and β is the angle between the incident direction of the illumination source and the spectral energy receiving direction. Limiting the range of the angle θ between the spectral energy receiving direction and the specular reflection direction allows the sample to have relatively high reflective energy, reflecting the color differences between different samples to a greater extent. Limiting the difference between the maximum and minimum values ​​of the angle β between the incident direction of the illumination source and the specular reflection direction facilitates a complete analysis of the entire color change range of the sample. Limiting the number of different measurement conditions for the β angle ensures that the measurement results are sufficient for analyzing the color differences of the samples while saving measurement and analysis time. Limiting the interval Δβ between adjacent measurement conditions for the β angle ensures that the measurement results uniformly reflect the color change process of the photochromic ink. Combining the θ and β angles into two or more measurement conditions (θ, β), and calculating the color difference of different samples under corresponding measurement conditions, provides a judgment on whether the sample is qualified. This has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0005] The purpose of this invention is to propose a method for measuring the color of photochromic inks. This method standardizes measurement conditions by setting the angle θ between the spectral energy receiving direction and the specular reflection direction, and the angle β between the incident direction of the illumination source and the spectral energy receiving direction. This limits the range of the angle θ between the spectral energy receiving direction and the specular reflection direction, ensuring the sample has relatively high spectral energy (reflection + interference energy), thus reflecting color differences between different samples to a greater extent. Limiting the difference between the maximum and minimum values ​​of the angle β between the incident direction of the illumination source and the specular reflection direction facilitates a complete measurement of the color change range of the photochromic ink samples. Limiting the number of different measurement conditions for angle β ensures that the measurement results are sufficient for analyzing color differences between samples while saving measurement and analysis time. Limiting the interval Δβ between adjacent measurement conditions for angle β ensures that the measurement results uniformly reflect the color change process of the photochromic ink. Furthermore, combining angles θ and β into two or more measurement conditions (θ, β), and calculating the color difference of different samples under corresponding measurement conditions, a judgment is made on whether the sample is qualified.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A method for measuring the color of optically variable ink includes the following steps:

[0008] (1) Use a multi-angle measuring instrument, connect the measuring instrument to the computer, and open the software on the computer that is compatible with the multi-angle measuring instrument;

[0009] (2) Place the sample to be tested flat on the measuring platform;

[0010] (3) Set different measurement conditions: the included angle θ between the spectral energy receiving direction and the specular reflection direction, and the included angle β between the illumination light source incident direction and the spectral energy receiving direction. The measurement conditions can reflect the color reproduction range of different photochromic ink samples and cover the entire color change range of the samples. While meeting the measurement efficiency, the set measurement angle range and quantity are within a reasonable range;

[0011] (4) Control the multi-angle measuring instrument through the software supporting the multi-angle measuring instrument to complete the measurement of all samples one by one;

[0012] (5) Use the software supporting the measuring instrument to export the L * a * b * chromaticity values of the measurement results, calculate the color difference of the multiple measurement results corresponding to the measurement conditions, and give an evaluation of whether the sample is qualified according to the actual requirements.

[0013] Preferably, in step (1), the multi-angle measuring instrument may be a spectral measuring instrument with different geometric measurement conditions such as a multi-angle spectrophotometer, an angular resolution spectral measurement system, etc.

[0014] Preferably, in step (1), the software supporting the multi-angle measuring instrument is used to control the setting of the measurement conditions and the output of the measurement results.

[0015] Preferably, in step (2), some multi-angle measuring instruments do not have a supporting measurement platform. At this time, the photochromic ink sample can be placed on any convenient flat surface for measurement.

[0016] Preferably, in step (3), a complete measurement condition includes the θ angle and the β angle, and the representation method is (θ, β). Generally speaking, the measurement conditions of the instrument are not set according to the θ angle and the β angle described in the present invention. It is necessary to represent the measurement conditions of the instrument in (θ, β). Let the included angle between the illumination light source incident direction and the normal direction of the sample plane be i r , and the included angle between the spectral energy receiving direction and the normal direction of the sample plane be i d , i r and i d are non-negative values in any case. The calculation methods of the θ angle and the β angle are as follows:

[0017] θ = |i r - i d |;

[0018] β = i r + i d ;

[0019] There are two measurement conditions where both angle θ and angle β are the same. These two measurement conditions are equivalent to interchange the incident direction of the illumination source and the direction of spectral energy reception. Due to the principle of optical path reversibility, these two measurement conditions are equivalent and can be reasonably selected according to the measurement conditions of existing instruments. In addition, in the existing methods of representing measurement conditions, the sign of angle θ depends on whether the incident direction of the illumination source and the direction of spectral energy reception are on the same side or opposite side of the specular reflection direction. In this invention, the sign of angle θ does not affect the measurement result. Therefore, θ is set as the absolute value of the angle difference between the direction of spectral energy reception and the direction of specular reflection.

[0020] Preferably, in step (3), when the θ angle of the measurement condition is greater than 15°, the direction of spectral energy reception is far from the direction of mirror reflection. As the θ angle increases, the collected interference light energy is weaker, and the micro-parameters of the photochromic ink, such as its microstructure and material properties, have little influence on the measurement results. When the θ angle is less than or equal to 15°, the direction of spectral energy reception is closer to the direction of mirror reflection, and the interference effect is stronger. At this time, the small differences in the micro-structure parameters of the photochromic ink correspond to the large changes in the measurement results. Therefore, the measurement results can reflect the color difference of the sample to a large extent.

[0021] Preferably, in step (3), the θ angle of all the measurement conditions is the same. When the θ angle is different, the spectral energy received in different directions will have a large difference. When the β angle of the measurement conditions is the same, the size of the θ angle does not have a significant effect on the hue. The sample can show an overall increase or decrease in color saturation and brightness. In order to make the measurement and analysis more convenient, there is no need to set measurement conditions with different θ angles.

[0022] Preferably, in step (3), two or more different β angles are set. Since the β angle affects the hue of the measured color, the photochromic ink can present more than one color under different viewing angles. Therefore, in order to comprehensively characterize the color change of the photochromic ink, two or more different β angles need to be set. It should be noted that while meeting the color chromaticity measurement requirements of the photochromic ink, the β angle should not be set too many times in order to save measurement and data processing time.

[0023] Preferably, in step (3), since the incident direction of the illumination source and the spectral energy receiving direction must be within 180° above the sample plane, that is, the variation range of the β angle is [0, 180°-θ], since the saturation of the color sample may be relatively low under measurement conditions where the β angle is very large or very small, and only a relatively weak color sensation can be produced, it is not recommended to use the color measurement conditions for photochromic inks.

[0024] Preferably, the minimum, maximum, difference between the maximum and minimum values ​​of angle β are defined as follows:

[0025] 1) The minimum value of the β angle is 20°. Under sunlight, the measurement conditions within this range of the β angle correspond to the observation of the sample perpendicularly, making it easier to form such an observation angle. When the θ angle of the measurement conditions varies from 5° to 30° and the β angle varies from 10° to 140°, the measurement result for a sample will have a... * b * The distribution shows that the spectral energy collected under measurement conditions with a β angle greater than 20° is relatively high, and the color stimulation is strong when it enters the human eye.

[0026] 2) The maximum value of the β angle is 120°. The hue of the measurement result will change with the change of the β angle. When the measurement conditions are θ = 15° and the β angle varies from 10° to 140°, the saturation C of the measurement result will vary when the β angle is at a low or high level. * The values ​​are all relatively low; however, when the β angle is in the middle range of 10° to 140°, the measured saturation C is higher. * The highest, that is, the measurement conditions within this β angle range, map the differences in the microstructure and material parameters of the photochromic ink into a relatively large difference in the measurement results;

[0027] 3) The difference between the maximum and minimum values ​​of the β angle should be greater than or equal to 60°. In order to make the measurement range as close as possible to the saturation change range of the color of the optically variable ink, the range of the β angle should not be too large or too small. When the minimum value of the β angle meets the requirements of this invention, the difference between the maximum and minimum values ​​of the β angle should be greater than or equal to 60°. The main color change process of the optically variable ink can be measured. This color change process has a relatively large impact on the color of the optically variable ink.

[0028] 4) When the measurement conditions include multiple different β angles, in order to increase the ability of the measurement conditions within the set β angle range to reflect the color difference of the sample and to ensure that the measurement conditions with different β angles measure an equally spaced hue change, it is necessary to control the interval Δβ of the β angle between adjacent measurement conditions. When the β angle is small, the hue of the photochromic ink does not change significantly with the change of β angle, while when the β angle is large, the hue changes more significantly with the change of β angle, making it difficult to collect the intermediate hue change process, which is not conducive to the microstructural composition analysis of the photochromic ink. Under the same θ angle measurement conditions, as the β angle increases, the hue change shows a state of slow first and then fast. Therefore, in order to enable the measurement results to better characterize the photochromic ink, when the β angle is small, Δβ should be relatively larger than when the β angle is large.

[0029] Preferably, in step (3), the applicable range of the measurement conditions includes, but is not limited to: conventional interference type optical variable ink, pearlescent interference type optical variable ink, liquid crystal type optical variable ink, and magnetic optical variable ink.

[0030] Preferably, in step (3), according to the current naming method of optically variable ink, the optically variable ink can realize the change from color A to color B. The range of β angle corresponding to color A is 0°-60°, and the range of β angle corresponding to color B is 60°-120° (when the β angle meets the requirements of the present invention for the minimum value, maximum value, and the difference between the maximum and minimum values, the measurement condition with the minimum β angle and the measurement condition with the maximum β angle among the multiple measurement conditions are respectively within the range of β angle corresponding to colors A and B, so colors A and B can be measured respectively).

[0031] Preferably, step (4) is used to detect the color uniformity of a sample. The method is as follows: select several sampling points on the sample surface, complete the measurement one by one, and obtain the color values ​​of different positions of a sample; the measurement results of the selected measurement points should be able to reflect the color characteristics of the entire sample, and the selection steps are as follows:

[0032] (1) Select the geometric center of the sample as the standard sampling point;

[0033] (2) The remaining test points should be evenly distributed within a 360° range around the test points and as close as possible to the edge of the sample.

[0034] Preferably, in order to ensure that the measurement results better reflect the uniformity of the color of the photochromic ink, the sampling points should be evenly distributed on the surface of the sample.

[0035] Preferably, in step (5), CIE1976L can be selected. * a * b * The color difference formula calculates the color difference of different optically variable inks under the same measurement conditions. The color difference calculation result is relatively large because the color information of the optically variable inks collected under the measurement conditions described in this invention is mainly caused by interference coloration. Small differences in the properties of the samples can cause large color differences. The tolerance can be reasonably set according to actual requirements.

[0036] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0037] The color measurement method for optically variable inks of this invention sets measurement conditions according to the angle θ between the spectral energy receiving direction and the specular reflection direction, and the angle β between the incident direction of the illumination source and the spectral energy receiving direction. The θ and β angles of the measurement conditions have a more obvious correspondence with the measured chromaticity values ​​of the ink. The color information obtained based on the θ and β angles specified in this invention can reflect the color differences between samples to a greater extent, which is beneficial for the color quality control of optically variable inks.

[0038] The color measurement method for optically variable ink of the present invention specifies the design principles of the measurement conditions θ angle and β angle, but does not specify the specific measurement angle. Therefore, the measurement conditions can be flexibly set according to the actual accuracy requirements, and there are many multi-angle measuring instruments that can meet the conditions.

[0039] The color measurement method for optically variable ink of the present invention requires at least two measurement conditions, has a small amount of measurement data, and has a strong ability to reflect the color differences of samples. While meeting the requirements of measurement accuracy, it improves work efficiency and can be used to test large batches of samples.

[0040] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but this does not imply any limitation on the scope of protection of the present invention. Attached Figure Description

[0041] Figure 1 This is a schematic diagram of the calculation method for angle θ and angle β in the color measurement method of the optically variable ink of the present invention.

[0042] Figure 2 This is a schematic diagram of the optical path in the color measurement method of the optically variable ink of the present invention, where θ = 15° and β angle varies from 10° to 140°.

[0043] Figure 3 In the color measurement method for photochromic ink of this invention, sample No. 0 is measured when the θ angle varies from 5° to 30° and the β angle varies from 10° to 140°. The measurement a for this sample... * b * Distribution of chromaticity values.

[0044] Figure 4 The color measurement method for photochromic inks of this invention refers to the measurement of photochromic ink samples No. 1-No. 5 under fixed measurement conditions where θ = 15° and β angle varies in steps Δβ = 10° within the range of 10° to 140°. * b * Colorimetric value distribution.

[0045] Figure 5 The color measurement method for photochromic inks of this invention refers to the saturation C of photochromic ink samples No. 1-No. 5 under fixed measurement conditions of θ = 15° and β angle varying in steps Δβ = 10° within the range of 10° to 140°. * Value distribution.

[0046] Figure 6 In Embodiment 1 of this invention, optically variable ink samples No. 6-No. 11 from the same manufacturer, measured under three conditions: (5°, 30°), (5°, 80°), and (5°, 120°), showed a... * b* Colorimetric value distribution.

[0047] Figure 7 In Embodiment 3 of this invention, the measurement positions of samples No. 12-No. 17 of the same type under the same measurement conditions (15°, 20°), (15°, 70°), and (15°, 110°) are a. * b * Colorimetric value distribution. Detailed Implementation

[0048] Unless otherwise specified, the instruments and measurement conditions used in the embodiments of the present invention are all conventional instruments and measurement conditions in the technical field of the present invention; the raw materials or samples used are all conventional models of raw materials or samples that are available on the market.

[0049] Example 1

[0050] like Figure 1 The diagram shown is a schematic of the calculation method for angles θ and β in the color measurement method of the optically variable ink of the present invention; let i be the angle between the incident direction of the illumination source and the normal direction of the sample plane. r The angle between the direction of spectral energy reception and the normal direction of the sample plane is i. d i r with i d The values ​​are non-negative under all circumstances. The calculation methods for angles θ and β are as follows:

[0051] θ=|i r -i d |;

[0052] β=i r +i d ;

[0053] There are two measurement conditions where both angle θ and angle β are the same. These two measurement conditions are equivalent to interchange the incident direction of the illumination source and the direction of spectral energy reception. Due to the principle of optical path reversibility, these two measurement conditions are equivalent and can be reasonably selected according to the measurement conditions of existing instruments. In addition, in the existing methods of representing measurement conditions, the sign of angle θ depends on whether the incident direction of the illumination source and the direction of spectral energy reception are on the same side or opposite side of the specular reflection direction. In this invention, the sign of angle θ does not affect the measurement result. Therefore, θ is set as the absolute value of the angle difference between the direction of spectral energy reception and the direction of specular reflection.

[0054] like Figure 2The diagram shows the optical path of the color measurement method for optically variable ink of the present invention, where the fixed measurement conditions are θ = 15° and the β angle is within the range of 10° to 140°, with a step size Δβ of 10°. The diagram marks the sizes of the θ and β angles under the measurement conditions, and different line colors represent different measurement results. It should be noted that, for the sake of readability, not all measurement conditions are shown. The optical path diagrams for three measurement conditions are shown: (15°, 10°), (15°, 70°), and (15°, 140°). The ellipsis indicates the omitted measurement conditions.

[0055] like Figure 3 The figure shows the measurement conditions for photochromic ink sample No. 0 in the color measurement method of photochromic ink of the present invention, with an θ angle ranging from 5° to 30° and a step size of 5°, and an β angle ranging from 10° to 140° and a step size of Δβ of 10°. * b * Colorimetric value distribution: Sample No. 0 is a conventional interference-type photochromic ink, which can exhibit a color change similar to golden yellow to green when the sample is tilted; Figure 3 Different line types and colors represent different θ angles, and different symbol types and colors represent different β angles. Measurement conditions with smaller θ angles are located further from the origin, and the sample receives stronger spectral reflection energy under measurement conditions with smaller θ angles. As the β angle of the measurement conditions changes, the hue of the measurement result changes, that is, the size of the β angle is related to the hue of the measurement result. In other words, measurement conditions with similar β angles have similar hues. Avoiding setting the β angles of measurement conditions too close can reduce the repetition rate of measurement information and allow a limited number of measurement conditions to play their full role. The β angles of different measurement conditions are distributed over a large range, which controls the color quality within the large color range of the optically variable ink.

[0056] like Figure 4 The figure shows the results for samples No.1-No.5 of this invention under fixed measurement conditions: θ = 15°, and β angle varying in steps Δβ = 10° within the range of 10° to 140°. * b * Colorimetric value distribution; among them, samples No.1-No.3 are from the same company and are all mica-type photochromic inks, showing red-yellow and yellow-green-yellow-green color changes respectively when the samples are tilted; samples No.4 and No.5 are from the same company and are both conventional interference-type photochromic inks, showing green-red and yellow-purple color changes respectively; the results show that the hue of photochromic ink samples of different types and different color-changing effects changes with the change of β.

[0057] like Figure 5The figure shows the saturation C of samples No.1-No.5 of this invention under fixed measurement conditions: θ = 15°, and β angle varying in steps Δβ = 10° within the range of 10° to 140°. * Distribution; saturation C when β is too large (greater than 120°) or too small (less than 20°) * The β value decreases significantly, and the ability to analyze sample differences under measurement conditions within this range is reduced. Therefore, it is not recommended to use measurement conditions within this range to analyze photochromic inks. When β is too large or too small, the hue changes faster while the saturation decreases. Therefore, the β value of the measurement conditions should not be too large or too small. Measurement conditions with a β angle that is too large or too small cannot reflect the differences between photochromic ink samples well.

[0058] Based on Figures 3-5 Based on the analysis, this invention proposes a method for determining the measurement conditions of optically variable ink color according to (θ, β). According to the limitations of the measurement conditions (θ, β) based on this invention, the following specific implementation scheme is designed:

[0059] A method for measuring the color of optically variable ink, comprising the following steps:

[0060] (1) Taking the angle-resolved spectral measurement system as an example, this measurement system consists of an angle-resolved spectral measurement platform, a deuterium-halogen combined lamp as the light source, an ultraviolet quartz fiber, and a cooled fiber optic spectrometer. Both the incident arm and the receiving arm can rotate 360°. It should be noted that the present invention is not limited to specific measuring instruments. Any measuring instrument that meets the design requirements of the present invention for the θ angle and β angle can be used.

[0061] (2) Place the sample horizontally on the measurement platform and fix it. The operator must ensure that the sample is not tilted relative to a certain angle or has uneven surface. The six optically variable ink samples used in this test are from the same batch of products from the same company. The samples are conventional interference-type optically variable inks printed on paper. The samples have no obvious differences in appearance and uniform thickness. When the samples are tilted for observation, they all achieve a yellow-green color change effect. These six samples are marked and numbered No.6-No.11. One sample (No.6) is used as the standard sample, and the other five samples (No.7-No.11) are used as test samples. The measurement results can be used to analyze the color differences between different optically variable ink samples.

[0062] (3) In this test, θ = 5°, β = 30°, 80°, 120°; a total of three measurement conditions: (5°, 30°), (5°, 80°), and (5°, 120°);

[0063] (4) Control the multi-angle measuring instrument with the software that is compatible with the multi-angle measuring instrument, and complete the color measurement of all samples one by one. The measurement point of each measurement should be at the same position of the sample, such as the geometric center of the sample, to avoid large differences in the results due to uneven coloring of the sample surface. It should be noted that the measurement system used in this test has a matching software, which is used to control the measurement conditions and derive the relative spectral reflectance. This invention does not specify that only the matching software can be used. Any method or other software that can control the measurement conditions and derive the experimental data can be used. The choice of matching software does not affect the final result of the experiment.

[0064] (5) Using this test, calculate the CIE1976L from the relative spectral reflectance. * a * b * The value is calculated using the CIE1964 standard observer and a D65 light source.

[0065] Table 1 shows the CIE1976L results for the six samples under the measurement conditions of this test. * a * b * value.

[0066] Table 1 Sample L * a * b * Value calculation results

[0067]

[0068] Appendix Figure 6 For the six samples in this embodiment, under the three measurement conditions of this test, a * b * The distribution of chromaticity values ​​is shown in the figure. Different symbol types and colors represent different measurement conditions, and different line types and colors represent different samples.

[0069] Calculate ΔE for different samples under corresponding measurement conditions. * ab Color difference, Table 2 shows the ΔE of sample No. 6 and the other five samples under the corresponding measurement conditions. * ab color difference calculation results.

[0070] Table 2. ΔE between sample No. 6 and samples No. 7-No. 11 * ab Color difference

[0071]

[0072] The color difference values ​​in Table 2 are mostly at a relatively high level. This is because the measurement conditions selected for this test have a small angle θ, which maps the differences in microscopic parameters between samples No.2-No.6 and sample No.1 to a relatively large ΔE. * ab Color difference. A reasonable tolerance should be set for parts where the manufacturing process precision cannot be achieved, based on the actual precision required. This can be addressed through ΔE. * ab The relative size is used to evaluate whether a sample is qualified.

[0073] For example, the color difference between sample No. 9 and standard sample No. 6 under the three measurement conditions of (5°, 30°), (5°, 80°), and (5°, 120°) is relatively small, indicating that sample No. 9 differs little from sample No. 6 within the main color-changing range. Conversely, sample No. 7 shows relatively significant differences from sample No. 6 within the main color-changing range, indicating a problem with the overall process of the photochromic ink. Furthermore, the color difference between sample No. 11 and the standard sample is relatively larger under the measurement condition of (5°, 30°), reminding operators to pay attention to the production process within a specific color-changing range of the photochromic ink.

[0074] The results of the tests in this embodiment can be used to analyze the color differences between different optically variable ink samples.

[0075] Example 2

[0076] Furthermore, the color uniformity of the surface of the optically variable ink sample is also an important quality attribute. In actual production, the printing of optically variable inks is quite difficult, and the ink layer is relatively thick, which easily leads to uneven printing on the surface and thus uneven color. This invention proposes a method for uniformly sampling and measuring the surface of the optically variable ink to reflect the color differences at different locations of the ink. The measurement results can be used to assess the color uniformity of different locations on the surface of the optically variable ink sample under corresponding measurement conditions.

[0077] A method for measuring the color of optically variable ink, comprising the following steps:

[0078] (1) The multi-angle measuring instrument and the parameter settings of the instrument used in this test are the same as those used in Example 1;

[0079] (2) Place the sample horizontally on the measurement platform and fix it. The operator needs to ensure that the sample has no inclination relative to a certain angle or unevenness on the surface. The sample used in this test is a color block printed with pearlescent interference type photovariable ink. When the sample is tilted, a similar blue - red change will occur. The appearance of the sample is uniform, and there is no obvious difference in color at different positions. Among them, the sampling point No.12 is the geometric center of the sample, and the sampling points No.13 - No.17 are at the same distance from the sample center and are evenly distributed in the 360° range outside the sample geometric center in a counterclockwise direction starting from directly above the sample. The measurement results can be used to analyze the color uniformity of the sample surface;

[0080] (3) In this test, set θ = 15°, β = 20°, 70°, 110°; there are a total of three measurement conditions: (15°, 20°), (15°, 70°), (15°, 110°);

[0081] (4) Control the multi - angle measuring instrument through the software supporting the multi - angle measuring instrument to complete the measurement of different positions of the sample one by one; the selection of the supporting software is the same as that in Example 1, and the selection of the software does not affect the final conclusion.

[0082] (5) Export the relative spectral reflectance through the software supporting the multi - angle measuring instrument, and calculate the CIE1976L * a * b * value. The calculation conditions are CIE1964 standard observer and D65 light source.

[0083] Table 3 shows the CIE1976 L * a * b * values of the sample in this example at different positions under different measurement conditions.

[0084] Table 3 Calculation results of LL * a * b * values at different positions of the sample

[0085]

[0086] Appendix Figure 7 shows the distribution of a * and b * chromaticity values of the sample in this example at different positions under the three measurement conditions of (15°, 20°), (15°, 70°), and (15°, 110°); different symbol types and colors in the figure represent different measurement conditions, and different line types and colors represent different measurement positions.

[0087] Sampling point No. 12 is the standard sampling point. Calculate the color difference between sampling point No. 12 and the five sampling points No. 13-No. 17. Each sampling point has three measurement results, corresponding to three measurement conditions. Calculate the ΔE between different sampling points and the standard sampling point. * ab Color difference, the measurement results can be used to analyze the differences in color properties at different locations of the same sample. Table 4 shows the ΔE between the middle of the sample and positions No.7 to No.12. * ab Color difference calculation results.

[0088] Table 4 shows the ΔE between the middle position and the other five positions of the sample. * ab Color difference

[0089]

[0090] The color difference values ​​in Table 4 are at a relatively high level, meaning that the measurement conditions set in this test mapped the minute differences in the microscopic parameters of the optically variable ink surface to a relatively large ΔE. * ab Color difference, ΔE * ab The relative magnitude of the color difference reflects the color uniformity of the surface of the optically variable ink sample; for example, the ΔE between sampling points No.13 and No.14 and the center sampling point in this embodiment. * ab The color difference is relatively small, and these two sampling points are located above the geometric center of the sample. The color difference in other directions is relatively large, which indicates that there is uneven printing in some directions during the printing process of the sample to be tested. Combined with the equipment accuracy, we can determine whether the sample is qualified.

[0091] The color measurement method for photochromic ink of the present invention determines the measurement conditions by using the angle θ between the spectral energy receiving direction and the mirror reflection direction, and the angle β between the incident direction of the illumination source and the spectral energy receiving direction. The measurement results under these conditions can reflect the color change differences of the photochromic ink under different observation angles, as well as the color reproduction consistency over a large color change range. By fixing the photochromic ink sample on a horizontal plane and setting multi-angle measuring instruments to measure the sample, the color measurement results of the sample are obtained. The color difference of different samples under corresponding measurement conditions, or the color difference of the same sample at different positions, can be analyzed.

[0092] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A color measurement method of optically variable ink, comprising the following steps: (1) using a multi-angle measuring instrument, connecting the measuring instrument with a computer, and opening the software matched with the multi-angle measuring instrument in the computer; (2) placing the sample to be measured on the measuring platform; (3) setting the measurement conditions: the angle θ between the spectral energy receiving direction and the mirror reflection direction, and the angle β between the illumination light source incident direction and the spectral energy receiving direction; (4) controlling the multi-angle measuring instrument through the software matched with the multi-angle measuring instrument to complete the measurement of all samples one by one; (5) Using the software that comes with the measuring instrument to export the measurement results L * a * b * chroma values, calculating the color difference of the corresponding measurement conditions of multiple measurement results, and giving an evaluation of whether the sample is qualified or not. The angle θ is less than or equal to 15° and greater than 0°; The minimum value, maximum value, range and interval of the angle β are limited as follows: 1) The minimum value of the angle β is 20°; 2) The maximum value of the angle β is 120°; 3) The difference between the maximum value and the minimum value of the angle β is greater than or equal to 60°; 4) The measurement conditions contain multiple different angles β, in order to increase the ability of the measurement conditions in the set angle β range to reflect the color difference of the sample, make the measurement conditions with different angles β measure an equal interval of color tone change, control the interval Δβ of the angles β of adjacent measurement conditions, and as the angle β increases, the color tone change presents a slow-then-fast state, set Δβ to decrease as the angle β increases.

2. The method of color measurement of optically variable ink according to claim 1, characterized in that: In step (1), the multi-angle measuring instrument is a multi-angle spectrophotometer or an angle-resolved spectral measurement system.

3. The method of color measurement of optically variable ink according to claim 1, characterized in that: In step (2), the multi-angle measuring instrument does not have a matched measuring platform.

4. The method of color measurement of optically variable ink according to claim 1, characterized in that: In step (3), the measurement conditions include the θ angle and the β angle, denoted as (θ, β), where i is the angle between the incident direction of the illumination light source and the normal direction of the sample plane r , i is the angle between the direction of the received spectral energy and the normal direction of the sample plane d , i r , and i d are all non-negative values, and the θ angle and the β angle are calculated as follows: θ = | i r - i d |; β = i r + i d .

5. The method of color measurement of optically variable ink according to claim 1, characterized in that: In step (3), the angle θ of all measurement conditions is the same.

6. The method of color measurement of optically variable ink according to claim 1, characterized in that: In step (3), the range of the angle β is [0, 180°-θ].

7. The method of color measurement of optically variable ink according to claim 1, characterized in that: In step (3), the applicable range of the measurement conditions includes but is not limited to: conventional interference type optically variable ink, pearl interference type optically variable ink, liquid crystal type optically variable ink or magnetic optically variable ink.

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