There are two ways to match the color of an original in print production. One method is to compare the color of the original with the color spectrum or standard ink matching guide, and select the closest color to match based on visual observation. Another method is to measure the target chromaticity value and use a mathematical model to convert the target chromaticity value into a dot coverage set. However, in the matching process, metamerism chromatic aberration will occur due to the change of the light source, and this chromatic aberration can be quantified by calculation.
1. Match with sample system
The so-called color sample system is a collection of fixed colors. These colors are arranged in order of hue, saturation and brightness, and Munsell chromatography is an example.
Munsell color spectrum is an absolute color reference system. Its response in the printing industry is limited due to the following reasons. First, there are very few people who own Munsell chromatograms; second, some colors in Munsell chromatograms cannot be reproduced by ordinary ink printing, while some The color that can be reproduced with printing ink is not found in the Munsell color spectrum; third, the method of copying the color is not described on the Munsell color spectrum. The printer must study the method of copying it after selecting a color; in addition, the color sample The interval between them is large, which is not enough to accurately match known colors.
The sample system commonly used in the printing industry is a relative sample system. In the relative sample system, the color of the sample is set relative to the ink, paper, and existing process conditions. After selecting a color, there will be a corresponding formula or a set of screen tone values ​​provided to the printer, which is important information on how to reproduce this color in practice.
There are two types of relative sample systems used in the printing industry. One is the ink matching guide, which is suitable for the copying of spot colors such as tables and advertisements; if you want to match the target color such as a special background color, you should use a mesh tone printing chromatogram.
1. Ink guide. A typical ink matching guide consists of a series of solid colors. These colors are made up by mixing different inks. Each color is composed of two of 8 or 10 color inks and white ink or black ink. Most of them are mixed with no more than 3 or 4 inks.
The ink distribution guide is printed with the standard ink layer thickness, and the deviation meets the stipulated standards. It is printed with coated paper and uncoated paper respectively.
2. Mesh chromatogram. The color spectrum is overprinted by 3 or 4 color ink dots. Each color spectrum is based on the commonly used basic percentage (such as cyan), and then the second color percentage of dots (such as magenta) is printed, increasing from 0% at equal intervals. To 100%; the percentage of halftone dots of the third color increases from 0% to 100% column by column. The next page of the chromatogram increases the percentage of the dots of the basic color (such as cyan), and the other remains unchanged, and so on, until the basic color reaches 100%.
Chromatography can be purchased, or can be purchased to make self-made plates. When making your own paper, you should use the paper and ink of our factory. In addition to considering the reflectance and smoothness characteristics of paper and ink, the solid density of the ink, the increase of the dots, the color sequence and the overprint rate must also match. A chromatogram that meets (the factory's standard) production conditions is very useful.
The overall design of the chromatogram is also very important. The design method can refer to the existing chromatogram, and several representative chromatograms are now introduced.
The standard Scott chromatography is an 11 × 11 grid format. The first page is overprint of yellow and magenta. The coverage of yellow in each column is the same, from left to right from 100% to 0%. The coverage of magenta in each row is the same, from top to bottom from 0% to 100%; next Each cell of the page is printed with the same cyan coverage, and the other is the same as the first page; the third page is not printed with cyan, and is replaced with 5% black. Continue in this mode until 80% of the black sum 100% green combination is completed. Then use cyan and black as the basis, and let yellow and magenta be overprinted in the order of increasing coverage of the same spot. Chromatographic paper is divided into coated paper and non-coated paper.
Kueppers chromatography is printed with a non-color structure. It mainly includes â‘ overprinting of black ink with yellow products, yellow cyan and magenta respectively; â‘¡overprinting of three-color inks; â‘¡theoretic explanation of non-color structure. A total of 5500 color blocks.
The ideal chromatographic design should meet the following requirements: â‘ Color sequence: color blocks with similar hues are adjacent to each other; â‘¡Black version: should include a combination of single color, two-color, three-color and black; â‘¢Small specifications: To reduce costs, Easy to carry and simple to use; â‘£ Level change interval: Standard accuracy can reach 9 to 11 levels, including 5% of steps, or 90% or 100% of steps should be removed, the interval should be as close as possible to the visual Equal steps.
Another form of chromatography is made of transmissive color plastic film. The film is a commonly used pre-proofing film, and the mode of increasing yellow, magenta, and cyan inks is still used. There are also disc-shaped combinations in which the dot percentage is changed by rotating the film.
The advantages of using a color transparent film to make a color spectrum: the color display is dynamic rather than static, which can easily simulate thousands of colors, the specifications are small and cheap, and the paper can be placed under the transparent film to simulate the effect of paper properties on color . The disadvantage is that it does not simulate colors as accurately as the actual printed color spectrum. [next]
Second, match mathematically according to process conditions
The chromaticity value obtained by color measurement is often RGB or tristimulus value or cmy value, and the color rendering of the dot image is finally implemented in the combination of color dots. Therefore, in the dot image measurement and control technology or the electronic color separation system, according to the measurement It is the most common thing to convert the obtained tristimulus value to match the dot coverage value of the color. There are three ways to convert these data: The first method is to use the mathematical model, namely the Newcastle equation, to convert the measured color block or pixel. This method is accurate, but the algorithm is inefficient; the second method is to use mathematics The analytical method is converted, but the accuracy is poor; the third method is to establish a lookup table through chromatographic measurement, and convert each measured color through the lookup table. This method has a fast conversion speed, but the conversion accuracy is directly the same as that of the lookup table. Precision is related.
1. Use the Newcastle equation for conversion. The original form of the Newburgh equation has limited accuracy, so some methods of modifying the Newburgh equation have emerged. Ilvon Bob Lavsky and Milton Pearson proposed a modified Newburgh equation, the formula is as follows :
Xe / Yf / Zg = ∑fnn = 8 n = 1 (Xn) e / (Yn) f / (Zn) g
Where X, Y, Z-the tristimulus value of the color to be matched;
Xn, Yn, Zn-the tristimulus value of Newcastle color element, if it is yellow, magenta, and cyan printing, there are 8 Newcastle color elements;
fn——The percentage of Newcastle color element in forming the target color;
e, f, g-are the correction coefficients of tristimulus value XYZ respectively.
When the 8 Newcastle color elements form the target color, the percentage of each component is represented by the following formula:
Paper white: n = 1 f1 = (1-c) (1-m) (1-y)
C youth: n = 2 f2 = c (1-m) (1-y)
M magenta: n = 3 f3 = (1-C) (1-Y)
Y yellow: n = 4 f4 = y (1-c) (1-m)
C + M cyan + magenta: n = 5 f5 = c · m (1-y)
M + Y magenta + yellow: n = 6 f6 = m · y (1-c)
C + Y green + yellow: n = 7 f7 = c · y (1-m)
C + M + Y cyan + magenta + yellow: n = 8 f8 = c · m · y
c, m, y—the coverage of the three primary color dots to be solved and constituting the color to be matched.
With the help of three correction coefficients, the dot coverage rate c, m, y in one measurement point can be corrected. The correction coefficients e, f, g can be calibrated according to (c, m, y) = (50, 50, 50).
The accuracy obtained by this correction method is medium. In order to improve the accuracy of the solution, the method of solving the Newburg equation can be used in a lattice. This method divides the CMY color space into 8 squares (Figure 2-10). These squares are the three primary color ink solid blocks and the three primary color ink 50% dot blocks in different ways combined and overprinted. The number of vertices is increased from 9 to 27 in the previous modified form, which greatly improves the accuracy of the solution, but the calculation workload is very large.
When using the Newcastle equation to convert, the calculation efficiency is very low, which is not suitable for real-time measurement and control. In order to meet the needs of real-time measurement and control, a matrix transformation method has emerged.
(Figure 2-10) [next]
2. Use matrix transformation to convert. Due to density addition failure and density proportional failure, the CMY space is nonlinear. But imitating the principle of the density balance equation, the absorption values ​​of the three primary color inks of yellow, magenta, and cyan are defined by tristimulus values, and the conversion between XYZ and CMY can be linear conversion. This conversion is not only fast in operation, suitable for real-time measurement and control, but also reversible.
When the effective absorption rate is defined according to the chromaticity value XYZ, the white of the paper itself and the black solidly printed on the three-color ink are used as the basic points for calculating the effective absorption rate. The conversion expression of effective absorption rate Ax, Ay and Az is as follows:
Ax = 1-X-Xs / Xw-Xs
Ay = 1-Y-Yx / Yw-Ys
Az = 1-Z-Zs / Zw-Zs
In the above formula, X, Y, and Z are the tristimulus values ​​of the converted color, Xw, Yw, and Zw are the tristimulus values ​​that are overprinted on the paper in the XYZ space, and Xs, Ys, and Zs are the three primary colors that are solidly printed on the paper Stimulus value. Therefore, the following linear conversion is established:
Ax Ay Az = (Aik) cmy (2-1)
The above formula includes the effective absorption rate Ax, Ay, Az and dot coverage rate cym, the conversion matrix (Aik) is determined according to the effective absorption rate of the three primary color inks, namely:
Aik = Axc100 Axm100 Axy100
Ayc100 Aym100 Ayy100
Azc100 Azm100 Azy100 (2-2)
Because the tristimulus values ​​of the color to be matched XYZ and the three primary colors of ink can be measured in advance and converted into an effective absorption rate, the dot coverage rate cmy can be obtained according to the formula (2-1).
This first-order matrix linear conversion method has a small amount of calculation and is suitable for real-time measurement and control, but its accuracy is not high. When the color to be matched is a monochrome, the linear conversion effect is very good. If the color to be matched is composed of two or three ink dots, the conversion will cause a large deviation. In order to improve the conversion accuracy, the second-order matrix conversion method can be further adopted. When converting from CMY to XYZ, the following quadratic interpolation is used:
Ax = Axc · c + Axm · m + Axy · y + A2xc · c2 + A2xm · m2 + A2xy · y2 + Axcm · c · m + Axcy · c · y + Axmy · m · y (2-3)
Similarly, Ay and Az formulas can be listed.
When converting from XYZ to CMY, use the following quadratic interpolation:
C = Acx · Ax + Acx · Ay + Acz · Az + A2cx · A2x + A2cy · A2y + A2cz · A2z + Acxy · AxAy + Acxz · Axz + Acyz · AyAz (2-4)
The quadratic interpolation of m and y can also be listed.
The second-order matrix conversion formula is as follows:
c / m / y / cm / cy / my / x2 / m2 / y2 = Aik · Ax / Ay / Az / AxAy / AyAz / AxAz / (Ax) 2 / (Ay) 2 / (Az) 2 (2-5 )
In the above formula: AxAyAz is the effective absorption rate converted from the XYZ tristimulus value, cmy is the dot coverage, and Aik is the coefficient to be determined. This is a 9 × 9 matrix and obtained from 9 calibration points
100% c, 100% m, 100% y
50% c, 50% m, 50% y
100% c + m, 100% c + y, 100% m + y
The second-order matrix conversion formula improves the conversion accuracy and is suitable for real-time measurement and control. However, except for the 11 calibration points, there are errors, and the largest deviation occurs in the overprint color.
In order to obtain higher conversion accuracy than the second-order matrix model, a fourth-order matrix model can be used. The fourth-order polynomial is expressed by the following matrix formula:
c / m / y / cm / cy / my / x2 / m2 / y2 / c2m2 / c2y2 / m2y2 = Aik · Ax / Ay / Az / AxAy / AyAz / AxAz / (Ax) 2 / (Ay) 2 / (Az ) 2 / (Ax · Ay) 2 / (Ax · Az) 2 / (Ay · Az) 2 (2-6)
Aik is a 12 × 12 matrix determined by 12 calibration points, they are:
3 solid colors: 100% c, 100% m, 100% y;
3 50% mesh toning: 50% c, 50% m, 50% y;
3 solid overprint colors: 100% c + m, 100% c + y, 100% m + y;
Three 50% mesh tone overlay printing colors: 50% c + m, 50% c + y, 50% m + y.
The fourth-order matrix model can also be used for real-time measurement and control, and the number of calibration points has been increased to 14. [next]
Three, using the look-up table method to convert
The density of basic points in the lookup table is very important. For example, the 27 vertices in Figure 2-10 can be used as the basic points of the lookup table. For a certain color to be matched, you can find the four nearest basic points, and then use linear interpolation to solve. The table look-up method can be used for real-time measurement and control with medium conversion accuracy.
4. The problem of metamerism between processes
If you observe a piece of yellow paper and a piece of white paper under yellow light, the colors of the two will be the same, this phenomenon is called metachromatic. Generally, when some objects with different spectral curves are illuminated with colored light, if the colors of the objects look the same, this form of metamerism will hardly cause any problems in practice. And when two objects look the same in daylight, if you change the lighting a little, it becomes an object with a different color, which will cause problems in practice. Only when the colors being compared are produced by different pigments, will the metamerism color difference occur.
If non-photographic manuscripts, such as textiles and metal surfaces are used for manuscript copying, the largest metamerism chromatic aberration will occur, and watercolor pigments are also sensitive to metamerism. Copying with photographic manuscripts, there is less risk of homochromatic color difference, because color photos and four-color prints are based on the three primary colors to express colors, this is similar, the same color may also occur between mechanical proofing and pre-proofing Heterospectral deviation. There will never be a problem of metamerism between mechanical proofing and printed matter, because the inks used in both cases have the same spectral properties. In the printing process, there are mainly two cases of metamerism deviation.
â‘ The first case: when copying a manuscript, if you observe it under the D50 light source, the color of the manuscript and the copy (which may be a pre-proof proof or a mechanical proof) is matched, and then observe under the D65 light source Originals and replicas will appear metamerism color difference (Figure 2-11). When discussing the problem of metamerism, it is assumed that the three primary colors of the original and the pigments of the ink have different spectral curves.
(Figure 2-11)
â‘¡The second case: a manuscript is copied into a pre-proofing proof by this method: Observation under D50 light source makes the color of the pre-proof proof match with the color of the original, and then prints on the basis of the pre-proof proof. Color matching is achieved under D50 light source. In this case, the printed image and the original are also accurately color matched under the D50 light source, but if the pre-proofing and the printed image are compared under the D50 light source, you will find metamerism and chromatic aberration (right in Figure 2-11).
In order to determine the two metamerism chromatic aberrations shown in Figure 2-11, four color measurements are required. The first and second times are to measure the original and its proof to identify the original and it under a certain light source. The color matching of proofing; the third and fourth color measurement involve the same color heterochromatic color difference caused by the second kind of lighting. The effect of proofing often loses the matching of the color of the original manuscript at the lower limit of another kind of lighting. There is another layer of metamerism color difference.
In order to evaluate the sensitivity of a metamerism phenomenon in the process of printing and copying, the colors listed in Table 2-5 can be used as test samples. These colors are common colors in life. There are a total of 17 colors in the table, and the average value of the metamerism of the 17 colors can be regarded as a measure of the sensitivity of the replication method to metamerism. This sensitivity depends on the choice of primary colors and the type of lighting.
Table 2-5 Color name ΔE (CIELAB) A light source D65 light source flesh color 3.74 0.77 light brown 4.08 1.55 yellow green 5.34 2.10 green 3.09 0.91 blue green 3.62 1.23 bright blue 2.02 0.83 light purple 2.51 1.30 red purple 0.98 0.71 standard red 1.51 0.59 standard yellow 2.88 1.11 Standard green 2.18 0.79 Standard blue 1.90 1.62 Skin color 2.38 0.82 Leaf green 6.72 2.63 Dark gray 5.59 2.25 Medium gray 4.84 1.90 Bright gray 0.74 O.26 Average 3.18 1.26
(Figure 2-12)
The theory points out that as long as the spectral curves of two object colors intersect at least 3 points, a metamerism phenomenon will occur (Figure 2-12), but the magnitude of the metamerism color difference cannot be derived only from the difference in the spectral curves. Table 2-5 lists the metamerism chromatic aberration that occurs when the illumination is changed from D50 light source to A light source and D65 light source, respectively. It can be seen from Table 2-5 that the conversion of the metamerism from the D50 light source to the A light source has a larger metachromatic deviation than the conversion to the D65 light source.
In most cases, the metamerism color difference is 1 to 5 units. If one color difference unit is equivalent to 1 unit of mid-tone dot coverage change, then the metamerism color difference is equivalent to 1% of dot coverage. 5% change. Some laws can be derived. The metamerism sensitivity of the three colors is relatively strong, such as light brown, green, flesh, leaf green, etc., and the gray tone depending on the color is also more sensitive; light cyan is less sensitive, such as light blue , Light purple, etc .; saturated colors are most sensitive to metamerism, such as red, yellow, green, and blue. There is a relationship between the dot coverage required to reproduce these colors and the metamerism sensitivity: when the dot area of ​​yellow, magenta, and cyan inks is relatively large, they are very sensitive to metamerism. If the dot area of ​​the three primary colors is small, then The sensitivity is less; if only one or two primary colors are printed, the metamerism sensitivity can be said to be very slight.
When evaluating printed reproductions, if the customer uses the internationally recommended matching light source D65, then because the illumination of the photo original matching is D50 light source, the change from D50 to D65 becomes an important issue, and the color difference generated in most cases is almost invisible Out, nevertheless, the D50 light source should be used to compare the manuscript and its replicas to achieve agreement with the customer.
When the pre-proofing method was adopted, it was suspected that it had stronger metamerism sensitivity than ink printing. In fact, this doubt was unfounded, because the pre-proofing system is similar to the printing method, and the colors are organized with three primary colors. .
In order to check the metamerism color difference between an original and its pre-proofing, experiments were conducted on the following pre-proofing systems to determine the extent to which they can produce metamerism color difference: Cromalin (DuPont), Matchprint (3M), Color ï¼Art (Fuji) (the above belongs to the photoelectric mechanical method); Stork Color Proofing (stock photography method); Hell CP403 (Hull Company) (digital proofing method); Ink Jet (Diablo) (inkjet printing method) ); Thermal transfer (Panasonic) (thermal transfer method).
Table 2-6 The average method obtained from the 17 colors in Table 2-5 ΔE (CIELAB) Lighting A Lighting D65 Matchprint 3.13 1.31 Cromalin 2.46 0.92 Color-Art 2.61 0.93 Stork 2.10 0.78 HellCP403 1.09 0.47 Ink Jet 2.56 1.01 Thermal transfer 2.38 0.93 Four-color printing 3.18 1.26
Table 2-7 Method ΔE (D65) Matchprint 0.07 Cromalin 0.45 Color-Art 0.36 Stork 0.70 Hell CP403 1.12 Ink Jet 0.53 Thermal transfer 0.49 Average 0.53
It can be seen from Tables 2-6 that the metamerism color difference generated when printing ink is greater than most other cases. It can be expected; the metamerism difference between the pre-proofing and the printed matter is small, because the three primary colors used in the pre-proofing are very similar to the spectral curve of the printing ink, and the metamerism deviation obtained by different pre-proofing systems and non-contact printing methods It is listed in Tables 2-7. From the table, the metamerism deviation is generally very low.
In short, there are two metachromatic color differences in printing, namely the color difference between the original and its replica and the color difference between different replicas, such as the color difference between the printed matter and the pre-proof.
The metamerism and chromatic aberration only cause interference in the first case.
The second metamerism color difference must be small. Studies have shown that in this case, no disturbing metamerism chromatic aberration occurs at all, which is also suitable for the conversion from D50 lighting to D65 lighting. Therefore, it is permissible to compare a manuscript with a proof sample under the D50 light source, and then compare a printed sample with the pre-proof sample under the D65 light source. There are some differences between different pre-proofing systems (and non-contact methods), because some methods are based on the three primary colors as printing inks, while others use three primary colors that are closer to the silver salt lighting method. The first case produces a very slight metachromatic chromatic aberration (eg Matchprint), the latter will produce a slightly larger metachromatic chromatic aberration.
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