JP4269719B2 - Color estimation method and color image forming apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a color estimation method for estimating a color obtained by overprinting and printing a color material and a color image forming apparatus to which the color estimation method is applied, and particularly to a case where the color material to be overprinted contains a pigment.
[0002]
[Prior art]
In recent years, with the spread of DTP (Desk Top Publishing) and the like, the work of image editing and page imposition on computer software has become common, and full digital editing has become rare.
[0003]
In these processes, aiming for further efficiency, image setter output that directly outputs image data that has undergone page editing on film, CTP (Computer to Plate) output that directly records images on a printing plate, and printing A CTC (Computer To Cylinder) is performed in which image recording is directly performed on a printing plate wound on a cylinder of the machine.
[0004]
In this case, film output or printing plate output is performed only for calibration confirmation, and printing calibration or calibration with other calibration materials has the problem that waste of film and printing plate and unnecessary work increase. .
[0005]
Therefore, in particular, in a process of creating and editing a full digital image by such a computer, a system for directly outputting a color image called DDCP (Direct Digital Color Proof) or DCP (Digital Color Proof) is required. .
[0006]
Such DDCP records digital image data processed on a computer on a plate-making film with an image setter or the like, performs a final printing operation for directly producing a printing plate with CTP, or a printing machine with CTC. Create a color proof that reproduces the output object indicated by the digital image processed on the computer before recording the image directly on the printing plate wound on the cylinder, and the design, color, text, etc. This is to confirm.
[0007]
Moreover, in the calibration process in such a printing process, (1) confirmation of internal errors in the work site, that is, internal school, (2) external school to be submitted to the orderer and designer for confirmation of finishing, (3) printing Proofs are created and used in three main applications: a print sample provided to the captain of the machine as a sample of the final print.
[0008]
As described above, when color printing is made, color calibration is performed at the stage of the original film, so that the Y (yellow) plate, the M (magenta) plate, the C (cyan) plate, and the BK (black) plate are obtained. Produce a proof (color proof) using each color separation net original film, and make the original printing plate, the color of the original film is correct, the color is correct, and there is no character error. Inspecting these items, etc., the finished product is confirmed in advance.
[0009]
On the other hand, based on halftone dot image data of each color separation halftone document, a silver spot color photosensitive material is exposed to a light spot composed of a combination of a plurality of lights having different wavelengths such as R, G, B, etc. There is a color proof creating apparatus that creates a color proof by reproducing each halftone image by developing the dots of basic colors Y, M, and C. In addition, the primary color (single ink color) of the printing press, the primary color (color obtained by overlaying the ink) and white (the color of the printing paper) and white (the color of the printing paper) that are obtained by overprinting the ink are reproduced. For each of color, multi-order color, and white, there are some that are output by adjusting and superimposing the output intensity of three basic colors (for example, Japanese Patent Application No. 2001-348371 (paragraphs [0119]-[0168]). ], FIG. 5))).
[0010]
The primary color, the multi-color, and white are output and measured by a printing press to obtain the characteristic values of the colors. That is, if the ink used in the printing press is Y (yellow), M (magenta), C (cyan), K (black), measurement is performed for 16 types of 1/2/3/4 color and white. It was. Further, in the case of using special colors, the measurement was performed for a total of 32 patterns including these 16 patterns and a color obtained by superimposing the respective special colors.
[0011]
[Problems to be solved by the invention]
However, the same spot color is not always used, and in most cases the printing amount is limited. Furthermore, since different colors are the same color or the same color is different in lot or the color changes, it is necessary to measure the color in which the special colors are overlaid on the above 16 patterns each time. I spent the measurement.
[0012]
Therefore, a color estimation method has been proposed in which special colors are overprinted in 16 types of 1/2/3/4 color and white. As an example, there is a method of estimating the color obtained by simply adding the spectral densities for each wavelength of light, and if the ink contains a dye, the measured value and the estimation are sufficiently close, but the ink is When a pigment is included, a close one cannot be obtained due to light scattering by the pigment. Of course, when the spectral density is added, the spectral density of the lower ink is corrected by the transparency coefficient of the upper ink, and the spectral density of the upper ink is superimposed on the lower ink. Correction and addition are performed using a trapping rate coefficient for correcting a change in glue, but the same transparency coefficient and trapping rate coefficient are used for a wavelength indicating a relatively high density and a wavelength indicating a relatively low density of ink to be overprinted. In some cases, the result is greatly different. For example, the obtained spectral density may be larger than the measured value at a wavelength indicating a relatively high density.
[0013]
The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an upper side of the spectral density of the ink which is the lower side when the ink containing the pigment is overprinted, and the upper side of the color density. It is an object of the present invention to provide a color estimation method that can be accurately and easily estimated from the spectral density of ink and the spectral density of printing paper, and a color image forming apparatus to which the color estimation method is applied.
[0014]
[Means for Solving the Problems]
In order to achieve the above object, the invention described in claim 1 is a color estimation method for estimating a color obtained by overprinting and printing when a plurality of color materials are overprinted on a recording medium. Measuring a spectral density D1 (λ) with respect to each wavelength λ of light when the first color material containing the pigment which is the lower side when printed is printed on the recording medium, and the printing Measuring a spectral density D2 (λ) with respect to each wavelength λ of light when the second color material containing the pigment that is on the upper side when printed is printed on the recording medium, and each of the light of the recording medium A step of measuring a spectral density DW (λ) with respect to a wavelength λ, a value proportional to a difference between the spectral density D1 (λ) and the spectral density DW (λ) for each wavelength λ of the light; A value proportional to the difference between the density D2 (λ) and the spectral density DW (λ), The spectral density DW (λ) is added, and a value proportional to the difference between the spectral density D1 (λ) and the spectral density DW (λ), the spectral density D2 (λ), and the spectral density DW (λ The value obtained by subtracting a value proportional to the product of the difference between the first color material and the second color material from the addition result, and printing the second color material on the first color material. And a step of setting the spectral density DM (λ) of the color.
[0015]
According to a second aspect of the present invention, the color system value of a color obtained by printing the second color material on the first color material from the spectral density DM (λ) is printed. The method further includes a step of calculating.
[0016]
The invention described in claim 3 is characterized in that the first color material includes a plurality of color materials each including a pigment.
[0017]
According to a fourth aspect of the present invention, a target halftone image line color obtained by printing each halftone image of a plurality of color materials on a recording medium is printed using a plurality of basic colors. A color image forming apparatus for reproducing and forming a color image, comprising: a reference table of combinations of output intensities of a plurality of basic colors with respect to a plurality of color characteristic values; and a lower pigment when overprinting Spectral density table storing a spectral density D1 (λ) for each wavelength λ of light and a spectral density DW (λ) for each wavelength λ of light of the recording medium when the first color material is printed on the recording medium. And an input means for inputting a spectral density D2 (λ) with respect to each wavelength λ of light when the second color material containing a pigment on the upper side when printed is printed on the recording medium; The spectral density D2 (λ) is input. The spectral density D1 (λ) and spectral density DW (λ) stored in the spectral density table and the input spectral density D2 (λ) are used for each wavelength λ of the light. A value proportional to the difference between the spectral density D1 (λ) and the spectral density DW (λ), a value proportional to the difference between the spectral density D2 (λ) and the spectral density DW (λ), and the spectral density The density DW (λ) is added, and a value proportional to the difference between the spectral density D1 (λ) and the spectral density DW (λ), the spectral density D2 (λ), and the spectral density DW (λ) Obtained by subtracting a value proportional to the product of a value proportional to the difference between the first color material and the second color material on the first color material based on the result. Color calculation means for obtaining the color characteristics of the color, color characteristics obtained by the calculation means, and the reference table Generating a color channel table by obtaining a combination of output intensities of the plurality of basic colors for reproducing the target color, and when forming the color image, based on the color channel table, Control means for changing the output intensity of the plurality of basic colors.
[0018]
Further, the invention according to claim 5 is characterized in that the first color material includes a plurality of color materials including a pigment printed on each other.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, in describing the color estimation method of the present invention, an embodiment applied to a color proof creating apparatus (color image forming apparatus) is shown as an example, and will be specifically described with reference to the drawings.
[0020]
(Control configuration of color proofing device)
First, the color proof creating apparatus according to the present embodiment targets a color of a halftone dot line portion obtained by printing each halftone dot image of a plurality of inks on a printing press, and the target color is a color proof. A plurality of basic colors (Y (yellow), M (magenta), C (cyan)) of the creation apparatus may be described in the order of “C, M, Y”, “YMC”, “CMY”. The color estimation method of the present invention is applied.
[0021]
FIG. 1 shows a functional block diagram of the color proof producing apparatus. As shown in FIG. 1, the color proof creating apparatus according to the present embodiment includes a control unit 1, a color calculation unit 2, a color channel creation unit 3, and an image recording unit 4. Further, details of each part will be described below.
[0022]
The color calculation unit 2 includes a color calculation unit 21, a spectral density table 22, a coefficient table 23, a color table 24, and an input unit 25.
[0023]
The spectral density table 22 is a CMYK1 / 2/3/4 color and white background (ground color of the printing paper) printed on a printing paper as a recording medium using a C, M, Y, and K ink. The spectral density measured in advance is stored.
[0024]
Here, the spectral density is, for example, the reflectance R is measured by changing the wavelength every 10 nm between 380 nm and 730 nm,
-0.5 x LOG₁₀(R)
The density is converted by the above formula, which is the density for each wavelength.
[0025]
The CMYK1 / 2/3/4 quaternary colors will be described. The primary color is one color in CMYK, the secondary color is a combination of two of CMYK, and the tertiary color. A color combining three of CMYK and a quaternary color are colors combining four of CMYK. C, M, Y, K, R (M + Y), G (C + Y), B (C + M) , ..., 15 colors of Y + M + C + K.
[0026]
Further, FIG. 2 shows the configuration of the spectral density table 22. As shown in FIG. 2, it is composed of 16 colors including a white background printed using C, M, Y, and K inks on a printing paper and their spectral densities. However, each density value is omitted because the figure becomes complicated.
[0027]
The coefficient table 23 stores a transparency coefficient a, a trapping rate coefficient b, and a correction coefficient c used for color estimation described later. The transparency coefficient a is a coefficient for correcting the spectral density when the spot color is printed over the spot color of the lower color, and the trapping ratio coefficient b is the value when printing on the printing paper. It is a coefficient for correcting a difference in spectral density generated due to a difference in so-called ink paste when spot color is overprinted on ink. The correction coefficient c is a coefficient used for a correction term included in an expression used for color estimation described later. Details of each correction coefficient will be described later.
[0028]
Further, the spectral density table 22 and the coefficient table 23 are, for example, those incorporated from the beginning in an HDD (Hard Disk Drive) provided in the color proof creating apparatus, FD (Floppy (registered trademark) Disk), or the like. It may be possible to put it in and out and attach it only when estimating the color.
[0029]
The input means 25 is for allowing the spectral density of the special color ink printed on the printing paper to be input, and may be an interface for connecting to a measuring device. The input means 25 is an FD (Floppy (registered trademark) Disk) to which the measurement result is input. ) Etc., and may be attached only when inputting the measurement result.
[0030]
The color calculation means 21 obtains each color system value L * a * b * value from the CMYK1 / 2/3/4 color and the white background spectral density stored in the spectral density table 22, and the color table 24. To store. Further, from the spectral density obtained by measuring the spot color, the CMYK1 / 2/3/4 color and the white background spectral density stored in the spectral density table 22, and the coefficients stored in the coefficient table 23. , The spectral density of the color obtained by overprinting the special colors on each of the CMYK1 / 2/3/4 quaternary colors is calculated, and the color system value L * a * b * is obtained. Store. A detailed calculation method will be described later.
[0031]
The color table 24 has values L * a * b * values of the above-described CMYK1 / 2/3/4 color and CMYK1 / 2/3/4 color and a color system of white color and a white color system. Are stored in a configuration as shown in FIG. FIG. 3 shows an example of L * a * b * values of the above-mentioned CMYK1 / 2/3/4 color and white background (displayed as W (white) in the figure). The numerical value is not shown.
[0032]
The color channel creation unit 3 includes a color channel calculation unit 31, a reference table 32, and a color channel table 33.
[0033]
The reference table 32 defines L * a * b that defines a correlation with a combination of output intensities of basic colors corresponding to color system values L *, a *, and b * values as output characteristics in the image recording unit 4. → CMY LUT is stored. This L * a * b → CMY LUT is a plurality of colors in which C, M, and Y output values of an image recording unit 4 to be described later are equally divided from a minimum value 0 to a maximum value and the respective outputs are combined. A color chart in which images are arranged is output, L * a * b * values of the plurality of color images are measured, and based on the combination and L * a * b * values, Japanese Patent Application No. 2002-090702 by the present applicant. It can be determined by the method described in the specification of the issue.
[0034]
The color channel calculation means 31 has a function as a generation means of the present invention, and features a special color for each of the CMYK1 / 2/3/4 color and the CMYK1 / 2/3/4 color stored in the color table 24. The image recording unit 4 uses the L * a * b * value of the color system and the color system of the white background and the L * a * b → CMY LUT stored in the reference table 32. A combination of CMY output intensities for outputting * a * b * values is obtained and stored in the color channel table 33. As a method for obtaining CMY using the L * a * b * value as an input value, a method described in the specification of Japanese Patent Application No. 2002-090702 by the present applicant can be used.
[0035]
The color channel table 33 is a CMYK1 / 2/3 / 4-order color and a CMYK1 / 2/3 / 4-order color, each of which has a special color printed on it and a CMY output from the image recording unit 4 corresponding to a white background. The intensity combinations are stored in a configuration as shown in FIG. FIG. 4 shows an example of a combination of the output intensity of the CMYK1 / 2/3/4 color and the white background (shown as W (white) in the figure) from the minimum value 0 to the maximum value. The output intensity of 140, M, and C is shown from the minimum value 0 to the maximum value 160, and the value related to the spot color does not indicate a numerical value because it changes depending on the spot color.
[0036]
The image recording unit 4 sensitizes the magenta coloring layer (M layer) of the photosensitive material with red (R) light, and sensitizes the cyan coloring layer (C layer) of the photosensitive material with green (G) light. The yellow color developing layer (Y layer) of the photosensitive material is exposed to light, and development processing is performed to reveal and output basic colors Y, M, and C, thereby forming a color image. The photosensitivity by the red (R) light, the green (G) light, and the blue (B) light is performed by simultaneously irradiating light of these colors for each pixel in a dot-sequential manner.
[0037]
The control unit 1 has a function of controlling each unit and a function as a control unit of the present invention, and is a halftone dot separated for each ink of C, M, Y, K, and special color from image data. Based on the image data, the color to be output by each pixel in the image storage unit 4 is obtained by printing a special color on each of the above-described CMYK1 / 2/3/4 color and CMYK1 / 2/3/4 color. A color or white background is obtained, a combination of CMY output intensities is obtained by referring to the color channel table 33, and the output intensities of basic colors Y, M, and C are changed in the image recording unit 4. The output intensity data of Y, M, and C obtained from the color channel table 33 is converted into exposure output intensity data of B, R, and G, and the light intensity of each color of the image recording unit 4 is changed. To control. In order to realize such functions, a CPU (not shown), a program for performing overall control, a program for changing the output intensity of Y, M, and C, and various data necessary for executing each program are stored. A system memory (not shown) that constitutes a work area for storing and executing each program is configured.
[0038]
In this way, the color proof making apparatus changes the output intensity of the basic colors Y, M, and C, and prints the halftone image of each ink of C, M, Y, K, and special colors on the printing press. Create a color proof by reproducing the colors that are created.
[0039]
(Color estimation method)
Next, the color estimation method of the present invention will be described. In order to make the description easy to understand, first, an outline of each step of the color estimation method will be described, and an example applied to a color proof creating apparatus will be described later. Further, the transparency coefficient a, the trapping rate coefficient b, and the correction coefficient c used for color estimation will be described later.
[0040]
FIG. 5 is a flowchart showing the procedure of the color estimation method of the present invention. First, the lower ink (first color material) when printing is printed on a printing paper, and the spectral density D1 (λ) and the spectral density DW (λ) of the ground color of the printing paper are measured ( 5 is abbreviated as S11 (hereinafter, the other steps are also shown). This lower ink corresponds to one of the CMYK1 / 2/3/4 quaternary colors printed using the above-described C, M, Y, and K inks. The spectral density is obtained by measuring the reflectance at each wavelength as described above and converting the reflectance. The same applies to the spectral density measurement performed below.
[0041]
Next, the upper ink (second color material) when printing is overprinted is printed on the printing paper, and its spectral density D2 (λ) is measured (S12). This upper ink corresponds to that printed using the above-mentioned special color ink.
[0042]
Then, the spectral density DM (λ) of the color obtained by printing the upper ink on the lower ink,
DM (λ) = a × (D1 (λ) −DW (λ)) + b × (D2 (λ) −DW (λ)) + DW (λ) −c × a × (D1 (λ) −DW (λ) ) × b × (D2 (λ) −DW (λ))
(S13).
[0043]
The first term a × (D1 (λ) −DW (λ)) indicates the spectral density of the lower ink itself obtained through the upper ink. First, by subtracting DW (λ) from D1 (λ), the spectral density of the lower ink itself excluding the spectral density DW (λ) of the printing paper is obtained, and the transparency of the upper ink is further obtained. Is multiplied by the transparency coefficient a to obtain the spectral density of the lower ink itself obtained through the upper ink. If the upper ink is completely transparent, the transparency coefficient a is “1”, and if it is completely opaque, the transparency coefficient a is “0”, so the transparency coefficient a is between 0 and 1. Is a constant.
[0044]
The second term b × (D2 (λ) −DW (λ)) indicates the spectral density of the ink itself, which is the upper side when superimposed on another ink. First, by subtracting DW (λ) from D2 (λ), the spectral density of the lower ink itself excluding the spectral density DW (λ) of the printing paper is obtained, and printing is further performed on the printing paper. By multiplying the so-called ink paste difference when printing on other ink as the trapping rate coefficient b, the spectral density of the ink itself on the upper side when overlaid on the other ink is obtained. In addition, since this ink paste is generally lower when printing on other inks than when printing on printing paper, the trapping rate coefficient b is between 0 and 1. It is a constant.
[0045]
Here, since the spectral density DW (λ) of the printing paper is excluded by the above two terms, the spectral density DW (λ) is added by the third term.
[0046]
The fourth term c * a * (D1 ([lambda])-DW ([lambda])) * b * (D2 ([lambda])-DW ([lambda])) is a correction term for calculating a correction value, and is the lower ink. Since the spectral density obtained by the sum of the above three terms is larger than the measured value at a wavelength at which the spectral density of the ink itself or the spectral density of the ink itself on the upper side is relatively high, the lower ink itself Or a × (D1 (λ) −DW (λ)) and b × (D2 (λ) so that the value to be reduced is increased when the spectral density of the ink itself or the spectral density of the upper ink itself is relatively high. ) −DW (λ)) multiplied by the correction coefficient c. In other words, the value is proportional to the product of (D1 (λ) −DW (λ)) and (D2 (λ) −DW (λ)).
[0047]
Further, the value of the color system such as the XYZ color system or the L * a * b * color system can be obtained from the spectral density.
[0048]
Further, a processing procedure in the color proof creation device of the present embodiment to which the above-described color estimation method is applied will be described with reference to the flowchart shown in FIG. Here, the transparency coefficient a, the trapping rate coefficient b, and the correction coefficient c are stored in the coefficient table 23 in advance. Further, the spectral density D1 (λ) of the lower ink and the spectral density DW (λ) of the ground color of the printing paper are measured in advance and stored in the spectral density table 22.
[0049]
First, when the measurement result of the spectral density D2 (λ) of the special color, which is the upper ink, is input (S21), the color calculation unit 21 refers to the spectral density table 22 and the coefficient table 23 (S22) and inputs them. The spectral density D2 (λ) of the special color, the spectral density D1 (λ) of each of the CMYK1 / 2/3/4 quaternary colors as the lower ink, and the spectral density DW (λ of the ground color of the printing paper ), Transparency coefficient a, trapping rate coefficient b, and correction coefficient c, the calculation formula of spectral density DM (λ) is used for each color obtained by printing a special color on the CMYK1 / 2/3/4 color. The spectral density DM (λ) is calculated (S23). Then, each color system value L * a * b * value is obtained from each spectral density DM (λ) and stored in the color table 24 (S24). Next, the color channel calculation means 31 uses the L * a * b * values stored in the color table 24 and the L * a * b → CMY LUT in the reference table 32 as a special color for CMYK1 / 2/3/4 colors. A combination of output intensities of a plurality of basic colors obtained by overprinting is obtained and stored in the color channel table 33 (S25). Then, the control unit 1 refers to the color channel table 33 and controls the image recording unit 4 so as to change the combination of the output intensities of a plurality of basic colors based on the image data, and the image recording unit 4 follows the control. Image formation is performed (S26).
[0050]
By performing color estimation when spot colors are overlaid in this way, a color proof is reproduced by recreating a color created by printing dot images of C, M, Y, K, and spot inks on a printing press. When creating an image, there is no need to measure the color (in this case, there are 15 patterns) by printing a spot color on each of the target CMYK1 / 2/3/4 quaternary colors, and only the spot colors to be printed are printed. It is only necessary to print on the printing paper and measure. Therefore, the working time can be shortened.
[0051]
In addition, in the case where special characteristics having different characteristics are used for the transparency coefficient a, the trapping ratio coefficient b, and the correction coefficient c, the coefficients are stored in the coefficient table 23 for each type as shown in FIG. In addition, when the spectral density D2 (λ) of the upper ink is input and printed, the type can be selected and the spectral density DM (λ) can be obtained.
[0052]
Further, the above color estimation or color channel table generation is performed using an information processing device such as a PC (Personal Computer), and the color proof creation device receives the color estimation result or the color channel table via a medium such as a network or an FD. It is also possible to obtain the image and use it for forming a color image.
[0053]
(Example)
Next, a specific example in which the color obtained by performing the overprinting of ink using the color estimation method of the present invention is estimated will be described. In this example, an example in which a special color (red) is printed over magenta (M) is shown as an example.
[0054]
First, FIG. 8 shows the measurement result of the spectral reflectance R, the spectral reflectance RW (λ) of the ground color of the printing paper, the spectral reflectance R1 (λ) of magenta (M), and the spectral reflectance of the special color (red). R2 (λ) was indicated. In addition, a result of actually printing a special color (red) on magenta (M) and printing it on a printing paper for a comparison of the spectral density described later between the color obtained by estimation and the color obtained by actual printing. Was also shown as the spectral density RT (λ). In addition, in FIG. 8, the result measured about the wavelength of 10 nm increments between 380 nm and 730 nm was shown.
[0055]
Next, the spectral density D is calculated from the spectral reflectance R.
D = −0.5 × LOG₁₀(R)
The spectral density DW (λ) of the ground color of the printing paper, the spectral density D1 (λ) of magenta (M), and the spectral density D2 (λ) of the special color (red) are obtained, and the magenta (M) Spectral density DM (λ) of the color obtained by overprinting the special color (red),
DM (λ) = a × (D1 (λ) −DW (λ)) + b × (D2 (λ) −DW (λ)) + DW (λ) −c × a × (D1 (λ) −DW (λ) ) × b × (D2 (λ) −DW (λ))
The calculation is performed for each wavelength by the following formula. At this time, each coefficient is
The transparency coefficient a = 0.8, the trapping rate coefficient b = 0.9, and the correction coefficient c = 0.75. Also, the spectral density DW (λ) of the ground color of the printing paper, the spectral density D1 (λ) of magenta (M), the spectral density D2 (λ) of the special color (red), and the special color (red) are estimated for magenta (M). FIG. 9 shows the spectral density DM (λ) of the color obtained by reprinting according to the above. Further, in FIG. 9, the spectral density DT (λ) obtained from the spectral reflectance RT (λ) for comparison between the color obtained by estimation and the color actually obtained by overprinting is obtained and written together. Further, in FIG. 9, in order to clarify the effect of the correction term, the spectral density obtained by simply adding together as an example of the conventional estimation method,
DM ′ (λ) = a × (D1 (λ) −DW (λ)) + b × (D2 (λ) −DW (λ)) + DW (λ)
The results of calculating the spectral density DM ′ (λ) for each wavelength are also shown. At this time, each coefficient is
Transparency coefficient a = 0.7, trapping rate coefficient b = 0.8
It was.
[0056]
Further, FIG. 10 shows the spectral density DW (λ), the spectral density D1 (λ), D2 (λ), and the spectral density DM (λ), and the spectral density DT (λ) and the spectral density DM ′ (λ). This is shown in the graph. As is apparent from FIG. 10, the spectral density DM (λ) obtained by the estimation of the present invention is close to the spectral density DT (λ) obtained by actual measurement. On the other hand, as described in [Prior Art], the spectral density DM ′ (λ) simply obtained by adding together has a large difference at a wavelength of about 530 nm where the density is highest. This is also the above correction term.
c × a × (D1 (λ) −DW (λ)) × b × (D2 (λ) −DW (λ))
Shows that it works effectively.
[0057]
Next, the value of the color system is obtained from the spectral density DM (λ). The process of obtaining the color system value from the spectral density is generally known, so only an outline will be described.
[0058]
First, X, Y, and Z of the XYZ color system are obtained. First, spectral reflectance RM (λ) of each wavelength from spectral density DM (λ)
RM (λ) = 10^{-2DM (λ)}
X is obtained by adding the value obtained by multiplying RM (λ) by x (λ) for all wavelengths (380 nm to 730 nm), and Y is obtained by multiplying RM (λ) by y (λ). The value is obtained by adding the values for all wavelengths (380 nm to 730 nm), and Z is obtained by adding the value obtained by multiplying RM (λ) by z (λ) for all wavelengths (380 nm to 730 nm). These x (λ), y (λ), and z (λ) are CIE1931 XYZ color system color matching functions x (λ), defined by CIE (Commission Internationale de l'Eclairage). y (λ), z (λ) (in CIE 1931, a bar (-) is attached above x, y, z) and the value of the D50 standard light source. is there. In this way, X, Y, Z are
(X, Y, Z) = (27.519916971, 13.61715009, 3.0355570972)
It is obtained.
[0059]
Furthermore, the L * value, a * value, and b * value of the L * a * b * color system are obtained as follows. First, X ′, Y ′, and Z ′ are obtained as follows.
(X / X₀) Is 0.008856 or less,
X ′ = (X / X₀) × 7.787 + 16/116,
(X / X₀) Is greater than 0.008856,
X ′ = (X / X₀)^1/3,
(Y / Y₀) Is 0.008856 or less,
Y ′ = (Y / Y₀) × 7.787 + 16/116,
(Y / Y₀) Is greater than 0.008856,
Y ′ = (Y / Y₀)^1/3,
(Z / Z₀) Is 0.008856 or less,
Z ′ = (Z / Z₀) × 7.787 + 16/116,
(Z / Z₀) Is greater than 0.008856,
Z ′ = (Z / Z₀)^1/3,
However, X₀, Y₀, Z₀Are values obtained by adding x (λ), y (λ), and z (λ) for all wavelengths (from 380 nm to 730 nm), respectively. And from X ', Y', Z '
L * = 116 × Y′−16
a * = 500 × (X′−Y ′)
b * = 200 × (Y′−Z ′)
It can be asked. In this way, from the spectral density DM (λ) shown in FIG. 9 and DT (λ) obtained by actual measurement, the L * value, a * value, and b * value are converted into LM *, aM *, bM *, and When calculated as LT *, aT *, and bT *,
(LM *, aM *, bM *) = (43.6679999, 71.96972009, 36.38036747),
(LT *, aT *, bT *) = (44.665544368, 72.0925997, 37.96175686)
Is obtained. The color difference ΔE is
ΔE
= ((LM * -LT *)²+ (AM * -aT *)²+ (BM * -bT *)²)^1/2= 1.8673522691
It can be seen that the color is sufficiently small and the color is estimated accurately.
[0060]
Here, an example of how to obtain the transparency coefficient a, the trapping rate coefficient b, and the correction coefficient c will be described.
[0061]
First, the spectral density DW (λ) of the ground color of the printing paper and the spectral densities D1 (λ) of the CMYK1 / 2/3/4 quaternary colors printed using inks of C, M, Y, and K colors. And a spectral density D2 (λ) printed with a special color, and a spectral density DT (λ) printed by printing a special color on a plurality or all of 15 colors of CMYK1 / 2/3/4. Is determined by measurement.
Furthermore, the equation for performing the above estimation
DM (λ) = a × (D1 (λ) −DW (λ)) + b × (D2 (λ) −DW (λ)) + DW (λ) −c × a × (D1 (λ) −DW (λ) ) × b × (D2 (λ) −DW (λ))
Is used to obtain the spectral density DM (λ) while changing the transparency coefficient a, the trapping rate coefficient b, and the correction coefficient c, and compares the spectral density DT (λ) with the value and spectral density. A transparency coefficient a, a trapping rate coefficient b, and a correction coefficient c are obtained for each CMYK1 / 2/3/4 color so that the shapes are closest. Further, the transparency coefficient a, the trapping rate coefficient b, and the correction coefficient c obtained for each are averaged to obtain a transparency coefficient a, a trapping rate coefficient b, and a correction coefficient c.
[0062]
The transparency coefficient a, the trapping rate coefficient b, and the correction coefficient c obtained in this manner can be used for the color estimation described above.
[0063]
These coefficients may be obtained at the work site. For example, these coefficients may be obtained by the manufacturer of the color proof producing apparatus or the manufacturer of the special color ink and distributed to the user. As a distribution method, the manufacturer of the color proof producing device may be shipped in preparation for the color proof producing device. Therefore, the estimation of the color when printing the spot colors can be performed only by measuring the spectral density of only one color in which the spot colors are printed independently.
[0064]
【The invention's effect】
As described above, according to the present invention, when the ink containing the pigment is overprinted, the color density of the ink on the lower side, the spectral density of the ink on the upper side, and the spectral density of the printing paper are Therefore, it is possible to provide a color estimation method that can be accurately and easily estimated, and a color image forming apparatus to which the color estimation method is applied.
[Brief description of the drawings]
FIG. 1 is a functional block diagram showing a configuration of a color proof creating apparatus according to the present invention.
FIG. 2 is an explanatory diagram showing an example of a configuration of a spectral density table according to the present embodiment.
FIG. 3 is an explanatory diagram showing an example of a configuration of a color table according to the present embodiment.
FIG. 4 is an explanatory diagram illustrating an example of a configuration of a color channel table according to the present embodiment.
FIG. 5 is a flowchart showing a procedure of a color estimation method according to the present invention.
FIG. 6 is a flowchart showing a processing procedure in a color proof creating apparatus to which the color estimation method of the present invention is applied.
FIG. 7 is an explanatory diagram illustrating an example of a configuration of a coefficient table according to the present embodiment.
FIG. 8 is a diagram showing measurement results of the spectral reflectance RW (λ) of the ground color of the printing paper, the spectral reflectance R1 (λ) of magenta (M), and the spectral reflectance R2 (λ) of the special color (red). is there.
FIG. 9 is a spectral density DM (λ) of a color obtained by the color estimation method of the present invention, a spectral density DT (λ) based on actual measurement, and a spectrum obtained by a color estimation method different from the color estimation method of the present invention; It is a figure which shows density | concentration DM '((lambda)).
10 is a spectral density D1 (λ) of magenta (M), a spectral density D2 (λ) of a spot color (red), a spectral density DM (λ) of a color obtained by the color estimation method of the present invention, and a spectral based on actual measurement; It is a graph which shows density | concentration DT ((lambda)) and spectral density DM '((lambda)) obtained by the color estimation method different from the color estimation method of this invention.
[Explanation of symbols]
1 Control unit 1
2-color operation unit
21 color calculation means
22 Spectral density table
23 Coefficient table
24 color table
25 Input means
3 Color channel creation part
31 Color channel calculation means
32 reference table
33 Color channel table
4 Image recording unit

Claims (5)

A color estimation method for estimating a color obtained by overprinting and printing a plurality of coloring materials on a recording medium,
Measuring a spectral density D1 (λ) with respect to each wavelength λ of light when the first color material containing a pigment which is a lower side when printed is printed on the recording medium;
Measuring a spectral density D2 (λ) with respect to each wavelength λ of light when the second color material containing the pigment on the upper side when printed is printed on the recording medium;
Measuring the spectral density DW (λ) for each wavelength λ of the light of the recording medium;
For each wavelength λ of the light, a value proportional to the difference between the spectral density D1 (λ) and the spectral density DW (λ), and the spectral density D2 (λ) and the spectral density DW (λ) A value proportional to the difference and the spectral density DW (λ) are added, and a value proportional to the difference between the spectral density D1 (λ) and the spectral density DW (λ) and the spectral density D2 (λ) Is subtracted from the addition result, and the result is printed on the first color material. And a step of setting the spectral density DM (λ) of the color obtained by overlapping printing to a color estimation method. 2. The method according to claim 1, further comprising calculating a color system value of a color obtained by printing the second color material on the first color material by printing from the spectral density DM (λ). The described color estimation method. 3. The color estimation method according to claim 1, wherein the first color material includes a plurality of color materials each including a pigment. Color image formation that reproduces the color of the target halftone image line area obtained by printing each halftone image of a plurality of color materials on a recording medium and using a plurality of basic colors to form a color image A device,
A reference table of combinations of output intensities of the plurality of basic colors for a plurality of color characteristic values;
Spectral density D1 (λ) with respect to each wavelength λ of light and the wavelength λ of light of the recording medium when the first color material containing the pigment which is the lower side when printed is printed on the recording medium A spectral density table storing spectral density DW (λ) for
An input means for inputting a spectral density D2 (λ) with respect to each wavelength λ of light when the second color material containing the pigment on the upper side when printed is printed on the recording medium;
When the spectral density D2 (λ) is input, the spectral density D1 (λ) and spectral density DW (λ) stored in the spectral density table and the input spectral density D2 (λ) are used. For each wavelength λ of the light, a value proportional to the difference between the spectral density D1 (λ) and the spectral density DW (λ), the spectral density D2 (λ), and the spectral density DW (λ) And a value proportional to the difference between the spectral density D1 (λ) and the spectral density DW (λ) and the spectral density D2 (λ). ) And a value proportional to the difference between the spectral density DW (λ) and a value proportional to the product is subtracted from the addition result, and the second color is formed on the first color material based on the result. Color calculation means for obtaining the color characteristics of the color obtained by overprinting and printing the material;
Generating means for generating a color channel table by obtaining a combination of output intensities of the plurality of basic colors for reproducing the target color based on the color characteristics obtained by the calculating means and the reference table;
A color image forming apparatus comprising: control means for changing output intensities of the plurality of basic colors based on the color channel table when forming the color image. 5. The color image forming apparatus according to claim 4, wherein the first color material includes a plurality of color materials including a pigment that are overprinted.

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