With the continuous development of computer-to-plate technology and digital image technology, the process known as G7 is quietly emerging. It was developed by the American Offset Commercial Printing Specification Organization (abbreviated as GRACOL), combined with CTP's many years of practice. It is a new printing process that mainly controls gray balance. Its purpose is to realize the “identity†effect of commercial offset printing with the help of CTP.
〖G7 Comparison with Traditional Printing Technology〗
The G7 process is a traditional process that has been developed under the auspices of GRACoL (General Requirements for Applications in Commercial Offset Lithography). The main method of the G7 process is to use a spectrophotometer to measure and control the neutral gray balance on the printed sheet, and use a new NPDC curve (Neutral Print Density Curve) to match the visual effect of the image. Where G represents the gray value to be corrected, 7 represents the seven basic colors defined in the ISO 12647-2 printing standard: cyan, magenta, yellow, black, red, green, and blue.
The main difference between the G7 process and the traditional printing process is that the focus of the original color copy is the bright and midtone adjustment of the image, and through the control of the gray balance (mainly the highlight range HR, the bright contrast HC and the dark tone The contrast Lab value of the SC3 parameter is controlled to achieve, while the traditional printing process takes the field density and dot gain as the main control object.
Five outstanding advantages of G7
1, gray balance printing density curve Neutral Print DensityCurve (NPDC)
The G7 does not use the TVI curve but uses its unique neutral gray print density curve NPDC. It is based on the specially designed file Press2 Proof of GRAC0L. The abscissa is the percentage of the dots set in P2P, and the ordinate is the density value after printing. There are two kinds of NPDC curves: one is CMY overprinted three-color gray, and the other is single black. For commercial printing, the maximum density of the overprint is generally between 1.37 and 1.90, which is the upper fan-shaped area in the black solid line in the NPDC diagram. At this point 50% C. The neutral gray areas at 40% M and 40% Y have a relative density of O.54. These curves actually begin to branch out, mostly at 0.6, where Munsell's brightness value is about 5.8th, and it is sufficient to ensure that the most sensitive 10 to 6 brightness values ​​of human eyes are well copied. In this way, even if the density of the field changes in a certain range, the bright tone and the middle tone will be perfectly reproduced. The NPDC curve is a characteristic replication curve at different field densities. With NPDC, HR has a guarantee that homomorphic reproduction with different materials and different on-site densities can be achieved.
The NPDC curve was completed in conjunction with P2P proofs. In G7's latest printing process—G7, there are two NPDCs that need to be measured: one is a three-color combination curve of CHY, and the other is a single black version of the NPDC curve. This is done using the fourth and fifth columns of P2P data, respectively. The main role of NPDC is to adjust the RIP output network point value, emphasizing that the adjustment of the output curve is based on the maximum density, and that the density at 50%C, 40%H, and 40%Y at the time of printing on all the paper is a fixed value O. 54. The above NPDC correction results for C, M, Y, and K are assigned as new target values ​​for the RIP or correction device. Some RIP devices need to enter "measured" values ​​instead of "needed" values. In any case, the new target value is the value that should be obtained for each curve point after correction.
2. Spectrophotometer instead of densitometer becomes the main measurement method
In the G7 process, a spectrophotometer was introduced into the process control management. Neutral ash is evaluated using the chromaticity coordinates a*, b* and the lightness value L* to characterize the observations of the human eye. The Lab value constitutes a uniform three-dimensional color space, which has the advantage of being able to visually perceive two color differences as represented by the distance between two points in the color model. Here, when ΔE=1, it is the smallest color difference that the human eye can discern. The values ​​produced during the printing process, such as density values, dot gains, etc., can be calculated from the optical formula. The spectrophotometer compares the reflected (transmitted) light energy of the sample with the standard reflected (transmitted) light energy under the same conditions to obtain the spectral reflectance of the sample at each wavelength, and then uses the standard observers and standards provided by the CIE. Light source, calculated as follows:
Thus, three stimulus values ​​X, Y, and Z are obtained. Then X, Y, Z are used to calculate the color coordinates X, Y, and CIELAB chromaticity parameters according to the formulas such as CIEYxy and CIELab. The printing process is controlled using a spectrophotometer to make the human eye The visual effects can be directly reflected in the numerical values, completing more scientific and more accurate reproduction. In traditional printing, the most commonly used tool is a densitometer, which is highly popular in factories because of its low cost and ease of use. The densitometer can measure the density of the field and the integral density of the dot. The dot enlargement and relative contrast can also be measured through the measurement and control strip. However, there is no direct correspondence between the density and the Munsell value. For example, the magnitude and density of the density are not linear but rather are in a curvilinear relationship. In color evaluation, the response value of the color filter is far from the human eye. Therefore, the density meter cannot simulate the observation power of the human eye, and the density value cannot reflect the visual effect of the human eye. Therefore, in the G7 process, the use of a spectrophotometer as the main measuring device, which subvert the basis of the traditional printing quality control process, makes the G7 quality control process more scientific and reasonable.
3, dark contrast Shadow Contrast (SC)
In G7, instead of using printed relative contrast K, dark contrast SC is used. The SC can perform a quick inspection of the dark gray balance during or after printing. The SC has two data: one for the CMY gray balance and the other for the monochrome black. Three-color gray SC, its three color combinations (75C, 66M, 66Y), monochrome black SC, the dot percentage of 75K, their relative density value depends on the actual SID. In fact, it is sufficient to control a field density and HR value, and then to increase the darkness of control SC, the requirement for printing control is high. Generally speaking, a general printing manufacturer may not be able to meet such technical requirements. In some specific high-quality printing, the control darkness SC value is used. In ordinary situations, only controlling the HR can meet the requirement.
4ã€Highlight Range(HR)
The traditional printing quality control methods can not make the printing effect of newsprint catch up with coated paper. The main reason is that the quality of newsprint paper is poor, and the quality of coated paper is good, resulting in different density on the spot when printing. The contrast of coated paper can reach 1.7, while the newsprint is only about 1.4. Conventional printing control, which usually controls the density in the field, reduces the value of the density in the field, and all other parts also decrease linearly. Newsprint printing is the same as the linear compression of coated paper. If the HR is set as a constant, it actually plays a selective compression effect on the news printing, that is to keep the bright tone, compression dark tone, G7 is to do so. In commercial printing, regardless of any substrate, the G7's requirement is: In the gray tone at the middle, ie 50% C, 40% M, 40% Y, the relative density value is 0.54, and the chroma is 0~2. Because the density and color of the intermediate tone are guaranteed, the density change in the bright tone is guaranteed, and only the dull tone of the human eye is unresponsive, thus achieving the “identity†printing of different printing materials. Under the premise of guaranteeing the maximum density, the density at 50%C, 40%H, 40%Y is guaranteed to be a fixed value of 0.54 when printed on all papers. Since dark tone is not the focus of copying, the relative K value of print contrast, which reflects changes in shadow tone, is not a measure.
Where is the G7's control point HRA? It is not difficult to see from the relationship between density and the brightness of Munsell that the HRA with a density of 0.54 is almost at the sixth level of Munseen. It is not difficult to observe from the above graph that the curve segment with the largest slope is at the 10th to 6th level of the Munsell brightness value, that is, the density value is at O~0.54. Therefore, this HR point represents a turning point in human visual observation. From this point to the left, the human eye feels stronger, showing a very short curve that corresponds to a 4-level lightness perception; and to the right, the observational power becomes sluggish, showing a very long curve that corresponds to a 6-level lightness perception. Therefore, the density segment of 0~0.54 is the part of the image that we need to protect in the printing. The highlight range is measured separately and it can be quickly checked in the middle during printing or after printing. In G7, it replaces TVI and checks overall darkness and gray balance. HR has two data: one for the gray balance of CMY, the other for monochrome black. Three-color gray HR, its three-color combination (50C, 40M, 40Y), monochrome black HR, the dot percentage of 50K, their relative density values ​​were 0.54 and 0.50. The bright-ranging range HR is also the core control part of the G7 control process and is the parameter most needed for control in a large number of experimental studies.
5. Contrast Highlights Highlight Contrast (HC)
Bright contrast HC can quickly check the gray balance of the bright adjustments during printing or after printing. HC has two data: one for the CMY gray balance and the other for the monochrome black. Three-color gray HR, its three-color combination (25C, 19M, 19Y), monochrome black HC, the dot percentage of 25K. Their relative density is 0.25 and 0.22 respectively.
〖G7 Comparison with Traditional Printing Technology〗
The G7 process is a traditional process that has been developed under the auspices of GRACoL (General Requirements for Applications in Commercial Offset Lithography). The main method of the G7 process is to use a spectrophotometer to measure and control the neutral gray balance on the printed sheet, and use a new NPDC curve (Neutral Print Density Curve) to match the visual effect of the image. Where G represents the gray value to be corrected, 7 represents the seven basic colors defined in the ISO 12647-2 printing standard: cyan, magenta, yellow, black, red, green, and blue.
The main difference between the G7 process and the traditional printing process is that the focus of the original color copy is the bright and midtone adjustment of the image, and through the control of the gray balance (mainly the highlight range HR, the bright contrast HC and the dark tone The contrast Lab value of the SC3 parameter is controlled to achieve, while the traditional printing process takes the field density and dot gain as the main control object.
Five outstanding advantages of G7
1, gray balance printing density curve Neutral Print DensityCurve (NPDC)
The G7 does not use the TVI curve but uses its unique neutral gray print density curve NPDC. It is based on the specially designed file Press2 Proof of GRAC0L. The abscissa is the percentage of the dots set in P2P, and the ordinate is the density value after printing. There are two kinds of NPDC curves: one is CMY overprinted three-color gray, and the other is single black. For commercial printing, the maximum density of the overprint is generally between 1.37 and 1.90, which is the upper fan-shaped area in the black solid line in the NPDC diagram. At this point 50% C. The neutral gray areas at 40% M and 40% Y have a relative density of O.54. These curves actually begin to branch out, mostly at 0.6, where Munsell's brightness value is about 5.8th, and it is sufficient to ensure that the most sensitive 10 to 6 brightness values ​​of human eyes are well copied. In this way, even if the density of the field changes in a certain range, the bright tone and the middle tone will be perfectly reproduced. The NPDC curve is a characteristic replication curve at different field densities. With NPDC, HR has a guarantee that homomorphic reproduction with different materials and different on-site densities can be achieved.
The NPDC curve was completed in conjunction with P2P proofs. In G7's latest printing process—G7, there are two NPDCs that need to be measured: one is a three-color combination curve of CHY, and the other is a single black version of the NPDC curve. This is done using the fourth and fifth columns of P2P data, respectively. The main role of NPDC is to adjust the RIP output network point value, emphasizing that the adjustment of the output curve is based on the maximum density, and that the density at 50%C, 40%H, and 40%Y at the time of printing on all the paper is a fixed value O. 54. The above NPDC correction results for C, M, Y, and K are assigned as new target values ​​for the RIP or correction device. Some RIP devices need to enter "measured" values ​​instead of "needed" values. In any case, the new target value is the value that should be obtained for each curve point after correction.
2. Spectrophotometer instead of densitometer becomes the main measurement method
In the G7 process, a spectrophotometer was introduced into the process control management. Neutral ash is evaluated using the chromaticity coordinates a*, b* and the lightness value L* to characterize the observations of the human eye. The Lab value constitutes a uniform three-dimensional color space, which has the advantage of being able to visually perceive two color differences as represented by the distance between two points in the color model. Here, when ΔE=1, it is the smallest color difference that the human eye can discern. The values ​​produced during the printing process, such as density values, dot gains, etc., can be calculated from the optical formula. The spectrophotometer compares the reflected (transmitted) light energy of the sample with the standard reflected (transmitted) light energy under the same conditions to obtain the spectral reflectance of the sample at each wavelength, and then uses the standard observers and standards provided by the CIE. Light source, calculated as follows:
Thus, three stimulus values ​​X, Y, and Z are obtained. Then X, Y, Z are used to calculate the color coordinates X, Y, and CIELAB chromaticity parameters according to the formulas such as CIEYxy and CIELab. The printing process is controlled using a spectrophotometer to make the human eye The visual effects can be directly reflected in the numerical values, completing more scientific and more accurate reproduction. In traditional printing, the most commonly used tool is a densitometer, which is highly popular in factories because of its low cost and ease of use. The densitometer can measure the density of the field and the integral density of the dot. The dot enlargement and relative contrast can also be measured through the measurement and control strip. However, there is no direct correspondence between the density and the Munsell value. For example, the magnitude and density of the density are not linear but rather are in a curvilinear relationship. In color evaluation, the response value of the color filter is far from the human eye. Therefore, the density meter cannot simulate the observation power of the human eye, and the density value cannot reflect the visual effect of the human eye. Therefore, in the G7 process, the use of a spectrophotometer as the main measuring device, which subvert the basis of the traditional printing quality control process, makes the G7 quality control process more scientific and reasonable.
3, dark contrast Shadow Contrast (SC)
In G7, instead of using printed relative contrast K, dark contrast SC is used. The SC can perform a quick inspection of the dark gray balance during or after printing. The SC has two data: one for the CMY gray balance and the other for the monochrome black. Three-color gray SC, its three color combinations (75C, 66M, 66Y), monochrome black SC, the dot percentage of 75K, their relative density value depends on the actual SID. In fact, it is sufficient to control a field density and HR value, and then to increase the darkness of control SC, the requirement for printing control is high. Generally speaking, a general printing manufacturer may not be able to meet such technical requirements. In some specific high-quality printing, the control darkness SC value is used. In ordinary situations, only controlling the HR can meet the requirement.
4ã€Highlight Range(HR)
The traditional printing quality control methods can not make the printing effect of newsprint catch up with coated paper. The main reason is that the quality of newsprint paper is poor, and the quality of coated paper is good, resulting in different density on the spot when printing. The contrast of coated paper can reach 1.7, while the newsprint is only about 1.4. Conventional printing control, which usually controls the density in the field, reduces the value of the density in the field, and all other parts also decrease linearly. Newsprint printing is the same as the linear compression of coated paper. If the HR is set as a constant, it actually plays a selective compression effect on the news printing, that is to keep the bright tone, compression dark tone, G7 is to do so. In commercial printing, regardless of any substrate, the G7's requirement is: In the gray tone at the middle, ie 50% C, 40% M, 40% Y, the relative density value is 0.54, and the chroma is 0~2. Because the density and color of the intermediate tone are guaranteed, the density change in the bright tone is guaranteed, and only the dull tone of the human eye is unresponsive, thus achieving the “identity†printing of different printing materials. Under the premise of guaranteeing the maximum density, the density at 50%C, 40%H, 40%Y is guaranteed to be a fixed value of 0.54 when printed on all papers. Since dark tone is not the focus of copying, the relative K value of print contrast, which reflects changes in shadow tone, is not a measure.
Where is the G7's control point HRA? It is not difficult to see from the relationship between density and the brightness of Munsell that the HRA with a density of 0.54 is almost at the sixth level of Munseen. It is not difficult to observe from the above graph that the curve segment with the largest slope is at the 10th to 6th level of the Munsell brightness value, that is, the density value is at O~0.54. Therefore, this HR point represents a turning point in human visual observation. From this point to the left, the human eye feels stronger, showing a very short curve that corresponds to a 4-level lightness perception; and to the right, the observational power becomes sluggish, showing a very long curve that corresponds to a 6-level lightness perception. Therefore, the density segment of 0~0.54 is the part of the image that we need to protect in the printing. The highlight range is measured separately and it can be quickly checked in the middle during printing or after printing. In G7, it replaces TVI and checks overall darkness and gray balance. HR has two data: one for the gray balance of CMY, the other for monochrome black. Three-color gray HR, its three-color combination (50C, 40M, 40Y), monochrome black HR, the dot percentage of 50K, their relative density values ​​were 0.54 and 0.50. The bright-ranging range HR is also the core control part of the G7 control process and is the parameter most needed for control in a large number of experimental studies.
5. Contrast Highlights Highlight Contrast (HC)
Bright contrast HC can quickly check the gray balance of the bright adjustments during printing or after printing. HC has two data: one for the CMY gray balance and the other for the monochrome black. Three-color gray HR, its three-color combination (25C, 19M, 19Y), monochrome black HC, the dot percentage of 25K. Their relative density is 0.25 and 0.22 respectively.
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