Four highlights of G7's new printing process
1. Gray balance printing density curve NeutralPrintDensityCurve (NPDC)
G7 does not use TVI (dot expansion) curve to control printing quality, but uses its unique neutral gray printing density curve NPDC. It is held based on the Press2Proof file specially designed by 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 trading printing, the maximum density of ordinary overprinting will be 1.37 ~ 1.90, which is the sector area above the black solid line in the NPDC diagram. At this time 50% C. The relative density of the neutral gray area at 40% M and 40% Y is 0.54. These curves actually start to bifurcate, mostly at 0.6, that is, where the Munsell brightness value is about 5.8, which can fully ensure that the most sensitive 10-6 brightness of the human eye is cost-effective to replicate. In this way, even if the density of the field changes in a certain range, its bright tone and middle tone will be perfectly reproduced. The NPDC curve is a characteristic replication curve under different field densities. With NPDC, HR has the guarantee that the same appearance replication under different materials and different field densities can be realized.
The NPDC curve is completed by combining P2P proofs. In G7's latest printing process method, G7, there are two NPDCs to be measured: one is the three-color combination curve of CHY, and the other is the NPDC curve of the single black version. Completed with the fourth and fifth column data of P2P respectively. The main function of NPDC is to adjust the RIP output dot value. It is emphasized that the adjustment of the output curve is under the premise of maximum density to ensure that the density at 50% C, 40% H, 40% Y is fixed value when printing on all paper. 54. The above NPDC correction results of C, M, Y, and K are used as RIP or correction equipment to assign new target values. Some RIP devices need to enter the "after measurement" value instead of the "required" value. No matter what the situation is, the new target value is the value that should be obtained for each curve point after correction.
2. Spectrophotometer replaced density meter as the main measurement method. In the G7 process, the spectrophotometer was introduced into the process control management.
For the evaluation of neutral gray, the chromaticity coordinate values ​​a *, b * and lightness value L * are used to characterize the observation effect of the human eye. The Lab value constitutes a uniform three-dimensional color space. Its advantage is that it can express the difference between two colors perceived visually as the distance between two points in the color model. Here, when ΔE = 1, it is the smallest color difference that the human eye can distinguish. The numerical values ​​established during the printing process, such as density values, dot expansion, etc., can be calculated by optical formulas. The spectrophotometer is measured by the light energy reflected (transmitted) by the sample and the light energy reflected by the standard under the same conditions (transmitted) to obtain the spectral reflectance of the sample at each wavelength. The light source is calculated as follows:
Three stimulus values ​​X, Y, and Z are obtained, and then the color coordinates X, Y, and CIELAB chromaticity parameters are calculated by X, Y, and Z according to the formulas such as CIEYxy and CIELab. The visual effect can be directly reflected on the numerical value to complete a more scientific and accurate copy.
In traditional printing, the most commonly used tool is the density meter. Due to its low price and ease of use, the penetration rate in the factory is high. The density meter can measure the on-site density and the integrated density of the dots, and can also measure the expansion and relative contrast of the dots through the measurement and control strip, but there is no direct correspondence between the density and the Munsell value. For example, the density and brightness are not linear, but curvilinear. In terms of color comment, the response value of the color filter is very different from the human eye observation. 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 spectrophotometer is used as the main measurement device, which subverts the basis of the traditional printing quality control process, making the G7 quality control process more scientific and reasonable.
3. Shadow Contrast (SC)
In G7, instead of printing relative contrast K, dark contrast SC is used. During the printing process or after the printing is completed, the SC can quickly check the gray balance of the dark tone. SC has two data: one for CMY gray balance and the other for monochrome black. For the three-color gray SC, the three-color combination is (75C, 66M, 66Y), and the single-color black SC has a dot percentage of 75K, and their relative density values ​​depend on the actual SID. In fact, it is enough to control a solid density and HR value, and then increase the control dark tone SC value, the requirements for printing control are very high. Generally speaking, ordinary printing manufacturers may not be able to meet such technical requirements. In some specific high-quality printing, the control of the dark tone SC value is used. In ordinary circumstances, as long as the HR is controlled, the requirements can be satisfied.
4. Bright Range (HR)
The traditional printing quality control method can not make the printing effect of newsprint catch up with art paper, mainly because the quality of newsprint paper is poor, and the quality of art paper is good, resulting in different solid density during printing. The contrast of printed paper on coated paper can reach 1.7, while that of newsprint is only about 1.4. Conventional printing control usually controls the solid density, and the solid density value decreases, and all other parts also decrease linearly. The linear compression of newsprint printing and coated paper printing is the same. If HR is set as a constant, it actually exerts a selective compression effect on news printing, that is, maintaining a medium-bright tone and compressing a dark tone, which is what G7 does. In buying and selling printing, regardless of any substrate, the G7's requirements are: in the mid-tone gray areas, that is, 50% C, 40% M, 40% Y, the relative density value is 0.54, and the chroma is 0 ~ 2. As the density and color of the middle tone are guaranteed, the density change at the mid-bright tone is also guaranteed. At the expense of the dark tone, which is unresponsive to the human eye, the "same look" printing of different printing materials is realized. Under the premise of ensuring the maximum density, it is guaranteed that the density at 50% C, 40% H, 40% Y is fixed at 0.54 when printing on all papers. Since dark tone is not the focus of reproduction, the relative contrast K value of printing reflecting changes in dark tone areas is not a measure.
Where is the control point HRA of G7? According to the standard measurement equation, HRA with a density of 0.54 is almost at the sixth level of Munsell brightness.
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