Gray balance means that the yellow, magenta and cyan colors produce neutral gray on the printed matter according to the ratio of different dot area ratios. The accuracy of the gray balance directly affects the reproduction of the tone level and color in the printing copy, so the correct gray Balance control is very important throughout the printing process and is a guarantee of acceptable print quality.
In the standardization, data and standardized printing production process, the quality control of the printing process is usually monitored by specially designed measuring and controlling bars. In addition to the CMYKRGB color standard, the gray balance color standard (such as C50M40Y40, etc.) is indispensable. During the use of the test strip, firstly, the CMYK solid density or chromaticity is within the standard range, and then the CMYK ink color is further finely adjusted by the gray balance color mark reproduction condition, so that the gray balance color mark reaches an accurate neutral gray.
However, in the printing production, we often encounter CMYK solid density or chromaticity is already within the standard range, but the neutral gray standard still has visual color cast phenomenon, although sometimes the neutral gray mark is changed by changing the CMYK ink color depth. Forcibly adjusted to gray phase, but at this time CMYK solid density or chromaticity has a large deviation from the standard tolerance, and some tones have a large abnormality, which can be said to be lost. What's more, even if you change the depth of CMYK ink, you still can't get accurate neutral gray reproduction!
What is the cause? The conclusion is that the CMY combination value of the gray balance color scale on the control strip is not suitable for the printing conditions at this time. In other words, the ash balance dot value configuration is unreasonable, which makes it difficult for the neutral gray to reach equilibrium, or does not follow the corresponding process requirements of the gray balance standard. control! If the gray balance dot value configuration is unreasonable, then how to set the correct gray balance CMY combination value to make the printing control more accurate, the answer is to standardize the management of the plate printing, and determine the accurate gray balance according to the process corresponding to different standards.
First, the definition of gray balance based on traditional printing standards
The traditional printing standard has no restrictions on the printing materials, and is suitable for various printing conditions. The gray balance is dynamic under different printing conditions, that is, the gray balance combination value changes with the printing conditions, but no matter how it changes, as long as it conforms to the neutral gray The required CMY combination value can be defined as the final gray balance value under the printing conditions, such as: C50M39Y39, C50M40Y40, C50M41Y41, etc. are possible. Once the gray balance combination value is determined, in the post-printing production, the CMY value of the neutral gray mark on the control strip is set to the value for gray balance printing control, thereby achieving good results.
Attachment: The main parameters of the process control under the traditional printing standard are as follows:
(1) Field density (△SID: ±0.05)
(2) Network expansion (△Dotgain: ±3%)
(3) Relative contrast (ΔK: ±10%)
(4) Overprint ratio (△T: ±10%)
1. Density method defines gray balance
According to the traditional printing standard process parameters, the printing process is controlled. According to the printed gray balance guide, the CMY color separation density of different color blocks is measured by using a spectrodensitometer (such as X-Rite518/528, etc.), and the CMY color separation density is the most equal. The CMY combination value corresponding to the color block is the gray balance value.
2, the chromatic method defines the gray balance
The printing process is controlled according to the traditional printing standard process parameters, and the L*a*b* chromaticity values ​​of different color blocks are measured by a spectrodensitometer (eg, X-Rite 528, etc.) according to the printed gray balance guide, a*, The b* chromaticity value is compared with the standard value calculated according to the paper, and the CMY combination value corresponding to the color block whose tolerance is within the range of Δa*:±1, Δb*:±2 is the gray balance value. Or use Eyeone to measure the P2P meter and automatically calculate the gray balance value by the relevant professional calculation tool.
Note: Gray balance chromaticity formula (calculated according to the paper a*, b* value)
Wanted a*= a*_paper * (100-C%)/ 100;
Wanted b* = b*_paper *(100-C%) / 100;
Second, based on the ISO12647-2 printing standard gray balance definition
ISO12647-2 is currently the international standard for offset printing, which is called “Printing Technology – Mesh Separation, Sample and Printed Process Control (Offset)â€, which was drafted and revised by the ISO/TC130 Printing Technical Committee. . Since the standard specifies the color of the printed materials (paper and ink), and also defines the expansion criteria and tolerances of the control tone, the ISO12647-2 standard recommends gray balance based on these preconditions. Value, which is a fixed CMY combination. In addition, ISO TS10128 recommends three control methods that meet the ISO12647-2 standard, namely the dot-expansion curve (TVI curve) fitting technique recommended by the European Fogra/Ugra PSO, and the neutral gray density curve (NPDC curve) recommended by the US IEDAliance/GRACol G7. Fitting technology and CMYK-CMYK color gamut conversion technology based on DeviceLink ICC, so the following mainly describes the gray balance defined by the TVI and NPDC control modes.
1. PSO-TVI control mode defines gray balance
The gray balance values ​​defined by the ISO 12647 (PSO-TVI) standard are: C25M19Y19, C50M40Y40, C75M64Y64, that is, as long as the control according to the ISO12647 (PSO-TVI) standard is controlled, the gray balance value is guaranteed to be neutral gray. However, since the dot gain value is a relative value and has a certain tolerance range, the ISO12647 (PSO-TVI) standard cannot specify the corresponding neutral gray density value (ND value), so even if the paper and ink chromaticity and dot gain value are Within the standard range, gray balance may still differ in terms of visual consistency.
Attached: The main parameters of the process control under the ISO12647 (PSO-TVI) printing standard are as follows:
(1) Paper chromaticity conforms to ISO12647-2 standard (△L: ±3, △a: ±2, △b: ±2)
(2) Ink chromaticity conforms to ISO2846-1 standard (△E≤5 for CMYK, △E≤6 for RGB)
(3) Dot enlargement (△Dotgain: ±3~4%)
(4) Gray balance (C25M19Y19, C50M40Y40, C75M64Y64)
2, G7-NPDC control mode defines gray balance
In order to make up for the lack of definition of gray balance by PSO-TVI control method, the G7-NPDC control method based on ISO12647-2 standard came into being, in which the standards of printed materials (paper and ink) were similar, and the control tone part was no longer It is a dot gain value, but a well-defined neutral gray density value (ND value), so the ISO12647 (G7-NPDC) standard not only specifies a fixed gray balance value, but also specifies the visual density of the gray balance. Therefore, as long as the process according to the ISO12647 (G7-NPDC) standard is controlled and meets the NPDC standard curve requirements, the gray balance is relatively best in terms of visual consistency.
Attachment: The main parameters of process control under the ISO12647 (G7-NPDC) printing standard are as follows:
(1) Paper chromaticity conforms to ISO12647-2 standard (△L: ±3, △a: ±2, △b: ±2)
(2) Ink chromaticity conforms to ISO2846-1 standard (△E≤5 for CMYK, △E≤6 for RGB)
Note: Paper △E≤3, CMYK△H≤2.5 is required to meet the G7 ColorSpace requirements.
(3) Gray balance
C2M1Y1, C4M3Y3, C6M4Y4, C8M5Y5, C10M7Y7, C15M11Y11, C20M15Y15, C25M19Y19, C30M23Y23, C35M27Y27, C40M31Y31, C45M36Y36, C50M40Y40, C55M45Y45, C60M50Y50, C65M55Y55, C70M60Y60, C75M66Y66, C80M72Y72, C85M78Y78, C90M84Y84, C95M92Y92, C100M100Y100
â– Neutral gray density control point HighlightRange (HR)
HR_cmy=ND(50c, 40m, 40y) - ND(paper) = 0.54 ; Absolute HC_cmy= L*57.5
â– Bright contrast Contrast (HC)
HC_cmy=ND(25c, 19m, 19y) - ND(paper) = 0.25; Absolute HC_cmy=L*75.7
â– Dark Contrast ShadowContrast (SC)
SC_cmy=ND(75c, 66m, 66y) - ND(paper) = 0.90; Absolute SC_cmy=L* 39.5
Note: The ND value measurement conditions are no polarizer, paper white and dry.
(4) Network expansion (△Dotgain: ±3~4%)
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