Re-discussion on three printing color calibration methods and their production applications
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The color management work of the printing house contains a lot of content. From the process of file receiving, modification processing, output and printing, it is roughly the color management of the file, the color management of the screen, the color management of the digital proofing machine, and the traditional printing. The color management of the machine, the factory of the digital printing machine also needs to do the color management of the digital printing machine, and the color change of the post-press processing is also one aspect. Maintaining the accurate delivery of color throughout the process, minimizing color loss and large variations, and maximizing color consistency is not only the work of color management workers, but also the color participants need to pay great attention.
The final output device is the terminal for realizing color, and all the work is limited by it. From this point, it can be said that the color management work of the printing device is the core and the key. This article will focus on the color correction method of printing equipment, or the method of color management.
In my opinion, the color management of traditional printing should include equipment adjustment, color calibration, production equipment characterization files, color conversion and post-maintenance maintenance. It can be said that color calibration is only part of color management.
The international printing standard ISO 12647-2 is the only standard for international lithographic offset printing, with broad consensus and application. It stipulates the factors that mainly affect the color, such as the color of the substrate (paper), the hue of the solid ink, and the tone reproduction curve (the dot enlargement). Based on these conditions, each printing company can get a factory standard, but each enterprise equipment, materials, practices, etc. are not the same, although there is a general guiding standard program, but still can not guarantee the color uniformity Sex.
Therefore, some internationally renowned regional printing organizations, including Fogra, Ugra, IDEAlliance, etc., have conducted extensive and rigorous printing tests in various regions based on ISO 12647-2, and summarized, analyzed and optimized the results. And to develop specific specifications, relevant organizations (such as ECI) will also publish ICC profile documents, because they are all authoritative and representative, usually regarded as the specific standard or regional norms of printing. They played a very important role in the specific advancement and implementation of ISO.
So how can these specific specifications be achieved? ISO/TS 10128 proposes three methods: one is the matching of tone value curves, and the other is the method of calibrating gray balance (achromatic tones). Use of near-neutral scales), the third is the use of CMYK to CMYK (Use of CMYK to CMYK multi-dimensional transforms).
Before introducing these methods, it is necessary to emphasize the calibration of the device itself, or debugging. This is the basis of the foundation, such as the adjustment of the pressure between the rollers, the standard of the roller diameter, whether the blanket or the liner is flat, whether it has been aged, whether the blanket is tightly fitted, and the dampening solution (water bucket) is blended. Is the ratio appropriate? These are basic work, but have a crucial impact on the final print quality. It is recommended that the printing factory adjust the equipment to the standard state as much as possible according to the requirements of the equipment supplier, and record the relevant parameters in detail, and perform color calibration and management on the basis of the standard state, so if the color is deviated, the printing equipment will have The basis for inspection and adjustment is also convenient for quick color recovery. In addition, maintaining equipment stability and standardization during testing and even production is critical and one of the important factors for successful color management. The following is based on this, and its impact will not be repeated.
The first method: matching the dot gain curve.
When the offset printing machine is embossed, the blanket will be deformed, and the edge of the dot will expand to the periphery, thereby generating a change in the dot area and forming a dot enlargement (also referred to as mechanical dot enlargement). The ISO 12647-2:2013 new standard specifies the dot gain curve under various conditions, so the calibration can be calibrated with reference to the corresponding TVI curve. For example, the printing condition PT1 specifies the corresponding CMYK and RGB hue. If AM printing is used, CMYK is specified to use the dot enlargement curve A, that is, 50% of the dots are expanded to 16%.
The expansion of outlets is affected by many factors, such as printing pressure, ink viscosity, paper condition, dampening solution, etc. Even if the equipment is carefully adjusted, it may not be guaranteed to be completely within the scope of ISO requirements. In particular, ink density is also a key factor.
At present, the confirmation of ink density has basically been referred to the ISO standard. In most cases, the printing contrast is not used to determine, but the ink density is adjusted according to the Lab value to achieve the closest target Lab value. density. The density required for different inks to reach the closest target Lab value is not the same, and objectively affects the dot gain value.
At this time, according to the measurement results of the printed test sheets, counter-compensation of the publishing outlets is an effective method. For example, in the figure below, blue is the ISO standard dot expansion target curve, orange is the measurement curve, how to calculate the 50% compensation value?
At the 50% horizontal axis, the vertical upward line intersects the target curve, then the horizontal measurement line is crossed to intersect, and finally the vertical axis is crossed downwards. The horizontal axis is 45%, which is the compensated dot value. Under the same conditions, 45% of the printed results just reach the target curve that should be reached by 50%, and the compensation data of the entire tone can be calculated in turn. This method is called "inverse function operation" in mathematics. This is only a calculation of the monochrome version, and the compensation curves of the other three color versions can be calculated in turn.
This method has been used for a long time and is calibrated according to the requirements of ISO12647. However, other parameters must be consistent with the requirements of ISO to get better results. For example, the color sequence must be KCMY, the materials used, the printing method, and the measurement method. Must be consistent. Especially for matching Fogra data sets, this method is preferred. For example, the printing conditions of Fogra 51 (released in 2015, according to ISO 12647-2:2013, which may gradually replace Fogra 39 in the future) are as follows:
This method is popular in Europe and China. For example, the Prinect process in Heidelberg defaults to front-end curve correction according to the way the dot is enlarged. Especially in recent years, along with the popularity of related digital printing evaluation software, such as PressSIGN, after measuring the ribbon, the TVI (Tone Value Increase) compensation curve can be directly generated, which greatly improves the efficiency and convenience of calibration.
Some industry certifications, such as Fogra/Ugra PSO and GMI certification, require that outlet expansion must be within tolerance. However, this method is based on the individual adjustment of the monochromatic printing plate. Although the relative difference of the CMY dot enlargement is also required, it is often troublesome in practical operation, and it is difficult to obtain a good gray balance performance. At the same time, due to different materials using different dot expansion curves, it also caused color inconsistency.
The second method: use gray balance (non-color tone) for color correction.
First of all, what is gray balance? Before we can answer what is gray? It is definitely not color, not a separate C/M/Y. Is that the middle note of pure black ink? Obviously not. The gray balance referred to here is the non-color tone achieved by overprinting the CMY three colors at an appropriate dot ratio over the entire tone range. If it is biased toward a certain color, it is considered to be out of balance.
ISO also defines the specific dot composition, but it is not detailed enough to explain the gray balance, but it is more troublesome in actual operation. Here, we have to mention a way to get the general recognition of the printing industry, that is, G7. This method originated in the United States and was developed by Don Hutcheson. It was supported and recognized by the GRACoL working group of IDEAlliance. It quickly became popular in North America and became a regional standard. In 2006, IDEAlliance and Hong Kong APTEC began testing in Greater China and got After the praise, it was quickly promoted.
At present, more than 4,000 companies in the world have obtained G7 Master certification. More than 280 companies in Greater China have successively conducted G7 certification, and more than 200 G7 experts have been trained to promote and certify G7 technology.
G7 clearly defines the gray calculation method, which is related to the substrate material. It also defines the gray dot ratio and designs the corresponding test chart P2P. After P2P23 and P2P25 development, the following figure is more suitable for large-format inkjet printing calibration. The latest version of P2P 51.
In addition, G7 also defines the gray tone, which determines the depth of the tone, ie the neutral print density curve (NPDC). The horizontal axis is the ratio of the original dot, and the vertical axis is the density value. The NPDC is divided into two pairs, one is the density curve corresponding to the three-color ash composed of CMY in the fifth column of P2P, and the other is the density curve of the single-K color corresponding to the fourth column of P2P. In fact, it reflects the tone reproduction characteristics under different field densities. It can be seen from the figure that although the density in the field varies greatly, the density of the highlights below 25% tends to be the same, that is, the same brightness contrast (HC), for most printing methods, the density is from 1.0-1.6, basically It can achieve the consistency of mid-tone and high-tone tone, that is, similar brightness range (HR). At the same time, G7's definition of gray balance is based entirely on paper, which gives a visual gray balance with respect to paper. This is the advantage of G7 calibration, which can achieve a shared appearance of various printing methods.
The G7 calibration method can be hand-painted, measure the density value of each tone and compare the target value, similar to the TVI method to calculate the C and K compensation curves, M and Y need to go to the specially designed gray search test chart GrayFinder Finding and plotting the compensation curve in a single point is relatively cumbersome and inefficient. Of course, it is also possible to calculate the software. Currently, in addition to the official original CHROMiX CurveTM software, more and more software supports G7 gray calibration methods, such as Bodoni PressSIGN, Heidelburg Color Toolbox, Alwan Printing Standardizer X, Caldera Print Stand Verifier G7, ColorGate Production Server, FUJIFILM ColorPath Sync. , KonicaMinolta ColorCare, Mutoh G7 Calibrator. In practice, most companies use software calibration. Simply measure the P2P data, then import the data into the software, and the software can calculate the compensation curve, which is very fast.
G7 adopts gray balance calibration method, which is widely used in various printing methods using CMYK. It is not limited to printing materials, and the printed materials have a shared visual appearance after being calibrated. It is also favored by printing buyers, due to the approval of printing buyers in recent years. With the promotion of G7 technology by organizations such as APTEC in Hong Kong, more and more companies are beginning to be keen on this approach. And the seven reference print condition data sets (CRPC) sets in ISO 15339 are based on this type of calibration, and I believe that in the future, more standardized applications will be obtained.
Newsprint 1, newsprint 2, matte uncoated 3, super calendered paper 4, magazine publishing 5, offset printing 6, digital printing 7,The third method: CMYK direct conversion CMYK method.
In most cases, this method requires a specific Device Link Profile, which is the device association profile. This profile is to calculate the ICC of two devices or printing conditions, one set as the source, ie the target color gamut, one Set as the output device, the intermediate color space PCS (Public Color Space) is omitted, and the conversion from the target color gamut to the output color gamut is directly realized.
For conventional ICC profiler color conversion, you need to convert the device color space, such as RGB or CMYK, to a device-independent color space, usually a Lab or XYZ color space, and then convert to another device color space, such as RGB or CMYK. . After two conversions, the color must be lost. The CMYK to CMYK method eliminates the intermediate PCS. Its advantage is that the color conversion is more precise, and CMYK can be controlled separately, keeping the purity of the primary colors, especially the black channel. This is significant and can avoid single black text. And the thin line is converted into four-color black and the overprinting fault is not allowed. In some professional software, such as CGS ORIS Pressmatcher, etc., it can also control the more useful parameters such as the starting point of black addition, black width, etc. The addition of color control makes the printing process easier, the gray balance performance is more stable, and the ink saving (GCR/UCR based) is achieved.
In the field of traditional offset printing, this conversion method can maintain the linearity of publication, that is, there is no need to make a publication compensation curve, and only need to convert the file to achieve accurate color matching.
Since the target can be set to the color gamut of the printing industry standard ICC or a certain printing condition, the output device is obtained in the field test, so that the gamut matching of different printing conditions can be realized. And because it is a point-to-point color accurate conversion, the entire IT8 or ECI2002 color block has more than 1000 color blocks one-to-one correspondence data generation table, different from the single channel control technology of the above two methods, so you can get closer color The matching effect is mainly applied to the color matching of high precision requirements of different equipments, different materials and different printing methods.
At present, in this way, there are only a few dozen large and medium-sized customers in the traditional offset printing field as high-end color management in production, such as Hong Kong-funded Lifeng Accor, Shenzhen Zhongwan. Of course, the benefits can be better than the curve adjustment results, the drawback is that it requires a lot of extra investment, and the production schedule should not change frequently.
The application of this method is more common in the field of digital printing, such as HP Indigo, Xerox, KM and so on. The reason is that digital printing does not require CTP platemaking. The software-converted files can be quickly printed to verify the actual effect. Multiple cycles of color calibration can be performed in software such as ORIS PMW.
The application of this method is more common in the field of digital printing, such as HP Indigo, Xerox, KM and so on. The reason is that digital printing does not require CTP platemaking. The software-converted files can be quickly printed to verify the actual effect. In ORIS PMW and other software, multiple cycle color calibrations can be performed to achieve closer color matching.
Since digital printing still has a certain degree of color fluctuation, the stability is not very satisfactory. In practical applications, this method can also maintain the color consistency of digital printing, and in some applications using digital printing machines for proofing, The color requirements are higher, and even ISO 12647-7 is used for evaluation. Due to the accurate color matching effect of this method, it plays an irreplaceable role.
In short, ISO TS10128 proposed and summarized three more effective color matching methods in 2009. It has been widely applied and popularized in recent years and is still very effective today.
All three methods have their own advantages and scope, and they all have their own limitations. See the following comparison table for details. In practical applications, it is necessary to combine and analyze the printed matter to control the color quality in the production process more effectively.