How does the print color calibration work? These 3 ways can help you solve it quickly
We are a big printing company in Shenzhen China . We offer all book publications, hardcover book printing, papercover book printing, hardcover notebook, sprial book printing, saddle stiching book printing, booklet printing,packaging box, calendars, all kinds of PVC, product brochures, notes, Children's book, stickers, all kinds of special paper color printing products, game cardand so on.
For more information please visit
http://www.joyful-printing.com. ENG only
http://www.joyful-printing.net
http://www.joyful-printing.org
email: info@joyful-printing.net
In my opinion, the color management of traditional printing should include equipment adjustment, color calibration, production equipment characterization files, color conversion and post-maintenance. It can be said that color calibration is only part of color management. Stabilization and standardization of equipment should also be maintained during testing and even during production. The three methods described below are based on this, and the influence of the printing device will not be described again.
Method 1: Match the dot gain curve
When the offset printing machine is embossed, the blanket will deform to make the edge of the printing dot spread to the periphery, resulting in a dot gain (also called "mechanical dot gain"). The new standard ISO 12647-2:2013 (hereinafter referred to as "ISO standard") specifies the dot gain curve under various conditions, in which A, B, C, D, E are the dot gain under different printing conditions. Curve), the calibration can be calibrated by referring to the corresponding dot gain curve during printing calibration.
The dot gain is affected by many factors, such as printing pressure, ink viscosity, paper condition, dampening solution, etc. Even if the device is carefully adjusted, it does not necessarily guarantee that it is completely within the scope of the ISO standard.
It is worth mentioning that the ink density is a key factor affecting the increase of printing dots. 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. Good density. The density required for different inks to reach the closest target Lab value is not the same, and objectively affects the dot gain.
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, the blue is the target point of the ISO standard to increase the target curve, and the orange is the curve obtained by actually measuring the printed matter. How to calculate the dot compensation data at the input dot percentage of 50%?
At the 50% horizontal axis, the vertical upward line intersects the target curve, and then the intersection point is horizontally drawn to the measurement curve to make the two intersect. Finally, the intersection point is vertically underlined to intersect the abscissa axis, and the intersection point is 45%. The dot value, that is, under the same conditions, the dot of the prepress file with a dot percentage of 45% is printed to just reach the 50% dot area rate, just to meet the target curve requirement. By analogy, the entire tonal compensation data can be calculated. This method is mathematically called "inverse function operation". According to this method, the dot compensation data of the other three color plates can be calculated.
This method has been used for a long time and is calibrated according to the requirements of the ISO standard. However, other parameters must be consistent with the requirements of the ISO standard in order to obtain better results. For example, the color sequence must be KCMY, the materials used, the printing method, The measurement method must also be consistent. This method should be preferred, especially when matching Fogra data sets. By the way, the Fogra 51, released in 2015, was established in accordance with ISO 12647-2:2013 and may gradually replace Fogra 39 in the future.
Method 2: Calibrate the gray balance
First of all, what is gray balance? Gray balance is definitely not color, not separate C, M, Y. Is that the middle note of pure black ink? Obviously not. The gray balance mentioned 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.
The ISO standard also defines the specific network composition, but it is not detailed enough. It also explains the gray balance, but it is more troublesome in practical operation. Here, we have to mention a method that is generally accepted by the printing industry, that is, the G7. 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 calculation method of gray balance, which is related to the substrate material. It also defines the percentage of dots of each color when reaching the gray balance, and designs the corresponding test chart P2P. After the development of P2P23 and P2P25, it is more suitable for large-format spray. The latest version of the ink color calibration P2P51 has also been introduced.
In addition, G7 also defines the gray tone, the density of the tone is determined in density, that is, the neutral printing density curve (NPDC), the horizontal axis is the percentage of the input dot, and the vertical axis is the printing 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 P2P51, and the other is the density curve corresponding to the single-color K in the fourth column of P2P51. 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 to 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 completely based on paper, and the relative gray balance of paper is obtained. This is the advantage of G7 calibration, which can realize the shared visual appearance of various printing methods.
G7 calibration can be performed in a hand-drawn manner. The density values of each tone are measured and compared with the target value. The compensation curves of C and K are calculated similarly to the way the dot is increased. M and Y need to be specially designed for gray search. The single point of the test chart GrayFinder finds and plots the compensation curve. The process is relatively cumbersome and inefficient.
Of course, G7 calibration can also be assisted by software calculation. Currently, in addition to the official original CHROMiX CurveTM software, more and more software supports G7 calibration methods, such as Bodoni PressSIGN, Heidelburg Color Toolbox, Alwan Printing Standardizer X, Caldera Print Stand Verifier G7, ColorGate Production Server, FUJIFILM ColorPath Sync, Konica Minolta ColorCare, Mutoh G7 Calibrator, etc. In practice, most companies use software calibration. This method only needs to measure the P2P data, and then import the data into the software to 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 after calibration, prints have a shared visual appearance, which is favored by printing buyers. Appreciation and the promotion of G7 technology by institutions such as APTEC in Hong Kong, more and more companies are keen on this approach. And the seven reference printing condition data sets (CRPC) in ISO 15339 are based on this calibration method, and I believe that in the future, more standardized applications will be obtained.
Method three: CMYK-CMYK conversion
In most cases, this method requires a specific Device Link Profile, which is the device association profile, which computes the ICCs corresponding to the two devices or printing conditions, one set as the source, ie the target color gamut, and one set to The output device eliminates the intermediate color space PCS (Public Color Space) and directly converts the target color gamut to the output color gamut.
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 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 direct conversion of CMYK to CMYK eliminates the need for intermediate PCS. The advantage is that 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 avoids the single black. Text and thin lines are converted to four-color black and the overprinting is not correct. In some professional software (such as CGS ORIS Pressmatcher, etc.), it is also possible to control more useful parameters such as the starting point of black addition, the width of black tone, etc., because of the addition of black, the color control in the printing process is easier. The gray balance performance is more stable, and it also achieves the goal of saving ink (based on GCR/UCR).
In the field of traditional offset printing, this conversion method can maintain the linearity of the 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 by field test, so that gamut matching of different printing conditions can be realized. And because it is a point-to-point accurate color conversion, the entire IT8 or ECI2002 color block has more than 1000 color blocks one-to-one correspondence data generation table, which is different from the single channel control technology of the above two methods, so that a closer color can be obtained. The matching effect is mainly applied to the color matching of high precision requirements of different equipments, different materials and different printing methods.
The application of this method is more common in the field of digital printing, such as HP, Fuji Xerox, Konica Minolta 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 more accurate color matching.
Since digital printing still has a certain degree of color fluctuation, the stability is not ideal, and in practical applications, this method can maintain the color consistency of digital printing. In some applications where the color requirements are higher and the digital printing machine is used for proofing, even ISO 12647-7 is used for evaluation. This method is used for accurate color matching and plays an irreplaceable role.
All of the above three methods have their own advantages and scope, and they all have their own limitations. See Table 1 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.