Screening is the basis of color printing reproduction. The color, gradation and clarity of the printed matter are reproduced by the dots with certain characteristics. The reason why the digital proofing is screened is to copy the proofs that can better predict the subsequent printing results, including whether there are moire, Rose spots, expanded outlets, etc., really do what you see is what you get.
1. Screening location
In the digital proofing workflow, the specific location of digital screening will vary with the post-RIP proofing and the pre-RIP proofing.
In the workflow of data proofing after receiving RIP, the digital proofing system directly receives the digital screen data for phototypesetting after RIP, and converts the extracted screen information data into digital screen data for inkjet printers, and controls the printing equipment to print Equivalent result of screen printing in China. That is, the digital proofing system after RIP needs to convert and control the screen after RIP.
2. Type of screening
At present, the screening technologies used in digital proofing generally include AM screening, FM screening, and hybrid screening.
1) AM screen
Amplitude modulation screening is a traditional screening method, and it is also called AM screening. Its characteristic is that the dot intervals are fixed, the ink dots are concentrated in the center of the grid, and its area is a function of tone information. In the case where the position and arrangement angle of the dots are fixed, the size of the dots is used to express the depth of the image.
Due to the fixed frequency, the amplitude modulation network forms a fixed arrangement of dot matrixes, and there are limitations on the number of dot lines and the dot angle. The screen angle of the two color versions needs to be controlled at 30 degrees or above to avoid moiré. Amplitude modulation dots not only have the advantages of stable printing reproduction, easy and good uniformity, but also have the problems of easy generation of net flowers, interference between moiré patterns, color jumps, text and subtle levels of edges and other problems. Overprint accuracy.
2) FM screening
FM screening is also called FM screening, which is a brand new technology after the AM network. It is characterized by randomly distributing inked particles of the same size in a unit area, where the density of distribution, that is, the spatial distribution frequency is a function of image tone information. The FM dots are the same size, with a diameter of 8-30 microns, which is much smaller than traditional dots. The distribution of dots is uneven, and the density of the dots within a unit area changes to express the depth of the image.
FM screening, as a newer technology that appears later than AM screening, has many advantages. In the FM screened image, there are only a certain number of randomly distributed element points in the unit grid, and the traditional dots cannot be seen on the whole. The distribution of element points is irregular, so there is no problem with the angle of the mesh, which fundamentally eliminates interference fringes and mid-level layer jump phenomena, and the layer reproduction is even and beautiful. Since the element points are very small, the angular power and sharpness of the image are greatly improved, and the subtle levels of the high-resolution AM network can be obtained with low-resolution equipment. Element points at high-profile have less overlap, which can increase the chroma of graphics. Since there is no concept of screen angle, the registration problem will not affect the color balance, the requirement for overprint accuracy of the device is reduced, and the possibility of copying high-quality extreme color samples for mid-range devices is provided. In addition, the FM screening file of sample image quality is smaller than the AM network file, which can shorten the transmission and processing time of the data back end.
Due to its outstanding advantages, FM screening has attracted people's attention as soon as it appeared, and is considered to be a better method to replace traditional AM screening. It has developed rapidly in the past few years. Many foreign companies have successively launched their own FM screening technology, which has excellent development prospects in offset flexographic printing, news printing and other fields. But in China, the popularization and application of FM screening technology is not satisfactory due to many reasons, especially the FM screening algorithm and other factors. The shortcomings of FM screening are:
The random distribution of dots makes the highlights and flat areas easy to produce rough and grainy. Especially in the quarter-tone area. This random noise is even more obvious and cannot be filtered out.
The perimeter of the mid-frequency FM network is larger than the perimeter of the AM network, and the density of the dots is extremely high, making it difficult to control the expansion of the dots. Although the final screen file generated is small, due to the small dots, the calculation speed of the screen is slower than amplitude modulation screen.
Due to the expansion of the dots and the smaller dots, the stability of the FM dots at the time of output is poor, and it is now more difficult to copy than the AM screen. [next]
Highlight areas are easy to lose, and dark areas are easy to paste.
When the same image is printed, the FM screening and AM sampling consume more ink.
The digital sample draft using FM screening is not satisfactory to people because of the above reasons, and it is also restricted by factors other than itself, so it encountered some obstacles in the promotion process.
Restricted by traditional printing amplitude modulation and screening methods. In traditional printing, it has become accustomed to use amplitude modulation dots, so the digital proofing that replaces the traditional proofing task must realize the seamless connection of the printing process to serve the main process of traditional printing. Especially in the transitional stage where CTP has not been widely used now, users also request to be able to obtain proofs of the screening characteristics. If the digital proofing is different from the AM and FM screening, it cannot accurately reflect the changes in the structure and size of the AM dots on the printing press, it cannot avoid the errors that may occur in the back-end printing, nor can it reflect the actual number of lines printed, which is not conducive to the best printing conditions The determination therefore restricts the popularity of proofs for FM network proofing.
Constrained by traditional offset proofing habits. Some printing staff of Printing 3 have been contacting AM sites for decades. After an experienced worker visually inspects a traditional offset proof, he knows where to add ink, and how much ink is added to control the FM network proofs. It is difficult to adapt and creates an instinctive resistance. Many printing companies even reject digital proofs using FM outlets. However, after being familiar with and accustomed to digital proofing and having some experience, workers in printing houses are also very welcome to digital proofing.
In response to the domestic and international demand situation, manufacturers that use color management software and color print tops to form digital proofing systems have focused their research on digital proofing software on how to realize amplitude-modulated analog outlets that meet printing requirements on color printers. For example, Blackmagic's imitation amplitude modulation real dot technology is the final printed proof and printed matter, consistent in color and dot conditions. Including the moire that will appear in the printing process, and the dot expansion, etc. can also be pre-checked before printing. This is a hybrid screening technology.
3) Hybrid screening
Hybrid screening is a screening technology that combines the advantages of AM and FM for high-precision printing. In the highlight area and the dark area, it is like FM screening, and the level changes in the picture are expressed by the density of fine dots of the same size. The positions of the dots are randomly distributed and subjected to special calculations so that the dots do not overlap each other or the distance is too large. In order to adapt to the printing, an additional calculation method is added. D? D uses multiple small dots to form a larger printing dot.
In the midtone area, the location of the dots is random, and at the same time, the size of the dots can be changed like AM screening. Therefore, the dots in the middle part have both the distribution characteristics of FM dots and the tone expression method of AM dots.
The hybrid screening combines the two screening structures AM and FM into one body and exhibits the following characteristics:
Due to the special treatment of the dot position in the highlight and dark areas, the particles have a lot of FM reduction. In addition, the combined calculation processing is performed on the small dots, which improves the recurrence and stability of replication.
The midtone area avoids the color jump in AM because of the randomness of the dot positions. In addition, since the number of halftone dots is fixed, the density of halftone dots can be calculated from the number of halftone dots contained in each square inch, so that the halftone dot density can be defined.
For the printing process, the production process and equipment with conventional precision can be adopted to realize the printing quality of high line number dots, and the production efficiency is not affected.
Hybrid screening technology came into being with the advent of the CTP era. For example, FAIRDOT and screen screens must be screened, which are two hybrid screen technologies for CTP that are aimed at high-precision printing. As the core of CTP technology, digital proofing has also launched its own hybrid screening proofing technology, which is imitation amplitude modulation real dot technology. This hybrid screening technology can not only show the colors vividly, but also show the flaws in the CTP process flow at a glance. Therefore, hybrid screening will definitely be the best choice for future image copying and digital process inspection.
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