FM screening, also called random screening (usually called "FM screening"), that is, all halftone micro-dots have very small and the same size, each point is as small as a single element of the recording device, it is a graphic recorder or The smallest dot that the printer can produce, but the average number (or frequency) of dots in the unit surface area of ​​the image varies with the copied tone value. In addition, their spatial distribution is carefully assigned by algorithms, and the ideas are distributed according to the statistical estimates of tones and the details of the adjacent parts of the image, without obvious accumulation or accumulation of unwanted micro-points. In fact, the location of the dots is based on "computed randomness". In the traditional amplitude modulation screening (usually called "AM screening") method, the distance between the dots is the same and the dot size is different; FM screening changes the distance between the center of the dots and the dot size is the same according to the brightness and blackness of the input pixels.
The concept of FM screening has been proposed many years ago, but it was not until 1993 that lithographers were able to obtain their commercialized products. Since the first manufacturer announced its FM network products in the spring of 1993, many new manufacturers have also launched their own The corresponding product. Although the advantages of FM screening are commendable, the actual use of this technology has been developing slowly. Many printing customers are interested in FM screening, but few printing companies and plate manufacturers use this process as a fixed part of production. .
FM screening technology, as a newer technology than AM screening technology, has many advantages:
â‘ The very small dot image produced by FM screening looks smoother, the boundary definition is enhanced, and more details can be displayed;
â‘¡ The size of the image file is 25% of the amplitude and screen file. (Same image quality);
â‘¢ No screen moire will appear, the color definition is clearer;
â‘£Low-grade scanning and lower output resolution;
⑤ It is faster to print on the printing machine without adjustment;
â‘¥ It is easier to maintain the balance of water and ink;
⑦ Thicker ink coverage can get a higher dynamic range;
⑧ Short drying time;
⑨Ink distribution is more uniform in the thinner layer;
â‘©The registration problem does not affect the color balance;
⑾The screen printing of spot color and primary color version of overprint will not appear moire, can achieve high fidelity color (HiFi color) and spot color printing plate to improve the color gamut of printed matter;
â‘¿ Eliminate the tone jump of the middle tone.
Because of these advantages, FM screening technology is considered to be a better method to replace traditional AM screening. But looking at the actual situation in China at present, you will find that the application of FM screening technology is still loud, and the rain is small. It takes a long period of difficult exploration to fully exploit the potential advantages of FM screening technology. Due to the algorithm of FM screening itself The reasons for this, as well as other factors, make FM screening technology difficult to popularize:
1. Expanded outlets. Dot expansion occurs around the dots, but a large dot is divided into several small dots. The perimeter and area ratio increase. The larger the perimeter, the more the dot expansion value increases. For any given dot density, the perimeter in the FM network Bigger than the circumference in traditional screens. Extra dot expansion is an inherent characteristic of small dots. The mid-tone dot circumference is larger than the 1 / 4-order tone, so there are more dot expansions. In the 200l screen, the perimeter of the 40% dot increases by 1μm, and the dot size increases by 1%.
The expanded range of dots in halftone images is 18% -25% according to SWOP (Web Offset Press Standard) regulations. The images with FM screen will have a particularly large dot expansion, which is 25% -35% when printed on coated paper, and as high as 50% on uncoated paper. This problem is difficult to overcome with FM screening, although some printing machinery manufacturers provide software-based conversion curves to enable their equipment to compensate for the expected expansion of the dot, it changes the reproduction curve of the imagesetter itself, and This is also only a link in the expansion of the dot. The basis of the compensation curve is only "expected", and it is always impossible to compensate according to the specific characteristics of each manuscript and the specific printing press, which leads to errors at the same time.
2. Has a grainy feel. The appearance of "grain" or sandpaper is the result of the random distribution of FM dots, especially in the 1 / 4th tone area of ​​the screen. The amplitude modulation grid is regular, so the noise generated by it is easy to filter out, while the FM grid is Random, random noise is not filterable. Ideas in the 1/4 tone area are easily noticeable and produce a grainy appearance, but they have not yet merged with each other. This situation is more likely to occur in image areas with an idea size greater than 20 μm and very smooth.
Combining AM and FM networks in one image may be the best way to reduce the graininess in the future. It is best to use an amplitude modulation screen for the smooth tone area of ​​the FM network image to maintain the smooth appearance of the screen. The tone transition area is best to use the FM network to copy as much detail as possible, especially in the highlights and about 25% tone. Since there is no special standard boundary for the manuscripts adapted to these two screening technologies, this combination method is not for professionals, and according to some professionals, the output errors caused by this combination are more difficult to troubleshoot.
3. The tolerance of the whole production process becomes smaller. Due to the expansion of the dots and the smaller dots, the FM dots cannot tolerate dust in the working environment when they are output, and the tolerance is usually smaller. The problems caused by dust and dirt in the amplitude modulation screening process are not large. Caused great problems. The entire work area must be kept clean to avoid the appearance of fine particles during reproduction. When the entire system uses small-sized dots, small dust particles will cause defects that cannot be ignored. The micro dots in the FM network are equal to or smaller than the dust in the AM network. According to the traditional method, overexposure plates can eliminate them, which is very obvious when the FM screen is added and cannot be eliminated by overexposure, because this It will cause small dots to be exposed together. When printing, you need consistent and uniform vacuum pressure to ensure consistency between plates and the entire plate. Slight changes in vacuum pressure will cause "hot spots", which is more than that in AM The same "hot spots" that are obtained are more striking. The environment of the printing workshop also becomes very critical when printing after the plate is released. Paper wool, particles in the air and flying fibers can cause the printing plate or rubber cylinder to become dirty and make the printed product obvious. Defects.
Because the effect of increasing the thickness of the ink film on the size of the FM dot is not as significant as the amplitude modulation screen, it is difficult to improve the quality of the image with poor color separation on the printing press through ink adjustment; in the printing process, adjust the ink to compensate for the scanning. Defects are simply impossible, because the dots are too small, and it is still difficult to print normally, and it is easy to paste. If you want to improve the density caused by scanning by adding ink, it is very easy to adjust by printing in AM screening because The amplitude modulation network has large ideas and large dot spacing, and there is room for adjustment, but the FM network does not. Having said that, I ca n’t help but want to mention a few questions about the working environment. I ’ve been to a lot of pre-press, printing, and post-press processing. The pre-press working environment is acceptable, but printing and post-press processing are difficult for people. Compliment, there are not only a few printing and post-press processing operations in a large workshop, nearly ten machines such as printing machines, paper cutters, die cutting machines, laminating machines, binding machines, etc. are gathered together, dust, smoke, The strange smell of chemical gas, static electricity, noise, etc., can not be imagined here how to stop the machine to remove dirt and the uneven ink color caused by the problem, how to make the printer master burnt, the same is true of other operators.
Although this is a digression, at least in such a "bad" environment, it is still difficult to print high-quality printed products with AM-screened printing plates, not to mention the printing plates that have been printed with FM-screened screens, which have strict environmental requirements. . This is like having a high-speed car but no highway, the car still can't speed up. While adopting new processes and new equipment, the printing industry is facing the challenge of environmental issues, which is also a serious topic of global concern.
4. Film characteristics. High-contrast film is critical to suppress dot expansion, because there may be partial exposure areas around the fine FM network. All films have a part of the exposed transition area around the fully exposed dot, which will cause the dot size to become larger. The FM outlet itself is very small, and its slight expansion will cause significant effects. The use of high-contrast film can reduce the partially exposed area, thereby reducing the potential factor of dot expansion at this stage of the process. Because the inherent latitude in the FM network replication process is very small, the film with the widest exposure range can get the best dot reproduction. However, no matter whether DuPont, Kodak and other films are using FM screening, these manufacturers are gradually improving their product performance.
5. Proofing. The current proofing technology is a conventional proofing technology that uses analog materials. It is used to accurately reflect the changes in the structure and size of amplitude modulation dots on the printing press. The behavior of amplitude modulation dots is fundamentally different from that of FM dots. behavior. Accurate FM network proofing is difficult to achieve, and users cannot adjust the traditional proofing system to reflect the behavior of the FM network point. Precise proofing of the FM screen image is still an unsolved problem, because the existing proofing system can not reproduce the extremely small dot size or reflect the excessive dot expansion, only to develop a suitable proofing method, FM Only screen technology can be recognized. Some analog systems predict the behavior of FM dots more accurately than others, but the granular toner used in these analog systems is too large to reproduce the fine printed dots used in FM screening and cannot be accepted as contract proofing. The future of FM network proofing lies in electronic proofing, because electronic meshless proofing is similar to the way in which dots are placed on substrates in FM screens. This may produce good results, but electronic proofing technology is still not perfect.
6. The dot size is too small. The FM screen processing method provided by the imagesetter manufacturer generally allows only one or two fixed dot sizes at a given output resolution. These dots are used in uncoated paper and newsprint or for screen printing The time is too small, and the expansion of the network at this time is difficult to control. Too small dots are also one of the reasons for noise interference in low-contrast images, but the increase in dots will make the graininess of the image stronger. The smallest micro-dot size is too small, and many printers will not reproduce correctly. Even if small dots can be obtained on the printing plate, it is difficult to accurately transfer the ink to the paper, because the small dot size is only operated by experienced operators on new and expensive printing machines. This kind of replication is feasible.
7. The point of view of printers and customers. For printers, the use of FM screening technology means that the purchase of RIP with the ability to calculate and randomly place dots requires an additional investment, and with it the additional harsh working conditions, environment and staff The improvement of the quality requirements has increased investment, and the printers also considered whether these additional investments will bring them greater profits when investing. They will not be easy for customers who do not have fixed requirements to use FM screening. Investment. Even if the printer invests the money, the investment cost usually exceeds the additional cost that the customer pays for the advantages of random screen output. Because the high quality required by most customers can be achieved through amplitude modulation screening technology with a high-quality output of more than 200LPI, they will not pay extra money for not providing them with many affordable methods. Even if the customer is willing to pay extra money, it is not that the final structure of the image output with FM screening is better than the result with AM screening. For example, when printing a large area of ​​flat solid color, the result of FM screening is The sensation of showing stains is not felt when printing with AM screen printing.
The above is an analysis of the potential shortcomings of FM screening technology and the reasons why it cannot be popularized. In fact, there are many other factors, such as the economic status of the country and the quality of the entire country. Although the use of FM screening technology has so many defects and factors It affects its popularity, but as a new technology, it has unparalleled advantages over traditional screening methods, but there are currently many external conditions that affect its use, and it still has many areas to be improved, such as Many people are still researching better screening algorithms. These algorithms are based on the advantages of combining AM and FM networks. Some algorithms have been well applied locally, such as those specifically used in screen printing. Wait.
We have reason to believe that FM screening technology, which is currently difficult to popularize, will eventually be recognized by most people with the development of science and technology, especially the development of printing technologies such as waterless offset printing and direct plate printing.
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